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209 Cards in this Set
- Front
- Back
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Polysomes, polyribosomes |
The name for the structure formed by a group of ribosomes and partially translated polypeptides attached to a single molecule of mRNA during the process of translation.
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positive inducible regulation
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A type of gene expression control in bacteria. Transcription of a gene under this type of control is induced only when an specific activator molecule becomes activated.
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|
post-transcriptional modification
|
The enzymatic processing of the eukaryotic primary RNA transcript to produce a mature transcript; production of a mature mRNA requires capping, polyadenylation and intron splicing.
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post-translational modification
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Changes occurring to proteins after they have been translated; may involve covalent modifications such as glycosylation and lipidation or proteolytic cleavage. [Book 2]
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post-translational modifications
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The enzymatic processing of polypeptides after translation to produce the mature functional protein. For example, proteins that are intended for export, or for incorporation into membranes, pass into the lumen of the ER, where they undergo post-translational modification including glycosylation; the addition of short sugar chains to form a glycoprotein. Other types of modification include the addition of other chemical groups (such as phosphates or lipids) or cleavage. [Book 1]
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predation
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The ingestion of one organism by another. Occurs when a heterotroph feeds on another living organism. The feeding heterotroph benefits by gaining organic carbon, other nutrients and energy.
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primary adaptive response
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Slow response occurring when a certain kind of pathogen infects its host for the first time. (cf. secondary adaptive response.)
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primary cells
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Cultured cells derived from a fresh tissue sample. They are only able to divide a certain number of times in culture.
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primary producers
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Autotrophs, mainly photosynthetic green plants and microbes but also chemotrophic microbes, are the primary producers of organic material (or biomass) in an ecosystem.
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primary RNA transcript
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A eukaryotic RNA transcript immediately after transcription in the nucleus, before any processing e.g. RNA splicing or polyadenylation.
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primary structure
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The linear order of amino acids in a polypeptide or protein commonly referred to as the amino acid sequence.
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Primase
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Enzyme that catalyses the synthesis of a short RNA molecule called an RNA primer that initiates the synthesis of a new strand of DNA.
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Primer
|
A short sequence of nucleic acids that serves as a starting point for DNA synthesis by DNA polymerase. In DNA replication, the primer is an RNA molecule. In molecular techniques, synthetic DNA or RNA primers are used to ‘prime’ synthesis of new nucleic acids in vitro, for example, in the PCR reaction, or DNA sequencing.
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Procaspase
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An inactive form of caspase present in cells all the time. In order to become an active enzyme, a procaspase requires proteolytic cleavage to reveal the enzyme’s active site. Involved in apoptosis.
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programmed cell death
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Also known as apoptosis. Type of cell death where particular cell populations die in a reproducible manner in every individual. Because of its predictable nature, this form of death was believed to occur as the result of a death ‘programme’, and so was named programmed cell death. Well-known examples are the loss of the cells between the digits (e.g. during the development of fingers), and in the tail of the tadpole, when it metamorphoses into a frog. In adult tissues, cell death usually balances cell division, ensuring that tissues and organs retain the same size and structure as old cells are replaced.
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Prokaryotes
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The mainly single-celled organisms in the domains Bacteria and Archae.
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prokaryotic cell
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Describing the type of organisation where cells have no nucleus or other membrane-bound organelles; therefore their DNA is not enclosed in a nucleus but is located in an area of the cytoplasm called the nucleoid.
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Promoter
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The nucleotide sequence, upstream of a gene to which RNA polymerase binds in order to initiate transcription.
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Prophase
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The phase of mitosis or meiosis in which the chromosomes start to condense (and become visible), the nuclear membrane disassembles and the mitotic spindle starts to assemble. During prophase, each chromosome consists of a pair of chromatids.
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Proteases
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Proteolytic (i.e. protein-digesting) enzymes that catalyse the cleavage of protein peptide bonds.
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Proteasome
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A large complex of proteins found in eukaryotic cells; the site where most cellular proteins are degraded into peptides and amino acids. Proteins to be degraded by proteasomes are identified by covalent attachment to the protein ubiquitin. [Book 1]
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proteasomes
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Large multi-subunit protease complexes, which are found in the cytosol of eukaryotic cells, where most unwanted proteins are broken down. Proteins to be degraded in proteasomes are identified by covalent attachment to the protein ubiquitin. [Book 2]
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protein kinase A (PKA)
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Also known as cAMP-dependent protein kinase; a serine-threonine kinase that is activated by the second messenger cAMP. It has many targets such as enzymes involved in glycogen metabolism and CREB (cyclic AMP response element binding protein).
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protein kinase C (PKC)
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A serine-threonine kinase that is activated by the membrane-associated second messenger 1,2-diacylglycerol (DAG). In the absence of stimulation, PKC is in the cytosol in an inactive form and activation requires the enzyme to translocate to the cell membrane. Activation of appropriate trimeric G proteins (via their GPCRs) results in the generation of the second messengers inositol 1,4,5-triphosphate (IP3) and DAG, through the action of phospholipase C. IP3 stimulates release of Ca2+ from intracellular stores. The increase in cytosolic Ca2+ levels causes PKC to translocate from the cytoplasm to the inner side of the plasma membrane, where it is activated by DAG. Once activated, PKC phosphorylates downstream targets.
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protein sorting
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Also called protein targeting. The delivery of newly synthesised proteins to the correct destination within the cell or, in the case of secreted proteins, to the exterior.
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Proteinopathies
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Diseases where structurally abnormal proteins within cells can lead to impaired functioning and frequently cell death. Huntington’s and Parkinson’s diseases are examples of proteinopathies.
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Proteins
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Large macromolecules consisting of one or more polypeptide chains built from amino acids. Proteins perform numerous functions in living organisms, for example, some are incorporated into cell structures while others act as enzymes that catalyse chemical reactions.
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proton motive force
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The free energy stored in an electrochemical proton gradient such as that which is established across the inner mitochondrial membrane or the thylakoid membrane in chloroplasts. It is this that drives synthesis of ATP in mitochondria and in chloroplasts.
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Proto-oncogene
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Normal gene that encodes a protein involved in cell proliferation. When mutated it can form an oncogene, which can cause abnormal cell proliferation and hence promote cancer.
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Psychrophiles
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‘Cold-loving’ microbes having very low temperature optima.
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Pure-breeding
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Organisms that possess a particular characteristic or trait and which, when interbred over a number of successive generations, produce offspring which all possess that same trait. The genotype of a true breeding organism is homozygous.
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pyruvate dehydrogenase complex (PDC)
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The large multienzyme complex that catalyses the conversion of pyruvate into acetyl CoA in the link reaction in the mitochondrial matrix.
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quaternary structure
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The highest level of organisation of protein structure; refers to the arrangement of the subunits in a multimeric protein.
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quorum sensing
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A density-dependent regulation system in bacteria. For example, the enzyme luciferase of bioluminescent bacteria is switched on and emits light, only when bacterial populations are of sufficiently high concentration, and the autoinducer reaches a critical concentration. Cultures of bioluminescent bacteria are not luminous at low cell density.
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quorum sensing crosstalk
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Communication between different species using quorum sensing via autoinducer signalling molecules. This allows coordination of bacterial activity within, for example, a biofilm, by regulation of gene expression.
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radioresistance
|
Feature of certain microbes, including the bacterium Deinococcus radiodurans and its relatives, that can survive exposure of up to 30 000 Gy of ionising radiation, a dose that would be expected to split its genome into small fragments.
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random coils
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Irregularly structured stretches of polypeptide that link regions of α-helical or β sheet structures. While coil conformations are highly varied between proteins, they nonetheless have a very consistent conformation in any particular protein.
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Ras protein
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A small (monomeric) GTPase, associated with the inside of the plasma membrane via a covalently-attached lipid group. Ras is activated upon binding GTP, in response to the activation of many types of enzyme-linked cell surface receptors (including most RTKs). It is a member of a very large family of monomeric GTP-binding proteins which resemble the subunits of trimeric G proteins.
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reaction centre (RC)
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Component of the photosystems that are found in the thylakoid membranes of chloroplasts and which are the site of the photochemical reactions. Excitation energy, derived from light energy captured by the light-harvesting complex (another component of the photosystem), is directed towards the reaction centre where it causes each of a specific pair of chlorophyll molecules to eject a high-energy electron which passes to an electron acceptor.
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receptor tyrosine kinases (RTKs)
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Type of cell surface receptor that has intrinsic tyrosine kinase activity associated with its cytoplasmic domain. These receptors typically bind growth factors and some cytokines. They usually act as dimers, each member of the pair phosphorylating the other. Upon activation, the receptors activate phosphorylation cascades, via intermediate adaptor proteins and enzymes that bind to the activated cytoplasmic domain.
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receptor-mediated endocytosis (RME)
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The selective uptake of particular macromolecules and macromolecular complexes into the cell. RME involves specific receptor proteins at the cell surface that recognise and bind the macromolecule. The occupied receptors become clustered together in clathrin-coated pits, which then pinch off to become vesicles in the cytosol, i.e. they are endocytosed. RME thus transports the ligand and the receptor together into the interior of the cell. An example of RME is the uptake of low-density lipoproteins (LDLs) by animal cells.
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Recessive
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Refers to a gene allele whose phenotype is not manifest in a heterozygote if it is masked by the presence of a dominant allele. A recessive phenotype will only be expressed in a homozygote carrying two copies of the recessive allele.
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Recombinant
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In genetics, a cell or an individual with a new combination of gene alleles not found in either parent; usually applied to linked genes (on the same chromosome). In gene cloning, the product of inserting a DNA fragment from one organism into the DNA molecule of another. The resultant recombinant DNA molecule is a combination of the two.
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Recombination
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The exchange of genetic material by the breaking and rejoining of two DNA molecules; can occur between similar molecules of DNA (for example, recombination or crossing over, of chromosomes during meiosis) or dissimilar molecules, as in DNA repair by non-homologous end-joining.
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recombination frequency
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The frequency at which recombination (crossing over) has occurred between two gene loci.
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Redox
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Term describing reactions that entail the oxidation of one substrate and the reduction of another.
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redox potential (E)
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A measure of how attractive a reactant is to electrons; i.e. its electron affinity. Low redox potential indicates low electron affinity, so electrons will be readily transferred from a reactant with low redox potential to one with a higher redox potential. The transfer of electrons from a reactant with a low redox potential to one with a higher redox potential is exergonic and energetically favourable (ΔG is negative); conversely, transfer of electrons from a reactant with a high redox potential to one with lower redox potential is energetically unfavourable (ΔG is positive).
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Reduction
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A reaction in which electrons are added to the reactant; also usually involves addition of hydrogen atoms and loss of oxygen atoms. Reductions tend to be endergonic (i.e. energy is consumed).
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regulatory DNA elements
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Short consensus sequences in DNA that are specifically recognised and bound by DNA binding proteins called transcription factors. These factors either promote the recruitment of RNA polymerase to the promoter, thereby increasing transcription of the gene (transcriptional activators), or repress recruitment of the RNA polymerase and decrease transcription of the gene (transcriptional repressors).
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relative molecular mass (Mr)
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The relative molecular mass (Mr) of a molecule is the ratio of the mass of that molecule to 1/12 the mass of an atom of carbon-12. This value can be calculated as the sum of the relative atomic masses of all the atoms that the molecule contains (where the relative atomic mass of an atom is the ratio of its mass to 1/12 the mass of an atom of carbon-12). Being a relative measure, Mr has no units (it is ‘dimensionless’). Note, however, that relative molecular mass is numerically equivalent to molecular mass which is given in units of Daltons (Da) or kilodaltons (kDa, equivalent to 1000 Da) and, in the case of proteins, it has become more common to indicate protein size in this way.
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replication complex
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A large protein complex comprising a number of molecules and enzymes that are needed for DNA replication and which recognises and binds to origins of replication.
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Polysomes, polyribosomes
|
The name for the structure formed by a group of ribosomes and partially translated polypeptides attached to a single molecule of mRNA during the process of translation.
|
|
|
positive inducible regulation
|
A type of gene expression control in bacteria. Transcription of a gene under this type of control is induced only when an specific activator molecule becomes activated.
|
|
|
post-transcriptional modification
|
The enzymatic processing of the eukaryotic primary RNA transcript to produce a mature transcript; production of a mature mRNA requires capping, polyadenylation and intron splicing.
|
|
|
post-translational modification
|
Changes occurring to proteins after they have been translated; may involve covalent modifications such as glycosylation and lipidation or proteolytic cleavage. [Book 2]
|
|
|
post-translational modifications
|
The enzymatic processing of polypeptides after translation to produce the mature functional protein. For example, proteins that are intended for export, or for incorporation into membranes, pass into the lumen of the ER, where they undergo post-translational modification including glycosylation; the addition of short sugar chains to form a glycoprotein. Other types of modification include the addition of other chemical groups (such as phosphates or lipids) or cleavage. [Book 1]
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predation
|
The ingestion of one organism by another. Occurs when a heterotroph feeds on another living organism. The feeding heterotroph benefits by gaining organic carbon, other nutrients and energy.
|
|
|
primary adaptive response
|
Slow response occurring when a certain kind of pathogen infects its host for the first time. (cf. secondary adaptive response.)
|
|
|
primary cells
|
Cultured cells derived from a fresh tissue sample. They are only able to divide a certain number of times in culture.
|
|
|
primary producers
|
Autotrophs, mainly photosynthetic green plants and microbes but also chemotrophic microbes, are the primary producers of organic material (or biomass) in an ecosystem.
|
|
|
primary RNA transcript
|
A eukaryotic RNA transcript immediately after transcription in the nucleus, before any processing e.g. RNA splicing or polyadenylation.
|
|
|
primary structure
|
The linear order of amino acids in a polypeptide or protein commonly referred to as the amino acid sequence.
|
|
|
Primase
|
Enzyme that catalyses the synthesis of a short RNA molecule called an RNA primer that initiates the synthesis of a new strand of DNA.
|
|
|
Primer
|
A short sequence of nucleic acids that serves as a starting point for DNA synthesis by DNA polymerase. In DNA replication, the primer is an RNA molecule. In molecular techniques, synthetic DNA or RNA primers are used to ‘prime’ synthesis of new nucleic acids in vitro, for example, in the PCR reaction, or DNA sequencing.
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|
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Procaspase
|
An inactive form of caspase present in cells all the time. In order to become an active enzyme, a procaspase requires proteolytic cleavage to reveal the enzyme’s active site. Involved in apoptosis.
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|
programmed cell death
|
Also known as apoptosis. Type of cell death where particular cell populations die in a reproducible manner in every individual. Because of its predictable nature, this form of death was believed to occur as the result of a death ‘programme’, and so was named programmed cell death. Well-known examples are the loss of the cells between the digits (e.g. during the development of fingers), and in the tail of the tadpole, when it metamorphoses into a frog. In adult tissues, cell death usually balances cell division, ensuring that tissues and organs retain the same size and structure as old cells are replaced.
|
|
|
Prokaryotes
|
The mainly single-celled organisms in the domains Bacteria and Archae.
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|
|
prokaryotic cell
|
Describing the type of organisation where cells have no nucleus or other membrane-bound organelles; therefore their DNA is not enclosed in a nucleus but is located in an area of the cytoplasm called the nucleoid.
|
|
|
Promoter
|
The nucleotide sequence, upstream of a gene to which RNA polymerase binds in order to initiate transcription.
|
|
|
Prophase
|
The phase of mitosis or meiosis in which the chromosomes start to condense (and become visible), the nuclear membrane disassembles and the mitotic spindle starts to assemble. During prophase, each chromosome consists of a pair of chromatids.
|
|
|
Proteases
|
Proteolytic (i.e. protein-digesting) enzymes that catalyse the cleavage of protein peptide bonds.
|
|
|
Proteasome
|
A large complex of proteins found in eukaryotic cells; the site where most cellular proteins are degraded into peptides and amino acids. Proteins to be degraded by proteasomes are identified by covalent attachment to the protein ubiquitin. [Book 1]
|
|
|
proteasomes
|
Large multi-subunit protease complexes, which are found in the cytosol of eukaryotic cells, where most unwanted proteins are broken down. Proteins to be degraded in proteasomes are identified by covalent attachment to the protein ubiquitin. [Book 2]
|
|
|
protein kinase A (PKA)
|
Also known as cAMP-dependent protein kinase; a serine-threonine kinase that is activated by the second messenger cAMP. It has many targets such as enzymes involved in glycogen metabolism and CREB (cyclic AMP response element binding protein).
|
|
|
protein kinase C (PKC)
|
A serine-threonine kinase that is activated by the membrane-associated second messenger 1,2-diacylglycerol (DAG). In the absence of stimulation, PKC is in the cytosol in an inactive form and activation requires the enzyme to translocate to the cell membrane. Activation of appropriate trimeric G proteins (via their GPCRs) results in the generation of the second messengers inositol 1,4,5-triphosphate (IP3) and DAG, through the action of phospholipase C. IP3 stimulates release of Ca2+ from intracellular stores. The increase in cytosolic Ca2+ levels causes PKC to translocate from the cytoplasm to the inner side of the plasma membrane, where it is activated by DAG. Once activated, PKC phosphorylates downstream targets.
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|
|
protein sorting
|
Also called protein targeting. The delivery of newly synthesised proteins to the correct destination within the cell or, in the case of secreted proteins, to the exterior.
|
|
|
Proteinopathies
|
Diseases where structurally abnormal proteins within cells can lead to impaired functioning and frequently cell death. Huntington’s and Parkinson’s diseases are examples of proteinopathies.
|
|
|
Proteins
|
Large macromolecules consisting of one or more polypeptide chains built from amino acids. Proteins perform numerous functions in living organisms, for example, some are incorporated into cell structures while others act as enzymes that catalyse chemical reactions.
|
|
|
proton motive force
|
The free energy stored in an electrochemical proton gradient such as that which is established across the inner mitochondrial membrane or the thylakoid membrane in chloroplasts. It is this that drives synthesis of ATP in mitochondria and in chloroplasts.
|
|
|
Proto-oncogene
|
Normal gene that encodes a protein involved in cell proliferation. When mutated it can form an oncogene, which can cause abnormal cell proliferation and hence promote cancer.
|
|
|
Psychrophiles
|
‘Cold-loving’ microbes having very low temperature optima.
|
|
|
Pure-breeding
|
Organisms that possess a particular characteristic or trait and which, when interbred over a number of successive generations, produce offspring which all possess that same trait. The genotype of a true breeding organism is homozygous.
|
|
|
pyruvate dehydrogenase complex (PDC)
|
The large multienzyme complex that catalyses the conversion of pyruvate into acetyl CoA in the link reaction in the mitochondrial matrix.
|
|
|
quaternary structure
|
The highest level of organisation of protein structure; refers to the arrangement of the subunits in a multimeric protein.
|
|
|
quorum sensing
|
A density-dependent regulation system in bacteria. For example, the enzyme luciferase of bioluminescent bacteria is switched on and emits light, only when bacterial populations are of sufficiently high concentration, and the autoinducer reaches a critical concentration. Cultures of bioluminescent bacteria are not luminous at low cell density.
|
|
|
quorum sensing crosstalk
|
Communication between different species using quorum sensing via autoinducer signalling molecules. This allows coordination of bacterial activity within, for example, a biofilm, by regulation of gene expression.
|
|
|
radioresistance
|
Feature of certain microbes, including the bacterium Deinococcus radiodurans and its relatives, that can survive exposure of up to 30 000 Gy of ionising radiation, a dose that would be expected to split its genome into small fragments.
|
|
|
random coils
|
Irregularly structured stretches of polypeptide that link regions of α-helical or β sheet structures. While coil conformations are highly varied between proteins, they nonetheless have a very consistent conformation in any particular protein.
|
|
|
Ras protein
|
A small (monomeric) GTPase, associated with the inside of the plasma membrane via a covalently-attached lipid group. Ras is activated upon binding GTP, in response to the activation of many types of enzyme-linked cell surface receptors (including most RTKs). It is a member of a very large family of monomeric GTP-binding proteins which resemble the subunits of trimeric G proteins.
|
|
|
reaction centre (RC)
|
Component of the photosystems that are found in the thylakoid membranes of chloroplasts and which are the site of the photochemical reactions. Excitation energy, derived from light energy captured by the light-harvesting complex (another component of the photosystem), is directed towards the reaction centre where it causes each of a specific pair of chlorophyll molecules to eject a high-energy electron which passes to an electron acceptor.
|
|
|
receptor tyrosine kinases (RTKs)
|
Type of cell surface receptor that has intrinsic tyrosine kinase activity associated with its cytoplasmic domain. These receptors typically bind growth factors and some cytokines. They usually act as dimers, each member of the pair phosphorylating the other. Upon activation, the receptors activate phosphorylation cascades, via intermediate adaptor proteins and enzymes that bind to the activated cytoplasmic domain.
|
|
|
receptor-mediated endocytosis (RME)
|
The selective uptake of particular macromolecules and macromolecular complexes into the cell. RME involves specific receptor proteins at the cell surface that recognise and bind the macromolecule. The occupied receptors become clustered together in clathrin-coated pits, which then pinch off to become vesicles in the cytosol, i.e. they are endocytosed. RME thus transports the ligand and the receptor together into the interior of the cell. An example of RME is the uptake of low-density lipoproteins (LDLs) by animal cells.
|
|
|
Recessive
|
Refers to a gene allele whose phenotype is not manifest in a heterozygote if it is masked by the presence of a dominant allele. A recessive phenotype will only be expressed in a homozygote carrying two copies of the recessive allele.
|
|
|
Recombinant
|
In genetics, a cell or an individual with a new combination of gene alleles not found in either parent; usually applied to linked genes (on the same chromosome). In gene cloning, the product of inserting a DNA fragment from one organism into the DNA molecule of another. The resultant recombinant DNA molecule is a combination of the two.
|
|
|
Recombination
|
The exchange of genetic material by the breaking and rejoining of two DNA molecules; can occur between similar molecules of DNA (for example, recombination or crossing over, of chromosomes during meiosis) or dissimilar molecules, as in DNA repair by non-homologous end-joining.
|
|
|
recombination frequency
|
The frequency at which recombination (crossing over) has occurred between two gene loci.
|
|
|
Redox
|
Term describing reactions that entail the oxidation of one substrate and the reduction of another.
|
|
|
redox potential (E)
|
A measure of how attractive a reactant is to electrons; i.e. its electron affinity. Low redox potential indicates low electron affinity, so electrons will be readily transferred from a reactant with low redox potential to one with a higher redox potential. The transfer of electrons from a reactant with a low redox potential to one with a higher redox potential is exergonic and energetically favourable (ΔG is negative); conversely, transfer of electrons from a reactant with a high redox potential to one with lower redox potential is energetically unfavourable (ΔG is positive).
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|
|
Reduction
|
A reaction in which electrons are added to the reactant; also usually involves addition of hydrogen atoms and loss of oxygen atoms. Reductions tend to be endergonic (i.e. energy is consumed).
|
|
|
regulatory DNA elements
|
Short consensus sequences in DNA that are specifically recognised and bound by DNA binding proteins called transcription factors. These factors either promote the recruitment of RNA polymerase to the promoter, thereby increasing transcription of the gene (transcriptional activators), or repress recruitment of the RNA polymerase and decrease transcription of the gene (transcriptional repressors).
|
|
|
relative molecular mass (Mr)
|
The relative molecular mass (Mr) of a molecule is the ratio of the mass of that molecule to 1/12 the mass of an atom of carbon-12. This value can be calculated as the sum of the relative atomic masses of all the atoms that the molecule contains (where the relative atomic mass of an atom is the ratio of its mass to 1/12 the mass of an atom of carbon-12). Being a relative measure, Mr has no units (it is ‘dimensionless’). Note, however, that relative molecular mass is numerically equivalent to molecular mass which is given in units of Daltons (Da) or kilodaltons (kDa, equivalent to 1000 Da) and, in the case of proteins, it has become more common to indicate protein size in this way.
|
|
|
replication complex
|
A large protein complex comprising a number of molecules and enzymes that are needed for DNA replication and which recognises and binds to origins of replication.
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replication fork
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The arrangement of DNA at the point where the two strands are being separated and replicated. Two replication forks are formed at an origin of replication, and proceed in opposite directions along the DNA.
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Residue
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In the context of the polypeptide chain, a single amino acid unit; short stretches of amino acid residues linked together are called peptides, and longer chains are polypeptides.
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Resolution
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The smallest distance between two objects that can be distinguished by a particular type of microscopy.
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respiratory complexes
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Each of the four large protein complexes of the mitochondrial electron transport chain (numbered I to IV) are referred to as respiratory complexes and consists of several proteins with associated prosthetic groups.
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respiratory control
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In mitochondria, the coupling of electron transport to ATP synthesis, so that the rate at which fuel is oxidised is matched to need.
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restriction endonucleases
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Enzymes present in many bacteria that recognise particular sequences in DNA and cut the nucleic acid within that sequence (or sometimes a fixed distance from it). The resulting double strand cuts usually have protruding single-stranded or 'sticky' termini which can be rejoined using the enzyme DNA ligase. Restriction endonucleases are widely used in gene cloning.
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restriction point
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The point in G1 phase of the eukaryotic cell cycle at which a cell becomes committed to the cell cycle. Once a cell passes the restriction point, it is committed to completing the rest of the cell cycle. The restriction point is therefore the first of several important checkpoints in the cycle and it monitors several factors, including nutrient availability, cell size and the presence of growth factors.
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retinoblastoma (Rb) protein
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Product of the tumour suppressor gene, Rb, present in the cell nucleus. In its unphosphorylated state, it acts as a brake on the cell cycle by binding to the transcription factors needed for the expression of the molecular machinery for cell proliferation, including those required for DNA replication. When Rb is phosphorylated by G1 cyclin-Cdks, it releases the transcription factors and the cell can progress through the cell cycle.
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reversible covalent modification
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Reversible addition of a small chemical group (e.g. phosphate, acetyl) to the side-chain of a particular amino acid residue in a protein. Such modifications are an important means of regulating protein function and the most common modification is phosphorylation.
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Rho-family GTPases
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A subgroup of the small GTPase family; activation of Rho family GTPases leads to reorganisation of the cytoskeleton as well as affecting cell adhesion, for example by influencing interactions between integrins and the actin network.
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Ribonucleases
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Enzymes that degrade RNA into smaller components, for example those removing nucleotides from the end of the poly(A) tail of an mRNA, gradually shortening it over time.
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ribonucleic acid (RNA)
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A type of nucleic acid which is usually single stranded and differs from DNA in that it contains the base uracil instead of thymine. RNA serves as the intermediary in transferring genetic information between DNA and the protein synthesising processes in the cell.
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ribosomal RNA (rRNA)
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The RNA that, together with proteins, makes up the structures called ribosomes, which are the sites of synthesis of proteins by the process of translation.
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Ribosome
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A particle composed of RNA and protein that is found in large numbers in all cells and is essential for protein synthesis. Using messenger RNA (mRNA) as a template, the ribosome pairs each mRNA codon with a transfer RNA (tRNA) carrying an appropriate amino acid to assemble a polypeptide chain.
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ribosome binding site
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A short consensus sequence on mRNA where the ribosome binds during the initiation of translation.
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RNA interference (RNAi)
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A cellular process that regulates gene expression by the action of small non-coding RNA molecules (microRNAs or small interfering RNAs (siRNAs)) that specifically suppress the expression of a gene containing the same sequence.
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RNA polymerase
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An enzyme that carries out the synthesis of RNA molecules during transcription. Using a DNA strand as template, RNA polymerase adds ribonucleotides to the RNA chain according to the rules of complementary base-pairing.
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RNA splicing
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A post-transcriptional process that removes the non-coding sequences (introns) from the transcribed mRNA to produce the mature mRNA molecule.
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root nodules
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Swellings on the roots of certain plants (e.g. legumes) within which nitrogen-fixing bacteria form endosymbiotic associations and supply the host plants with fixed nitrogen.
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rough endoplasmic reticulum (RER)
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Part of the endoplasmic reticulum where protein synthesis takes place. Ribosomes are hence attached to the RER, giving it a ‘rough’ appearance. It is the site of synthesis of membrane and lysosomal proteins, and also proteins that are to be exported from the cell.
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sarcomeres
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Repeating structural unit in the myofibrils of striated muscle, readily identified by electron microscopy.
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Saturated
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In relation to an organic compound, particularly a fatty acid, this term indicates that it contains only single bonds between the carbon atoms.
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Scanning electron microscopy (SEM)
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A type of microscopy used to study the surface of intact cells and tissues. A sample is coated with a thin metallic layer that deflects an electron beam onto a detector.
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SDS polyacrylamide gel electrophoresis (SDS–PAGE)
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Technique that is used to analyse proteins in a mixture (e.g. a cell homogenate). Proteins are denatured by heating with sodium dodecyl sulfate (SDS) and are then resolved by electrophoresis in a polyacrylamide gel. The negatively charged dodecyl sulfate ions give the unfolded polypeptides a uniform negative charge that masks any intrinsic charge due to charged amino acid residues. This means that the proteins carry a charge that is approximately proportional to their Mr.
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secondary adaptive response
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Greatly enhanced response to the introduction of a pathogen to its host for a second time. It develops sooner, lasts longer and displays greater levels of activity than the primary adaptive response.
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secondary producers
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Heterotrophs growing and producing biomass by metabolising carbon-based compounds derived from other organisms in an ecosystem.
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secondary structure
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Term used to describe a limited number of regular stable three-dimensional arrangements of a polypeptide of which the two most common forms are α‑helix and β pleated sheet. A protein may contain numerous elements of secondary structure and different parts of the chain can adopt different secondary structures.
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segmentation genes
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Collectively, the gap genes, pair-rule genes, and segment polarity genes in Drosophila, so called because they are involved in setting up the location of body segments.
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sequential induction
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Arises from a series of signals between cells, where a signal from one cell determines the fate of a neighbouring cell, which in turn induces the differentiation of a third type of cell, and so on.
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Shut-off
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Thirty minutes after entry of the foot-and-mouth disease virus (FMDV), the host cell’s ribosomes are no longer producing any cellular proteins. This loss of the cellular protein-making machinery results ultimately in cell death.
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signal recognition particle (SRP)
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A protein complex that recognises a specific sequence of translated amino acids emerging from a free ribosome. The sequence is a signal that the protein is to be translated on the membrane of the rough endoplasmic reticulum (RER). Translation is interrupted whilst the whole complex (including the ribosome, mRNA and partially translated polypeptide) is moved to the RER.
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signal sequence
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A short, N-terminal sequence at the end of a protein molecule that serves as its ‘address label’, directing proteins to specific targets in the cell.
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signal transduction
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The process by which an extracellular signal is communicated or transmitted from the exterior of the cell to inside the cell and converted into a form that can bring about a specific cellular response.
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Silencers
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Regulatory sequences in DNA that, when bound by transcriptional repressor proteins, reduce the rate of transcription of a gene or genes; typically located some distance away from the gene(s).
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single nucleotide polymorphisms (SNPs)
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A type of genetic variation consisting of single nucleotide differences between the genomes of individuals.
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single-pass transmembrane protein
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Integral membrane protein that spans the lipid bilayers of the membrane and in which the polypeptide crosses the membrane once only.
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single-strand DNA binding proteins (SSBs)
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Proteins that bind to unwound stretches of the DNA helix during DNA replication, thereby preventing the strands from reannealing while they are being used as a template for DNA synthesis by DNA polymerase.
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site-directed mutagenesis (SDM)
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A technique for examining the role of individual amino acid residues in proteins. It involves the use of recombinant DNA techniques to selectively replace the residue of interest with a different amino acid with critically different properties; the resulting protein can then be tested functionally. SDM is most commonly used in the study of enzymes or to identify key residues in protein–protein interactions.
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sliding filament model
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A model to describe the process by which skeletal muscle contracts. The interdigitating actin and myosin filaments of myofibrils slide past each other, without shortening of either. The myosin heads act as ‘cross-bridges’ between the myosin filament and the actin filament, applying power-strokes to pull the actin filaments towards the centre of the sarcomere. As the actin filaments are anchored at the Z disc, the sarcomere is effectively shortened.
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smooth endoplasmic reticulum (SER)
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Part of the endoplasmic reticulum where phospholipids and steroids are produced, and where detoxification of drugs occurs.
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sodium pump
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A membrane carrier protein that uses the energy of ATP hydrolysis to pump sodium (Na+) ions out of the cell and at the same time pump in potassium (K+) ions, hence its alternative name: Na+–K+ ATPase. The sodium pump maintains a high intracellular K+ concentration and a low intracellular Na+, relative to the external medium.
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somatic cells
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Body cells that are not germ cells in multicellular eukaryotes. In most cases somatic cells are diploid.
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Speciation
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The evolutionary process by which a new species arises by the divergence of one species to give rise to two or more distinct species.
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Species
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The lowest rank of taxonomy (the basic unit of classification). A population of organisms that have similar characteristics and often (but not always) are able to breed with each other to produce fertile offspring.
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specific activity
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When a given enzyme is assayed, specific activity is, by convention, expressed relative to the amount of protein (in mg, which can be measured chemically) that is contained in the preparation. Specific activity of an enzyme preparation is therefore defined as micromoles of substrate converted into product per minute per mg protein (i.e. μmol min-1 mg-1).
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specific transcription factors
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Molecules that bind to regulatory DNA elements and have an effect on gene expression.
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stationary phase
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The part of a cell population growth curve where the number of new cells is balanced by the number of deaths, so the size of the population remains constant. Growth rate is slow because nutrient availability is depleted and inhibitory waste products have accumulated.
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steric interference
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When two atoms are brought close together such that their electron clouds overlap, repulsive forces act to drive the atoms apart. This effect is known as steric interference (or steric hindrance) and it influences the conformations that a molecule can adopt.
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Stomata (Sing. Stoma)
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Specialised pores in the epidermis of leaves and stems, bordered by guard cells. The pores control movement of water vapour and gases into and out of stem and leaf tissue.
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Substrate
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The chemical upon which an enzyme acts is known as the substrate. The substrate binds to a binding site on the enzyme and is chemically transformed to form a product or products.
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substrate level phosphorylation
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The synthesis of ATP by transfer of a phosphate group from a substrate molecule to ADP. Cf. oxidative phosphorylation, where the oxidation of reduced coenzymes provides the energy needed to generate ATP from ADP and free inorganic phosphate.
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Subunit
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A single polypeptide component of a multimeric protein (a protein composed of more than one polypeptide). The arrangement of subunits in a multimeric protein is referred to as quaternary structure.
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Supercoiling
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The structural configuration of prokaryotic circular DNA which, due to the torsional stress of coiling tends to wind up on itself.
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Suppuration
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Pus formation, often resulting from inflammation. Pus consists of dead cells, dead phagocytes, debris, fibrin (blood clotting protein), plasma and living and dead bacteria.
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Symbiosis
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Close associations (literally living together) between individuals of two species which last for most or all of their lives. The term itself does not imply any cost or benefit to either partner.
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Symport
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Cotransport of two different solutes across a membrane at the same time and in the same direction. (Note that the term is used to describe both the type of transport and the carrier protein responsible.)
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synapsis
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The physical pairing together of homologous chromosomes during prophase I of meiosis.
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Syncytia (Sing. syncytium).
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Membrane-bound masses of cytoplasm containing multiple nuclei, formed by fusion of cells (usually during embryogenesis). Skeletal muscle fibres are syncytia, formed by the fusion of hundreds of individual cells called myoblasts.
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Syntrophy
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Also called ‘cross-feeding’; a specialised form of mutualism when the product of one species is used as the nutrient of another.
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TATA box
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A consensus sequence present in the core promoter of most eukaryotic genes; the site where the transcription initiation complex (including general transcription factors and RNA polymerase II) assembles.
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Taxonomy
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The science of describing, classifying and naming organisms based on their shared characteristics or phylogenic relationships.
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T cells
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One of two main types of lymphocyte (cf. B cells). T cells recognise virus-infected cells and carry antigen receptors on their surface; they can bind to specific antigens expressed on the surface of virus-infected cells and then destroy the cell.
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Telomerase
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An enzyme that adds additional nucleotide repeats to the end of the chromosomes, thereby constantly restoring telomere length.
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Telomere
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Regions at the ends of eukaryote chromosomes consisting of short non-coding nucleotide repeats. Their presence avoids the loss of coding regions during DNA replication (the 3' end of the lagging strand cannot be replicated all the way to its end).
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Telophase
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The phase of mitosis or meiosis in which a complete set of chromosomes gathers at each end of the cell and a nuclear envelope reforms around each set prior to cell division.
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template strand
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The DNA strand that is used as the template for RNA synthesis during transcription. The other DNA strand, which is not used as a template in RNA synthesis, is termed the non-template strand.
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Termination
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Transcription can be divided into three stages. Termination is the third stage in which mRNA synthesis (elongation) ceases and the transcript dissociates from the template. Translation also has a termination stage.
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tertiary structure
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Term used to describe the arrangement of secondary structures in a folded polypeptide.
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Thermoacidophiles
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Polyextremophilic organisms adapted for growth in at least two extreme conditions and requiring both high temperatures and low pH for growth.
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Thermoalkaliphiles
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Polyextremophilic organisms adapted for growth in at least two extreme conditions and requiring both high temperatures and high pH.
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Thermophiles
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‘Heat-loving’ microbes having high temperature optima. See also hyperthermophiles.
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Thylakoids
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Internal membrane system of chloroplasts, where photosynthesis takes place.
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tight junctions
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A type of junction that forms tight contacts between cells in animal tissues such as epithelia. Tight junctions prevent the lateral movement of other membrane proteins, thus allowing different regions of the cell membrane to have different functional properties (e.g. transport characteristics).
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Totipotent
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A cell which has the potential to form an entire organism.
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Traits
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Genetically determined attributes or characteristics.
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Transcription
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The process of making an RNA copy of genetic information stored in DNA. Using one DNA strand as a template, RNA polymerase adds ribonucleotides to the RNA chain according to the rules of complementary base-pairing.
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transcription factors
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Proteins that bind to specific DNA sequences, promoting (activators) or inhibiting (repressors) the recruitment of RNA polymerase to specific gene promoters, thereby controlling the transcription of genes.
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transcription initiation complex
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A complex assembly of proteins, including transcription factors and RNA polymerase, that binds to DNA at a specific site upstream of a gene called a promoter, where it initiates transcription of the gene sequence into RNA.
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transcription start site
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The site in a gene promoter sequence at which the DNA starts to be copied into RNA sequence during transcription.
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transcription terminator
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A sequence in RNA being transcribed from a DNA template that, when encountered by the RNA polymerase enzyme, signals the termination of RNA synthesis.
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Transduction
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The transfer of DNA from one cell to another mediated by a virus.
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transfer RNA (tRNA)
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Small RNA molecules involved in protein synthesis at the ribosome. Each tRNA molecule carries an amino acid and a particular anticodon (a triplet of bases), which binds to a complementary codon in mRNA, adding the appropriate amino acid to the growing polypeptide chain.
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Transformation
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Genetic alteration of a bacterial cell by the uptake and incorporation of 'naked' DNA from the environment. Transformation occurs naturally in some species of bacteria and can be induced artificially in other cells. Because transformation has an alternative meaning in animal cells (indicating progression to a malignant, cancerous state) the term transfection is often used for this type of DNA uptake in animal cells.
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transition temperature (Tm)
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The characteristic ‘melting’ temperature of a lipid bilayer, below which it changes from a fluid state to a solid gel-like state. Tm is directly related to the fluidity of the lipid bilayer: the lower the Tm the more fluid the bilayer is at any given temperature.
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Translation
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The process of protein synthesis that occurs at the ribosome; the information in an mRNA is used to specify the sequence of amino acids in the polypeptide chain.
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transmembrane proteins
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Integral membrane protein which interacts with and spans the hydrophobic core of the lipid bilayer; may be described as single-pass or multipass, depending on whether the polypeptide crosses the membrane once or more than once.
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transmission electron microscopy (TEM)
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A type of microscopy used to study the internal organisation of cells. A beam of electrons is passed through a very thin tissue section and an image obtained by focussing them on a fluorescent screen that emits light when electrons hit it.
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transport vesicles
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Small membrane-bound sacs that bud off from one membrane, move through the cytosol and fuse with another membrane. For example, vesicles move from ER to Golgi and from Golgi to the cell membrane and to lysosomes.
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Transposons
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A small genetic element that can move, or transpose, from one position to another in a genome; usually by copying itself and inserting into a second site, but sometimes by splicing out of the original site and inserting in a new location.
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trinucleotide repeat
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A sequence of three nucleotides is repeated over and over again in a gene. For example, the sequence existing within a particular location in the normal huntingtin gene (Hd), where the repeat is the triplet CAG in the non-template strand and GTC in the template strand.
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Tubulin
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Protein subunit of microtubules.
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Tumbling
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Changes in direction of swim of motile bacteria in response to chemotactic signals, caused by reversal of direction of rotation of bacterial flagella. Re-orientation occurs more often when repellents are sensed, and is reduced when attractants are sensed.
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tumour suppressor
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Genes whose products act to suppress cell proliferation and appear to prevent the formation of a cancer. An example is the Rb gene and its product the Rb protein.
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uncoupler
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A molecule that increases the proton permeability of the inner mitochondrial membrane and hence severs the link between electron transport and ATP synthesis. The energy is instead released as heat.
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Unicellular
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An organism consisting of a single cell.
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Uniport
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Type of carrier protein that transports only one type of solute across the membrane.
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Unsaturated
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In relation to an organic compound, particularly a fatty acid, this term indicates that it contains one or more C=C double bonds.
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vacuole
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Membrane-bound space, prominent in all plant and fungal cells (but usually not in animal cells), which acts as a storage compartment for water, ions and small organic molecules. In mature plant cells the vacuole fills most of the centre of the cell and together with the cell wall plays a role in determining the shape of the cell.
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variable expressivity
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Individuals with the same genotype can demonstrate a different phenotype.
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vascular tissue
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One of the three main tissue types in flowering plants comprising xylem, which transports water and dissolved ions from the roots to the rest of the plant and phloem, which transports the products of photosynthesis around the plant.
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Virions
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Mature virus particles, consisting only of a small amount of genetic material, which is complexed with proteins and packaged into a protein coat called a capsid.
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Virulence
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The ability of a microbial pathogen to infect a host organism and cause disease. Virulence is determined by factors affecting the microbe’s ability to attach to and enter the host’s body and/or its cells and its ability to evade the host’s immune defences.
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virulence factors
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Properties of a given microbial pathogen that determine its virulence. These affect the pathogen’s ability to attach to, enter and reproduce inside its host and its capacity to evade the host’s immune defences.
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Vmax
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The maximum initial rate of an enzyme-catalysed reaction, vmax (pronounced ‘vee max’).
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xylem
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Plant vascular tissue, which transports water and dissolved ions around the plant.
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zygote
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A diploid cell produced by the fusion of two haploid gametes during sexual reproduction.
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Zymogens
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Secreted proteins that remain inactive until the required extracellular activation has taken place. Examples include digestive enzymes, whose secretion as zymogens ensures that the cells secreting them are not damaged by digestion in the process. |
