Chromosomes

Chromosomes

     Chromosomes (chroma = colour, soma = body) are tiny thread-like structures found in the nucleus of a cell. In a non-dividing cell they appear as a chromatin network while during cell division they become condensed to form short and thick chromosomes. Chromosomes are unique cell structures which are capable of replication. They store and transmit the coded information which is responsible for all the life processes of an organism. Hence, chromosomes are commonly described as carriers of heredity. The term "chromosome" was coined by Waldeyer in 1888 since these structures easily take up dye stains.

Bacterial Chromosome

     In bacteria, which being prokaryotic organisms, the entire hereditary material is packed into a single, irregularly folded compact mass called nucleoid or genophore or bacterial chromosome. It is short and simple consisting of a single DNA molecule. The DNA is in the form of a double helix which forms a closed ring or circle with no free ends. It is permanently attached to a mesosome, an infolding of the plasma membrane. The bacterial chromosome lacks a protein coat and it is in direct contact with the cytoplasm, since a nuclear membrane is absent. A small amount of protein, mainly in the form of an enzyme called RNA polymerase, may be found associated with the bacterial chromosome.

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.

Structure of Chromosomes

     The most ideal stage to study the structure of chromosomes is the metaphase of mitosis. A metaphase chromosome consist of two identical components called chromatids, which are held together at a specific region called primary constriction. It is usually found in the centre and hence it is commonly described as centromere. It shows a plate-like proteinaceous structure called kinetochore where the microtubules of the spindle become attached during cell division. The portions of a chromatid found on either side of a centromere, are called arms which may be equal (isobrachial) or unequal (heterobrachial), based on the position of centromere.

Chromosomes and Centromere

     Centromere is the primary constriction in the chromatid of a chromosome where the spindle tubules become attached during cell division. A chromosome generally has only one centromere and such a chromosome is described as monocentric. Sometimes two centromeres may occur in the chromosome as in the case of maize plant. Such a chromosome is described as dicentric. In such a chromosome each chromatid will have three arms. In the round worm Parascaris equorum the chromosomes have more than two centromeres and are hence described as polycentric. Very rarely, a centromere may be absent and such a chromosome may be described as acentric or holocentric. In such chromosomes the entire surface of the chromatid functions as the centromere.

Giant Chromosomes

     There are chromosomes which are extremely large compared to normal chromosomes. Such chromosomes, called giant chromosomes occur in some animal cells.

Autosomes and Allosomes

     In unisexual organisms, the chromosomes can be distinguished into autosomes and allosomes. Autosomes or somatic chromosomes carry genes which determine the somatic characteristics and do not have any influence on determining the sex of the organism. Allosomes are sex chromosomes which carry genes responsible for sexual characteristics and as such have a significant role in the determination of sex. Generally, one specific pair of chromosomes in the diploid number is identified as the allosomes.

Sex Determination

     It is the phenomenon where in the sex of an individual organisms becomes fixed soon after the formation of a zygote, due to the specific composition of sex chromosomes.

Linkage

     Every individual organism bears several heritable characters. Which are represented by the innumerable genes present on the chromosomes. During meiosis, the chromosomes move into the gametes as units, all the genes present on any given chromosome will segregate as a group and move together from generation to generation. This tendency of the genes located on the same chromosome, to stay together in hereditary transmission, is known as linkage. The genes located on the same chromosome are called linked genes.

Crossing Over

     Genetic recombination crossing over is the mutual exchange of the corresponding parts of the adjacent paternal and maternal chromatids of the pachytene of meiosisI, producing new combinations of genes. The chromatids resulting from the interchange of segments are known as the cross over recombinants and the chromatids that remain intact are called non-crossover parental chromatids.

Chromosome Mapping

     It has been noted that in the dihybrid genetic crosses, a recombination frequency of less than 50% indicates linkage of the two genes involved. Morgan and Sturtevant found that the recombination frequency varies for different mutant gene pairs. The difference in the recombination frequency was found to be associated with the distance between the genes.

Genetic Variation

     The variations at the level of the gene which form the basis of the differences between related organism as well as the diversity among organisms is called gene mutation. Gene mutation is defined as a sudden discrete change in the genetic material of gene which is heritable.

Summary

     Chromosomes store and transmit the coded information which is responsible for all the life processes of an organism. Hence, chromosomes are commonly described as carriers of heredity.