The DNA double helix model
In 1953 James Watson and Francis Crick made a scientific breakthrough when they discovered the structure of DNA. From this structure they inferred that DNA contains information written in a genetic code, is passed from one generation to the next. This discovery was to be a key step in the genetics revolution DNA has four molecular building blocks called nucleotides; adenine (A), thymine (T) guanine (G) and cytosine (C). Purine base always pairs (by hydrogen bonding) with a pyrimidine base thus adenine always pairs with thymine and guanine always pairs with guanine. Guanine and cytosine have 3 hydrogen bonds while adenine and guanine have 2. The three-dimensional model created by Watson and Crick is composed of a nucleotide base strand twisted into the shape of a double helix and held together by hydrogen bonds forming a spiral staircase like structure. The backbone of each strand has phosphate and deoxyribose sugar units that are connected by phosphodiester linkages. Another prediction of the Watson–Crick model is DNA replication, the two strands of DNA separate and newly synthesized nucleotides are deposited on the old strands, each exposed base will pair only with its complementary base pair i.e. A with T and G with C. the hydrogen bonds form and keep the base pairs intact, also the newly formed base will contain a phosphate and deoxyribose sugar unit too. In the end, there will be two DNA molecules, each carrying one of the old separated strands plus a newly synthesized strand. This DNA replication process takes place every time somatic cells divide and also when sex cells are formed. DNA, as part of the chromosome, is passed from one generation to the next thus all cells in each generation contain the same set of DNA with the same nucleotide sequences. The story begins in the ﬁrst half of the twentieth century, when the results of several experiments led scientists to conclude that DNA is the genetic material. The model of the...
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