Question

If in a dsDNA, 100 bp are present and number of adenine in it are 20. Then, what would be the number of cytosine in this DNA?​

Answer 1

• In double stranded DNA, purines (Adenine and Guanine) pair with pyrimidines (Thymine and Cytosine).
• Adenine (A) pairs with Thymine (T) with two hydrogen bonds and Guanine (G) pairs with Cytosine (C) with three hydrogen bonds.
• Thus, amount of A is equal to T and amount of G is equal to C
• (i) So, out of 100 base pairs, if 40 are G, then C will also be 40
• As (A + T) + (G+C) = 100, so A + T = 100 - (40 + 40) = 100 - 80 = 20
• and A = T = 20/2 = 10
• So, nucleotides having A are 10 and nucleotides having C are 40
• (ii) In this DNA, base pairs G and C are maximum as they are present in 40 nucleotides.

Answer 2

Answer:

The number of cytosine in the given DNA is equal to 80.

Explanation:

• We know that. in a double stranded DNA, purines which are Adenine and Guanine pair with pyrimidines Thymine and Cytosine.

• Adenine (A) pairs form two hydrogen bonds with Thymine (T) and Guanine (G) pairs form three hydrogen bonds with Cytosine (C).

• Therefore number of A is equal to number of T and the number of  G is equal to the number of C.

We have given the number of base pairs = 100

The number of nucleobases $= 100\times2 = 200$

And, the number of adenine (A) = 20

It means number of Thymine (T) = 20

We can write total number bases as:

$A + T + C+G =200$

$20+20+C+G = 200$

$C+G = 160$

As the number of Guanine (G) = number of cytosine (C)

$2C= 160$

$C= 80$

Therefore, number of cytosine in given DNA is equal to 80.

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