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: C**

**hargaff'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%

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**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.

(A+G)/(T+C) in the complementary strand will be (T+C)/(A+G) =10:7 or 10/7 = 1.4285 =

(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

**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

here A+G =7 and T+C= 10

as A=T, G=C in the complementary stand

**1.43**

**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