3.2.2 Genes and polypeptides

In this lesson we covered the following points:

  • A gene occupies a fixed position called a locus on a particular strand of DNA
  • Genes code for polypeptides
  • The base sequence of a gene can change resulting in a mutation producing one or more alleles of the same gene.
  • A sequence of three bases called a triplet, codes for a specific amino acid. The base sequence of a gene determines the amino acid sequence of a polypeptide.
  • Eukaryotic DNA contains introns and exons.
  • In eukaryotes DNA is linear and is associated with proteins. In prokaryotes DNA molecules are smaller, circular and are not associated with proteins.


chromosomeIn the image the numbers 3 and 4 each represent the locus of particular genes. Locus = the location of a gene on a chromosome. Remember – you have two copies of every chromosome which means two copies of every gene. Sometimes these two copies of the same gene produce a different variation of a characteristic. In this case the genes are alleles of each other. Even though they are different variations of a gene they still occupy the same gene locus.



Remember that in eukaryotes (animals, plants and fungi – cells that have a nucleus) DNA is associated with proteins called histones. Histones work like little packaging elements which DNA winds around to reduce the amount of size it takes up in the cells.histones






The Genetic Code

Ben, Jack and Blomers supplied the following (which I have edited a bit to make it fit the video)

The genetic code is read in groups of three nucleotides.

A strand of DNA contains a string of nucleotides arranged into triplets. There are 64 different combinations of base when grouped together in threes. This gives 64 different triplets. These sequences of triplets correspond to a sequence of amino acids.

Mutations can occur and this may lead to changes in the amino acids associated with the triplets. There are 64 triplet codes and only 20 amino acids, meaning some of the amino acids can correspond to up to six different triplets. This acts as a defense mechanism against mutations.

The genetic code is non-overlapping, this means that successive triplets are read in order.

genetic code



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