What is the structure of DNA or DNA double helix?
James Watson and Francis Crick deciphered DNA structure in 1951. They used findings of Rosalind Franklin and Maurice Wilkins in X-ray crystallography technique.
They published their findings i.e. DNA model concept in the British Journal Nature with heading “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid” [25th April, 1953, volume 171:737 – 738]
| Nucleotides, phosphates and N-bases | We have seen these (sugar + base + phosphate) while studying DNA components. |
|---|---|
| DNA chain | Many nucleotides are joined to form a chain. 5’ carbon of sugar from one nucleotide and 3’ carbon from sugar of next nucleotide bond with each other by phosphate (phosphodiester bond). DNA molecule contains two such chains. |
| DNA double helix | These two chains are helically wrapped around each other. This forms DNA double helix. Sugar-phosphate backbone lies to the outer side of helix. To the inner side, nitrogen bases from opposite stands form pairs. |
| Base pairing | Adenine pairs with thymine and guanine with cytosine. These base pairs are held together by hydrogen bonds, two hydrogen bonds between adenine and thymine, three between guanine and cytosine. |
| Antiparallel strands | The two strands run in opposite directions i.e. they are antiparallel to each other. Phosphodiester linkage of one strand is in 3’ to 5’ direction and that of others in 5’ to 3’ direction. |
| Complementary strands | The two strands are complementary to each other. This means if we know base sequence of one strand we can understand what the sequence on the other is. Complementation is due to specific base pairing (A to T and G to C), this is referred as complementary base pairing. |
| Width of helix | It is 20A°. This means the two strands remain 20A° apart. |
| Inter-nucleotide distance and complete turn | Inter-nucleotide distance is 3.4 A°. Complete turn of helix occurs after every 34 A° nucleotides. Hence, there are 10 nucleotides per turn. |
| Base pair rotation | Every base pair rotates at 36° with respect to adjacent pair. |
| Grooves | These grooves allow contact of protein molecules with nitrogen bases. DNA has two types of grooves, major (22A° wide and deep) and minor (12A° wide i.e. narrow and shallow). |