Question

a scientist designs a primer, which is a short oligonucleotide, for use in a dna sequencing reaction. the primer is complementary to the dna template shown below.
3-agctagcgatcggacgat-5
which of the following shows the sequence and orientation of the primer?
choose 1 answer:
a 3-tcgatcgctagcctgcta-5
b 5-tcgatcgctagcctgcta-3
c 5-ctagctatcgattcatcg-3
d 3-ctagctatcgattcatcg-5

Explanation:

Step1: Recall DNA base pairing and directionality

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

Step2: Find the complementary sequence

To find the complementary sequence, we replace each base with its complement: A→T, G→C, C→G, T→A. So the complementary sequence (without considering direction yet) of \( \text{AGCTAGCGATCGGACGAT} \) (from 3' to 5') would be \( \text{TCGATCGCTAGCCTGCTA} \) (since A pairs with T, G with C, etc.).

Step3: Determine the direction of the primer

Since the template is \( 3'\) to \( 5' \), the primer (complementary strand) should be in the \( 5'\) to \( 3' \) direction (antiparallel). So we need to write the complementary sequence with a \( 5' \) to \( 3' \) orientation. The complementary sequence we found was \( \text{TCGATCGCTAGCCTGCTA} \), so the primer should be \( 5'\text{-TCGATCGCTAGCCTGCTA-}3' \), which matches option B.

Answer:

B. \( 5'\text{-TCGATCGCTAGCCTGCTA-}3' \)