In this article we will discuss about:- 1. Meaning of Chromosomes 2. Number of Chromosomes 3. Size 4. Chemical Composition 5. Functions 6. Special Types.
Meaning of Chromosomes:
Chromosomes are the most important constituents of the nucleus and were first observed by Holfmeister (1848) in pollen mother cell of Tradescantia and called it nuclear filaments. Chromosomes are filamentous bodies which are typically present in the nucleus and become visible during cell division. They are units of heredity or the carriers of genes.
Chromosomes are not clearly visible in prophase stage of the cell division due to high water content but are clearly visible during metaphase and anaphase respectively. Most of the chromosomes in a cell are called autosomes.
There are one or two of sex-chromosomes or allosomes or heterosomes which carry the genes for determination of sex. The chromosomes of eukaryote organisms are being described only. The term ‘chromosome’ was given by Waldeyer (1888). Boveri and Sutton described chromosome as bearer of hereditary characters. According to them, the chromosomes are physical structures responsible for transmission of hereditary characters.
Number of Chromosomes:
It has been generally observed that all the individuals of a species have the fixed and identical number of chromosomes —Beneden and Boveri (1887). The species which are closely related exhibit or show the similar chromosome number. Presence of whole sets of chromosome is referred to as euploidy. It includes haploids, diploids, triploids, tetraploids, pentaploids, hexaploids etc.
Normally a gamete contains only one set of chromosome. This number is called haploid number (n). Somatic or mitotic cells normally contain two sets of chromosomes. This number is called the diploid number (2n). Triploids, tetraploids, pentaploids, hexaploids have three, four, five, six sets of chromosomes respectively.
An organism having more than two sets of chromosomes is said as polyploid and the phenomenon polyploidy. When the change in chromosome number does not involve complete sets of chromosome, the condition is called aneuploidy.
The example of aneuploidy is as follows:
The minimum haploid chromosome number has been recorded in eukaryotes is 2. e.g., Mesotoma (flatworm) and Ophryotrocha puerillis (polychaeta). In Ascaris megalocephala also chromosome number is found 2. In higher plants the minimum number is 4 i.e., Haplopappus gracilis (Compositae) and maximum number 1266 in Pteridophyte (Ophioglossum reticulatum).
There are a few species where haploid chromosomes exceed more than 15 whereas in animals up to 50 are usual. In most plants and animals number of haploid chromosomes ranges between 6-25. In animals the highest recorded chromosome number is 1600. Generally, plant chromosomes are bigger than the animal ones and chromosomes of monocots are bigger than those of dicots and other plants.
The following table gives the diploid (= somatic) and gametic number of chromosome in some common plants and animals:
Size of Chromosome:
The length of chromosome ranges from 0.25 µm (fungi and birds) to 30 µm (Trillium, Liliaceae). It does not include salivary gland chromosomes of diptera which may be 2 mm long. All the chromosomes of a species are of similar size (symmetrical karyotype). In the asymmetrical case there may be two size groups e.g., Yucca or a gradual series of different sizes e.g., man.
Size differences may be found in the different species of a genus. For example, the chromosomes of Allium porum are half the size of the chromosomes of Allium sativum. Chromosome size may also vary within the species or in different varieties. In the plant Mediola, for example, the chromosomes of root tip are 50% longer than the chromosomes of shoot tip.
Heterochromatin and Euchromatin:
Chromosomes are composed of materials called as chromatin. Chromatin may be classified in two groups on the basis of its stain-ability with basic dyes and other characters during the various stages of cell cycle.
Chemical Composition of Chromosome:
1. DNA — 40%
2. RNA — 1.5% (Helps in synthesis of chromosomal fibres)
3. Histone (Protein) — 50%.
4. Non-Histone (Protein) — 8.5%;
5. Some amount of calcium which is attached with DNA. DNA is the chief and most significant part of chromosome)
According to Mirsky and Ris (1945), the chemical composition of an isolated chromosome is as follows:
Protein in the chromosome probably acts as a framework to which the different nucleic acids are attached. The electron microscopic studies of chromosomes demonstrated that chromosomes contain very fine threads having a thickness of 2nm-4nm.
It can be considered that the thickness of a chromosome is usually hundred times more than that of DNA and the length of DNA present in chromosome is many hundred times more than the length of chromosomes themselves.
Functions of Chromosomes:
1. The chromosomes carry genes or DNA molecules, hence are carriers of heredity.
2. During total life of any organism chromosomes preserve or maintain their identity and continuity.
3. It indirectly helps in protein synthesis. Nucleolus contains RNA and this serves as a means of transmission of genetic information’s of cytoplasm.
4. The sex chromosomes or allosomes decide the sex of the individual.
5. The number of chromosomes always remain constant for a particular species.
6. One chromosome is related with only one set of physiological functions while other chromosomes will have other pairs of other character. The autosome may under go any change but sex chromosome or allosome will remain unchanged. Sex chromosome, if changed by mutation or aberration, will give rise to intersex or gynandrosporous individual.
Special Types of Chromosomes:
Two different types of chromosomes which are much larger in size than the mitotic or meiotic chromosomes are called giant chromosomes.
(a) Lampbrush Chromosome:
Ruckert (1892) was the first to observe these chromosomes in the oocyte nucleus of both vertebrates and invertebrates. They are thin but very long and possess radiating hairs or side loops due to which said as lampbrush or hairy chromosomes. These chromosomes have been observed in birds, amphibia, fish under vertebrates and their function is synthesis of RNA and protein by the loops. They are also related with the formation of yolk in the egg.
The main axis of the lampbrush chromosome is formed of DNA and protein and is continued with the axis of loops, which also appear to be composed of DNA and protein. A fuzzy look of the loops is due to the presence of matrix around their axes. The matrix is made up of protein and RNA.
According to some cytologists, the lateral loops or hairs develop from chromomeres, which take a deep stain and represents tightly coiled parts of the chromosomal axis. Loop axis is composed of two double helices of DNA. The diameter ranges from 30-50 Å.
A study of the developing oocytes demonstrate that the loop grow in number and size up to a maximum in diplotene and then decrease and disappear up to metaphase. This decrease may occur either by disintegration or by reabsorption in to the chromomere. On this basis, loops may be considered to be formed of chromatin material synthesized by chromomeres and not an integral part of chromonemata.
There are no lateral loops in the centromere region. However, Gall (1956) and Ris (1957) believed by observing it under electron microscope that the loops are a part of chromonemata. These chromosomes are remarkably elastic and regain their size without any distortion after stretching by micro needle.
At the base of loop near the chromonema, matrix is thick at one end than the other end and called thick and thin insertion respectively. The loops represent the modified chromosome structures of the loci of active genes.
(b) Salivary Gland or Polytene Chromosome:
It was observed for the first time by E.G. Balbiani (1881) in the salivary gland cell of the larvae of Diptera e.g., Drosophila. The name ‘polytene chromosome’ was suggested due to the presence of many chromonemata in them. Polytene chromosomes are also found in some other organs as foregut, midgut and malpighian tubules of the dipteran insects as Drosophila besides the salivary glands.
The salivary gland chromosomes are the largest chromosome known so far and consist of class pairing of a pair of homologous chromosomes. Several duplication of the chromosomes has been found without cell division.
Along the complete length of each of the chromosome are dark bands and light inter-bands. The dark bands are Fuelgen positive which are made up of DNA, proteins and take darker stain than others. The inter-bands are Fuelgen negative having affinity for basic stains and are made up of fibrils. The bands pattern is specific.
The bands represent genie loci. It is possible to associate some gene loci with particular band. The inter-bands have also been found to be genetically active portion of the chromosome.
During development of dipteran larvae some of the bands and inter-bands show swellings or puffs which depends upon the stage of larval development. Some portion show larger puff than the others and is called as chromosomal puff. According to Beermam (1956) the chromonemata laterally form series of loops which make these rings.
These Balbiani rings are present around the chromosome and are supposed to be formed by chromonemata. It is possible that the metabolic activities required for the formation of puffs are related to the secretory function of the salivary glands.
These puffs increase the thickness of chromosome and give it fuzzy appearance. The formation of these puffs are controlled by some specific genes and are related with the synthesis of RNA and protein. The very large puffs are called Balbiani rings. These rings are rich in DNA and mRNA.
The main work of polytene chromosome is to carry the genes which are necessary for controlling the physiology of an organism. The bands have greatly helped in chromosome mapping during cytogenetics studies. It has been observed the shifting of heterochromatin in to euchromatin brings about giant changes. These are called Position effects. These position effects cause mutation.
(Change in the phenotypic expression of a gene due to a change in its position in the chromosome; usually, when gene is placed within or close to a heterochromatin segment; due to heterochromatinization, the gene is not expressed.)