Question

Explain the double helix structure of a DNA molecule.

Answer 1

Hint: The name double helix is given because the structure of DNA molecules is such that it has two strands that wind with each other and looks like a twisted ladder. Each strand is composed of smaller units which are termed as nucleotides.

Complete answer:

- The double helix structure is proposed by two scientists, Watson and Crick and therefore it is also called Watson and Crick model.
- DNA molecule, also known as deoxyribonucleic acid is the genetic material present in all humans and most of the other organisms. Maximum amount of DNA is present in a cell’s nucleus where it is termed as nuclear DNA. Very less DNA is also present in the Mitochondria which is commonly known as the powerhouse of cell.
- DNA is composed of nucleotides which are also called building blocks. These nucleotides have three main parts namely; phosphate group, sugar group and one of nitrogen bases among Adenine (A), Guanine (G), Cytosine (C) or Thymine (T). Based on the number of rings present in the structure, these nitrogen bases are divided into purines and pyrimidines.
- Adenine and Guanine have two rings and they fall under the category of purines. The Thymine and Cytosine have one ring and they are called pyrimidines.
- The Adenine of one strand always pairs with Thymine of the other. And the Guanine of one strand will always pair with Cytosine on the other strand. This is called complementary base pairing.
- There are two ends of the Double stranded DNA molecule. 5’ end is called the phosphate end and the other is the 3’ end which has the hydroxyl part.

Note: The understanding about composition of DNA was given by Chargaff. According to him, the A, T, G and C bases are not equally distributed in all organisms. The proportion of Adenine is similar to Thymine whereas, the proportion of Cytosine is equal to that of Guanine. Another important feature of DNA is its replication power. Each strand of DNA can be used for duplicating the base sequences. 3 billion base pairs of DNA consist of the human genome.