In this article we will discuss about:- 1. Definition of Chromosomes 2. Types of Chromosomes 3. Functions.
Definition of Chromosomes:
Chromosomes are rod shaped or threadlike deeply stainable condensed chromatin fibres which are hereditary vehicles as they store and transmit coded hereditary information. Chromosomes appear only during karyokinesis. They are meant for equitable distribution of genetic material. The number is fixed and is the same in all the individuals of a species.
There is a single set in gametophytic or haploid forms and two sets in sporophytic or diploid forms. Size and shape of individual chromosomes are quite distinct. The shape is more clearly visible in late prophase and metaphase (as well as anaphase) when primary constriction or centromere becomes distinct.
During prophase and metaphase, the chromosomes are replicated. There are two chromosome halves or chromatids. The two chromatids are attached to each other by a narrow area called centromere or primary constriction. Anaphasic chromosomes do not have chromatids.
The two parts of a chromosome or chromatid on either side of primary constriction are called arms. The two arms are equal in isobrachial chromosomes and unequal in heterobrachial chromosomes. The ratio between the two arms of a chromosome is called centromeric ratio.
Types of Chromosomes:
Centromere terminal in the area of telomere,
Centromere inner to telomere (= sub-terminal).
Besides primary constriction or centromere, a chromosome may have one or more secondary constrictions. A secondary constriction present near the distal part of an arm may develop a small outgrowth or fragment called satellite. Satellite is connected to secondary constriction through a chromatin thread.
A chromosome having satellite is called sat chromosome. Sat chromosomes are called marker chromosomes. Other secondary constrictions can also function as markers because they occupy a constant position.
Under light microscope, cytologists found that a chromosome contains a coiled filament called chromonema. Chromonema was thought to be gene bearing part.
Some workers thought that a chromosome may have several chromonemata. Electron microscope has revealed that a chromosome is actually formed by direct condensation of loops of chromatin fibre attached to a scaffold. It is 30 nm in diameter and contains a single DNA duplex.
They are of two types, polytene and lampbrush.
Polytene Chromosomes (Gk. polys- many, tainia- threads; KQllar, 1882):
Polytene chromosomes were first reported by E.G. Balbiani in 1881. They are quite common in salivary glands of insects and are, therefore, popularly called salivary chromosomes. Polytene chromosomes also occur in other organs of insects, antipodal cells (of embryo sac), endosperm cells and suspensor cells of embryo (Nagl, 1974; Malik and Singh, 1979).
The chromosomes can reach a length of 2000 µm and contain 1000 (Drosophila) to 16000 0Chironomus) times DNA as compared to the ordinary somatic chromosomes. Polytene chromosomes are multi-stranded. They are in permanent prophase stage. The giant chromosomes are formed by somatic pairing between homologous chromosomes and repeated replication (endomitosis) of their chromonemata.
All the polytene chromosomes may remain attached to one another at a common point called chromo Centre. It represents pericentromeric heterochromatin which is slow to replicate. Polytene cells and their nuclei are large-sized. Polytene cells cannot divide further. They ultimately die. The adult organs develop from some small-sized diploid cells lying nearby.
Depending upon their reaction to basic dyes, the polytene chromosomes bear a number of dark bands of various sizes and intensity. They are separated by light areas called inter-bands. The dark bands are presumed to be formed by the juxtaposition of chromomeres of the different chromonemata of a polytene chromosome (Fig. 8.57 B).
In certain developmental stages the polytene chromosomes bear conspicuous swellings called chromosome puffs (Fig. 8.57 C). The larger swellings are called Balbiani rings. In the region of a puff or Balbiani ring, the DNA strands uncoil, become active and produce number of copies of messenger or mRNA.
The mRNAs may remain temporarily stored in the puff. Puffs are not permanent. At different physiological or developmental stages different bands uncoil to produce puffs. Puffs are withdrawn after the completion of the stage. By correlating puffs with different physiological or developmental processes scientists have been able to locate genes on the polytene chromosomes and prepare chromosome maps.
Lampbrush Chromosomes (Fig. 8.58):
The lampbrush chromosomes are highly elongated special kind of synapsed mid-prophase or diplotene chromosome bivalents which have already undergone crossing over. They were first seen by Flemming (1882) but were described by Ruckert (1892).
Lampbrush chromosomes occur in diplotene stage of most animal oocytes, spermatocytes of many and even giant nucleus of unicellular alga Acetabularia (Spring et al, 1975). The lampbrush chromosomes are larger than even polytene chromosomes. Their total length in a urodele oocyte may be upto 5900 pm or three times the aggregate length of total polytene chromosomes.
Lampbrush chromosomes occur in pairs. The pair consists of homologous chromosomes which are joined at certain contact points called chiasmata. Each chromosome has a double main axis due to presence of two elongated chromatids.
Both the chromatids bear rows of large number of chromomeres. Two adjacent chromomeres are separated by interchromomeric stretches. Many of the chromomeres give out lateral projections or loops.
The lateral loops provide a test tube or lampbrush-like appearance to the chromosome pair. Length of a lateral loop may vary from 5-100 pm. Loops are uncoiled or expanded parts of a chromomere with one to several transcriptional units.
Usually a lateral loop has a thin or uncoiling part and a thick or coiling part. Lateral loops take part in rapid transcription of mRNA meant for synthesis of yolk and other substances required for growth and development of meiocytes. RNA synthesis begins at the thinner end.
It progresses towards the thicker end. The transcripts along with their binding proteins remain attached to the loop and give it a fine fibrillar appearance. Some mRNAs produced by lamp-brush chromosomes may be stored as informosomes (mRNA + protein) for producing bio-chemicals during early development of embryo. After the full development of meiocytes, the lateral loops are withdrawn and the chromosomes shorten.
Functions of Chromosomes:
1. Chromosomes contain genes. All the hereditary information is located in the genes.
2. Chromosomes control the synthesis of structural proteins and thus help in cell division and cell growth.
3. They control cellular differentiation.
4. By directing the synthesis of particular enzymes, chromosomes control cell metabolism.
5. Chromosomes can replicate themselves or produce their carbon copies for passage to daughter cells and next generation.
6. Sat chromosomes produce nucleoli for synthesis of ribosomes.
7. Their haploid or diploid number respectively bring about gametophytic and sporophytic characteristics to the individual.
8. Chromosomes form a link between the offspring and the parents.
9. Some chromosomes called sex chromosomes (e.g., X and Y or X and 0) determine the sex of the individual.
10. Through the process of crossing over, chromosomes introduce variations.
11. Mutations are produced due to change in gene chemistry.