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67 Cards in this Set

  • Front
  • Back
function of a gene
dictate the production of a specific enzyme
a nutritional mutant that is unable to synthesize and that cannot grow on media lacking certain essential molecules normally synthesized by wild type strains
the one gene-one enzyme hypothesis
3 diff. classes of mutants could not synthesize arginine, the last step in the pathway. each had a different metabolic block along the way that prevented them from reaching the end.
one gene- one polypeptide hypothesis
many proteins are constructed from two or more polypeptide chains, but each chain has its own gene
differences between DNA and RNA
1. RNA almost always has 1 strand
2. Has the sugar ribose instead of deoxyribose
3. has the base uracil instead of thymine
monomers of geners are bases, monomers of proteins are amino acids
synthesis of RNA under the direction of DNA. DNA provides a template, and RNA makes a complementary copy
messenger RNA
carries a genetic message from the DNA to the protein-synthesizing machinery of the cell
actual synthesis of a polypeptide under the direction of mRNA. ribosomes translate base sequence into amino acid sequence
primary transcript
pre mRNA, initial RNA transcript
RNA processing
modification of RNA before it leaves the nucleus, unique to eukaryotes
translation and transcription in a prokaryotic cell
since there is no nucleus, mRNA produced by transcription is immediately translated without additional processing
triplet code
the flow of info from gene to protein is based on this code: the genetic instructions for a polypeptide chain are written in the DNA as a seris of three nucleotide words
template strand
for each gene, this is the only strand transcribed. provides the template for ordering sequence of nucleotides in RNA transcript. the other strand is used for DNA replication
mRNA base triplets
codon AUG codes for methionine and a start signal for ribosomes to begin translating the mRNA
UAA, UGA, UAG are stop codons, and dont code for an amino
RNA polymerase
enzyme that pries the two strands of DNA apart and hooks together the RNA nucleotides as they base pair along the DNA template. can only add nucleotides to the 3' end
transcription unit
the stretch of DNA that is transcribed into an RNA molecule
a region of DNA where RNA polymerase attaches and initiates transcription. includes transcription startpoint and extends upstream. the promoter determines which strand is the template
transcription factors
collection of proteins that mediate the binding of RNA polymerase and the initiation of transcription
transcription initiation complex
completed assembly of transcription factors and RNA polymerase bound to the promoter
TATA box
a nucleotide sequence typically somthing like TATAAAA, located about 25 nucleotides upstream from the startpoint
RNA polymerase II
enzyme that initiates RNA synthesis at promotes that commonly include a TATA box. also transcribes protein coding genes into pre-mRNA
DNA sequence. the transcribed sequence (RNA) functions as the actual termination signal. in euks, mRNA is cut free from the enzyme 10-35 nucleotides past the terminator
5' cap
5' end is capped with a modified form of guanine. this helps protect the mRNA from degradation by hydrolytic enzymes. it is also an attach here sign for ribosomes
poly(A) tail
on 3' end, 30-200 adenine nucleotides. serves the same purpose as 5' cap. facilitates the export of mRNA from the nucleus
RNA splicing
the removal of non coding portion (introns) of the RNA molecule after initial synthesis
sequences of codons that are expressed. these are interspersed with introns
several different small nuclear ribonucleoproteins (snurps) joined with additional proteins. within the splicosome, the RNA of certain SNURPS base pairs with certain nucleotides at the ends of the intron. the RNA transcript is cut to release the intron, and the spliceosome comes apart, releasing the exon mRNA
RNA molecules that function as enzymes
1. regulate cell 2. regulate passage of mRNA from nucleus to cytoplasm 3. determine sex in fruit flies 4. evolution of new useful proteins 5. increase corssing ove rbetween two alleles of a gene
many proteins have a modular architechture consisting of discrete structural and functional components
transfer RNA (tRNA)
transfer amino acids from the cytoplasm's amino acid pool to a ribosome. anticodon on one end, amino acid on the other
nucleotide triplet that binds according to base pairing rules to a complementary codon on mRNA.
structure of tRNA
bases in certain regions of the tRNA strand form hydrogen bonds with complementary bases of other regions. 4 base paired regions and 3 loops. 3' end is amino acid attachment site, middle loop is the anticodon triplet, unique to each tRNA type
a violation of the base pairing rules in that third nucleotide (5' end) of a tRNA anticodon can from H bonds with more than one kinds of base in the third position (3' end) of a codon
inosine (I)
a modified base formed by enzymatic alteration of adenine after tRNA is synthesized that leads to the most versatile tRNAS
aminoacyl-tRNA synthetase
joins amino acid to the correct tRNA. catalyzes the covalent attachment of the amino acid to the tRNA, which is endergonic and requires ATP
ribosomal RNA (rRNA)
the most abundant type of RNA, together with proteins, it forms the structureof ribosomes that coordinate that sequential coupling of tRNA molecules to the series of mRNA codons. has a binding site for mRNA, 3 tRNA binding sites.
P Site
peptidyl tRNA site, holds the tRNA carrying the growing polypeptide chain
A Site
aminoacyl-tRNA site, holds the tRNA carrying the next amino acid to be added to the chain
E Site
exit site, where discharged tRNAs leave the ribosome
brings together mRNA, a tRNA bearing the first amino acid of the polypeptide, and the two subunits of a riubosome
steps of initiation
1. a small ribosomal subunit binds to both mRNA and a special initiator tRNA. The small rib. attaches to the leader segment at the 5' end of the mRNA. the 5' cap tells the small subunit to attach to the 5' end of the mRNA. 2. The initiator tRNA, which carries the amino "Met" attaches to the initiation codon.
3. Finally, large rib. subunit is attached.
initiation factors
proteins that bring the components of initiation together. cell spends GTP energy in forming the initiation complex
amino acids are added one by one to the first amino acid. aided by proteins called elongation factors
Elongation 1: Codon Recognition
The mRNA codon in the A site forms H bonds with the anticodon of the incomiing tRNA carrying its amino. Elongation factors help tRNA into A site. Requires hydrolysis of GTP
Elongation 2: Peptide Bond Formation
An rRNA molecule of the large ribosomal subunit, functioning as a ribozyme, catalyzes the formatin of a peptide bond that joins the polypeptide extending from the P site to the newly arrived amino acid in the A site. amino acid at carboxyl end is attached to amino in the A site
Elongation 3: Translocation
the tRNA in the A site is translocated to the P site, taking the mRNA along with it. Meanwhile, the tRNA in the P site moves to the E site and is released from the ribosome. In translocation, the mRNA shifts its position on the ribosome by one codon.
When a ribosome reaches a termination codon on a strand of mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. The relase factor hydrolyzes the bond between the tRNA in the P site and the last amino of the polypep chain. The polypep is freed from the ribosome. The 2 subunites dissociate.
an mRNA molecule is usually translated simultaneously by several ribosomes in clusters
posttranslational modifications
certain aminos may be chemically modified, or enzymes may remove a leading amino acid, a single chain may be cleaved into two, or two subunits may join
signal peptide
a stretch of amino acids on polypeptides which targets the protein to the ER
signal recognition particle (SRP)
functions as an adaptor that brings the ribosome to a receptor protein built into the ER membrane.
Why RNA is so useful
can H bond to other nucleic acid molecules and can assume a specific 3D shape by forming H bonds between bases in diff parts of its polynucleotide chain.
changes in the genetic material of a cell
point mutations
chemical changes in just one or a few base pairs in a single gene
base pair substitution
replacement of one nucleotide and its partner in the complementary DNA strand with another pair of nucleotides.
missense mutations
substitution mutations: the altered codon still codes for an amino acidand makes sense, although not necessarily the right sense.
nonsense mutations
alterations that change the amino acid codon to a stop signal--leads to nonfunctional proteins
insertions and deletions
additions or losses of one or more nucleotide pairs in a gene
frameshift mutation
occurs whenever the number of nucleotides deleted is not a multiple of 3, the entire rest of the message is wrongly coded
spontaneous mutations
errors from DNA replication, repair or recombination
physical and chemical agents that interact with DNA to cause mutations
ames test
used to identify chemicals that may cause cancer. uses easily grown bacteria that is already mutated. the only way for it grow more is if it mutated again to start growing once more.
what is a gene
a region of DNA whose final product is either a polypeptide or an RNA molecule