Deoxyribonucleic acid (DNA)
Chromosomes are mainly made up of DNA.This acid was discovered by the Swiss biochemist, Frederick Miescher in 1869 while studying white blood cells. Initially this acid was reported to be only in the nucleus of cells.Hence, it was named nucleic acid.
However, it was later realized that it is present in other parts of the cell too. Molecules of
DNA are present in all organisms from viruses and bacteria to human beings. These molecules control the functioning, growth and division (reproduction) of the cell and are therefore called ‘Master Molecules’.
The structure of the DNA molecule is the same in all organisms. In 1953, Watson and
Crick produced a model of the DNA molecule. As per this model, two parallel threads of nucleotides are coiled around each other. This arrangement is called a ‘double helix’. This sturcture can be compared with a coiled and flexible ladder.
Each strand in the molecule of DNA is
made up of many small molecules known as ‘nucleotide’. There are four types of nitrogenous bases adenine, guanine,
cytosine and thymine. Adenine and guanine are called as ‘purines’ while cytosine and thymine are called ‘pyrimidines’.
In the structure of the nucleotide, a molecule of a nitrogenous base and phosphoric acid are each joined to a molecule of sugar.As there are four types of nitrogenous bases, nucleotides also are of four types.
Nucleotides are arranged like a chain,
in a molecule of DNA. The two threads of the DNA molecule are comparable to the two rails of a ladder and each rail is made up of alternately joined molecules of sugar and phosphoric acid. Each rung of the ladder is a pair of nitrogenous bases joined by hydrogen bonds. Adenine always pairs with thymine and cytosine always pairs with guanine.
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