L3 - Translation

Question 1

Features of the genetic code

Answer 1

• degenerate (more than one codon per amino acid)
• non overlapping (single base mutation sonly ever affect one amino acid)
• universal
• a triplet code

Question 2

Summary of Crick, Brenner at al’s genetic code experiment

Answer 2

• Generated mutants in rII gene of bacteriophage T4 using proflavin
• proflavin is a mutagen
• intercalates between base pairs in DNA
• can cause insertion of a single extra base
• or can cause a deletion of a single base
• double frameshift mutants of same type have mutant phenotype
• triple frameshift mutants of same type have wild type phenotype

Question 3

Features of codons

Answer 3

• REDUNDANCY / DEGENERACY - some A.A. have more than one tRNA
• Wobble - some tRNAs can tolerate a mismatch at the third codon position
• STOP CODONS - three codons do not code for amino acids

Question 4

Why are amino acids given a single letter code

Answer 4

Bioinformatics - allows protein sequences to be analysed digitally

Question 5

Exception to the genetic code being universal

Answer 5

variations in mitochondria and some in nuclear genomes

Question 6

Roles of mRNA, tRNA and rRNA

Answer 6

mRNA - carries the genetic information
tRNA - deciphers the codons of the mRNA
rRNA - makes up the ribosome

Question 7

How does tRNA decipher codons?

Answer 7

1. Specific enzymes couple tRNAs to the correct amino acid (tRNA becomes charged)
2. The anticodon in the charged tRNA pairs with the codon in the mRNA, delivering its amino acid in the correct sequence
   - high energy ester bond broken

Question 8

Ribosome structure

Answer 8

• large complex
• 4 RNAs
• more than 80 proteins

A site - aminoacyl-tRNA site
P site - peptide-tRNA site
E site - exit site

Question 9

What area of the nucleus are ribosomes assembled

Answer 9

nucleolus

Question 10

3 steps of translation

Answer 10

1. Initiation
2. Elongation
3. Termination

Question 11

Initiation

Answer 11

• small subunit of ribosome attaches to methylated 5’ cap
• migrates up the untranslated leader between the cap and the ignition site
• in eukaryotes usually initiates at the first AUG codon
• AUG codon bound by the initiator tRNAMet
• the first amino acid (amino terminal) in the methionine will always be methionine
• binding of the initiator tRNA to the small subunit allows the large subunit to bind, ribosome assembles
• P-site is filled with the initiator, A site is empty

Question 12

Termination

Answer 12

• STOP codon at the end of coding region of mRNA
• there are no tRNAs with anticodons that bind to stop codons
• stop codons are bound by termination factors
  (termination factors also called release factors, two known types that interact with stop codons)
• the release factor recognises the stop codon and signals an end to peptide synthesis
• UAG is one of three stop codons
• interaction with release factors = cleavage of the complex protein chain from the tRNA from the P-site
• protein leaves the ribosome
• release factors encourage dissociation of ribosomal subunits, terminal tRNA and mRNA (components are recycled)

Question 13

Key points of translation

Answer 13

• translation occurs on ribosomes
• mRNA is a template
• tRNAs act as adaptors
• the result is a particular protein made up of a certain order of amino acids
• mRNA synthesised 5’ to 3’
• polypeptide synthesis is N to C (amino-terminal to carboxyl-terminal)

Question 14

Elongation

Answer 14

• the tRNA with a corresponding anticodon to the codon in the A site binds
• with two tRNAs bound to the ribosome the two amino acids begin to react
• ester bond broken between tRNA and amino acid
• the amino acid on the tRNA in the P site (methionine) is transferred onto the amino acid on the tRNA in the A site
• now empty tRNA moves to exit site, is released
• there is a peptide tRNA in the A site
• ribosome slides down the mRNA
• tRNA with the growing peptide is in the P-site
• A site left free to bind to another aminoacyl-tRNA
• growing peptide chain is transferred to the aminoacyl tRNA, converting it to a peptide tRNA
• cycle continues with the ribosome sliding, a new aminoacyl tRNA bonding, and peptide transfer, exit of the empty tRNA
• ribosome moves along at the rate of one codon per peptidyltransferase reaction
• as each reaction completes, a new charged tRNA enters the ribosome