Biology – Structure of nucleic acids and replication of DNA | e-Consult

DNA exists as a double helix, a spiral staircase-like structure. This structure is primarily due to two polynucleotide strands winding around each other. The two strands are antiparallel, meaning they run in opposite directions. One strand runs in the 5′ to 3′ direction, while the other runs in the 3′ to 5′ direction. This directionality is crucial for DNA replication and transcription.

Complementary base pairing is a fundamental aspect of DNA structure. Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This pairing is due to hydrogen bonds between the bases. The specific number of hydrogen bonds is different for each pair: two hydrogen bonds between A and T, and three hydrogen bonds between G and C. The difference in the number of hydrogen bonds contributes to the stability of the DNA double helix; G-C pairs are held together more strongly than A-T pairs.

The 5′ and 3′ designations refer to the carbon atoms in the deoxyribose sugar of the nucleotide. The 5′ end has a phosphate group attached to the 5′ carbon, while the 3′ end has a hydroxyl (-OH) group attached to the 3′ carbon. The antiparallel arrangement ensures that the sugar-phosphate backbone of one strand runs in the 5′ to 3′ direction, while the other runs in the opposite direction. This arrangement is essential for the correct sequence of base pairing during DNA replication and transcription.