Question

If the length of DNA strand is 68 $A^{\circ }$∘ then, what will be the number of nucleotides present in DNA molecule
(a) 20
(b) 40
(c) 60
(d) 80

Answer 1

Hint: Typically a DNA molecule is 20 $A^{\circ }$ in diameter. The average distance between it's two adjacent base pairs is 3.4 $A^{\circ }$, and length of a complete turn is nearly 34 $A^{\circ }$ i.e 10 nucleotide residues.

Complete answer:
As We already said that the distance between adjacent nitrogenous bases is 3.4 $A^{\circ }$. The length of a complete turn is 34 $A^{\circ }$. Here the length of the DNA is 68 $A^{\circ }$ that means the DNA strand has 68 ÷ 3.4 = 20 base pairs. Each base pair has 2 bases, so there are 40 bases. We know 4 nucleotides form 2 base pairs and so 40 nucleotides can form 20 base pairs. Thus, in this case the DNA strand has 40 nucleotides. DNA pairing always occurs between adenine and thymine and also between guanine and cytosine.DNA pairing always occurs between adenine and thymine and also between guanine and cytosine. The number of Adenines is equal to Thymines and that of cytosine is equal to guanine..

Additional Information:
- DNA is an information tape in which the hereditary messages are encoded in the form of a base-pair sequence.
- It constitutes the essential substance of genes and the apparatus by which genes act.
- DNA is primarily concerned with the storage, transmission, and expression of vast genetic information.
So, the correct answer is '(b) 40'.

Note:
- The nitrogenous bases of nucleic acids are heterocyclic, nitrogen-containing organic compounds.
- They are of two kinds, purines, and pyrimidines. The commonest purines of DNA and RNA are adenine and guanine.
- The commonest pyrimidines of DNA include cytosine and thymine and those of RNA include cytosine and uracil.