DNA translation

Question 1

what is a codon?

what does degenerate mean?

what is the translation initiator & its code?

what are the 3 stop codons?

Answer 1

3 nucleotide bases red in a triplet & code for 1 amino acid

more than 1 codon can be used to specify an amino acid

methionine- AUG

UAA, UGA, UAG

Question 2

What is the mRNA sequence in prokaryotes that initiates translation?

How long is it?

Where is it located?

what molecule is it recognised by?

Answer 2

Shine-Dalgarno

8 nucleotides long

upstream of methionine/AUG

16s rRNA

Question 3

what is the mRNA sequence in eukaryotes that starts translation?

where is it located?

where does the ribosome bind? how is this different to the prokaryotic sequence?

what is tRNA’s function? how many are there?

what is aminoacyl-tRNA synthetases function? how many are there?

what are the 3 steps in forming an aminoacyl-tRNA?

Answer 3

Kozak sequence

spans methionine/AUG

5’ cap upstream of the sequence- whereas in prokaryotes it binds directly on the Shine-Dalgarno sequence (as it is upstream of methionine)

pairs with codons in mRNA & covalently links to an amino acid- MANY/more than aminoacyl-tRNA synthetases (for all different codons)

catalyses bonding between tRNA & amino acid- only 20 as it recognises 1 amino acid but is compatible with all tRNAs

1. tRNA has anti-codon complementary to codon sequence on mRNA
2. hydrolysis of ATP into AMP (release 2Pi) & with catalysis, high energy ester bond forms between amino acid & tRNA to form aminoacyl-tRNA
3. aminoacyl-tRNA finds complementary sequence on mRNA

Question 4

what are the 3 basic steps in translation?

what are the 3 different sites of the ribosome?

what are the 3 release factors & what do they do/recognise?

how does the circularisation of mRNA occur? what does this ensure?

Answer 4

1. initiation: aminoacyl-tRNA for methionine binds & ribosome subunits assemble
2. elongation: ribosome translocates across mRNA & incoming amino acid joined in peptidyl bond while other tRNA exits
3. termination: release factor binds to stop coding causing dissociation of subunits & release of polypeptide

A = aminoacyl site
P = peptidyl site
E = exit site

RF1 = UAG/UAA
RF2 = UGA/UAA
eRF3 = reads stop codon correctly

polyA binding protein I binds to poly A tail & also to transcription factors at 5’ end - makes it easier for ribosomal subunits to re-assemble at 5’ end after termination