| Direction: Read the following and answer the questions from given below Erwin Chargaff was one of those men, making two discoveries that led James Watson and Francis Crick to the double helix structure of DNA. At first Chargaff noticed that DNA whether taken from a plant or animal-contained equal amounts of adenine and thymine and equal amounts of cytosine and guanine. These equalities provides clues into the chemical pairing that make up the double helix. In addition Chargaff also found that amounts of guanine, cytosine, adenine and thymine vary by species are indication that DNA, not protein might be the genetic material for life. |
| (iii) In sea urchin DNA which is double stranded 17% of the bases were shown to be cytosine. The percentages of the other three bases expected to be present in this DNA are |
A) G/34%, A/24.5% T/24.5%
B) G/17%, A/16.5%, T/32.5%
C) G/17%, A/33%, T/33%
D) G/8.5%, A/50%, T/24.5%
Correct Answer: B
Solution :
[b] Chargaffs rule states that purine and pyrimidine base pairs are present in equal amount in ADNA i A=T, G=C i.e. A+T=G+C \[\frac{A+T}{G+C}\]=1 If cytosine = 17%, then G = 17% If A+G+C+T=100 and G=C, A=T then A + 17 +17+T =100 A+T+34=100 A+T =100-34 A+T =66 A =T =667 2 =33% Hence, if cytosine is 17% then G =17% and A and T will be 33% each.
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