7.3 Translation

Question 1

First step in translation

Answer 1

initiation
the start of translation brings together mRNA with a start codon (AUG), a tRNA bearing the start amino acid of a polypeptide (methionine) , and ribosomes
i.e. involves the assembly of the components that carry out the process

Question 2

Second step in translation

Answer 2

in the elongation stage, amino acids are added one by one by tRNAs as the ribosome moves along the mRNA
- tRNA brings the appropriate amino acid up to the ribosome
- the anticodon on the tRNA matches with the codon on the mRNA
- the new amino acid is then joined to the polypeptide chain
- as each new amino acid is joined, the ribosome nudges the mRNA strand along three notches and the next tRNA falls into place
- a peptide bond forms between adjacent amino acids
- the tRNA is released

Question 3

Third step in translation

Answer 3

the final stage of protein synthesis occurs when the ribosome reaches a stop codon
the polypeptide chain disconnects from the ribosome
i.e. disassembly of the components

Question 4

translation in prokaryotes

Answer 4

can occur immediately after transcription due to the absence of a nuclear membrane

Question 5

which ribosomes synthesise proteins for what?

Answer 5

free ribosomes synthesise proteins primarily for use within the cell
whereas bound ribosomes (rough ER) synthesise proteins primarily for secretion or for use in lysosomes

Question 6

Polysomes

Answer 6

several ribosomes can translate mRNA at the same time
meaning that it is possible to produce many polypeptides simultaneously from a single mRNA

Question 7

tRNA Activation

Answer 7

each tRNA is recognised by a tRNA activating enzyme that attaches a specific amino acid to the tRNA
this requires ATP
tRNA activating enzymes illustrate enzyme -substrate specificity and the role of phosphorylation