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Nucleotides

Among the key biopolymers, nucleic acids can store and transfer hereditary information; they are the genetic molecules. This is why people say that many factors in the way you look and your instincts are determined by your DNA. Life on Earth uses two different types: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). As suggested by their names, the polymer chains of RNA and DNA are made out of the same basic building blocks (monomers). The monomers for RNA and DNA are called nucleotides.

A nucleotide has three distinct structural components: a molecule of sugar, at least one phosphate group, and a base, as shown in the figure below. (Sugars are a chemical subclass of carbohydrates.) The polymer chain is formed by linking the sugars and phosphates, leaving the base groups available to interact with others.

RNA has a single polymer chain as its backbone, while DNA has two separate polymer chains that form a helix shaped backbone, allowing the bases to connect within the helix. The names of RNA and DNA come from the names of the sugar molecules in their backbones, ribose sugar in RNA, and deoxyribose sugar in DNA.

The bases are always attached to the sugar molecules and positioned along the chain. They are members of a much larger class of molecules, but only five are used by life in DNA (guanine, adenine, cytosine, and thymine) and RNA (uracil, instead of thymine). These bases are capable of bonding with each other, using hydrogen bonds. They do so only as follows: guanine with cytosine, and adenine with thymine or uracil. Each one of those possible pairs is called a "base pair." This complementary base-pairing is the essence of how the DNA molecule is capable of replicating a string of information, and its function as a genetic molecule (see figure below).

The two sugar-phosphate backbones of the DNA molecule run in opposite directions so that the bases on one can face and bond to their complementary bases on the other. The two backbones are stable when twisted like a helix with 10 base pairs per turn. The interior, where the base pairs are, is hydrophobic (repels water) which makes the DNA very stable in liquid water.

RNA does not form a double helix; it is a single strand with exposed base pairs. It can fold in different shapes, often pairing its own bases. In today's living cells RNA can perform a multitude of different functions, mostly to do with the transfer and implementation of genetic information. Though much less stable than DNA, the RNA molecule has all the necessary ingredients to play the role of a genetic molecule, and thus might have played an important role in very early life. You will see more about this in our section on the RNA

Солнечная система и ее тайны