Translation

Question 1

how is genetic code read

Answer 1

in groups of three -> codons (64 different codons)

Question 2

what is the purpose of mRNA, where is it’s ribosome binding site + start codon

Answer 2

is the coding sequence for the protein
- the ribosome binding site (RBS) is in the 5’ untranslated region
start codon is a few nucleotides away from RBS, and the stop codon is at the end of the coding sequence, and has the 3’ untranslated region following it

Question 3

what is the structure + function of tRNA

Answer 3

has 3 stem loops and a fourth stem with a single stranded region.
- an anticodon is at the end of stem-loop 2 which reads the mRNA, whic is a three base sequence complementary to a codon in the mRNA

Question 4

what are charging tRNAs

Answer 4

are enzymes called aminoacyl-tRNA synthetases which catalyse the attachments of amino acids to tRNA- there are 20 for each amino acid

Question 5

how are ribosomes arranged

Answer 5

have a small subunit, large subunit and then the assembled ribosome
bacteria: 30S, 50S and 70S
eukaryotes: 40S, 60S and 80s

Question 6

where does the mRNA lie in the ribosome (bacteria)

Answer 6

in a space between the 30S and 50S subunit, and the polypeptide exits through a site in the 50S subunit

Question 7

what are the stages of translation

Answer 7

initiation, elongation, termination

Question 8

describe initiation

Answer 8

a complexis formed between the mRNA, tRNA and ribosomal subunits.
in bacteria , assembly starts with mRNA binding the small ribosomal subunit and this happens because of the RBS sequence at the 5’ end of the mRNA pairing with a sequence in the 16S rRNA

Question 9

how does initiation differ in eukaryotes from bacteria

Answer 9

• eukaryotic mRNA have 5’ guanosine cap that is recognised by cap binding proteins and this promotes binding of mRNA to the ribosomal subunit
• the start codon can be a variable distance from the RBS
• starting tRNA is methionine compared to a modified methionine in bacteria called formyl-methionine

Question 10

describe the process of elongation

Answer 10

this involves the covalent bonding of amino acids one at a time to form a polypeptide

there are three sites on the ribosome: A, P and E. the polypeptide being synthesised occupies the P site attached to a tRNA. charged tRNA binds to mRNA in the A site. Peptide bond forms between the amino acid in the A site and the polypeptide chain. the polypeptide is then removed from the P site and moved to the amino acid on the A site by peptidyl transerase enzynme

tRNA in P and A sites shift to E and P sites respectively
process repeats

Question 11

describe termination

Answer 11

there are three stop codons and these are recognised by release factors, not tRNA. the finished polypeptide is in the P site and is released by the tRNA. the mRNA, ribosome subunits and the release factors dissociate