Which of the following represents the complementary strand of a DNA sequence 3′ TCGATCGCA 5′?
A.
3′ AGCTAGCGT 5′
B.
5’ AGCTAGCGT 3’
C.
5′ UCGAUCGCA 3′
D.
3′ TCGUTCGCU 3′
The correct answer and Explanation is :
The correct answer is B. 5′ AGCTAGCGT 3′.
Explanation:
DNA (Deoxyribonucleic acid) consists of two complementary strands that are arranged in an antiparallel fashion. Each strand is made up of nucleotides, which include a sugar, phosphate group, and a nitrogenous base. The bases pair according to specific rules:
- Adenine (A) pairs with Thymine (T).
- Guanine (G) pairs with Cytosine (C).
These base pairings occur due to hydrogen bonding: A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C. The complementary strands of DNA are always in opposite directions, meaning the 5′ end of one strand pairs with the 3′ end of the other.
In the given question, the original DNA strand is:
3′ TCGATCGCA 5′
To determine the complementary strand, follow these steps:
- Match the complementary bases:
- T → A
- C → G
- G → C
- A → T
- T → A
- C → G
- G → C
- C → G
- A → T
Thus, the complementary strand is:
AGCTAGCGT
- Determine the direction:
The direction of the complementary strand must be antiparallel to the original strand. Since the original strand is in the 3′ to 5′ direction, the complementary strand must run in the 5′ to 3′ direction.
Thus, the final complementary strand is:
5′ AGCTAGCGT 3′, which corresponds to option B.
Other options are incorrect because:
- A shows the complementary bases but has the wrong direction.
- C contains RNA bases (U instead of T), which is not applicable to DNA.
- D contains incorrect bases (U instead of T) and mismatched base pairs.