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MBG 2040 > translation and the genetic code > Flashcards

translation and the genetic code Flashcards

1
Q

the purpose of translation

A

to decode the mRNA and make the functional protein product of the gene

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2
Q

why does mRNA last longer in eukaryotes

A
  • translation occurs in the cytoplasm but transcription occurs in the nucleus
  • in prokaryotes both happen in the cytoplasm
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3
Q

“one gene, one collinear polypeptide”

A

the sequence of base pair triplets in the coding region of a gene specify a collinear sequence of amino acids in its polypeptide product

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4
Q

structure and function of proteins

A
  • made of polypeptides which are long chains of amino acids
  • amino acids have a free amino group, free carboxyl group and an R group (decides chemical nature)
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5
Q

peptide bonds

A
  • join amino acids
  • the carboxyl group of one AA is covalently attached to the amino group of another AA
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6
Q

4 levels of organization in proteins

A

primary: linear arrangement of amino acids
secondary: determined by the spatial organization of amino acids
tertiary: overall folding of the complete polypeptide
quaternary: only in some proteins, more than one polypeptide interact to make a functional protein

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7
Q

how many nucleotides are required to specify a single amino acid?

A

3 - triplet codon
- there are 64 possible codons, so some amino acids are specified by more than one codon

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8
Q

discovering the genetic code through homopolymers

A
  • used an artificial mRNA containing only one repeating base pair
  • the polypeptide contains a single amino acid
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9
Q

discovering the genetic code through mixed mRNAs

A
  • used random copolymers
  • there are 3 possible reading frames in the synthetic mRNA
  • a ribosome that starts at a different frame will read the code for different amino acids
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10
Q

with the use of tRNAs what was determined between matches of codons and amino acids

A

short mRNAs of known sequence stimulated the binding of ribosomes and corresponding amino acid bound tRNA
- assisted in the identification of amino acids

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11
Q

degeneracy in the genetic code

A
  • there are 64 codons and only 20 amino acids
  • has to due with the wobble position
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12
Q

Wobble position

A
  • there is flexibility in binding at the 3rd codon position (1st anti-codon position)
  • this codon can be changed and still produce the same amino acid
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13
Q

anticodon

A
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14
Q

Crick’s wobble hypothesis

A

to account for degeneracy, tRNAs must exist for certain amino acids, and some tRNAs must recognize more than one codon

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15
Q

codon recognition and the wobble hypothesis

A
  • stringent base pairing between the codon in mRNA and the anti-codon only occurs for the first 2 bases of the codon
  • ## antiparallel base pairing happens between the anti-codon in the tRNA and the codon in the mRNA
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16
Q

the macromolecules of translation

A
  • ribosomes, made up of polypeptides
  • amino-acid activating enzymes
  • tRNA molecules
  • soluble proteins
17
Q

ribosomes

A
  • composed of proteins and several different rRNAs
  • composed of 2 subunits - a large and small
  • “RNA machine” with key roles in protein synthesis, including the formation of peptide bonds between amino acids
18
Q

the role of tRNAs in translation

A
  • the adapters between amino acids and the codons in mRNA
  • anticodon of tRNA pairs with the codon of mRNA
  • the AA is covalently attached at the 3’ end of the tRNA
  • the normal ribonucleotides in tRNAs are often modified post-transcriptionally by enzymes
19
Q

when do amino acids attach to tRNA

A
  • tRNA is only attached to amino acids when you need to do translation
  • you need energy (ATP) to attach the amino acid to tRNA
  • the longer an amino acid is attached to a tRNA, the greater chance of degradation
20
Q

activation of tRNA by aminoacyl tRNA synthetase

A
  • cells contain at least one tRNA synthetase per AA
  • the amino acid reacts with ATP producing AMP and PPi
  • the amino acid is transferred to the appropriate tRNA and AMP is released
21
Q

steps in translation (protein synthesis)

A
  1. initiation: transitional complex forms and tRNA brings the first AA to bind to start codon on mRNA
  2. elongation: tRNAs bring AAs one by one to add to polypeptide chain
  3. termination: release factor recognizes stop codon, translational complex dissociates and polypeptide is released
22
Q

initiation in translation - prokaryotes

A
  • IF3 binds to the small subunit and attaches it to the mRNA
  • a tRNA charged with N-formylmethionine forms a complex with IF2 and GTP
  • binds to the initiation codon while F1 joins the small subunit
  • all initiation factors dissociate and GTP becomes GDP
  • large subunit then joins initiation complex
23
Q

initiation factors in translation - prokaryotes

A

IF-3: required to inhibit large (50s) subunit form binding (30s) small subunit
IF-2 and IF-1: position the fMet-tRNA to the ribosomal P site
- IF-2 GTP hydrolysis signals complex is ready for initiation by binding 2 subunits together

24
Q

initiation complex in translation - prokaryotes

A

consists of large and small ribosomal subunits, initiation factors 1-3, and GTP

25
Q

sites at the ribosome

A

A: aminoacyl site - site for incoming tRNA
P: peptide site - where you start protein synthesis
E: exit site - for ejecting tRNA
- fMet(or just Met is eukaryotes) is always the first AA to get incorporated and occupies the P site

26
Q

the shine-dalgarno sequence

A

tells the ribosomes where to bind on mRNA in prokaryotes

27
Q

16S rRNA

A
  • component of the 30s small ribosomal subunit and contains the sequence complementary to the shine-Dalgarno sequence in the mRNA
  • pairing between these 2 sequences positions ribosome near the AUG start codon
28
Q

how does translation initiation differ in eukaryotes

A
  • the first methionine is just Met not fMet
  • no shine-dalgarno sequence
  • ribosome initiation complex binds the 5’7-MG cap of the mRNA
  • the Kodak sequence influences the efficiency of which AUG is used to start translation
  • the polyA tail interacts with the 5’cap via cap-binding protein complex to promote initiation
29
Q

kozak sequence

A
  • translation start point in eukaryotes
    5’GCC(A or G)CCAUGG3’
  • the AUG of this sequence is the start codon
30
Q

Elongation in translation

A
  • tRNA binds to the A site of the ribosome
  • the AA is transferred from the tRNA in the P site to the tRNA in the A site by the formation of a peptide bond
  • ribosome moves along mRNA to position the next codon in the A site, results in the polypeptide tRNA being moved from A to P
  • the uncharged tRNA is translocated from P to E and is removed
  • a new tRNA binds in the A site and the cycle is repeated
31
Q

translation termination

A
  • occurs when a chain-termination codon enters the A site of the ribosome
  • when a stop codon is reached a release factor (RF) binds to the A site
  • RF1 or RF2 binding alters enzyme activity resulting in an H2O being added instead of an AA being added to the carboxyl terminus
  • the binding of RF3 and GTP to the ribosome assists in dismantling the complex
32
Q

release factors

A
  • triggers translation termination
  • attach a water molecule instead of incoming amino acid
  • RF1 recognizes UAG and UAA stop codons
    -RF2 recognizes UAA and UGA stop codons
    -RF3 assists in dismantling the entire complex
33
Q

quality control of RNA and protein

A

mRNA is eliminated that…
- has nonsense mutations
- which a stalled ribosome cannot complete proper translation]
- damaged by chemicals
- have unusual secondary structures

  • molecular chaperones assist the proper folding of newly-synthesized proteins and are associated with the ribosome during translation
  • the AAs of many proteins are modified post-translationally, can modify protein function and activity

MBG 2040 (23 decks)

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