Given below is a diagram representing a stage during the mitotic cell division. Study the diagram and answer the following questions: Given below is a diagram representing a stage during the mitotic cell division. Study the diagram and answer the following questions:
Name the four nitrogenous bases.
Topic: Nitrogenous bases
Answer
ADENINE, GUANINE, THYMINE and CYTOSINE.
These are the four bases of DNA. They divide into two kinds by the structure of their rings, and it is that division which governs the pairing: PURINES (double-ringed) — ADENINE and GUANINE PYRIMIDINES (single-ringed) — THYMINE and CYTOSINE
CHARGAFF’S RULE: a purine always pairs with a pyrimidine — A with T (two hydrogen bonds), G with C (three). This keeps the two strands an even distance apart all along the helix, since two double rings would bulge and two single rings would pinch.
A memory aid: the two CURVED letters C and G pair together; the two ANGULAR letters A and T pair together.
In RNA, URACIL replaces thymine — worth knowing, since papers ask for the difference.