nitrogenous bases
Summary
Nitrogenous bases are the nitrogen-containing rings in nucleotides: purines (A, G; two rings) and pyrimidines (C, T, U; one ring). In double-stranded DNA, A pairs with T and G with C (A with U in RNA), so A = T and G = C.
Detail
Meaning: Nitrogenous bases are the nitrogen-containing rings in nucleotides that carry genetic information. Purines, adenine (A) and guanine (G), have two fused rings. Pyrimidines, cytosine (C), thymine (T) and uracil (U), have one ring. DNA uses A, G, C, T; RNA uses A, G, C, U. The base attaches to carbon 1' of the sugar (base + sugar = nucleoside; add phosphate = nucleotide). In standard pairing a purine hydrogen-bonds to a pyrimidine: A with T (or U) by two bonds, G with C by three.
Equation: [A] = [T] and [G] = [C] in double-stranded DNA, so [A] + [G] = [C] + [T]. Each symbol is that base's mole fraction (unitless) or concentration (mol/L).
On the MCAT: A passage gives one base's percentage and asks for the rest: 20% A means 20% T, and the other 60% splits as 30% G and 30% C. Another compares melting temperatures: DNA richer in G–C pairs needs more heat to separate its strands because of the third hydrogen bond.
Common confusion: Students swap the ring counts. Pyrimidines are the single-ring bases despite the longer name (mnemonic: CUT the Py). Also, Chargaff's rules hold for double-stranded DNA, not for single-stranded DNA or most RNA.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.