Q.Write the sequence of the complementary strand for the following segments of a DNA molecule.
a. 5' - CGTTTAAG - 3'
b. 5' - CCGGTTAATACGGC - 3'

Step 1. The complementary strand of a DNA segment runs antiparallel (3' to 5' beneath the original's 5' to 3'), with each base following the A-T / G-C pairing rule (section 14.4.3), so it can be found by complementing every base of the original and then reversing the order (equivalently: reading the reverse of the original and complementing each base) to express the answer in its own conventional 5' to 3' direction.

Step 2 (a). Original: 5'-C G T T T A A G-3'. Complementing each base in place (A<->T, G<->C) gives, aligned 3' to 5': G C A A A T T C. Reversing this to write the complementary strand's own 5' to 3' sequence gives C T T A A A C G.

Step 3 (a, check). Verify by reverse-complementing directly: reverse of CGTTTAAG is GAATTTGC; complementing each base of that gives C,T,T,A,A,A,C,G = CTTAAACG. Matches.

Step 4 (b). Original: 5'-C C G G T T A A T A C G G C-3' (14 bases). Reverse the string: C G G C A T A A T T G G C C. Complement each base of this reversed string (A<->T, G<->C): G,C,C,G,T,A,T,T,A,A,C,C,G,G = GCCGTATTAACCGG.

For each segment, take the reverse of the printed sequence and replace every base with its Watson-Crick complement (A<->T, G<->C) to get the complementary strand written 5' to 3'.

• Complementing each base but forgetting to reverse the order (giving the complementary strand's sequence in the wrong, 3'-to-5', direction instead of the conventional 5'-to-3' direction).
• Mis-pairing a base (e.g. pairing G with A, or C with T) instead of the correct A-T / G-C rule.