Deoxyribonucleic acid, or DNA, is a polymer of nucleotides linked together by specific bonds known as phosphodiester bridges. The phosphate group (PO3) of one nucleotide links to the hydroxyl group (OH) of the following nucleotide. Hundreds of millions of these linkages occur within the DNA polymer. However, only four bases: A (adenine), C (cytosine), G (guanine), and T (thymine) constitute the backbone of the molecule.
The DNA molecule is made up of two polynucleotide strands intertwined together to form a long helical structure, the DNA double helix. This structure is very stable and it occurs because the DNA base pairs are able to interact with other bases in a very specific pattern: an A base on one strand will always pair with a T base on the other strand, the C base will always pair with a G. These combinations (A/T) and (G/C) are called base pairs. The two DNA strands are held together by interchain hydrogen bonds. H-bonds pair the bases in one chain to the complementary bases in the other chain. (A/T and G/C).
Hydrogen Bonds in DNA
A number factors are responsible for the stability of the DNA double helix structure, among them hydrogen bonds. Internal and external hydrogen bonds stabilize the DNA molecule. The two strands of DNA stay together by H bonds that occur between complementary nucleotide base pairs. Two hydrogen bonds occur between the adenosine and the thymine base pairs, and between the cytosine and the guanine there are three.
While each hydrogen bond is extremely weak (compared to a covalent bond for example), the millions of H-bonds together represent an extremely strong force that keeps the two DNA strands together. In addition, other groups of the base rings (polar groups) can form external hydrogen bonds with surrounding water that give the molecule extra stability.
Additional Forces help the DNA Double Helix to be a Stable Structure
The hydrogen bond is not the only force that confers stability to the DNA structure. The negatively charged phosphate group is free to interact with positively charged atoms in electrostatic forces. Also, hydrophobic forces (those that arise because of an antagonism with water) may contribute significantly to the stability of the DNA double strand.
D Voet, JG Voet, CW Pratt (1999). Fundamentals of Biochemistry: Biochemical Interactions CD-ROM – J. Wiley & Sons
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