DNA is a double-stranded molecule. Each strand of the DNA is formed by the alternative combining of four DNA nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are purines while cytosine and thymine are pyrimidines. Each DNA nucleotide is composed of a nitrogenous base and a phosphate group attached to a deoxyribose sugar. The two strands are held together by the hydrogen bonds between the nitrogenous bases of the DNA nucleotides. Generally, purines pair with pyrimidines. Thus, 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. How Do the Nucleotides in DNA Pair
– Pairing of Purines with Pyrimidines
Key Terms: Adenine, Cytosine, DNA, Guanine, Hydrogen Bonds, Thymine
What is DNA
DNA (deoxyribonucleic acid) is the hereditary material of most organisms. In eukaryotes, the majority of DNA is located in the nucleus. Some may remain inside mitochondria and chloroplasts as well. In prokaryotes, DNA can be found within a special region known as the nucleoid in the cytoplasm. DNA carries the genetic instructions for development, function, and reproduction of a particular organism.
Generally, DNA is a double-stranded molecule. The backbone of DNA is formed by the alternative combining of DNA nucleotides: A, G, C, and T. Each DNA nucleotide consists of a nitrogenous base and a phosphate group attached to deoxyribose. The formation of phosphodiester bonds between the phosphate group of the incoming nucleotide and the 3′ OH group of the deoxyribose sugar in the existing nucleotide forms the backbone of each DNA strand and it is known as the sugar-phosphate backbone. The structure of DNA is shown in figure 1.
Figure 1: DNA
The two DNA strands are held together by the hydrogen bonds between the nitrogenous bases of the two strands. The two strands are further coiled to form a DNA double-helix. Each strand in the double helix runs 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.
How Do the Nucleotides in DNA Pair
The double-strand DNA is formed by the hydrogen bonds between the complementary nucleotides of the two strands. Generally, purines pair with pyrimidines. Thus, adenine pairs with thymine while cytosine pairs with guanine. The resultant hydrogen bonds between complementary nucleotides of the two DNA strands are shown in figure 2.
Figure 2: Hydrogen Bonds Between Complementary Nucleotides
Generally, adenine forms two hydrogen bonds with thymine while cytosine forms three hydrogen bonds with guanine. Therefore, the interaction between adenine and thymine is weaker than the interaction between cytosine and guanine.
Conclusion
DNA is a double-stranded molecule made up of combining four DNA nucleotides alternatively. The two strands are held together by the hydrogen bonds formed between purine and pyrimidines. Generally, 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 simple2” By Forluvoft – Own work (Public Domain) via Commons Wikimedia
2. “DNA Nucleotides” By OpenStax College – Anatomy & Physiology, Connexions Web site. Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia
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