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Overview

Nucleic acids are long chains of nucleotides linked together by phosphodiester bonds. There are two types of nucleic acids: deoxyribonucleic acid, or DNA, and ribonucleic acid, or RNA. Nucleotides in both DNA and RNA are made up of a sugar, a nitrogen base, and a phosphate molecule.

Nucleic Acids Are the Genetic Material of the Cell

A cell’s hereditary material is comprised of nucleic acids, which enable living organisms to pass on genetic information from one generation to next. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA and RNA differ very slightly in their chemical composition, yet play entirely different biological roles.

Nucleic Acids Are Polymers of Nucleotides

Chemically, nucleic acids are polynucleotides—chains of nucleotides. A nucleotide is composed of three components: a pentose sugar, a nitrogen base, and a phosphate group. The sugar and the base together form a nucleoside. Hence, a nucleotide is sometimes referred to as a nucleoside monophosphate. Each of the three components of a nucleotide plays a key role in the overall assembly of nucleic acids.

As the name suggests, a pentose sugar has five carbon atoms, which are labeled 1^o, 2^o, 3^o, 4^o, and 5^o. The pentose sugar in RNA is ribose, meaning the 2^o carbon carries a hydroxyl group. The sugar in DNA is deoxyribose, meaning the 2^o carbon is attached to a hydrogen atom. The sugar is attached to the nitrogen base at the 1^o carbon and the phosphate molecule at the 5^o carbon.

Nucleotides Are Linked Together by Phosphodiester Bonds

The phosphate molecule attached to the 5^o carbon of one nucleotide can form a covalent bond with the 3^o hydroxyl group of another nucleotide, linking the two nucleotides together. This covalent bond is called a phosphodiester bond. The phosphodiester bond between nucleotides creates an alternating sugar and phosphate backbone in a polynucleotide chain. Linking the 5^o end of one nucleotide to the 3^o end of another imparts directionality to the polynucleotide chain, which plays a key role in DNA replication and RNA synthesis. At one end of the polynucleotide chain, called the 3^o end, the sugar has a free 3^o hydroxyl group. At the other end, the 5^o end, the sugar has a free 5^o phosphate group.

Pyrimidines and Purines Are the Two Major Classes of Nitrogen Bases

The nitrogen bases are molecules containing one or two rings made up of carbon and nitrogen atoms. These molecules are called “bases” because they are chemically basic, and can bind to hydrogen ions. There are two classes of nitrogen bases: pyrimidines and purines. The pyrimidines have a six-membered ring structure, whereas the purines are comprised of a six-membered ring fused to a five-membered ring. The pyrimidines include cytosine (C), thymine (T) and uracil (U). The purines include adenine (A) and guanine (G).

Cytosine, adenine, and guanine are present in both DNA and RNA. However, thymine is specific to DNA, and uracil is found only in RNA. The purines and pyrimidines can form hydrogen bonds with each other in a particular pattern, based on the presence of complementary chemical groups that are analogous to pieces of a jigsaw puzzle. Under normal cellular conditions, adenine forms hydrogen bonds with thymine (in DNA) or uracil (in RNA), whereas guanine forms hydrogen bonds with cytosine. This complementary base pairing is critical to DNA structure and function.

Structure of DNA and RNA

DNA adopts a double helical structure within the cell. A double helix is composed of two polynucleotide chains, called strands, that wind around each other in a helical (i.e., spiral) manner. The two strands are in opposite orientations, or are “antiparallel” to each other, meaning the 5^o end of one strand is close to the 3^o end of another. The two strands are held together through complementary base pairing (e.g., cytosine with guanine).

In a DNA double helix, the sugar-phosphate backbone is present on the outside, whereas the hydrogen-bonded bases are on the inside. RNA mostly occurs as a single-stranded molecule. The single RNA strand can form localized secondary structures through intra-strand complementary base pairing. Different types of RNA secondary structures have distinct functions within the cell.