STRUCTURE. FUNCTIONS AND PROPERTIES OF CHROMOSOMES

What are chromosomes? These are thread-like structures found in the nucleus. They are normally very thin and coiled and are not easily visible unless the cell is dividing.

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When a cell is about to divide, the chromosomes uncoil and thicken. Their structure, number and behaviour is clearly observed during the process of cell division.

The number of chromosomes is the same in all the body cells of an organism.In the body cells, the chromosomes are found in pairs.

Each pair is made up of two identical chromosomes that make up a homologous pair. However sex chromosomes in human male are an exception in that the Y-chromosome is smaller.

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Number of Chromosomes

Diploid Number (2n)

This is the number of chromosomes found in somatic cells.
For example, in human 2n = 46 or 22 pairs (44 chromosomes) are known as autosomes (body chromosomes”)
While 1 pair is known as the sex chromosomes.
In Drosophila melanogaster, 2n = 8.

Chromosome Structure

All chromosomes are not of the same size or shape.
In human beings; each of the twenty¬ three pairs have unique size and structure.
On this basis they have been numbered 1 to 23.
The sex chromosomes form the 23rd pair.

Properties of Chromosomes

Chromosomes are very long and thin.
They are greatly and loosely coiled and fit within the nucleus.
During cell division they shorten, become thicker and are easily observable.
Each consists of two chromatids.
The two chromatids are held at same position along the length, at the centromere.
Chromatids separate during cell division in mitosis and in the second stage of meiosis.
Chromosomes take most dyes and stain darker than any other part of the cell.
This property has earned them the name “chromatin material”
Each chromosome is made up of the following components:
Deoxyribonucleic acid (DNA) – this carries the genes.
It is the major component of the genetic material.
Protein e.g. histones.
Ribonucleic acid (RNA) is present in very small amounts.
Enzymes concerned with DNA and RNA replication – these are DNA and RNA polymerases and ligases.