DNA (Deoxyribonucleic Acid) is a double-helical molecule made of two antiparallel polynucleotide strands held together by hydrogen bonds between complementary base pairs. It is important for NEET Biology because NTA frequently tests Chargaff's rules, Watson-Crick model dimensions, antiparallel polarity, and semiconservative replication.
Sugar-phosphate backbone
Two antiparallel strands of alternating deoxyribose sugar and phosphate groups linked by phosphodiester bonds. Forms the structural framework of DNA.
Adenine (A)
Purine base that pairs with thymine via two hydrogen bonds. Along with guanine, adenine has a double-ring structure.
Thymine (T)
Pyrimidine base that pairs with adenine via two hydrogen bonds. Thymine is unique to DNA — RNA has uracil instead.
Guanine (G)
Purine base that pairs with cytosine via three hydrogen bonds. G-C pairs are stronger than A-T pairs due to the extra bond.
Cytosine (C)
Pyrimidine base that pairs with guanine via three hydrogen bonds. Single-ring structure like thymine.
Hydrogen bonds
Weak bonds between complementary bases that hold the two strands together. A=T has 2 bonds, G≡C has 3 bonds.
Major groove
Wider groove in the helix where proteins (like transcription factors) bind to read the DNA sequence without unwinding it.
Minor groove
Narrower groove in the helix. Some proteins and drugs bind here; provides less sequence information than the major groove.
Phosphate group
Negatively charged group that links sugars in the backbone via 3'-5' phosphodiester bonds. Gives DNA its acidic nature.
Deoxyribose sugar
Five-carbon sugar lacking an -OH group at the 2' carbon (unlike ribose in RNA). Connects phosphate above and base to the side.
Nitrogenous base
Purines (A, G) or pyrimidines (T, C) attached to the 1' carbon of deoxyribose. Bases face inward and pair with complementary bases.
Chargaff’s rules (A=T, G=C)
In any DNA molecule, the amount of adenine equals thymine and guanine equals cytosine. The ratio A+T/G+C varies between species but is constant within a species. This is a frequently tested concept in NEET.
Antiparallel strands and polarity
The two strands run in opposite directions: one 5’→3’ and the other 3’→5’. This antiparallel orientation is essential for base pairing and is critical for understanding replication (leading vs lagging strand).
Watson-Crick model dimensions
DNA double helix is 2 nm wide, has a pitch (one full turn) of 3.4 nm, with 10 base pairs per turn. Distance between adjacent base pairs is 0.34 nm. These values are directly asked in NEET.
Replication fork and semiconservative replication
DNA replicates semiconservatively — each new molecule has one old and one new strand, proven by Meselson and Stahl experiment using 15N-labeled E. coli DNA and CsCl density gradient centrifugation.
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