The Day is past when the cell can be considered a bag containing a nucleus and a host of enzymes swimming freely within the cytoplasm. It is now clear that the cell is a highly organized, carefully integrated and delicately controlled synthetic and catabolic organ with well defined particulate structures to which many specific functions may be assigned.
The nucleus contains two specialized structures, the genetic material or the chromosomes, and the nucleolus. The chromosomes are strands of desoxyribopolynucleotide (DNA) and histone-like proteins which are invisible as organized structures until mitosis is well established (Fig. 1). The DNA consists of double strands of polynucleotide with specific configuration wound in the form of a helix. The chromatin during mitosis can be visualized as multiple indented strips, which have sufficient individuality as to appear unique. Each chromosome strand is made up of many molecules; a locus on a chromosome with specific genetic potential has been termed a cystron, and a structurally related group of cystrons form a chromomere. The individual cystrons have not been clearly visualized but it seems reasonable to suppose that each represents a single molecule. Why particular genetic material is associated with specific chromosomes is not known.
The number of chromosomes appears to be characteristic of the species, and within a given organism all resting nuclei, except those of the germ cells, contain the same number of chromosomes and the same amount of DNA. Certain nuclei, for example a small number of hepatocytes, contain multiples of diploid DNA.
The lateral portion of each chromosome has in association with it a ribose polynucleotide (RNA or PNA), and that region is a site of rapid synthesis of this substance.
- Copyright © 1960 by the American Academy of Pediatrics