If a DNA strand has a base sequence TCA, its complementary strand must be?

Analyze the question: You're given one DNA strand, TCA, and asked for its complementary strand.

Concept: base pairing in DNA

• Adenine (A) pairs with Thymine (T)
• Guanine (G) pairs with Cytosine (C)

(In RNA, thymine is replaced by uracil (U), but this question is about DNA, so we use T.)

Step-by-step solution:

Original strand: T C A

1. T pairs with A
2. C pairs with G
3. A pairs with T

Complementary strand: A G T

Check against the options:

• A. ATG: wrong, because C should pair with G in the second position, not T
• B. TAG: wrong, because it doesn't pair correctly at any position
• C. GAT: wrong, because T should pair with A in the first position, not G
• D. AGT: correct

Exam tip: Remember "Apples in the Tree, Cars in the Garage": A-T and C-G. Replace each base with its partner, one at a time. Also note that the complementary strand is written antiparallel in real molecules (so it would read 3'→5' alongside the original). JAMB usually just wants the direct base-by-base pairing, as here.