Watson and crick model of DNA & Chemical composition of DNA

 What is DNA ............?

DNA stands for deoxyribonucleic acid, which is a molecule that contains the instructions an organism needs to develop, live and reproduce.

-  These instructions are found inside every cell and are passed down from parents to their offspring

 - DNA was first recognized and identified by the Swiss biologist, Johannes Friedrich Miescher in 1869 during his research on white blood cells.

- The double helix structure of a DNA molecule was later discovered through the experimental data by James watson and Francis Crick February 28, 1953, Cambridge University .

-  Finally, it was proved that DNA is responsible for storing the genetic information in living organisms.

Structure of DNA   ( Watson and crick model ):- 

EXPLANATION :- 

- The double helix is a description of the molecular shape of a double-stranded DNA molecule.

 - In 1953, Francis Crick and James Watson first described the molecular structure of DNA, which they called a "double helix," in the journal Nature. 

- For this breakthrough discovery, Watson, Crick, and their colleague Maurice Wilkins won a Nobel Prize in Physiology, or Medicine, in 1962. 

- However, a crucial contribution that enabled this discovery was made by Rosalind Franklin, who was not acknowledged at that time. After her death, Crick said that her contribution had been critical.

- The double helix describes the appearance of double-stranded DNA, which is composed of two linear strands that run opposite to each other, or anti-parallel, and twist together.

 - Each DNA strand within the double helix is a long, linear molecule made of smaller units called nucleotides that form a chain. 

- The chemical backbones of the double helix are made up of sugar and phosphate molecules that are connected by chemical bonds, known as sugar-phosphate backbones. 

- The two helical strands are connected through interactions between pairs of nucleotides, also called base pairs. 

- Two types of base pairing occur: nucleotide A pairs with T, and nucleotide C pairs with G.

- The DNA molecule consists of 4 nitrogen bases, namely adenine (A), thymine (T), cytosine (C) and Guanine (G) which ultimately forms the structure of a nucleotide. The A and G are purines and the C and T are pyrimidines.

- The two strands of DNA run in opposite directions. These strands are held together by the hydrogen bond that is present between the two complementary bases.

- The strands are helically twisted, where each strand forms a right-handed coil and ten nucleotides make up a single turn.

- The pitch of each helix is 3.4 nm. Hence, the distance between two consecutive base pairs (i.e., hydrogen-bonded bases of the opposite strands) is 0.34 nm.

Chemical composition of DNA :- 

Chemically DNA consist of - 

1) Deoxyribose sugar 

2) Nitrogen bases 

3) Phosphate group 

1) Deoxyribose sugar :- 

- The sugar present in the DNA is 2'deoxyribose, a five carbon monosaccharide, which is devoid of oxygen in its 2' position, hence the name deoxyribonucleic acid. 

- The carbon atoms present in the deoxyribose are numbered 1', 2', 3', 4' and 5'.

- The five-carbon sugar in DNA is called deoxyribose, while in RNA, the sugar is ribose. 

- These two are very similar in structure, with just one difference: the second carbon of ribose bears a hydroxyl group, while the equivalent carbon of deoxyribose has a hydrogen instead.

2) Nitrogen bases :- 

- A molecule that contains nitrogen and has the chemical properties of a base. The nitrogenous bases in DNA are in two category 

1) Purines -

 adenine (A), guanine (G)

2) Pyrimidines - 

thymine (T), and cytosine (C).

- The nitrogenous bases in RNA are the same, with one exception: adenine (A), guanine (G), uracil (U), and cytosine (C).

3) Phosphate group :-

- The phosphate group is attached to the 5′ carbon of one nucleotide and the 3′ carbon of the next nucleotide. 

- A sugar-phosphate backbone (alternating grey-dark grey) joins together nucleotides in a DNA sequence. 

- The sugar-phosphate backbone forms the structural framework of nucleic acids, including DNA and RNA. 

- The phosphate group in DNA is present in the form of phosphoric acid ( H3PO4).

- It has three reactive OH group .