DNA

//**__A. General Information/Structure__**// Deoxyribonucleic acid (DNA) is a biological molecule that is classified as a nucleic acid. It is also a polymer that is made up of monomers called nucleotides. A nucleotide is made up of the pentose sugar deoxyribose, a phosphate group, and one of four nitrogenous bases. The nitrogenous base is attached to the primary carbon of the sugar while the phosphate group is attached to the terminal carbon of the sugar. The bases include two purines, adenine (A) and guanine (G), and two pyrimidines, cytosine (C) and thymine (T). Adenine forms two hydrogen bonds with thymine while guanine forms three hydrogen bonds with thymine. The entire nucleotides are covalently bonded to each other to form a alternating sugar-phosphate backbone. This backbone is held together by phosphodiester linkages formed between the sugar of one nucleotide and the phosphate of the adjacent nucleotide. Most DNA molecules found in cells can be millions of bases long. DNA is made of two polynucleotide strands intertwined to form a double helix. Each strand has a 5’ end, which has a 5’carbon attached to a phosphate group, and a 3’ end, which has a 3’ carbon attached to an exposed hydroxyl group. The two strands run in opposite directions, so they are considered to be antiparallel to each other. Each strand is complementary to the opposite strand in that the bases of one strand bond with the bases of the opposite strand according to the base-pairing rules previously mentioned.

Scientists discovered that DNA replicates itself by a semi conservative method, meaning that new DNA is made up of one parental strand and one newly synthesized strand. DNA helicase creates multiple replication forks that open up the double helix. Another enzyme, DNA polymerase, now links nucleotide subunits to form a new DNA strand from the original strand, which acts as a DNA template. However, a short piece of RNA, known as a RNA primer is synthesized by DNA primase to initiate elongation, the process of making a chain of nucleotides longer. The primer is eventually degraded when the leading strand, which grows towards the replication fork, is created. On the other strand, short strands called Okazaki fragments are formed and eventually connected by DNA ligase molecules. Synthesis continues at each replication fork until it meets a newly synthesized strand coming from the opposite direction.
 * //__B. DNA replication__//**

A change in one of the bases of a strand of DNA may cause a point mutation. Recombinant DNA technology can reproduce a gene or other DNA segment of interest to be used in the replacement of portion with the point mutation. A portion of DNA that is of interest and not afflicted with the point mutation may be cut out of the organism with the assistance of restriction enzymes. This segment may be placed in a suitable vector, such as bacteria; so that it may reproduce the recombinant DNA. The segment with the mutation may be cut out with restriction enzymes and replaced with the gene taken from the recombinant DNA that was produced. A gene that codes for enzymes that are directly involved in DNA repair can also be added by these means
 * //__C. Role of Mutations__//**

See also: Genetic Engineering (GE) and DNA carries Hereditary Information
 * //__Sources:__//**
 * Solomon Biology Textbook, 8th Edition
 * Barron's AP Biology Review Book, 2008 Edition
 * Microsoft Encarta Encyclopedia, 2006 Edition