DNA Structure
DNA அமைப்பு
DNA is not just a molecule; it is the blueprint of your existence. If you confuse the 3' and 5' ends, you fail the entire Molecular Basis of Inheritance chapter. Precision in orientation is the difference between a doctor and a dropout.
The idea
DNA is a long polymer of deoxyribonucleotides. Each nucleotide consists of a nitrogenous base, a deoxyribose sugar, and a phosphate group. Two polynucleotide chains run anti-parallel, forming a double helix. Hydrogen bonds connect complementary nitrogenous bases: Adenine with Thymine via two bonds, and Guanine with Cytosine via three.
Why it matters
The double-helical structure provides a stable mechanism for storing genetic information and allows for accurate replication through complementary base pairing.
NEET lens
NEET focuses on the anti-parallel polarity and the specific number of hydrogen bonds. Expect questions on Chargaff’s rule: A+G = T+C. Do not confuse the glycosidic bond with the phosphodiester bond. Memorize the 0.34 nm distance between base pairs and the 3.4 nm pitch of the helix.
Clinical significance
Errors in DNA structure lead to mutations, the root cause of genetic disorders like Sickle Cell Anemia. If the hydrogen bonding or sugar-phosphate backbone integrity is compromised, the cell cycle halts or triggers apoptosis. DNA stability is the only barrier between health and malignancy.
Introduction
DNA is a long polymer of deoxyribonucleotides arranged as a double helix, as proposed by Watson and Crick (1953) using Wilkins and Franklin's X-ray diffraction data.
Core concept
Nucleotide = nitrogenous base + pentose sugar (deoxyribose) + phosphate. Bases: purines — adenine, guanine; pyrimidines — cytosine, thymine (uracil in RNA). N-glycosidic bond links base to sugar; phosphodiester bond links nucleotides (3'-5'). Double helix: two antiparallel strands (one 5'→3', the other 3'→5'); sugar-phosphate backbone outside, bases inside. Base pairing: A=T (two hydrogen bonds), G≡C (three hydrogen bonds). Chargaff's rule: A = T and G = C, so purines = pyrimidines. Right-handed coil; pitch 3.4 nm; about 10 bp per turn; distance between base pairs 0.34 nm. Length of DNA = number of bp × 0.34 × 10^-9 m (human diploid: 6.6 × 10^9 bp ≈ 2.2 m).
Go deeper
Because A pairs only with T and G only with C, knowing one strand tells you the other — the basis of replication. Two chains have anti-parallel polarity, which is why polymerases work differently on the two strands.
From Class 10
Class 10 described DNA as the genetic material in chromosomes. Here you learn its actual chemical shape — the double helix.