Clinical Microbiology

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haploid

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single copy of each chromosome.

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DNA gyrase

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DNA is extensively folded, coiled, and supercoiled by DNA gyrase

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plasmids

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Prokaryotic cells may also contain one or more extrachromosomal DNA molecules called plasmids, which contain genes that regulate nonessential life functions

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fertility plasmids

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bacterial conjugation

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resistance plasmids

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resistance to one or more antimicrobial drugs, heavy metals, or toxins

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bacteriocin plasmids

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carry genes for toxins that kill competing bacteria

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virulence plasmid

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carry code for virulence structures, enzymes, or toxins.

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diploid

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Many eukaryotes are diploid, having two copies of each chromosome.

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nucleosomes

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The DNA of eukaryotic chromosomes is wrapped around proteins called histones, to form nucleosomes (“beads”) that bind to other proteins to form chromatin fibers.

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euchromatin

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Loosely packed DNA, euchromatin, are regions where genes are active.

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heterochromatin

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Tightly packed DNA is heterochromatin

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semiconservative

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eachdaughter DNA molecule is composed of one original strand and one new strand.

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Deoxyribonucleotide triphosphates

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the building blocks and the source of energyfor DNA replication.

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origin

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DNA replication starts at a specific nucleotide sequence

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DNA helicase

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unzips the double helix, breaking hydrogen bonds between complementary base pairs, toform a replication fork.

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leading strand

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DNA polymerases always move in the 5' to 3' direction, so theleading strand is synthesized toward the replication fork.

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primase

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Synthesis is initiated by a primase,and continued by DNA polymerase

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DNA polymerase

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proofreads the pairing of new nucleotides.

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lagging strand

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discontinuously synthesized in a direction away from the replication fork inseries of Okazaki fragments. It always lags behind the process occurring in the leading strand.

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DNA ligase

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seals the gaps between adjacent Okazaki fragments to form a continuous DNAstrand.

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DNA replication is ______; that is, it proceeds in both directions from the originas two replication forks

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bidirectional

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Gyrase and topoisomerase enzymes

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remove supercoils that createtension in the DNA as it is unwound and would stop the replication process.

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methylation

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After bacterial DNA replication, methylation occurs. In methylation, a cell adds a methylgroup to one or two bases that are part of specific nucleotide sequences. In some cases, genesthat are methylated are “turned off” and are not transcribed, whereas in other cases, they are“turned on” and are transcribed. In some bacteria, methylated nucleotide sequences play arole in initiating DNA replication, repairing DNA, or recognizing and protecting againstviral DNA.

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central dogma of genetics

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states that DNA is transcribed to RNA, which is translated toform polypeptides. That is, transcription of the DNA produces RNA molecules, and translationof the RNA sequence by ribosomes produces polypeptides.

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Cells may regulatesynthesis by

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initiating (induction) or blocking (repression) transcription or by stoppingtranslation directly

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operon

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Transcription-level regulation in prokaryotes often involves operons. An operon consistsof a promoter, an adjacent regulatory element called an operator, and a series of genes whoseexpression is controlled by a regulatory gene located elsewhere.

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polygenic

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Operons coding for morethan one polypeptide

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Inducible operons

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such as the lactose (lac) operon arenot usually transcribed and must be activated by inducers

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Repressible operons

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such as thetryptophan operon are transcribed continually until deactivated by repressors.

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Lac operon regulation

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is complex, involving a repressor protein that binds the operatorsequence and prevents RNA polymerase binding

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an inducer

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(allolactose) that binds repressorprotein and prevents it from binding the operator sequence

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catabolite activator protein(CAP)

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that binds the DNA and facilitates the binding of RNA polymerase when cyclic adenosinemonophosphate (cAMP) is present

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The tryptophan (trp) operon

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The tryptophan (trp) operon is normally expressed. When tryptophan is abundant, it acts as a corepressor, binding inactive repressor protein, activating it. The activated repressor protein binds the trp operator, preventing RNA polymerase binding and transcription.

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Translation can be controlledby

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micro RNA (miRNA) or short interference RNA (siRNA); miRNA and siRNA molecules are micro RNA (miRNA) or short interference RNA (siRNA) complementary to a portion of an mRNA and prevent its translation when bound;

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miRNA-induced silencing complex (miRISC)

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miRNAs and regulatory proteins form a miRNA-induced silencing complex (miRISC), which then binds to mRNA, either blocking translation or cutting the molecule

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siRNAs

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are double-stranded RNAs that complex with regulatory proteins and cut the target RNA mole- cule.

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riboswitch

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is a sequence of mRNA that changes shape in response to changes in the environment, and may favor or block translation of the polypeptide.

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mutagens,

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Physical or chemical agentscalled mutagens, which include radiation and several types of DNA-altering chemicals,induce mutations.

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pyrimidine dimers

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nonionizing radiation in the form of ultraviolet light causes adjacent pyrimidine bases tobond to one another to form pyrimidine dimers. The presence of dimers prevents hydrogenbonding with the nucleotides in the complementary strand, distorts the sugar-phosphate backbone,and prevents proper replication and transcription.

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nucleotide analogs

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compounds that are structurally similar tonormal nucleotides but, when incorporated into DNA, may interfere with DNA polymerasefunction or cause mismatching.

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nucleotide-altering chemicals

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change the structure ofnucleotides, causing base-pair substitution mutations.

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Aflatoxins

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are nucleotide-alteringchemicals that result in missense mutations and cancer

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Frameshift mutagens

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are chemicalsthat insert or delete nucleotide base pairs, resulting in frameshift mutations. Benzopyrene,ethidium bromide, and acridine are examples of frameshift mutagens.

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Positive selection

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which involves selecting a mutant by eliminating wild-typephenotypes

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Negative selection (also called indirect selection)

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a process in which a researcherattempts to culture auxotrophs which have non–wild type nutritional requirements.Colonies of bacteria are allowed to grow on complete medium and then are replica platedto a medium lacking one or more nutrients.

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Ames test

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is used to identify potential carcinogens (cancer-causing agents). Salmonellaauxotrophs are grown in the presence of possible mutagens and then are screened forreversion to wild type.

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Genetic recombination

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refers to the exchange of nucleotide sequences between two DNAmolecules often mediated by segments composed of identical or nearly identical nucleotidesequences called homologous sequences

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recombinants

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DNA molecules that contain new arrangements ofnucleotide sequences

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Vertical gene transfer

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is the transmission of genes from parents to offspring.

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horizontalgene transfer

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DNA from a donor cell is transmitted to a recipient cell

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recombinant cell

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results from genetic recombination between donated and recipient DNA.

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transformation

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a competent recipient cell takes up DNA from the environment.

Competency results from changes in the cell wall and cytoplasmic membrane that allowextracellular DNA to enter cells. It is found naturally in some genera of bacteria and canbe created artificially in others. It is an important tool of genetic engineering.

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transduction

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a virus such as a bacteriophage (or phage) carries DNA from a donorcell to a recipient cell. Donor DNA is accidentally incorporated in transducing phagesduring assembly

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generalized transduction

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the transducing phage carries a randomDNA segment from the donor to the recipient

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specialized transduction,

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only specifichost sequences are transferred.

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conjugation

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a bacterium containing an F (fertility) plasmid forms a conjugation pilusthat attaches to an F– recipient bacterium. Plasmid genes are transferred as single-strandedDNA to the recipient, which synthesizes the complementary strand and becomes F+ as aresult.

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Hfr

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(high frequency of recombination) cells result when an F plasmid integratesinto a prokaryotic chromosome. Hfr cells form conjugation pili and transfer chromosomalsequences as well as plasmid genes.

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Transposons

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are DNA segments that code for the enzyme transposase and contain palindromicsequences known as inverted repeats (IR) at each end.

Transposons move from one location to another in chromosomes and plasmids in eukaryotesand prokaryotes. Regions of sequence homology are not required for transposition. Thesimplest transposons, known as insertion sequences (IS), consist only of inverted repeatsand transposase. Complex transposons contain other genes as well.