Question

multiple choice question
one strand of dna has the base sequence 5-a a t c g g t g c-3. what is the base sequence of the complementary dna strand?

○ 3-t t a g c c a c g-5

○ 3-a a u c g g u g c-5

○ 3-u u a g c c a c g-5

○ 3-a a t c g g t g c-5

Explanation:

Step1: Recall DNA base pairing rules

In DNA, adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). Also, the two strands of DNA are antiparallel, meaning if one strand has a 5' to 3' direction, the complementary strand has a 3' to 5' direction.
The given DNA strand is 5'-AATCGGTGC-3'.

Step2: Pair each base

• A (adenine) pairs with T (thymine)
• A pairs with T
• T (thymine) pairs with A (adenine)
• C (cytosine) pairs with G (guanine)
• G (guanine) pairs with C (cytosine)
• G pairs with C
• T pairs with A
• G pairs with C
• C pairs with G

Now, writing the complementary bases in the 3' to 5' direction (since the original is 5' to 3'):
Starting from the 3' end (opposite of the original 5' end), the complementary bases for 5'-AATCGGTGC-3' will be 3'-TTAGCCACG-5' (because we reverse the order of pairing to account for antiparallelism? Wait, no. Wait, the original strand is 5' to 3', so the complementary strand should be 3' to 5', and each base is paired as A-T, T-A, C-G, G-C. Let's do it step by step:
Original sequence (5' to 3'): A A T C G G T G C
Complementary bases (3' to 5'): T T A G C C A C G (because A pairs with T, A with T, T with A, C with G, G with C, G with C, T with A, G with C, C with G. Then, since the complementary strand is antiparallel, its direction is 3' to 5', so the sequence is 3'-TTAGCCACG-5'.
Now let's check the options:

• Option 1: 3'-TTAGCCACG-5' – matches our calculation.
• Option 2: 3'-AAUCGGUGC-5' – has U (uracil), which is in RNA, not DNA. Eliminate.
• Option 3: 3'-UUAGCCACG-5' – has U, RNA base. Eliminate.
• Option 4: 3'-AATCGGTGC-5' – same as original, no pairing. Eliminate.

Answer:

A. 3'-TTAGCCACG-5'