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The word ‘chromosome’ was coined by Waldeyer and means coloured body. Chromosomes are found in viruses, prokaryotes and eukaryotes.
1. Prokaryotic Chromosome:
Prokaryotic cells do not possess nuclear membrane and the genetic material is found in a compact structure called nucleoid. A prokaryotic cell possesses a single chromosome which is irregularly folded into a compact mass.
The chromosome consists of a double stranded helix of DNA. It is circular in shape and attached to the plasma membrane by an in folding called the mesosome. The DNA is naked and is not associated with histone proteins. However, small amounts of protein, mainly RNA polymerase is found to be associated with it.
The chromosome of prokaryotic cells is simpler than the eukaryotic chromosome, so it is called pro-chromosome. The bacterial chromosome has much less DNA and codes for far fewer proteins than the eukaryotic chromosomes in a cell. Escherichia coli has about 2500 genes. The 1100 mm long DNA molecule, is packed by coiling in a space of just 1 mm.
Other than the chromosome, the bacterial cell also possesses plasmids, which are small, circular self-duplicating extra chromosomal DNA molecules. The plasmids can replicate on its own or they may get temporarily integrated to chromosomal DNA and replicate with it. Such plasmids are called episomes.
They may pick up genes from one bacterium and transfer them to another. Plasmids may help in providing variability to asexually multiplying bacteria. They also carry genes for fertility and antibiotic resistance. They are also important tools for genetic engineering.
2. Eukaryotic Chromosome:
The structure of the eukaryotic chromosome is more complex than the prokaryotic chromosome. Eukaryotes possess many chromosomes and each chromosome possess large amount of DNA and positively charged histones and non-histone proteins. In the interphase stage the chromosome is made of long thin chromatin fibre. This replicates during the S phase and becomes differentiated into two chromatids.
The chromosome is differentiated into the following parts:
a. Pellicle,
b. Matrix,
c. Chromonemata,
d. Primary constriction,
e. Secondary constriction,
f. Satellite and
g. Telomere.
a. Pellicle:
It is the outer, thin but doubtful covering or sheath of the chromosome.
b. Matrix:
Matrix or ground substance of the chromosome is made up of proteins, small quantities of RNA and lipid. The double stranded DNA known as chromonema is embedded in the matrix.
c. Chromonemata:
They are coiled threads, which form the bulk of chromosomes. A chromosome may have one or two chromonemata. Each chromonema consists of DNA duplex surrounded by a protein cover.
A chromonema is made of:
i. A single molecule of DNA of enormous length with a diameter of only 2 nm.
ii. It is associated with 5 types of histone proteins. Four histone proteins (H2A, H2B, H3 and H4) are grouped together and bear a coil of DNA over them. This bead like structure is known as nucleosome.
iii. Two adjacent nucleosomes are connected by linker DNA. The latter possesses the fifth type of histone called HI. The fifth histone also helps in further coiling of the beaded DNA.
iv. The nucleosomal organisation makes the chromatin fibres approximately 10 nm thick, which gets further condensed to produce a solenoid of a 30 nm diameter. This solenoid structure undergoes further coiling to produce a chromatin fibre of 200 nm and then a chromatid of 700 nm diameters, which can be seen under the light microscope. All the folded loops of chromatin are held by a nuclear scaffold formed by non-histone proteins (Fig. 8).
d. Primary Constriction:
It is a narrow non-stainable area where the two chromatids are attached in the prophase. The primary constriction is also called centromere. The surface of the centromere bears a specialised plate called kinetochore. The latter helps in the formation of microtubules.
e. Secondary Constriction:
They are narrow areas other than the primary constriction. The secondary constrictions are metabolically active and function as nucleolar organisers. The nucleolar organisers give rise to nucleoli during interphase. Different nucleolar organisers function at different periods in the cell.
The chromosomes having nucleolar organiser regions are called nucleolar chromosomes. At least one pair of homologous chromosomes possesses nucleolar organiser regions. In human beings, 6 chromosomes (13, 14, 15, 20, 22 and Y) have nucleolar organiser regions.
f. Satellite:
The area of a chromosome distal to a nucleolar organiser is called satellite. Commonly, a satellite is knob-like in outline. The chromosome bearing a satellite is known as sat chromosome. The word ‘sat’ is not derived from satellite but from poor staining ability of the nucleolar organiser region as its DNA content is low.
g. Telomere:
The terminal ends of chromosome are named as telomeres. A telomere is a special area of the chromosome, which allows the latter to get attached to the nuclear envelope, but not to any other chromosome. Even when a chromosome breaks, the separated segment fuses in the region other than telomere.