The definition of a gene

• Gene - A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA molecule
• A gene is a base sequence of DNA that codes for:

  • The amino acid sequence of a polypeptide
  • A functional RNA
• The coded information in a gene is in the form of a specific sequence of bases along the DNA molecule
• A gene is a section of DNA that occupies a fixed position, called a locus on a particular DNA molecule
• Functional RNA molecules are required for protein synthesis:

  mRNA - The base sequences on messenger RNA molecules are used by ribosomes to form polypeptide chains
• Functional RNA molecules are required for protein synthesis:

  tRNA - Amino acids are carried to the ribosome by transfer RNA molecules
• Functional RNA molecules are required for protein synthesis:

  rRNA - ribosomal RNA molecules form part of the structure of ribosomes
• The shape and behaviour of a protein molecule depends on the exact sequence of amino acids

  • (the initial sequence of amino acids is known as the primary structure of the protein molecule)
• Polypeptides make up proteins and so genes determine the proteins of an organism

  • Therefore, the genes in DNA molecules control protein structure (and as a result, protein function) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell
• Allele - An alternative form of a gene
• Each gene can exist in two or more different forms called alleles

  • Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome