4. Genes

• DNA contains genes that determine the types of proteins made in the cell.

• A gene is a specific sequence of nucleotides on a DNA that codes for a specific sequence of amino acids on a polypeptide chain.

• Each specific sequence of nucleotides occupies a segment of the DNA molecule.

Top: An example of a gene which is a segment of the DNA.

Bottom: When the gene is unwound, there are two strands. The template strand is the reference used to synthesise the messenger RNA (mRNA) which is a temporary molecule that guides the synthesis of the polypeptide chain. The coding sequence is the sequence present in the mRNA except that thymine is replaced by uracil. More would be discussed in Section 5.

• • The sequence and types of bases on the template strand of the gene would determine the type and sequence of amino acids found on the polypeptide chain.

  • The gene shows 24 nucleotides, ie. 24 nitrogenous bases. Every 3 nitrogenous bases would code for 1 amino acid. Each triplet of bases is known as the codon. Usually the last codon (triplet of base) is the stop codon which does not represent an amino acid but instead acts as a signal that the gene stops there.