Answer:
From top to bottom, the nitrogenous bases needed to complement the DNA strand shown are adenine, thymine, guanine and cytosine (option B).
Explanation:
The DNA molecule is structured as two interlaced strands, linked by their nitrogenous bases through hydrogen bonds. The way the strands are linked depends on the complementarity of bases, where a purine binds to a pyrimidine.
In the DNA the nitrogenous bases are:
- Purines: adenine and guanine
- Pyrimidines: thymine and cytosine.
Adenine is complementary to thymine, with which it forms double bonds, while guanine is complementary to cytosine, and has a triple bond.
According to this, given a sequence of four bases, the complementary bases are arranged as follows
T=A
A=T
C≡G
G≡C
Then, the answer is that the nitrogenous bases that complement the given sequence are adenine, thymine, guanine and cytosine.
Obviously, the other options do not comply with the complementarity of bases, so they are incorrect.