According to Cohn (1964), the term chromatin refers to the Feulgen positive materials observed in the interphase nucleus and later during the division of nucleus. they are long, fine thread-like structures 40 to 150 A0 in diameter. Ris (1969) has observed that chromatin fibers contain only single DNA molecule. The term chromosome was coined by W.Waldever in 1888.
Sutton of Columbia University published the behavior of chromosomes of cells. Morgan and Sturtevant showed that the units hereby must be arranged in linear order on the chromosomes. The chromosomes have the ability of self-reproduction and keep its form and the physiological properties through the cell division. They play important role in variation, heredity, mutation etc.
Number of Chromosomes
In every species, the number of chromosomes is generally constant containing diploid number of chromosomes in their somatic cells and a haploid number of chromosomes in their gametes.
The number of chromosomes contributes to the determination of the taxonomic position of a plant or animal species. The chromosome number differs from species and it may range from two to several hundred. The smallest number of chromosomes is found in Ascaris megalocephaly, having two somatic chromosomes i.e. one haploid chromosome. The maximum number of chromosomes is found in protozoan. The group aggregate possesses more than 300 chromosomes while in Radiolaria the number even reaches up to 1600.
|Organism||Species||Diploid (2N) Chromosome Number|
|Fruit fly||Drosophila melanogaster||8|
|House fly||Musca domestica||12|
|Red ant||Formica sanguinea||48|
|Rhesus monkey||Macaca mulatta||42|
|Organism||Species||Diploid (2N) Chromosome number|
|Broad bean||Vicia faba||12|
|Corn (maize)||Zea mays||20|
|Cotton, American-Egyptian||Gossypium barbadense||52|
|Cotton, upland||Gossypium hirsutum||52|
|Garden pea||Pisum sativum||14|
|Kidney bean||Phaseolus vulgaris||22|
|Oats, white||Avena sativa||42|
|Oats, red||Avena byzantina||42|
|Sugar beet||Beta vulgaris||18|
|Sugar cane||Saccharum officinarum||80|
|Wheat, durum||Triticum durum||28|
|Wheat, common||Triticum vulgare||42|
|Wheat, club||Triticum compactum||42|
The shape and Size
The shape of chromosomes depends up to the position of the centromere, on the secondary constriction and on the localization of satellites. The chromosomes are generally rod-like and acrocentric, sometimes like V-shaped as amphibians. Metaphase or early anaphase is the best condition to observe the shape of the chromosome in an organism cell. The length of a chromosome varies from 0.1µ to about 30 µ and the diameter 0.2 µ to 2 µ.
The general structure of somatic chromosomes can be studied best at the metaphase and anaphase of mitosis.
Each comprises following parts:
- Pellicle and Matrix
- Chromonemata (Chromatid during Metaphase)
- Satellite bodies
1. Pellicles and Matrix:
- Pellicles: It is the outer most covering of the chromosome, formed of non-genetic material. Darlington (1935) and Ris (1945) discarded the presence of pellicle in the chromosome.
- Matrix: Inside the pellicle has filled a jelly-like substance called Matrix. It is also composed of non-genetic material.
- The matrix is formed from the nucleus during telophase of cell division. The structure and function of the matrix are not fully known.
- Embedded in the matrix chromosome, there are two similar and spirally coiled chromonemata.
- These are so intimately held together with each other that they appear to be one.
- There is present a long thread throughout the chromosome which is called Generally, the chromonema of one chromosome is identical in nature to the chromonemas of other chromosomes in the same cell.
- Each chromonema bears a number of bead-like bodies called chromomeres. The regions in between chromomeres are inter-chromosomes. The position of the chromomeres is relatively constant for a given chromosome.
- Some authors are of the opinion that chromomeres represent a condensation of nucleoprotein material.
- Others agree with the view that there are regions in which there is a superimposition of coils. DeRobertis in 1956 Ris in 1957 by electron microscopic studies proved that there is nothing like matric between the coils of chromonema and a pellicle surrounding each chromosome.
- In every chromosome, there is present generally a non-staining region which looks like a constriction. It is called as Centromere or kinetochore.
- During cell division, the spindle fibers remain attached to this region. This point also plays an important role in the chromosomal movement during mitosis.
- Darlington and Schrader suggested that the centromere is very similar to the centriole.
- The centromere and centriole behave similarly during mitosis; they appear similar in the cell and show similar reactions towards the stains.
Types of Chromosome
On the basis of position and number of centromeres, the chromosomes may be of following types:
- Acentric: The chromosome which is without centromere is called Acentric.
- Monocentric: When chromosomes have only one centromere, it is known as monomeric.
- Dicentric: A chromosome with two centromeres is termed
- Polycentric: A chromosome possessing more than two centromeres is called polycentric. White in 1936 reported polycentric chromosomes is Ascaris
- Metacentric: When the centromere is present about the middle of chromosome then the chromosome is termed as metacentric.
- Acrocentric: When centromere is located at the end of the chromosome in the chromosome is termed as telocentric.
- Satellites: The chromosome may have a rounded or elongated body at its end. The rounded body is connected by a thin chromatic filament. It is called a satellite. In diameter, it is equal or smaller than that of the chromosome on which it is present. Satellite bearing chromosomes are called SAT (sine acido thymonucleinico)
- Telomere: The chromosomal terminations at either end are known as Telomeres (Mullur, 1938). They have specific uniqueness. A broken end of one chromosome can fuse with that of the other chromosome but intact parts of telomeres can’t fuse with that of the other chromosome but the intact parts or telomeres can’t fuse in any case. Because of this type of polarity, the telomere prevents the other chromosomal parts to fuse with them.
The chromosomes are mainly composed of nucleic acids and proteins. The DNA and hist-protein combine together and form deoxyribonucleoprotein, which is 90% of the chromosome, DNA being 35%and histone protein about 55%. The remaining 10% part of a chromosome is termed as a residual chromosome. Stedman (1934) reported another type of protein called Chromosomin in addition to histone or protamines.
Functions of Chromosomes
The chromosome functions are given below
- They control the physiological behavior of an organism with the help of genes present in them.
- We know each chromosome is made up of DNA and this DNA by replication gives rise to messenger RNA (mRNA) which carry the genetic information in the form of code. This mRNA comes out of the nuclear wall into the cytoplasm where it helps to form a particular kind of protein needed by the cell or body.
- Sometimes regions within the chromosomes change their position which leads genetic effect or mutations. Such an effect is termed as position effect which is due to shifting in position of heterochromatic and euchromatin parts.
- Heterochromatin aids in the formation of the nucleus.
- They are a hereditary vehicle carrying the genetic information from one generation to the other.