Gene Expression

Translation

“Translation by ribosomes allows assembly of amino acids into polypeptides according to the original DNA code”: second stage of protein synthesis.

  1. mRNA attaches to a ribosome.
  2. tRNA has an anticodon on one end and an amino acid bonded to the other, which it carries to the ribosome.
  3. The tRNA anticodon binds itself to the first codon on the mRNA due to complementary base pairing
  4. Another tRNA molecule binds to the second codon of the mRNA.
  5. The amino acids join by a peptide bond and the tRNA molecules detach themselves.
  6. This process is repeated, leading to the formation of a polypeptide chain. This occurs until a stop codon is reached.
Which amino acid will join the polypeptide chain?

Codon: the three nucleotides inmRNA. Each codon codes for a single amino acid.

Anticodon: the three nucleotides in tRNA that pair to a codon.

Codon charts are used to find the amino acid that corresponds to DNA and RNA to produce a polypeptide chain. The charts use mRNA, but you may well be given a DNA sequence and have to deduce the mRNA sequence before working out the amino acid chain.

First, you go to the left side of the chart and find the first letter (base) of the codon. You then go to the top of the chart and find the second letter of the codon, before finally going to the right and finding the last letter that is in the same row as the first letter of the codon.

For example, the codons UUA – CGC – AGU would produce the amino acids leu – arg – ser.

Explain the process of translation, leading to polypeptide formation.

A. consists of initiation, elongation and termination;

B. mRNA is used as a template / guide;

C. mRNA translated in a 5′ to 3′ direction;

D. binding of ribosome to mRNA;

E. small sub-unit then large;

F. first / initiator tRNA binds to start codon / to small subunit of ribosome;

G. AUG is the start codon;

H. second tRNA binds to ribosome;

I. large subunit moves down mRNA after a second tRNA binds;

J. mRNA “read” in base triplets / codon;

K. each codon specifying addition of a particular amino acid to the growing polypeptide;

L. tRNA bring amino acids (to mRNA-ribosome complex);

M. amino acid / polypeptide on first tRNA is transferred / bonded to amino acid on second tRNA;

N. peptide bonds between amino acids;

O. movement of ribosome / small subunit of ribosome down the mRNA;

P. loss of tRNA and new tRNA binds;

Q. reach a stop codon / termination

R. stop codon has no corresponding tRNA / amino acid / causes release of polypeptide