What are the Base Pairing Rules for DNA

The base pairing rules for DNA are often called Chargaff’s rules of DNA base pairing. The two strands of DNA are held together by the hydrogen bonds formed between complementary nucleotides, forming the double-stranded molecule of DNA. Each strand is made up of alternative combining of DNA  nucleotides. These nucleotides can be either purines or pyrimidines. The purines are adenine (A) and guanine (G) while the pyrimidines are cytosine (C) and thymine (T). Generally, adenine pairs with thymine while cytosine pairs with guanine. Adenine forms two hydrogen bonds with thymine while cytosine forms three hydrogen bonds with guanine.

Key Areas Covered

1. What is DNA
      – Definition, Structure, Function
2. What are the Base Pairing Rules for DNA
     – The Pairing of Purines with Pyrimidines

Key Terms: Adenine, Chargaff’s Rules, Cytosine, Double-Stranded DNA, Guanine, Hydrogen Bonds, Thymine 

What is DNA

DNA (deoxyribonucleic acid) is the hereditary material of most organisms. It carries genetic instructions for development, functioning, and reproduction of a particular organism. The backbone of the double-stranded DNA molecule is formed by the alternative combining of DNA nucleotides: A, G, C, and T. DNA nucleotide are composed of a nitrogenous base and a phosphate group attached to deoxyribose. Each nucleotide is linked together by phosphodiester bonds formed between the phosphate group of the incoming nucleotide and the 3′ OH group of the deoxyribose sugar in the existing nucleotide. Since sugar and phosphate molecules are alternately held in the DNA backbone, it is known as the sugar-phosphate backbone. The structure of DNA is shown in figure 1.

Figure 1: DNA

The hydrogen bonds between the complementary nucleotides hold the two strands together. The double-stranded DNA is further coiled to form the DNA double-helix. Each strand in the double helix run in opposite directions. One strand runs from 5′ to 3′ direction while the other strand runs from 3′ to 5′ direction. This makes the two strands antiparallel.

What are the Base Pairing Rules for DNA

The base pairing rules of DNA is called the Chargaff’s rules of DNA base pairing. The four types of DNA nucleotides are adenine, guanine, cytosine, and thymine. Adenine and thymine are purines while cytosine and guanine are pyrimidines. Generally, purines base pair with pyrimidines. Therefore, adenine pairs with thymine while cytosine pairs with guanine. Adenine forms two hydrogen bonds with thymine while cytosine forms three hydrogen bonds with guanine. Thus, adenine is the complementary base of thymine while cytosine is the complementary base of guanine.

Figure 2: Hydrogen Bonds Between Complementary Nucleotides

The interaction between adenine and thymine is less strong than the interaction between cytosine and guanine due to the less number of hydrogen bonds formed between adenine and guanine.

Conclusion

The two strands of DNA are held together by hydrogen bonds between complementary nucleotides of the two DNA strands. The four nucleotides in DNA are adenine, guanine, cytosine, and thymine. Adenine forms two hydrogen bonds with thymine while cytosine forms three hydrogen bonds with guanine.

Reference:

1. Alberts, Bruce. “The Structure and Function of DNA.” Molecular Biology of the Cell. 4th Edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.

Image Courtesy:

1. “DNA double helix (13081113544)” By Genomics Education Programme – DNA double helix (CC BY 2.0) via Commons Wikimedia
2. “DNA chemical structure” By Madprime (talk · contribs) – Own worki.The source code of this SVG is valid.This vector image was created with Inkscape (CC BY-SA 3.0) via Commons Wikimedia