Q.A DNA fragment has 2000 nucleotides, out of which 140 are Adenine. How many bases does this DNA segment possess that have triple hydrogen bonds between them ? (A) 280 (B) 860 (C) 1720 (D) 1860

Chargaff's equivalence rule: A = T and G = C, so total purines (A+G) = total pyrimidines (T+C).

Erwin Chargaff analysed base compositions of DNA and found that in double-stranded DNA the amount of adenine equals thymine and guanine equals cytosine. Consequently the sum of the purine bases (adenine + guanine) equals the sum of the pyrimidine bases (thymine + cytosine). This base pairing rule was a key clue used by Watson and Crick for the double-helix model.

By Chargaff's base-pairing rule, Adenine always pairs with Thymine in double-stranded DNA, so the number of Thymine bases equals the number of Adenine bases — 30.

DNA is a double-stranded, antiparallel helical molecule in which the two strands are held together by hydrogen bonds between specifically paired nitrogenous bases: Adenine (A) always pairs with Thymine (T) (via two hydrogen bonds), and Guanine (G) always pairs with Cytosine (C) (via three hydrogen bonds). This is known as the base-pairing rule / Chargaff's rule, and it means that in any double-stranded DNA molecule, the total number of A bases equals the total number of T bases (A = T), and the total number of G bases equals the total number of C bases (G = C).

Given: Adenine (A) = 30.

Since A pairs only with T, Thymine (T) must also = 30.

By Chargaff's rule, %A = %T and %G = %C; since A + T = 55%, each is 27.5%, leaving G + C = 45%, so Cytosine = 22.5%.

In double-stranded DNA, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is Chargaff's rule, which means:

$\%A = \%T \quad \text{and} \quad \%G = \%C$

Given: $\%T + \%A = 55\%$.

Since $\%A = \%T$, each individually is:

$\%A = \%T = \dfrac{55}{2} = 27.5\%$

The remaining percentage is made up of G and C:

$\%G + \%C = 100\% - 55\% = 45\%$

Since $\%G = \%C$:

$\%C = \dfrac{45}{2} = 22.5\%$

This DNA segment contains 1720 bases that form triple hydrogen bonds, specifically Guanine and Cytosine.

By Chargaff's rule, %C = %G and %A = %T, and all four bases sum to 100%.

According to Chargaff's rule for double-stranded DNA, the amount of cytosine equals the amount of guanine, and the amount of adenine equals the amount of thymine. Given %C = 15%, therefore %G = 15% as well, so %(G+C) = 30%. The remaining bases, adenine and thymine, must together make up 100% − 30% = 70%, and since %A = %T, each equals 35%.

By Chargaff's rule (A = T, G = C) in double-stranded DNA, 20% cytosine gives 30% adenine.

In double-stranded DNA, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This means:

$\%A = \%T \quad \text{and} \quad \%G = \%C$

Given %C = 20%, therefore %G = 20% as well.

$\%G + \%C = 40\%$

$\%A + \%T = 100\% - 40\% = 60\%$

Since %A = %T:

$\%A = \frac{60\%}{2} = 30\%$

Using Chargaff's base-pairing rule, C = G = 18%, so A + T = 100 - 36 = 64% and A = T = 32%.

Chargaff's rule for double-stranded DNA states that adenine pairs with thymine (A = T) and guanine pairs with cytosine (G = C).

Given cytosine = 18%, therefore guanine = 18%.

So, C + G = 18 + 18 = 36%.

The remaining bases are A and T: A + T = 100 - 36 = 64%.

Since A = T, adenine = 64 / 2 = 32%.

By Chargaff's rule, %G = %C and %A = %T; so %C = 20% gives %G = 20%, leaving %A = %T = 30% each.

In a double-stranded DNA molecule, base pairing is complementary: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This means the percentage of A equals the percentage of T, and the percentage of G equals the percentage of C (Chargaff's rule).

Given %Cytosine = 20%, therefore %Guanine = 20% as well (since G = C).

Together, G + C = 20% + 20% = 40% of the total bases.

The remaining bases, A + T, must make up the rest: 100% - 40% = 60%.

Since %A = %T, each is 60% / 2 = 30%.

By Chargaff's rule, in double-stranded DNA, Adenine = Thymine and Guanine = Cytosine, so total bases = 2×(A) + 2×(G).

Given: Adenine (A) = 200, Guanine (G) = 400.

Since DNA is double-stranded and base pairing is A=T and G≡C:

Thymine (T) = A = 200

Cytosine (C) = G = 400

Total nitrogenous bases = A + T + G + C = 200 + 200 + 400 + 400 = 1200.

Chargaff's equivalence rule shows that purines and pyrimidines occur in a fixed 1:1 ratio in double-stranded DNA because of complementary base pairing.

According to Chargaff's rule, in any double-stranded DNA molecule the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C), because A pairs specifically with T (via 2 hydrogen bonds) and G pairs specifically with C (via 3 hydrogen bonds) in the double helix. Consequently, the ratio of total purines (A+G) to total pyrimidines (T+C) is constant, and always equal to 1, regardless of the species or the actual GC/AT content of the DNA. This constant ratio was one of the key pieces of evidence Watson and Crick used to deduce the base-pairing rules of the DNA double helix.

Chargaff's rule: purines (A+G) always equal pyrimidines (T+C) in double-stranded DNA, because of complementary base pairing.

Erwin Chargaff analysed the base composition of DNA from many species and found two consistent rules: (1) the amount of adenine (A) always equals the amount of thymine (T), and the amount of guanine (G) always equals the amount of cytosine (C) — i.e., A = T and G = C; and (2) consequently, the total amount of purines (A + G) always equals the total amount of pyrimidines (T + C). This became known as Chargaff's rule and was one of the crucial clues Watson and Crick used to deduce the double-helical, base-paired structure of DNA, in which A always pairs with T (via 2 hydrogen bonds) and G always pairs with C (via 3 hydrogen bonds) across the two antiparallel strands.