According to Chargaff’s rule, the concentration of guanine always equalled the concentration of cytosine.
In other words
In other words
The amount of purine = The amount of pyramidine in a given DNA molecule.
[A]+[G]=[C]+[T]
Adenine (A) is paired to Thymine (T) by 2 hydrogen bonds
Cytosine (C ) is paired to Guanine (G) by 3 hydrogen bonds
Genetics Problems: Chargaff's Rule Questions

1. A segment of DNA has 120 adenine and 120 cytosine bases. The total number of nucleotides present in the segment is
a) 60 (b)120 (c) 240 (d) 480
According to Chargaff rule,
- Adenine (A) is paired to Thymine (T)
- Cytosine (C ) is paired to Guanine (G)
Here adenine residues =120, cytosine residues = 120
[A]=[T][A= 120]=[T=120][C]=[G][C= 120]=[G=120]
there fore total number of nucleotides = [A] + [T]+ [C]+[G] =120 X 4 = 480
Ans: d) 480
2. In humans, there is approximately 30% adenine. What is the percentage of other nitrogenous bases?
According to Chargaff’s rule, [A]+[G]=[C]+[T]
[A]+[G]+[C]+[T]=100%
Adenine (A) is paired to Thymine (T), A=T
Cytosine (C ) is paired to Guanine (G), G=C
Here [A]=30% therefore % of [T] is also 30%.
therefore [G]+[C]=100 - 60 = 40 %.
[G]=20% and [C]=20%
Ans. is [A]=30%, [T]=30%,[G]=20%, [C]=20%
3. In a DNA sample, the percentage of adenine is 40% and percentage of thymine is 60%. What is your inference?
According to Chargaff’s rule, Concentration of adenine=concentration of thymine. therefore if [A]=40% then the [T] will also be 40%. But this rule rule is applicable only for double stranded DNA molecule.
The possibility of Adenine=40% and Thymine=60% is only in single stranded DNA molecule.
In the question, the DNA sample referred is a single stranded DNA.
4. In DNA, guanine is 10%. The content of adenine is
4. In DNA, guanine is 10%. The content of adenine is
a) 90% (b) 80% (c) 40% (d) 20% (e) 10%
If guanine=10% as guanine will always pair with cytosine,
i.e.cytosine is also 10% then adenine and thymine accounts 80% as adenine
will always pair with thymine.
So adenine=40%, thymine=40%, guanine=10% &
cytosine=10%
OR
Cytosine =10%
A+G+C+T=100%
A+10%+10%+T=100%
A+T=100%-20%=80%
A/T=80%/2=40%
Ans: 40%
5. If the ratio (A+G)/(T+C) in one strand of DNA is 0.7, what is the same ratio in complementary strand.?
Ans: (A+G) / (T+C)= 0.7 or 7/10 or other wise 7:10
(A+G)/(T+C) in the complementary strand will be (T+C)/(A+G) =10:7 or 10/7 = 1.4285 = 1.43
(A+G) / (T+C)= 0.7 or 7/10
A+G =7 (A=4, G=3)
T+C= 10 (T=8, C=2)
complementary strand (T+C)/(A+G) = (8+2)/(4+3) =10/7 = 1.4285 = 1.43
5. If the ratio (A+G)/(T+C) in one strand of DNA is 0.7, what is the same ratio in complementary strand.?
Ans: (A+G) / (T+C)= 0.7 or 7/10 or other wise 7:10
here A+G =7 and T+C= 10
as A=T, G=C in the complementary stand
OR
put any value for A, G, T, C but should match with the sum
here A+G =7 and T+C= 10
(A+G) / (T+C)= 0.7 or 7/10
A+G =7 (A=4, G=3)
T+C= 10 (T=8, C=2)
complementary strand (T+C)/(A+G) = (8+2)/(4+3) =10/7 = 1.4285 = 1.43