Question

A segment of DNA molecule contains 200 guanine and 200 thymine bases. What will be the total number of nucleotides in this segment of DNA?
A. 400
B. 200
C. 800
D. 100

Answer 1

Hint: According to Chargaff's rules, DNA from any cell of any organism must possess a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases, and the amount of guanine is equivalent to cytosine and the amount of adenine is equivalent to thymine.
This rule is named after the Austrian-American biochemist Erwin Chargaff (1905-2002), who discovered it at Columbia University.

Complete Step by Step Answer:
Each DNA strand is formed by the alternate combination of 4 DNA nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). Purines are adenine and guanine, whereas pyrimidines are cytosine and thymine. Each DNA nucleotide is made up of a nitrogenous base and a phosphate group that is joined to a deoxyribose sugar. The hydrogen bonds in between nitrogenous bases of the DNA nucleotides hold the two strands together. Purines typically pair with pyrimidines. As a result, adenine tends to form 2 hydrogen bonds with thymine and 3 hydrogen bonds with guanine.
According to Chargaff's rule, the number of adenosine residues in all cellular DNAs, regardless of species, is equivalent to the number of thymidine residues, implying that A=T; and also the number of guanosine residues is equivalent to the number of cytidine residues, implying that G= C. As a result, the sum of purine residues corresponds to the total of pyrimidine residues, i.e., A + G = T + C.
As a result, a DNA strand containing 200 thymine and 200 guanine bases will also contain 200 adenine and 200 cytosine bases. This adds up to a total of 800 bases and thus 800 nucleotides in that DNA molecule.
The correct option is C.

Note: In general, adenine forms 2 hydrogen bonds with thymine and 3 hydrogen bonds with guanine. As a result, the adenine-thymine interaction is weaker than the cytosine-guanine interaction.