Annex: Introduction to genes, the DNA molecule and other molecules
    Genes are composed of a molecule called DNA (deoxyribonucleic acid).  DNA is a molecule composed of four types of building stones (nucleotides) called A, C, G and T (Fig.A).  In the DNA molecule, two chains (strands) of nucleotides are interacting by highly stable pairings between the nucleotides.  Consequently, the DNA molecule is double stranded.  The sequence of these four nucleotides on one strand is the code of the DNA molecule, while the sequence on the other strand is complementary (like a mirror image) and among other helps to stabilize the DNA molecule.  The A's on one strand always pair with T's on the other strand and vice versa, and similarly C's and G's always pair.  The total amount of DNA in a cell is called the genome of the cell.  Genes are like codes that must be read and translated into functional gene products.  Gene products are usually amino acids that form proteins, but gene products can also be RNA (ribonucleic acid) molecules.  RNA molecules, although very similar to DNA molecules are usually single stranded, and much more unstable.  Gene products are responsible for biochemical reactions that in turn yield the features that can be observed, e.g. chemical defence against insects, nutritional value, colour, size etc.