Читать книгу Horse Genetics - Ernest Bailey - Страница 58

Horse genes are packaged into 64 DNA molecules called chromosomes that are found in the nucleus of nearly every cell. Chromosomes can be seen with the aid of a microscope and dyes that bind to DNA or to the proteins associated with DNA. The genetic information of all domesticated horses is nearly identical and, not surprisingly, horses of all breeds have the same number, size, and shape of chromosomes.

Before a cell divides, chromosomal DNA is in an extended form and the areas active in transcription are exposed for action by polymerases. Electron micrographs show an image not unlike tangled spaghetti. When a cell starts the process of division into two daughter cells, chromosomes condense by supercoiling about chromatin proteins to form discrete rod-shaped bodies we recognize as the classical image of chromosomes. Careful cutting and matching of stained chromosome images obtained from microscopic examination of a cell in the process of division shows that the 64 chromosomes can be arranged as a series of 32 pairs of chromosome structures (Fig. 4.4). This array of paired and condensed chromosomes is visualized as a karyotype. The only distinguishing feature between most horse karyotypes is a difference between males and females seen in a single pair of chromosomes (the sex chromosomes), which are discussed in a later section.

Fig. 4.4. Microscopic images of dye-stained nuclei from horse lymphocytes (white blood cells) undergoing cell division (image: T.L. Lear).