The number of chromosomes is constant for all individuals in a species and each one of them has a fixed and equal number of chromosomes. The number of chromosomes in the somatic cells of higher plants and animals is known as diploid number represented by 2n. In gametes (sperm and egg), it is known as haploid number and is represented by n.
Chromosomes differ greatly in their size and shape. A person may have different sizes of chromosomes. The chromosomes may differ in the position of the centromere. Centromere is the point on the chromosome where spindle fibres are attached during cell division.
If the centromere is near the middle, the chromosome is metacentric. If the centromere is towards one end (away from the centre), the chromosome is acrocentric. If the centromere is very near the end, the chromosome is telocentric.
In addition to a centromere, there may also be a secondary constriction on a particular chromosome. The part of the chromosome located distal to (far from) a secondary constriction is called a chromosome satellite.
As already mentioned in the previous section, each chromosome at the start of cell division consists of two chromatids joined at some point along the length. At the point of joining a constriction is formed, called centromere.
The spindle fibres are also attached to the centromere at the time of cell division. These spindle fibres contract and help in separation of two sister chromatids towards the opposite poles in a dividing cell.
On completion of cell division (mitosis followed by meiosis), the chromatids are called chromosomes. These chromatids de-condense and form the very thin thread-like chromatin fibres. Each chromatin fibre is made up of DNA, and histones with small amount of RNA.
The chromatin material consists largely of two strands of Deoxyribonucleic acid (DNA), and proteins.
i. DNA (Deoxyribonucleic acid)
ii. Histones (Protein)
DNA forms about 40% while histones form about 60% of the overall part of the chromosome. DNA has negative charges along its length while histones are positively charged basic protein molecules which are bound to it. This DNA-histone complex is called chromatin.
It is shown that the DNA helix combines with groups of eight histone molecules to form a structure known as nucleosome. It has an appearance of ‘beads on a string’. In a human cell, there are about two million nucleosomes among 46 chromosomes.
Rosalin Franklin for the first time in 1953 studied the shape of the DNA molecule. Then, Watson and Crick in 1953 worked out the structure of DNA. Watson and Crick showed that DNA is a large molecule (macro molecule) consisting of two polynucleotide strands complimentary in nature wound around each other in a double helix.
The strands run in opposite directions, i.e. they are antiparallel. Each single DNA strand is composed of repeating nucleotides. Nucleotides are made of three components, a phosphate, a sugar (pentose) arranged lengthwise and a nitrogenous base attached to the sugar inwards.
The sugar-phosphate backbone has nitrogenous bases arranged at right angles giving a ladder-like arrangement. There are two purine bases and two pyrimidine bases. The bases are Guanine (G), Thymine (T), Adenine (A) and Cytosine (C). The Guanine is complementary to Cytosine and Thymine to Adenine.
How are New DNA Strands Formed?
Formation of a new DNA molecule is called DNA replication. During replication, DNA double helix opens at one end, freeing the strands. To each of these few strands, new complementary strands begin to form in the opposite direction. This process takes place for the whole length of DNA.
Sex Chromosomes and Autosomes:
In human beings, out of 23 pairs of chromosomes, a specific pair, i.e. the 23rd pair of chromosomes, determines the sex of the individual. These are called sex chromosomes. All other 22 pairs of chromosomes are termed as autosomal chromosomes or autosomes. The autosomes carry genes which control somatic traits and have no bearing on the sex.
The two members of each pair of homologous autosomes are similar in size and shape, but this may not be true for the sex chromosomes. In human males, one sex chromosome is smaller than the other. The larger one is known as X chromosome and the smaller one as Y chromosome.
Thus, the condition in the male may be briefly expressed as XY and in female as XX. The sex chromosomes of human females are described as homomorphic and that of human males are heteromorphic. The human females produce only one type of gametes (all with X) and are said to be homogametic. The human males produce two types of gametes (X type and Y type) and are described as heterogametic.
Functions of Chromosomes:
i. Chromosomes are hereditary vehicles that contain genes. All the hereditary information is located on the genes.
ii. Chromosomes control the synthesis of structural proteins and thus help in cell division, cell repair and cell growth.
iii. By directing the synthesis of enzymatic proteins, chromosomes control cell metabolism.
iv. Chromosomes guide development and control cell differentiation.
v. Sex chromosomes (XX and XY or XX and XO chromosomes) determine the sex of individuals.