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Line 4: − ==History== − In the late 19th century [[Friedrich Miescher]], a Swiss [[biochemist]], discovered several different proteins and an unusual [[acid]] in the [[nucleus]] of cells. The acid was named deoxyribonucleic acid, or DNA. In 1944 two American [[biologist]]s, [[Alfred Hershey]] and [[Martha Chase]] used experiments with [[bacteria]] and [[bacteriophage]]s to show that DNA passed [[gene]]s from one generation to the next. − At that time, it was unclear how this simple molecule could hold all the complex information controlling the development of humans, animals and plants. Scientists knew it was made of four chemical bases called [[adenine]] (A), [[thymine]] (T), [[guanine]] (G) and [[cytosine]] (C), plus [[phosphoric acid]] and a [[sugar]]. They also knew that there was always the same amount of A as T and of G as C in cells, but they did not know the rules that controlled the arrangement. − − British scientists [[Rosalind Franklin]] and [[Maurice Wilkins]] passed [[X-ray|X-rays]] through DNA to study the patterns made when the crystals diffracted them. From studying photographs of patterns, Rosalind Franklin concluded that DNA must be be a [[helix]]. [[James Watson]] and [[Francis Crick]], working in [[Cambridge]], used this information to help them solve the puzzle of DNA structure. They built a model showing that if A always paired with T and G paired with C, DNA must be like a ladder made of two strands twisted together in a double helix. The sugar and phosphoric acid were the sides of the ladder, and the rungs were the paired bases. − − Watson and Crick suggested that DNA could unzip itself into two separate strands, and each strand could act as a pattern to grow a new strand. Crick showed later that each pair of genes worked in a group of three making the code for amino acids, the building blocks of proteins. These groups are called [[codon|codons]]. They make about fifty thousand different types of [[protein]], which make all the different types of cell in the body. Indian biochemist [[Har Gobind Khorana]] made all the possible codons and worked out which codons controlled which [[amino acid]]. − − If the DNA in one cell was stretched out, it would be about three feet long. Although DNA has a very simple structure, it can carry an enormous amount of [[information]]. Scientists do not yet understand all the DNA they see, but in 1991 a project called the [[Human Genome Project]] began to use computers to map the three billion base pairs which make up the 46 human [[chromosome|chromosomes]].
→History
DNA carries the [[gene|genet]]ic coding for the [[protein]]s that form the bodies of living [[organism]]s. All organisms contain DNA, and the DNA of a sexually reproducing organism is a mix of its parents' DNA.
DNA carries the [[gene|genet]]ic coding for the [[protein]]s that form the bodies of living [[organism]]s. All organisms contain DNA, and the DNA of a sexually reproducing organism is a mix of its parents' DNA.
==Modern understanding==
==Modern understanding==