DNA the blueprint for life
This molecule can act as a blueprint for the manufacture of amino acids and has the capacity to replicate itself. Such properties occur in all life as we know it including the simplest forms such as bacteria. DNA's ability to replicate itself is due to its double helix structure. During cell division, the DNA molecule splits longitudinally, and each side acts as template to which simpler molecules become attached until each half has once more become a double helix. The simple molecules from which DNA is built are of four kinds and are grouped in trios, and these can be abbreviated A, T, C, and G representing Adenine, Thymine, Cytosine and Guanine respectively. These arranged in particular and significant orders. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand. Because each strand of DNA has a directionality, the sequence order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C; For each given base, there is just one possible complementary base, so naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence. These sequences of amino acids on the immensely long DNA molecule specifies how various amino acids are arranged in a protein, and how much protein is to be synthesized. A length of DNA bearing the information for an unbroken sequence of manufacture is called a gene.
Occasionally, the DNA copying process goes wrong. A mistake may be made at a single point on the length of the DNA and a particular molecule may become temporarily dislocated and be re-inserted in the wrong place. The copy is then imperfect and the protein that it synthesizes will be different. Such mistakes are sources of variation from which natural selection can produce evolutionary change. We now know that photosynthesising organisms had evolved as long ago as 3700 million years.