Staining

KARYOTYPE

Genetic Significance of Chromosomes

The chromosomes are considered as the organs of heredity because of following reasons: (i) They form the only link between two generations.

(ii) A diploid chromosome set consists of two morphologically similar (except the X and Y sex chromosomes) sets, one is derived from the mother and another from the father at fertilization.

(iii) The genetic material, DNA or RNA is localized in the chromosome and its contents are relatively constant from one generation to the next.

(iv) The chromosomes maintain and replicate the genetic informations contained in their DNA molecule and this information is transcribed at the right time in proper sequence into the specific types of RNA molecules which directs the synthesis of different types of proteins to form a body form like the parents.

A karyotype is the characteristic chromosome complement of a eukaryote species. The preparation and study of karyotypes is part of cytogenetics. The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n=46. In the germ-line (the sex cells) the chromosome number is n (humans: n=23). So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size. Karyotypes can be used for many purposes; such as, to study

chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.

Idiogram

The study of karyotypes is made possible by staining. Usually, a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. For humans, white blood cells are used most frequently because they are easily induced to divide and grow in tissue culture. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn fetus can be determined by observation of interphase cells (see amniotic centesis and Barr body).

Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for females contain two X chromosomes and are denoted 46, XX; males have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.

Six different characteristics of karyotypes are usually observed and compared:

1. differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication.

2. differences in the position of centromeres. This is brought about by translocations.

3. differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.

4. differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.

5. differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.

6. differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.

A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information. Variation is often found:

1. between the sexes

2. between the germ-line and soma (between gametes and the rest of the body)

3. between members of a population (chromosome polymorphism)

4. geographical variation between races

5. mosaics or otherwise abnormal individuals

Дата добавления: 2014-11-16