Question

In the DNA segment of six coils, 22 BP are linked by two hydrogen bonds. How many cytosine bases would be present?
(a) 22
(b) 38
(c) 44
(d) 76

Answer 1

Hint: This molecule is made up of two polynucleotide chains that twist around each other to form a double helix. It carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.

Complete answer:
As we know, one complete turn of the double helix has 10 base pairs, therefore 6 coils of DNA will have 10x6= 60 base pairs. Adenine and thymine bases form 2 hydrogen bonds. Thus, the total number of cytosine and guanine base pairs is 60 - 22 = 38. As the number of cytosine bases is equal to the number of guanine bases, both equal to 38 bases.

 Additional Information:
1. DNA consists of four bases; two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). Adenine in one DNA strand makes two hydrogen bonds with thymine and in another DNA strand guanine makes three hydrogen bonds with cytosine.
2. According to Chargaff’s rule, regardless of the species, in DNA of all species, the number of adenosine and the number of thymidine residues equal, which means that A = T; and the number of guanosine and the number of cytidine residues equal; which means that G = C.

So, the correct answer is ‘38’.

Notes:
1. DNA was first isolated in the late 1860s by Swiss chemist Friedrich Miescher. He first isolated something which he called "nuclein" inside the nuclei of human white blood cells.
2. At the Cavendish Laboratory within the University of Cambridge in 1953, Francis Crick and James Watson first identified the molecular structure of DNA. Their model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, who was a postgraduate student of Rosalind Franklin at King's College London.