Question

A segment of DNA has 120 adenine and 120 cytosine bases. The total number of nucleotides present in the segment is
A. 60
B. 240
C. 120
D. 480

Answer 1

Hint: DNA has a double-stranded structure that is helical in orientation. A chromosome has DNA and histone proteins as components. Nucleotides are the building blocks that form the structure of DNA.

Complete answer:
A nucleotide is made of three components- a nitrogenous base, a phosphate and a pentose sugar. There are four types of nucleotides- Adenine(A), thymine(T), guanine(G), and cytosine(C). Adenine and guanine are purines. Cytosine and thymine are pyrimidines.
In a DNA strand, the nucleotides are arranged in a manner that Adenine is always paired with thymine and Cytosine is always paired with guanine. Adenine and Thymine are joined by two hydrogen bonds and guanine and cytosine are always joined by three hydrogen bonds. This means that Adenine is complementary to Thymine and guanine is complementary to cytosine.
If Adenine is present in a strand at a locus, thymine will be present on the other strand at the same locus.
The Chargaff’s rule of base pairs states that the base pairs are present in a one to one ratio in a DNA segment. According to this, if there are 120 adenine, there have to be 120 thymines also present. If there are 120 cytosines, there should be 120 guanines also present. So there are in total 480 nucleotides in the given segment.

Hence, the correct answer is option (D).

Note: The structure of RNA has nucleosides instead of nucleotides. Nucleosides are nucleotides-a phosphate. A DNA is double-stranded but an RNA is single-stranded.