Question

The number of purines in a segment of the DNA molecule is 68. What will be the number of pyrimidines in this segment?
(a)34
(b)43
(c)68
(d)86

Answer 1

They are organic molecules with a nucleoside and a phosphate part and act as the monomeric units of DNA and RNA. Purines consist of adenine, guanine and pyrimidines consist of cytosine and thymine.

Complete answer:
Applying the Chargaff rule of base pairing, the purine, and pyrimidine ratio of DNA should be in a 1:1 ratio, which means the ratio of the adenine - thymine bonding and guanine-cytosine bonding will remain the same all the time. Therefore the number of pyrimidines in the DNA molecule will be equal to the number of purines, which is 68 in the question.

Additional Information: -The entire size length of the genome or a single gene is usually measured in the number of base pairs. The twenty-three chromosomes in a human cell have approximately 3.0 to 3.2 billion base pairs.
-The base pairing of nucleotides is achieved by the chemical interaction between the bases. The nucleobases are bonded by hydrogen bonding and the right pairs will provide stability to DNA.
-Adenine and guanine are comparatively larger than cytosine and thymine, where purines have double-ringed chemical structure and pyrimidines have single ringed chemical structures.
So, The correct option is ‘C’.

Note: A double-stranded DNA is completely stable at room temperature and the two strands will remain bonded but as the temperature increases and crosses the melting point, the strands will separate.
-Certain chemical and physical factors can influence the base pairing and can create abnormality. For example, with their interaction UV rays (a common physical mutagen) can cause thymine dimer where thymine bonds with thymine instead of adenine.
-Formation of thymine dimer causes mutation, and when usually these abnormalities occur our body repairs it. The repair mechanism of thymine dimer in Prokaryotes is known as photoreactivation and in eukaryotes, it is an Excision repair mechanism.