Building a Scaff old: Genes Within Chromosomes

Abstract Between 1865 and 1900 major developments occurred in the study of cells, resulting in a field called cytology. It was in the structure and function of the nucleus where important discoveries were made. It was shown that during part of the life cycle of a cell, material present in the nucleus condensed into thread-like structures called chromosomes. The number of chromosomes was the same in all cells of an organism and was characteristic of the species. Prior to cell division, the chromosomes divided in a process called mitosis and each daughter cell received one complete set of chromosomes. In the formation of the male and female germ cells a reduction occurred in the number of chromosomes. During one cell division, the chromosomes did not divide and their number was reduced to half. This reduction division was later called meiosis. At fertilization, male and female germ cells were united and the full complement of chromosomes was restored. All these observations on mitosis and meiosis were made possible by improved techniques of microscopy and by the development of staining techniques by which chromosomes could be visualized. The word chromosome, introduced in 1885, reflects this technique (chromo = color; soma = body).