The nitrogenous bases in the opposite strands of DNA always pair in a specific combination: adenine (A) with thymine (T), guanine (G) with cytosine (C).
Adenine (A) and guanine (G) are called purine bases as they have two carbon-nitrogen rings. On the other hand, cytosine (C) and thymine (T) are pyrimidines with a single carbon-nitrogen ring. As it has been established DNA has a uniform diameter, purines must always pair with pyrimidines.
The constraint that specifically favors A-T and G-C base pairing is a chemical one. DNA has two strands that stay together due to hydrogen bonds between paired nitrogenous bases. Chemical side groups of each base allow hydrogen bond formation with only a specific nitrogenous base. Thus, A can form two hydrogen bonds only with T. On the other hand, G can form three hydrogen bonds only with C.
All DNA follows Chargaff’s Rule: the total number of purines in a DNA molecule is equal to the total number of pyrimidines. So, for every A in one strand there is a T in the opposite strand (or vice versa) and the same for G and C. If the sequence of one strand is 5′-ATGC-3′, the complementary strand would have the sequence 5′-GCAT-3′.
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• Due to base-pairing, adenine (A) always pairs with thymine (T) and guanine (G) with cytosine (C) in the opposite strand of DNA.
• Specific base pairing allows DNA to have a uniform diameter and the maximum number of hydrogen bondings between opposite strands.
• Adenine (A) and guanine (G) are purine bases with two carbon-nitrogen rings.
• Cytosine (C) and thymine (T) are pyrimidines with a single carbon-nitrogen ring.
Nitrogenous bases: Organic molecules, which are part of the nucleotides in DNA, showing base-like chemical properties.
Purines: Nitrogenous bases with two carbon-nitrogen rings (e.g. adenine, guanine).
Pyrimidines: Nitrogenous bases with one carbon-nitrogen ring (e.g. thymine, uracil, cytosine).