![]() These variations can be seen either in local structural parameters (Figure 4) or in the DNA handedness, base-pairing, or number of strands. However, depending on the DNA sequence and the environmental conditions in the biological systems, the geometries and dimensions of the double helix can vary thereby leading to DNA structural polymorphism, which is important for its biological function (discussed in later section). The most recognized DNA form in living cells, and the one modelled by Watson and Crick, is widely known as B-DNA (Figure 3). Geometrical parameters and DNA structural conformations (B) G.C and A.T Watson-Crick base pairing with the base atoms numbered in accordance with the standard nomenclature (image from Ghosh A, Bansal M. The four terminal bases are flattened instead of twisted to show the blown-up view of the hydrogen-bonded bases (image from Pray, L. 2008). The sugar-phosphate backbone of the DNA molecule is shown as grey ribbons and the arrows running oppositely on the ribbons indicate that the two strands of DNA are anti-parallel. The outer edges of the nitrogen-containing bases are exposed.įigure 3: (A) Simplified illustration of the double helical structure of DNA and its complementary base pairing.Nucleotides are linked to each other by their phosphate groups, but this linkage runs in opposite directions on the different strands making the DNA double helix anti-parallel.Most DNA double helices are right-handed with ten nucleotides per turn, separated by a 3.4 Å translation along the helix axis.Adenine pairs with Thymine, and Cytosine pairs with Guanine. DNA is a double-stranded helix, with the two strands connected by hydrogen bonds.The major features of Watson-Crick double helical DNA model are (Figure 3): In 1962, they were awarded the Nobel Prize along with Maurice Wilkins for solving the structure of DNA. This was a historical milestone and gave rise to modern molecular biology. Watson and Crick obtained Franklin's X-ray diffraction data and used it in creating their 3D model of double-helix DNA. This picture clearly indicated that DNA is a helical structure (‘X’ in the diffraction pattern), with a repeat of 10 units (reflected as layer lines) and repeat unit distance is 3.4Å, and the phosphates were on the outside of a helical structure. The famous X-ray diffraction image of DNA, used to determine its structure and widely known today as photograph 51, was taken by Raymond Gosling under the supervision of Maurice Wilkens and Rosalind Franklin in 1952. This rule has been described as “Chargaff's rule". (ii) The amount of adenine (A) and the amount of guanine (G) in a DNA molecule is approximately equal to the amount of thymine (T) and the amount of cytosine (C), respectively (i.e., As=Ts Gs=Cs). The same nucleotides do not repeat in the same order. (i) The DNA composition among species varies. In the year 1950, an Austrian biochemist Erwin Chargaff made two major conclusions. These building blocks of DNA are linked together by covalent bonds between the (deoxy)ribose sugar of one nucleotide and the phosphate of the next nucleotide, constituting the sugar-phosphate backbone (Figure 3 blown-up view).įigure 2: Components of nucleic acids and their chemical structure. In 1919, Phoebus Levene proposed that nucleic acids are composed of a series of nucleotides made of nitrogen-containing bases (purines - A and G Pyrimidines - C and T), a sugar molecule, and a phosphate group (Figure 2). ![]() Timeline showing the DNA structure discovery. However the idea of double helix was stemmed by scientific discoveries sprouted from the 18th century (Figure 1).įigure 1. In the 1950s, American biologist James Watson and English physicist Francis Crick were widely credited for discovering the double helical structure of DNA. The history behind determining molecular structure of DNA is shown in Figure 1. ![]() Interestingly, it is because of the differences in one's DNA which accounts for all the variations seen within us and other lifeforms around us. DNA provides instructions for the cells to grow, mature, divide, die, thus serving as the cell’s command centre, akin to how the brain functions controlling our body’s. One of its major components is the nucleus, which contains DNA (deoxyribonucleic acid) - the cell's hereditary material. ![]()
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