The first descriptions of the chromosomes of eukaryotic cells appeared between 1840 and 1880, but it was not until 1888 that Waldeyer introduced the term chromosome (“colored body”) for these structures.
Chromosomes are composed of chromatin, which readily binds basic stains. Because the chromatin is highly condensed during cell division, the chromosomes are easily seen and described by light microscopy.
The number of chromosomes in the cell nucleus varies considerably among different animal and plant species; however, each species has a specific chromosome number.
For example, cells of the nematode Ascaris megalocephala have only 2 chromosomes, human cells have 46, and certain protozoa have over 300. Unrelated organisms may have the same chromosome number. The potato plant has 48 chromosomes, but so do plum trees and chimpanzees.
Within a species, each chromosome exhibits a specific and characteristic shape during the metaphase of cell division. The unique appearance of the metaphase chromosomes is retained from one generation of cells and organisms to the next (Fig. 20-2).
Chromosome shape and size change during the stages of nuclear division. Most chromosomes have two arms, one on each side of the primary constriction or centromere (also called kinetochore). Metaphase chromosomes already have undergone replication so that each chromosome consists of sister chromatids and therefore appears to have two sets of arms (Fig. 20-3).
The centromere is the site of attachment of the chromosome to the microtubules of the spindle and acts as the focus of chromosome (or chromatid) movement during the anaphase of division. Chromosomes that lack a centromere are said to be acentric and fail to segregate normally during division. Secondary and tertiary constrictions may also be identified. Secondary constrictions are associated with nucleoli and are called nucleolar organizer regions (NOR). The significance of the tertiary constrictions is unclear, but their characteristic disposition in each chromosome helps to distinguish one chromosome from another.
Fine Structure of Chromosomes:
The condensed chromosomes visible during mitosis are composed of an organized array of chromatin fibers, but in their de-condensed state give rise to a highly disperse network (Fig. 20-4). Each chromatin fiber is believed to contain one molecule of DNA.
The average diameter of Type A chromatin fibers is about 10 nm, whereas the diameter of the DNA double helix is only 2 nm. The difference is due to the coiling of the DNA molecule and the presence of large quantities of proteins. Type B fibers are even thicker (20-30 nm), the increased thickness resulting from additional levels of coiling. Chromatin of varying thickness exists in the interphase nucleus and is presumed to consist of alternating areas of type A and B fibers.
An indication of the extent of DNA coiling that occurs in a chromosome is obtained from the packing raio (i.e., the length of DNA divided by the length of the chromatin fiber or whole chromosome). For decondensed interphase chromatin this ratio is about 10:1, but for metaphase chromosomes it is more than 1000:1. The extensive amount of coiling and folding that characterizes DNA permits storage of genetic information in a highly compact form.