Genes and Genetic Codes
Gene = a length of DNA that codes for a specific polypeptide. Some genes can code for more than one polypeptide (or protein).
Polypeptide = polymer, chain of amino acids residues, which are joined by peptide bonds.
Genes code for the particular polypeptides that make up the enzymes, which are needed in the biochemical pathways to make that characteristic.
Genes code for: Haemoglobin, Immunoglobulins (antibodies), actin, myosin, electron carries, enzymes, keratin and cell surface receptors.
Different versions of the same gene are called Alleles. Example: in pea pods there is an allele for green pods and an allele for yellow pods. The colour of the seed pods is controlled by the gene.
The sequence of nucleotide bases on the gene provides a code for the construction of the protein. The genetic code:
- A triplet code = a mixture of the four nitrogenious bases
- Degenerate code = amino acids that can be encoded by more than one codon (The genetic code is generally universal, however, there are a few exceptions e.g. two codes for methionine)
- Codon = a sequence of three bases
- Non-overlapping = all of the bases are definite
- Some codes indicate "stop" = which is the end of the polypeptide chain