Question

In $68{\text{ }}nm$ long DNA segment, $30\%$ adenines is present, and then calculate how many hydrogen bonds are present in this segment?

Answer 1

DNA is composed of nucleotides and is a nucleotide consisting of three basic components-the nitrogenous bases, a pentose sugar and a phosphate group. There are four types of nitrogenous bases which are found in DNA namely, adenine, thymine, guanine and cytosine. Adenine forms two hydrogen bonds with thymine and guanine forms three hydrogen bonds with cytosine.

Complete answer:
Concept to be applied:
To solve this question, we need to apply the basic rules given by Sir Erwin Chargaff which helps in understanding the total content of various nitrogenous bases in DNA segments.
According to Chargaff’s rules:
Purines (double ringed nitrogenous bases) and Pyrimidines (single ringed nitrogenous bases) occur in equal amounts in DNA.
Purines found in DNA are Adenine and Guanine while pyrimidines are Thymine and Cytosine.
Since the average length of nucleotides is $3.4$ angstroms, there are $200$ nucleotides in a DNA with a length of $68nm$.
$2$ nucleotides form a base pair, so there are $100$ base pairs. Thymine forms a double bond base paired with hydrogen and guanine-Cytosine forms a base pair with a triple bond.
Now, according to our question, $30\%$ of the DNA molecule is composed of adenine, which means that thymine exists in equal parts and forms $60\%$ of DNA in the form of adenine-thymine base pairs ($60$ base pairs).
The remaining $40\%$ of DNA is composed of guanine-cytosine ($40$ base pairs).
According to this number of existing hydrogen bonds, they are -
$2{\text{ }} \times {\text{ }}60{\text{ }} + {\text{ }}3{\text{ }} \times 40{\text{ }} = {\text{ }}240$

Hence, $240$ hydrogen bonds is the answer.

Note:
In DNA, the number between bases A and T and between bases G and C is always equal (A is adenine, T is thymine, G is guanine, and C is cytosine). American biochemist Erwin Chargaff ($1905 - 2002$) from Columbia University discovered this rule, also known as the Chargaff ratio. Complementary nucleotide pairing strongly indicates that the DNA molecule can be divided into two. Only complementary bases can be combined and assembled into a new DNA strand.