Complete the following statements by choosing the appropriate option for each blank:
The nitrogenous base Adenine always pairs with __________
Topic: Base pairing
Try it
Pick an option and check your answer. The full worked answer is below either way.
Answer
(a) Thymine.
By CHARGAFF’S RULE, ADENINE always pairs with THYMINE (by two hydrogen bonds), and GUANINE with CYTOSINE (by three).
THE SPELLING TRAP is option (d): THIAMINE is vitamin B1, an entirely different substance that merely looks like THYMINE. One letter apart, and it catches candidates who recognise the shape of the word rather than reading it.
THE RULE BEHIND THE PAIRING: a PURINE (double-ringed: adenine, guanine) always pairs with a PYRIMIDINE (single-ringed: thymine, cytosine). This keeps the two strands an even distance apart all along the helix — two double rings would bulge, 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.
Thymine.