Morphology of chromosome:
The morphology of chromosome can be best studies at the metaphase or anaphase of mitosis when they are present as definite organelles, being most condensed or coild.
Number:
The number of chromosomes in a given species is usually constant containing diploid number (2n) of chromosomes in their somatic cells and haploid (gametic or reduced) number (n) of chromosomes in their sex cells (sperms and ova). The number of chromosomes is variable from one to several hundred among different species
For example, in Ascaris megalocephala it is 2, while in certain protozoans (Aggreta), there are more than 300 chromosomes, in Paramecium 30 to 40, in radiolarians as many as 1600, in Hydra vulgaris 32, Musca domestica 12, Rana esculenta 26, Columba livia 80, Oryctolagus cuniculus 44, Gorilla gorilla 48 and Homo sapiens (man) 46.
The chromosome numbers are also helpful for taxonomy. In the angiosperms the most frequent haploid number is 12 and members of this group have a range from 3 to 16. Similarly, in fungi, haploid number ranges from 3 to 8.
In primates this haploid number is from 16 to 30. This haploid set of chromosomes present in the nucleus of gametes in while in a diploid cell there will be two genomes. The diploid cells are the somatic cells in body. The diploid cells get the diploid set of chromosomes by the union of the haploid male and female gametes in the sexual reproduction.
Size:
Chromosomes range, on an average from 0.5 to about 30µ in length and from 0.2 to Зµ in diameter. The relative number of chromosomes generally differ in the nucleus but at a time all chromosomes of a cell may be of the same size. Plant cells normally posses larger chromosomes then animal cells.
Trillium has chromosomes which may reach up to the length of 32µ at metaphase. Monocotyledon plants usually have larger chromosomes than the dicotyledon which contain greater number of chromosomes. Among the animals, grasshoppers, crickets, mantids, newts and salamanders have large chromosomes.
Variation in size of the chromosomes can be induced by a number of environmental agents:
1. Cells dividing at low temperature have shorter, more compact chromosomes than those dividing at high temperature.
2. Colchicine is an alkaloid which interferes with spindle formation and cell division. It tends to shorten the chromosomes.
3. Rapid and repeated division tends to result in smaller chromosomes. It appears that rate of cell division proceeds more rapidly than the formation of chromatin material as usual.
4. In plants, the amount of phosphate in the nutritional https://somabest.com medium has marked effect on the size of the chromosomes; high concentration gives larger chromosomes than those plants deficient of phosphates. Since the phosphate is an integral part of nucleic acid molecule, it would seem that the amount of nucleic acid in the chromosome can be varied to give alterations in size.
Shape:
The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division. In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread-like stainable structures, the chromatin threads.
In the metaphase and the anaphase, the chromosomes become thick and filamentous. Each chromosome contains a clear zone, known as centromere or kinetochore, along their length. The centromere divides the chromosomes into two parts, each part is called chromosome arm.
The position of centromere varies from chromosome to chromosome and it provides different shapes to the later which are following:
1. Telocentric:
The rod-like chromosomes which have the centromere on the proximal end are known as the telocentric chromosomes.
2. Acrocentric:
The acrocentric chromosomes are J-like in shape but these have the centromere at one end and thus giving a very short arm and an exceptionally long arm. The locusts (Acrididae) have the acrocentric chromosomes.
3. Sub-metacentric:
The sub-metacentric chromosomes are L-shaped. In these, the centromere occurs near the centre or at medium portion of the chromosome and thus forming two unequal arms.
4. Metacentric:
The metacentric chromosomes are V-shaped and in these chromosomes the centromere occurs in the centre and forming two equal arms. The amphibians have metacentric chromosomes.
Karyotype
A karyotype is simply a picture of a person’s chromosomes. In order to get this picture, the chromosomes are isolated, stained, and examined under the microscope. Most often, this is done using the chromosomes in the white blood cells. A picture of the chromosomes is taken through the microscope. Then, the picture of the chromosomes is cut up and rearranged by the chromosome’s size. The chromosomes are lined up from largest to smallest. A trained cytogeneticist can look for missing or extra pieces of chromosome.
There are 22 numbered pairs of chromosomes called autosomes. The 23rd pair of chromosomes are the sex chromosomes. They determine an individual’s gender. Females have two X chromosomes, and males have an X and a Y chromosome.
Idiogram is a diagram or graphic representation of a karyotype showing the number,relative sizes,and morphological characteristics of the chromosomes of a species,individual,or cell.
Ideograms are diagrammatic or idealized representations of chromosomes, showing their relative size, homologous groups and cytogenetic landmarks. Chromosomal analysis of cytological preparations involves calculation of karyotypic parameters and generation of ideograms.
