Erwin Chargaff is considered one of the pioneering scientists in the field of molecular biology. In one of his studies, Chargaff observed that a sample of human DNA contained approximately 30% adenine (A). Based on DNA structure, what are the expected percentages of the other nitrogenous bases in the sample?

A.

10% thymine, 40% cytosine

B.

20% thymine, 30% cytosine

C.

30% thymine, 20% cytosine

D.

40% thymine, 10% cytosine#

30% thymine, 20% cytosine

Explanation:

In  a typical DNA double helix pyrimidine; Cytosine and thymine always pair with Purine ;Adenine and Guanine. This pairing ensure that the double helix of DNA is maintained  by the presence of Hydrogen between these organic bases.

For correct pairing all adenine must pair with thymine,and all Cytosine with guanine.Therefore for this to occur if 30 % of adenine is present,the  composition and number of thymine(30%) must be available.If this percentage is not available the pairing  will not occur.

Like wise  20% Cytosine must be present because ,then same amount of  Guanine molecule 20% in the DNA molecule. must be present Thus Guanine percentage is 20%.

This give a total of 100(60+40)

This based on the Erwin Chargaff  stoichiometric ratio  of 1:1 .This  states that the percentage of adenine =thymine,and that of Guanine=Cytosine,in the DNA of living cells should be equal. Thus; [A]+[G]=[C]+[T]=1:1

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