Nucleotides are the building blocks of polymers called polynucleotides. Each nucleotide monomer consists of a pentose (five-carbon) sugar, to which is attached two other groups; a phosphate group and a nitrogenous base.

The nitrogenous base is either a double ringed structure known as a purine or single ringed structure known as a pyrimidine. There are five common nitrogenous bases; adenine, guanine, thymine, cytosine and uracil.

Nucleotides are joined together by covalent bonds between the phosphate group of one nucleotide and the third carbon atom of the pentose sugar in the next nucleotide. This produces an alternating backbone of sugar - phosphate - sugar - phosphate all along the polynucleotide chain.

The simplest of the polynucleotides is a single chain in which the pentose sugar is always ribose. The name of this polynucleotide comes from the sugar ribonucleic acid, abbreviated to the three letters RNA. Adenine, guanine, cytosine and uracil are the four nitrogenous bases always found in RNA.

There are several different forms of RNA, each playing a slightly different role in the cell.

• mRNA: messenger RNA - these molecules are complementary copies of genetic messages taken from the DNA genes. They deliver their "messages" to the protein synthesizing machinery in the cytoplasm of the cell.

• rRNA: ribosomalRNA - these molecules are critical structural components in cellular ribosomes (tiny structures important in protein synthesis). There are several kinds of rRNA, some are found in the large ribosomal subunit and some in the small subunit.

• tRNA: transferRNA - the smallest type of RNA. There are more than 20 types of tRNA. They act as the link between the genetic code and the process of joining amino acids together to form polypeptides.

Deoxyribose is the pentose sugar found in this type of polynucleotide, hence its name Deoxyribonucleic Acid, or DNA. The nitrogenous bases found in DNA are, adenine, guanine, cytosine and thymine. DNA molecules have two polynucleotide chains, held together in a ladderlike structure. The sugar phosphate backbones of the two chains run parallel to each other in opposite directions. Each "rung" of the ladder is a pair of nitrogenous bases, one purine and one pyrimidine extending into the center of the molecule.

The pairing of these bases is always adenine with thymine
(A - T)
and guanine with cytosine
(G - C).

The sugar-phosphate backbones of the two polynucleotide chains coil around one another (making the "ladder" into a spiral "staircase"). This superstructure is known as a "double helix".

DNA acts as the store of genetic information. The sequence of bases along its length are the "language" of the cell and code for all its proteins. DNA is also the molecule of heredity. When a cell or a multicellular organism reproduced either sexually or asexually, the genetic information stored in the DNA molecules is faithfully copied and exact copies of these DNA molecules passed along from one generation to the next.