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  4. In case of double-stranded DNA, which of the following base ratios are equal to one ? (1) ( G + T ) /( A + C ) (2) ( A + G ) /( C + T ) (3) C / G (4) ( A + T ) /( G + C )

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Q. In case of double-stranded DNA, which of the following base ratios are equal to one ?
(1) $( G + T ) /( A + C )$
(2) $( A + G ) /( C + T )$
(3) $C / G$
(4) $( A + T ) /( G + C )$

BHUBHU 2010

Solution:

Chargaff $(1950)$ discovered the equivalence rule, which suggested that despite wide compositional variations exhibited by different types of DNA, the total amount of purines equals to the amount of pyrimidine $(A+G=T+C)$. The amount of adenine equals to the amount of thymine $(A=T)$ and the amount of guanine equals to the amount of cytosine $( G = C ) .$ In contrast, the $( T + A / G +$ C) ratio varied widely in DNAs of different species. In case of double-stranded DNA $( A + G ) /( C + T ) C / G$ and $( G + T ) /( A + C )$ ratios are always or almost equal to one.