What is DNA? — Definition, Types & NEET Biology

DNA — Explained for NEET

DNA is a polymer of deoxyribonucleotides. Each nucleotide consists of three components: a nitrogenous base (purine — Adenine or Guanine; pyrimidine — Cytosine or Thymine), a pentose sugar (deoxyribose), and a phosphate group. The nucleotides are linked by 3'–5' phosphodiester bonds to form a polynucleotide chain.

Watson and Crick proposed the double helix model of DNA in 1953, based on X-ray crystallography data from Rosalind Franklin and Maurice Wilkins. The key features of this model are: two antiparallel polynucleotide chains coiled around a common axis, bases face inward and sugar-phosphate backbone faces outward, the two strands are held together by hydrogen bonds between complementary base pairs (A=T with 2 H-bonds, G≡C with 3 H-bonds), and one complete turn of the helix is 3.4 nm (34 Å) with 10 base pairs per turn.

Chargaff's rules state that in any DNA molecule, the amount of adenine equals thymine (A=T) and the amount of guanine equals cytosine (G=C). This means the total purines equal total pyrimidines.

DNA replication is semiconservative — each new DNA molecule has one parental strand and one newly synthesized strand. This was experimentally proved by Meselson and Stahl (1958) using heavy nitrogen (¹⁵N) and density gradient centrifugation. Replication requires DNA polymerase III (main replicating enzyme), helicase (unwinds DNA), primase (synthesizes RNA primer), and ligase (joins Okazaki fragments on the lagging strand).

DNA functions as the genetic material in most organisms (except some RNA viruses). It stores genetic information, replicates to pass information to daughter cells, and serves as a template for transcription (RNA synthesis).

Key Points from NCERT

• DNA is a double helix with antiparallel strands; A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
• One complete turn = 3.4 nm (34 Å) with 10 base pairs; distance between base pairs = 0.34 nm
• Chargaff's rule: A=T and G=C; total purines = total pyrimidines in any DNA molecule
• DNA replication is semiconservative — proved by Meselson and Stahl experiment using ¹⁵N and CsCl density gradient
• Key enzymes: helicase (unwinds), primase (RNA primer), DNA polymerase III (synthesis), ligase (joins Okazaki fragments)

Related Diagram