Chargaff’s rule

Chargaff’s rule

Chargaff’s rule was given by Erwin Chargaff an Austrian American biochemist. He did a detailed study of nucleic acids and DNA using chromatographic techniques. There are two parity rules based on his experiments. One of them forms the basis of Watson – Crick base pairing in DNA and its double helical structure. This disapproves the tetranucleotide hypothesis of  P. Levene. The other is used in characterizing a species. In this topic, we will know in detail about his rules.

Definition

Chargaff’s rule states that in the base composition of DNA, the quantity of adenine equals the quantity of thymine. The amount of guanine equals that of cytosine.

First parity rule

It states that in a double-stranded DNA, the number of guanine units equals the number of cytosine units. Number of adenine units equals the number of thymine units.  The amount of purine is equal to the amount of pyrimidine. Thus

• Adenine pairs with Thymine and Cytosine pairs with Guanine.
• % A = % T and % G = % C ( percentage base pair equality)
• Also A+G /T+C =1 or % A/T = % G/ C i.e., percentage of purines/percentage of pyrimidines =1 for an organism.

Second parity rule

The ratio of Adenine (A) to Thymine (T)  and cytosine (C ) to guanine (G) is approximately equal in each DNA strand.

• % A ~ % T  and  % G ~ % C are valid for each of the two DNA strands.
• The composition of DNA varies from species to species.
• The DNA types with which Chargaff worked, the percentage of A was equal to T. The Percentage of G was found to be equal to C.
• This rule applies to four of the five types of double-stranded genomes of bacteria, viruses, and eukaryotes.
• The rule does not apply to the organellar genomes ( mitochondria and plastids).
• It does not apply to single-stranded DNA viral genomes or RNA genomes
• However, this was disproved by Szybalski.

Szybalski’s  rule

In bacteriophage coding sequences purines (A and G) exceed pyrimidines (C and T).

• The biological basis for this is not yet clear.

Difference between Chargaff’s rule and Szybalski’s rule

Q&A

1. What is Chargaff’s rule of DNA?

In the base composition of DNA, the quantity of adenine equals the quantity of thymine. The quantity of guanine equals that of cytosine

2. What is Chargaff’s rule in biology example?

If Cytosine is 20 %, Guanine will also be 20 %. (C=G)

(A+T) = (100-40)%= 60% . (A+G+T+C=100%)

Adenine is 30% and Thymine is 30%

3. Why is Chargaff’s rule not applicable to RNA?

RNA being single-stranded may not contain equal amounts of A, U, G, Cand. Chargaff’s rule applies to double-stranded DNA only.

4. How can Chargaff’s rule be used to infer?

Chargaff’s rule can be used to infer the complementary base pairing of purine and pyrimidine in the DNA helix. It is helpful in the characterization of a species.

Summary

• Chargaff’s rule states that in the base composition of DNA, the quantity of adenine equals the amount of thymine. The quantity of guanine equals that of cytosine.
• First parity rule: In a ds DNA % A = %T and % G = % C.
• Second parity rule: The ratio of Adenine (A) to Thymine (T)  and cytosine (C ) to guanine(G)  is approximately equal in each DNA strand. % A ~  % T  and  % G ~ % C are valid for each of the two DNA strands. It varies from species to species but is more or less constant for different sources of DNA in a species.

• The second parity rule however does not conform with organellar DNA (mitochondria, plastid). Small genomic sequences, RNA, and bacteriophage DNA also don’t obey this rule.
• Szybalski’s rule: In bacteriophage coding sequences purines (A and G) exceed pyrimidines (C and T).
• Szybalski’s rule applies to all genomic sequences.

Reference

Biochemistry Simplified: Textbook of Biochemistry by P. Manjeshwar

Written By: Ahana Mitra