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Genetics is the study of inherited phenotypic traits that are controlled by biological information units called genes — what they are, what they do, and how they work. This information is carried inside the cells of all living organisms. For example, offspring produced by sexual reproduction usually look similar to each of their parents because they have inherited some of each of their parents' genes. Genetics identifies which features are inherited, and explains how these features pass from generation to generation. In addition to inheritance, genetics studies how genes are encoded inside the cell (DNA and RNA), turned on and off to control what substances are made in a cell (gene expression), and how genes are copied when a cell divides (DNA replication).

History and Basic Information
Originally, genetics originated in the work of Gregor Mendel (1822-1864), an Austrian Augustinian monk who worked on inheritance in plants. Mendelian Inheritance was worked out and published in 1866, but was not widely noticed at the time. Mendel selected the term gene as the theoretical units that controlled traits that could be inhereted. His laws of inheritance were re-discovered in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak. They did research and published results that confirmed Mendel's work and established the rules of genetic inheritance. At the time, there were various conjectures about how genes were stored inside the cells, but that remained a mystery until the middle pf the 20th century when DNA was conclusively identified as the carrier of genetic information. DNA was discovered in 1869 by Friedrich Miescher who called the substance “nuclein”. In 1878, Albrecht Kossel isolated the non-protein component of "nuclein", nucleic acid, and later isolated its five primary nucleobases. DNA was fairly quickly identified as the carrier of genetic information; however, its structure was unsure until 1953, when X-ray diffraction crystallography by Rosalind Franklin working with James Watson and Francis Crick revealed the double helix structure.

The term double helix describes the manner in which DNA curls up in its natural form. Along two parallel backbone structures formed by the linking of the ribose sugar and phosphate parts of nucleic acids (Adenine and Guanine - the purine nucleotides; Cytosine and Thymine and Uracil - the pyrimidine nucleotides) in a form resembling a ladder. Each purine acid will only link to a corresponding pyrimidine unit. This ladder form naturally coils: the first coiling is a rotation around an axis between the parallel backbones, the second wraps the twisted strand in a broader staircae form. RNA is similar to DNA but is slightly modified in shape, and uses the uracil nucleotide instead of thymine.

The DNA molecule can be very long. If the DNA in a human cell were to be uncoiled and laid out end to end, it would be about 6 meters long. To pack all of this DNA into a tiny cell, further wrappings, twistings, and foldings are used to organize DNA into a structure called a chromosome. Usually chromosomes are only loosely curled and generally not visible in the cell nucleus. When the cell is dividing for growth, the chromosomes condense around various proteins and form the pictured X-shaped structures. Humans have 23 pairs of chromosomes. One of each pair inhereted from the mother, and the other from the father. Chromosomes are found in eukariotic cell nuclei. In Archea and Bacteria, which have no nucleus, DNA comes in the form that used to called a plastid. These prokaryote circular bacterial chromosomes are usually loops of DNA materials that concentrate in a nucleoid.

Genes consist of sequences of DNA base pairs at a specific place along a long DNA molecule.