How does Base Pairing work in Nucleic Acids?
The basic biological unit that makes up nucleic acids are nucleotides. Nucleotides are composed of a set group of components: a pentose sugar (ribose in RNA and deoxyribose in DNA), a nucleobase, and a phosphate group. These nucleotides also come in the form of nucleoside triphosphates (such as UTP, GTP, CTP, or ATP) in which the nucleotide has three phosphate groups attached which can be used for purposes such as cellular signaling or energy production. Lacking any phosphate group, the nucleobase and sugar are known as a nucleoside. Phosphate groups are capable of forming bonds with the two, three, or five carbons of the sugar molecule, with the five carbon location being the most common of the three. Nucleotides which feature ribose as the sugar are known as ribonucleotides. Nucleotides which feature deoxyribose as the sugar are known as deoxyribonucleotides.
Nucleotides are also responsible for the base pairing in nucleic acids. Nucleotides are either pyrimidines or purines. Pyrimidines are organic compounds that are comprised of a carbon ring with nitrogens replacing carbons at positions one and three in the ring. Purines are organic compounds that are composed of a pyrimidine ring which is fused to an imidazole ring. The entire compound has four nitrogen atoms. When nucleotides bind together, they become known as base pairs. These base pairs are the primary building blocks of the double helix of DNA and are also responsible for folded structures within both single stranded DNA and RNA.
Base pairing always occurs between a pyrimidine and a purine. In DNA, the pyrimidines are cytosine and thymine. The purines in DNA are adenine and guanine. In RNA, thymine is replaced by uracil. Together, cytosine, adenine, guanine, thymine, and uracil make up the nucleobases which base pair to form nucleic acid double helices and other structures. Adenine will base pair with thymine or uracil (depending on whether the nucleic acid in question is DNA or RNA). Cytosine will base pair with guanine. These base pairs are ubiquitous amongst biological organisms.
Often, base pairing is used to help determine the size of genes or genomes. This is because the total number of base pairs in DNA will be equal to the number of nucleotides in one strand (because DNA typically comes in the form of a double helix). The bonds between different nucleobases are hydrogen bonds. Multiple hydrogen bonds are strong enough to hold the molecules together but they are also weak enough to be (somewhat) easily taken apart if need be.
Typically, the bonds between guanine and cytosine feature three hydrogen bonds rather than the two that are found between adenine and thymine or uracil. The higher number of hydrogen bonds between guanine and cytosine allow areas of DNA or RNA featuring higher guanine and cytosine bonding to be more stable. This is due to both the hydrogen bonding and to other interactions involved in the stacking of the nucleotides on top of each other.
Base pairing plays a very important role in the function of DNA and RNA molecules. It is used to help with both stabilization of nucleic acids and the formation of structures. Base pairing is also a way for the redundancy of having two strands of DNA to be packaged into a smaller unit in order to be stored until expression is needed.
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Edited by: Rajesh Bihani ( Find me on Google+ )