Organism variation : DNA – Genes and Polypeptides

Organism variation : DNA – Genes and Polypeptides

A chromosome is made up of genes. Human cells contain 46 of these thread-like structures. Human cells are diploid because they contain two sets of matching chromosomes. Gametes, however, are haploid because they only contain one set (23 chromosomes).

Pairs of chromosomes are homologous pairs because they’re the same. Each chromosome in a pair contains genes which code for the same characteristic but each chromosome may differ in how they code for it. Different versions of the same gene are known as alleles.

Genes are sections of DNA and take a specific position or locus on a particular strand. Each gene is a specific sequence of bases that contain coded information to create particular polypeptides which create an organism and determine its development.

A specific amino acid is coded for by a sequence of three bases known as a base triplet. The base sequence of a gene then determines the sequence of amino acids in a polypeptide.

Within eukaryotes most nuclear DNA doesn’t actually code for the production of polypeptides. However, if there’s a difference in the base sequences of alleles of a gene this can lead to non-functional proteins, like enzymes.

Look below for an a level biology revision summary:



  • DNA is a polymer composed of nucleotides.
  • Nucleotides are made of a phosphate group, pentose sugar (deoxyribose) and an organic base.
  • There are four bases: adenine and guanine are purines and cytosine and thymine are pyrimidines.
  • Adenine pairs with thymine and guanine pairs with cytosine.
  • The sugar-phosphate backbone is composed of strong phosphodiester bonds.
  • The bases are connected by weak hydrogen bonds.
  • DNA is made from two polynucleotides which run antiparallel to each other.
  • Genes are sections of DNA at a specific locus.
  • An amino acid it coded for by a base triplet.
  • A mutation is a change in genetic material which can occur due to a mutagen.
  • A gene mutation can be due to a base addition, deletion or substitution.
  • Base additions and deletions can cause a frame shift in the sequence.
  • Base substitutions can have no effect: silent mutations.
  • Eukaryotes contain linear DNA while prokaryotes contain circular DNA and plasmids.
  • Meiosis produces genetically different cells called haploids.
  • Crossing over between chromatids aids genetic variation.