Chromosome: The Basis of Inheritance

Chromosomes are the carriers of genes. They are located in the nucleus of a cell and can only be seen with the aid of a microscope during cell division.

All organisms of the same species have the same number of chromosomes. The number of chromosomes in the body cell is known as the diploidA diploid cell has two complete sets of chromosomes. Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total. This is 22 pairs of autosomes and... More number (2n), while we have the haploidHaploid refers to the presence of a single set of chromosomes in an organism's cells. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans,... More number (n) in a gamete.

Examples of chromosome numbers in the body cells of some organisms are shown in the table below.

Organism	Chromosomes (2n)
Man	46
Mosquito	6
Fruitfly	4
Maize	20

The chromosome is made up of DNA (Deoxyribonucleic acid) and protein. DNA is a very large molecule made up of repeating units called nucleotides consisting of: 

• Deoxyribose, a sugar molecule
• Phosphate
• Nitrogenous bases, which may be adenine, guanine, thymine, and cytosine.

DNA is a double helix-shaped like a twisted ladder. It consists of complementary strands running in opposite directions from each other.

The nucleotides of opposite chains are paired to one another by hydrogen bonds.

Adenine and guanine are both purine bases, this means that they have a double-ringed structure. Cytosine, uracil (only present in RNA) and thymine are pyrimidines and have single-ringed structures. These bases contain nitrogen in their ring compounds. 

Purines only ever pair with pyrimidines and pyrimidines only ever pair with purines, therefore, adenine always bonds with thymine while cytosine always bonds with guanine.

Two classes of genetic materials are responsible for the transfer of information from one generation to another in animals:

• DNA or deoxyribonucleic acid.
• RNA or ribonucleic acid.

It is in the DNA or RNA sequences that biological information is stored and passed on.

RNA is a single-stranded helix. It is a polymer with repeating units called nucleotides. Each nucleotide consists of a 5-carbon sugar (ribose), a phosphate, and a nitrogenous base.

Each nucleotide contains one of the four possible nitrogenous bases. Adenine(A), Uracil( U) , Cytosine (C) and guanine (G). There is no thymine in RNA. Uracil replaces thymine.

There are three(3) types of RNA

i. Messenger RNA.
ii. Transfer RNA.
iii. Ribosomal RNA.