What are the Chargaff's rules? & It's importance for DNA

Chargaff's rules were given by Austrian-American biochemist Erwin Chargaff (1905-2002) in 1950. Chargaff's rules are universal. Most forms of life obay this rules. This rules are applicable on the double stranded DNA, in both circular and linear form.

DNA contains four types of Deoxyribonucleotides having  Adenine, Guanine, Cytosine and Thiamine. In DNA structure Adenine pairs with Thiamine and Guanine pairs with Cytosine. So there are two hydrogen bond present between Adenine and Thiamine(A=T) and three hydrogen bond present between Guanine and Cytosine(G≡C).

Studies of Chargaff on DNA chemistry (Chargaff's rules) -

• Rule -1

All DNA possess purine and pyrimidine in equal proportions (1:1 ratio). All DNA have A=T and G=C.
Explanation -
  According to this rule the concentration of Adenine is always equal to the concentration of Thiamine and the concentration of Guanine is always equal to the concentration of Cytosine. (%A=%T & %G=%C)
  Hence, this rule says that total number of Adenine always equal to the total number of Thiamine and the total number of Guanine always equal to the total number of Cytosine.
The total number of Purine ( Adenine & Guanine) will always be equal to the total number of Pyrimidine (Thiamine & Cytosine). This means A+G = T+C, the ratio of Adenine and Guanine is equal to the ratio of Thiamine and Cytosine.

• Rule -2
  DNA from different sources have a characteristic A+T/G+C ratio and AT≠GC.
Explanation -
This rule only the balance of AT pair and GC pairs varies between species, means the percentage of G+C does not necessarily equal to the percentage of A+T.

Some important question about Chargaff's rules

Q1). If a double stranded DNA has 21% of Cytosine, calculate the Adenine.
Q2). In an experiment, DNA was found to have 31% Adenine and 19% Guanine. The quantity of Cytosine shall be
Q3). A segment of DNA has 120 Adenine and 110 Cytosine bases. The total number of nucleotide present in the segment is

Ans. 1)  29% Adenine.
Ans. 2)  19 % Cytosine.
Ans. 3)  460.

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