INTRODUCTION All cells come from pre-existing cells

Shortly after the beginning of the twentieth century the chromosome was shown to be the cellular structure responsible for heredity. The stage was then set for the extraordinarily rapid growth in under-

standing cell division and heredity in the twentieth and twenty-first centuries. In 1944 Oswald T. Avery and co-workers showed that DNA was the material that carried specific inherited characteristics, but it was

not until 1953 that James Watson, Francis Crick and Rosalind Franklin showed that DNA encoded information by the precise sequence of nucleotide bases in the DNA molecule.