DNA

DNA is the basic hereditary material in all cells and contains all the information necessary to make proteins.

.DNA is a linear polymer that is made up of nucleotide units . The nucleotide unit consists of a base, a deoxyribose sugar, and a phosphate. There are four types of bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Each base is connected to a sugar via a ß glycosyl linkage. The nucleotide units are connected via the O3' and O5' atoms forming phosphodiester linkages.

Click on the links for 3-D computer visualization of the molecules

In normal DNA, the bases form pairs: A to T and G to C. This is called complementarity. A duplex of DNA is formed by two complementary chains that are arranged in an anti-parallel manner.

Base pairs in DNA bond together to form a ladder-like structure. Because bonding occurs at angles between the bases, the whole structure twists into a helix.

Image courtesy of howstuffworks

The results of fiber and single crystal x-ray crystallographic studies have shown that DNA can have several conformations. The most common one is called (click here for 3D computer visualization of B-DNA). B-DNA is a right-handed double helix with a wide and narrow groove. The bases are perpendicular to the helix axis.

DNA can also be found in the A form in which the major groove is very deep and the minor groove is quite shallow.

A very unusual form of DNA is the left-handed Z-DNA. In this DNA, the basic building block consists of two nucleotides, each with different conformations. Z-DNA forms excellent crystals.

Several years ago it was discovered that nucleic acids can form four stranded structures and a few examples of these molecules now exist. Occasionally mutations occur in which a base is changed. Base pairs still form, but they are not in the usual Watson-Crick geometry. Examples of these mismatches have been characterized.

DNA structures have appeared that are very unusual in that the end pairs are flipped out or there are bulges.

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image courtesy of: U.S. Department of Energy Human Genome Program

All living organisms are composed largely of proteins. Proteins are large, complex molecules made up of long chains of subunits called amino acids. Twenty different kinds of amino acids are usually found in proteins. Within the gene, each specific sequence of three DNA bases (codons) directs the cells protein-synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since 3 bases code for 1 amino acid, the protein coded by an average-sized gene (3000 bp) will contain 1000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.

click on the picture for larger image
image courtesy of: U.S. Department of Energy Human Genome Program

Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Strict base-pairing rules are adhered to adenine will pair only with thymine (an A-T pair) and cytosine with guanine (a C-G pair). Each daughter cell receives one old and one new DNA strand. The cells adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.

Web Author: Tug Sezen