Protein synthesis

Question 1

genetic code

Answer 1

• genetic info carried on DNA
• predicts AA sequence
• codons are non-overlapping triplets
• start codon is AUG
• code is degenerate: more than one triplet for some AA’s
• translation of mRNA is 5–> 3’

Question 2

how mutations effect AA sequence

Answer 2

1. insertion or deletion - can change reading frame. often get truncated protein
2. point/substitution - may change AA but not reading frame. some are “silent” but change rate of translation and protein folding

Question 3

transcription vs translation

Answer 3

-info transcribed (copied) from DNA to RNA
-info translated (interpreted) into protein. tRNA in the middle of mRNA and AA’s
amino-acyl tRNA synthetases - transfer AA to tRNA

Question 4

tRNA structure

Answer 4

-anticodons on loop end. AA on 3’ end

translation in ribosome - has 3 spots for tRNA binding

Question 5

initiation of translation

Answer 5

• mRNA needs to bind tRNA for methionine start codon but there are many AUG
• in prokaryotes: recognize ‘Shine Delgarno’ sequence (a G-rich area)
• in eukaryotes: recognize 5’ guanine cap and 3’ poly A tail that were added during mRNA processing
• EIF2 is an initiation factor that brings Met tRNA and GTP to ribosome. leaves with GDP to allow initiation

Question 6

transpeptidation

Answer 6

• tRNA breaks peptide bond to release its AA

- rxn doesnt require additional energy because incoming tRNA is “charged” –has a high energy bond

Question 7

termination

Answer 7

• stop codon has no tRNA. instead, a release factor binds (molecular mimacry) in A site
• GTP hydrolysis–> tRNA, release factors and mRNA are released from ribosome. so is peptide chain

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Question 8

Regulation of translation

Answer 8

1. phosphorylation of EIF2 inhibits translation during initiation step
   - EIF2 carries initiation tRNA and GTP to ribosome
   - ex: Heme controlled inhibitor will phosphorylate EIF2 when no heme is present, to stop translation of globin because dont need globin if no heme
   - when heme is present, it inactivates the inhibitor. thus EIF2 is not phosphorylated and will initiation transcription of globin.

2: regulation by mRNA sequences. poly A tail gets degraded. once it is sufficiently degraded the whole mRNA is degraded and will not be translated.
ex: if low iron: bind 5’ end of transferrin mRNA to stop transferrin from being made.
- excess iron binds repressor and allowed transferrin to be made.