| |
|
|
| |
 |
| Eukaryotic Chromosome |
|
| |
| The eukaryotic cells show a varied number of chromosomes. It is presumed to be the result of breaking up of a single long chromosome into several short units, in the course of evolution, to accommodate the increase in the amount of genetic information. The eukaryotic chromosomes occur inside the nucleus of a cell, separated from the cytoplasm by a distinct nuclear membrane. |
| |
| The eukaryotic chromosomes become clearly visible when the cell enters the phase of division. It is particularly in the metaphase stage that chromosomes can be clearly studied for their number, shape, size and structure. |
| |
|
| The number of chromosomes is fixed for a given species. It varies from a minimum of two to several hundreds in different species of plants and animals. All individuals of a given species show the same chromosome number in all their body cells. The number of chromosomes in a species has no specific significance nor does it indicate any relationship between two species which may have the same chromosome number. |
| |
| The chromosome number of a given species is generally represented as the diploid number (2n) since chromosomes occur in pairs. The following table lists the diploid chromosome number of some familiar examples. |
| |
| Chromosome number (2n) of some organisms. |
| |
| Animal Examples |
| |
 |
| |
 |
| |
| Plant Examples |
| |
 |
| |
| In some cases, males have one chromosome less than females. In the round worm Coenorhabditis males and females have 11 and 12 chromosomes respectively. In the cockroach Blatta orientalis males and females have 23 and 24 chromosomes respectively. This is the case of XX-XO sex determination. |
| |
| In some cases, the chromosome number will be haploid (n) indicating the presence of unpaired chromosomes. Haploid number is a characteristic feature of gametes: the sperm and the ovum in animals or the gametophytic generation in plants. Haploid chromosome number may be seen in some adult organisms also. Yeasts show both haploid and diploid cells. Chlamydomonas, a unicellular green alga has a eight chromosomes (n=8). The male honeybees are haploid (n=16). |
| |
|
The size of chromosomes varies from species to species. In the same species, all the chromosomes occurring in a given cell will not be of the same size. However, in most organisms, chromosomes fall in the size range of 0.1 to 30  in length and 0.2 to 2.0  in thickness. Chromosomes are relatively larger in size where the number is less. In general, chromosomes are larger in size in plants than in animals. Among plants, cells of monocots contain larger chromosomes than cells of dicot plants. |
| |
|
|
| |
|
|
| |
|
|
|
|
|
