THE STRUCTURE OF DNA

Well before biologists understood the structure of DNA, they had recognized that inherited traits and the genes that determine them were associated with the chromosomes. Chromosomes were discovered in the nineteenth century as threadlike structures in the nucleus of eukaryotic cells that become visible as the cells begin to divide.As biochemical analysis became possible, researchers learned that chromosomes contain both DNA and protein.But which of these components encoded the organism’s genetic information was not clear.

DNA carries the hereditary information of the cell and the protein components of chromosomes function largely to package and control the enormously long DNA molecules. Biologists in the 1940s had difficulty accepting DNA as the genetic material because of the apparent simplicity of its chemistry .DNA, after all, is simply a long polymer composed of only four types of nucleotide subunits, which are chemically very similar to one another.

In 1950s, DNA was examined by X-ray diffraction analysis,a technique for determining the three-dimensional atomic structure of a molecule .The early results indicated that DNA is composed of two strands wound into a helix. The observation that DNA is double-stranded was of crucial significance. This structure immediately suggested how DNA could encode the instructions necessary for life, and how these instructions could be copied and passed along when cells divide.

 A DNA molecule Consists of Two Complementary Chains of Nucleotides

A molecule of deoxyribonucleic acid (DNA) consists of two long polynucleotide chains. Each chain, or strand, is composed of four types of nucleotide subunits, and the two strands are held together by hydrogen bonds between the base portions of the nucleotides.Each nucleotide is composed of a sugar– phosphate covalently linked to a nitrogenous base.

The nucleotides are covalently linked together into polynucleotide chains, with a sugar– phosphate backbone from which the  nitrogenous bases extend.

DNA molecule  is composed of two polynucleotide chains held together by hydrogen bonds between the paired bases. The arrows on the DNA strands indicate the polarities of the two strands, which run antiparallel to each other in the DNA molecule.

The two polynucleotide chains in the DNA double helix are held together by hydrogen-bonding between the bases on the different strands. All the bases are therefore on the inside of the double helix, with the sugar–phosphate backbones on the outside The bases do not pair at random, however: A always pair with T, and G always pairs with C  In each case, a bulkier two-ring base is paired with a single-ring base (a pyrimidine). Each purine–pyrimidine pair is called a base pair, and this complementary base-pairing enables the base pairs to be packed in the energetically most favorable  arrangement in the interior of the double helix. In this arrangement, each base pair has a similar width, thus holding the sugar–phosphate backbones an equal distance apart along the DNA molecule.The members of each base pair can fit together within the double helix because the two strands of the helix run antiparallel to each other—that is, they are oriented with opposite polarities.The antiparallel sugar–phosphate strands then twist around each other to form a double helix containing 10 base pairs per helical turn. This twisting also contributes to the energetically favorable conformation of the DNA double helix.

A consequence of the base-pairing requirements is that each strand of  a DNA double helix contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand—an A always matches a T on the opposite strand, and a C always matches a G. This complementarity is of crucial importance when it comes to both copying and repairing the DNA.