Deoxyribonucleic Acid

Introduction:

Nucleic acids are the macromolecules of the almost biological important. They form genetic material of all living organism, including the simplest virus. The term nucleic acid comes from the fact that they are found mainly in the nucleus. Elements taking part in the constitution of nucleus acids are C, H, O, N and P. In almost all nucleic acids there are approximately 15-16%N and 9-12%P. Nucleic acids were first reported by Friedrich Miescher in 1871 from nuclei of pus cells and named as nuclein, which was renamed nucleic acid by Altman in 1889.

There are 2 types of nucleic acids found in the living organisms as a genetic material. They are:

1)Deoxyribonucleic Acid (D.N.A)

2)Ribonucleic Acid(R.N.A)

1) DeoxyribonucleicAcid(D.N.A) :

D.N.A is formed by the end to end polymerisation of large numbers of reported units called nucleotides. Each nucleotide is formed by the cross-linking of three substance:

a) Deoxyribose sugar:-

D.N.A consists of deoxyribose sugar(C₅H₁₀O₄). Deoxyribose is a pentose sugar with five carbon atoms.

b) Nitrogenous base:-

Nitrogenous bases are of two types:

I) Purines:

They are double ring compounds and comprises of Adenine(A) and Guanine(G).

II)Pyrimidines:

They have one carbon-nitrogen ring and comprises of Cytosine(C) and Thymine(T).

c)Phosphoric acid:-It is made up of 3 acid groups. The phosphate in the D.N.A is present as H₃PO₄. It contains a phosphate group. It combines two nucleotides by a phosphodiester bond.

Complimentary Base:

A purine base is attached to its specific pyrimidine base with hydrogen bond, that is:

• A combines to T by double hydrogen bond. ( A=T)
• C combines to G by triple hydrogen bond. (G≡C)

This is called complementary bases.

Nucleotides:

These are basic units of nucleic acids. A nucleotide is a compound formed by the union of a nitrogen base, a pentose sugar, and phosphate. It is acidic in nature.

Nitrogenous base + sugar + phosphate→ Nucleotide

OR

Nucleoside + phosphate→ Nucleotide.

Nucleosides:

It is a compound formed by the union of a nitrogen base with a pentose sugar. It is a component of nucleotide and is slightly basic in nature.

Watson and Crick model of D.N.A (double helical structure):-

Watson and Crick established the structure of D.N.A in 1953 on the basis of X-ray diffraction and were awarded the noble prize in 1962. According to them:

1) D.N.A is a polymer of about 10¹⁰deoxyribonucleotides.

2) D.N.A is a macromolecule made up of helically twisted two anti-parallel strands. (One strand moves from 3'-5' direction and other moves from 5'-3' direction).

3) These two strands of D.N.A are called D.N.A duplex which is spirally twisted around the central axis in a right-handed manner.

4) The D.N.A has got many complete turns. The length of one complete turn is 34°A or 3.4nm and has about 10 nitrogen base pairs. The distance between two base pairs is 3.4°A or 0.34nm.

5) Each spirally coiled complete turn has one minor or shallow groove and one deeper or major groove.

6) The strands(backbones) of D.N.A are known as structural units formed by the combination of deoxyribose sugar and phosphate groups by phosphodiester bonds. The distance between two strands or diameter of D.N.A is 20°A.

7) Each deoxyribonucleotide of D.N.A is made up of an outer phosphate group, middle deoxyribose sugar and inner nitrogen bases.

8)Nitrogen bases are of two types: purine and pyrimidine. These bases are known as the functional unit of D.N.A and lie inner side of the helix.

9) A purine base is attached to its specific pyrimidine base. I.e:Adenine combines to Thymine by double hydrogen bond. ( A=T) and Cytosine combines to Guanine by triple hydrogen bond. (G≡C)

10) A D.N.A looks like a ladder as both strands are joined together by weak hydrogen bonds in their nitrogen bases.

11) The total number of purine is equal to the total number of pyrimidine. (Charguff's rule).

i:e A+G=C+T

Significance of D.N.A :

The significance of the D.N.A are as follows:

1)It carries genetic information from parents to the offsprings.

2)It controls all the biological activities of all the cells.

3)It controls the development of the individuals by intrinsic and extrinsic information flow.

4)D.N.A performs an autolytic function, that is, replication and heterolytic function, that is, transcription for the synthesis of proteins, hormones, enzymes e.t.c.

Types of D.N.A:

I)A-D.N.A: This type of D.N.A has 11 base pairs in a complete turn and twisted around the central axis in a right-handed manner.

II)B-D.N.A: This type of D.N.A has 10 base pairs in a complete turn and twisted around the central axis in a right-handed manner.

III)C-D.N.A:This type of D.N.A has 9 base pairs in a complete turn and twisted around the central axis in a right-handed manner.

IV)D-D.N.A: This type of D.N.A has 8 base pairs in a complete turn and twisted around the central axis in a right-handed manner.

V)Z-D.N.A:This type of D.N.A has 12 base pairs in a complete turn and twisted around the central axis in a left-handed manner in a zigzag fashion.

Reference:

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.