Q. 20. While analyzing the DNA of an organism a total number of 536 nucleotides were found out which the proportion of different bases were Adenine=29%, Guanine=17%, Cytosine=32%, Thymine=17%.
Considering the Chargaffs rule it can be concluded that
(a) It is a double-stranded circular DNA
(b) It is a single-stranded DNA
(c) It is a double-stranded linear DNA
(d) No conclusion can be drawn
Ans. (b). According to Chargaff's rule of base pairing,
(i) The amount of adenine is always equal to the amount of thymine and the amount of guanine is always equal to the amount of cytosine.
(ii) Adenine is joined to thymine with two hydrogen bonds guanine is joined to cytosine by three hydrogen bonds.
(iii) The ratio of adenine to thymine and that of guanine to cytosine is always equal to one,
i.e.,
In the given organism, the DNA is not following the Chargaff'srule, hence it can be concluded that it is a single-stranded DNA, not double-stranded.
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