Abi Starr
Chapter 14.3 Polypeptide synthesis – translation
The triplet code of DNA is transcribed into a sequence of codons (genetic code) on mRNA. The next
stage is to translate the codons on the mRNA into a sequence of amino acids that make up a
polypeptide.
Although the basic structure of tRNA is always the same, the sequence of three bases on the
anticodon loop varies. There are at least 60 variants, each of which corresponds to a codon of three
bases on the mRNA. At the other end of the tRNA molecule, there is a point of attachment for an
amino acid. Therefore, each amino acid has its own tRNA molecule, with its own anticodon of bases.
Synthesising the polypeptide
Once mRNA has passed out of the nuclear pore it determines the synthesis of a polypeptide. The
process occurs as follows:
A ribosome, attaches to the starting codon at one end of the mRNA molecule
The tRNA molecule with the complementary anticodon sequence moves to the ribosome
and pairs up with the sequence on the mRNA. This tRNA carries the amino acid, methionine.
A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA.
This tRNA molecule carries another amino acid, threonine.
The ribosome moves along the mRNA, bringing together two tRNA molecules at any one
time, each pairing up with the corresponding two codons on the mRNA
By means of an enzyme and ATP, the two amino acids on the tRNA are joined by a peptide
bond.
The ribosome moves on to the third codon in the sequence on the mRNA, thereby linking
the amino acids on the second and third tRNA molecules
As this happens, the first tRNA is released from its amino acid and is free to collect another
from the amino acid pool in the cell
The process continues in this way, with up to 15 amino acids being linked each second, until
a complete polypeptide chain is built up
Up to 50 ribosomes can pass immediately behind the first, so that many identical
polypeptides can be assembled simultaneously
The synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this
point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide
chain is complete.
In summary, the DNA triplets that make up a gene determine the codons on mRNA. The codons on
mRNA determine the order in which the tRNA molecules line up. They, in turn determine the
sequence of amino acids in the polypeptide. In this way genes precisely which proteins a cell
manufactures. As many of these proteins are enzymes, genes effectively control the activities of
cells.
Assembling a protein
Sometimes a single polypeptide chain is a functional protein. Often, a number of polypeptides are
linked together to give a functional protein (quaternary structure). What happens to the polypeptide
next depends upon the protein being made, but it usually involves the following:
The polypeptide is coiled or folded, producing its secondary structure.
The secondary structure is folded, producing the tertiary structure
Page 1 of 3
Chapter 14.3 Polypeptide synthesis – translation
The triplet code of DNA is transcribed into a sequence of codons (genetic code) on mRNA. The next
stage is to translate the codons on the mRNA into a sequence of amino acids that make up a
polypeptide.
Although the basic structure of tRNA is always the same, the sequence of three bases on the
anticodon loop varies. There are at least 60 variants, each of which corresponds to a codon of three
bases on the mRNA. At the other end of the tRNA molecule, there is a point of attachment for an
amino acid. Therefore, each amino acid has its own tRNA molecule, with its own anticodon of bases.
Synthesising the polypeptide
Once mRNA has passed out of the nuclear pore it determines the synthesis of a polypeptide. The
process occurs as follows:
A ribosome, attaches to the starting codon at one end of the mRNA molecule
The tRNA molecule with the complementary anticodon sequence moves to the ribosome
and pairs up with the sequence on the mRNA. This tRNA carries the amino acid, methionine.
A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA.
This tRNA molecule carries another amino acid, threonine.
The ribosome moves along the mRNA, bringing together two tRNA molecules at any one
time, each pairing up with the corresponding two codons on the mRNA
By means of an enzyme and ATP, the two amino acids on the tRNA are joined by a peptide
bond.
The ribosome moves on to the third codon in the sequence on the mRNA, thereby linking
the amino acids on the second and third tRNA molecules
As this happens, the first tRNA is released from its amino acid and is free to collect another
from the amino acid pool in the cell
The process continues in this way, with up to 15 amino acids being linked each second, until
a complete polypeptide chain is built up
Up to 50 ribosomes can pass immediately behind the first, so that many identical
polypeptides can be assembled simultaneously
The synthesis of a polypeptide continues until a ribosome reaches a stop codon. At this
point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide
chain is complete.
In summary, the DNA triplets that make up a gene determine the codons on mRNA. The codons on
mRNA determine the order in which the tRNA molecules line up. They, in turn determine the
sequence of amino acids in the polypeptide. In this way genes precisely which proteins a cell
manufactures. As many of these proteins are enzymes, genes effectively control the activities of
cells.
Assembling a protein
Sometimes a single polypeptide chain is a functional protein. Often, a number of polypeptides are
linked together to give a functional protein (quaternary structure). What happens to the polypeptide
next depends upon the protein being made, but it usually involves the following:
The polypeptide is coiled or folded, producing its secondary structure.
The secondary structure is folded, producing the tertiary structure
Page 1 of 3
