Watson and Crick model of DNA
This model of DNA proposed by Watson and Crick and supported by Wilkins et. al., by x-ray crystallography of DNA.
According to this model.
- DNA consists of two, polynucleotide chains which are twisted around each other and forming a right-handed double helix around the same central axis.
- These chains are antiparallel, means one chain (3! 5) appears as ascending and another one (5, 3′) as descending.
- In both the chains orientation of the sugar molecule is in the opposite direction.
- In this the phosphate groups of lies on the outside of the deoxyribose and the nitrogenous bases are stacked inside the helix..
- The nitrogenous bases of the two chains are linked through hydrogen bonds formed between oxygen and nitrogen atoms of the adjacent bases.
- Size of one turn of helix measures about 34 Å.
- One helix contains 10 base paired nucleotides.
- The distance between two nucleotide is 3.4 Å.
- Each successive base pair in the stack turns 36° in a clock-wise direction.
- The double helix, therefore, makes a complete turn´of 360° every ten base pairs like circular staircase.
- The adenine (A) base always pairs with thymine (T) by two H-bonds.
- (C) pairs with guanine (G) by three H-bonds .
- Due to this type of pairing, the two chains or strands are complementary to each other.
- for example :
- I strand 5′ A-T-G-A-C-T-A-G 3
- II strand 3 T-А-С-Т-G-A-T-C 51
- Due to this property, during DNA replication the two dissociated chains serve as templates for the synthesis of new complementary chains.
- The diameter of the DNA molecule is about 20 A or 2 nanometers (1 nanometer = 1 billionth of a meter).
- The twisting of the strands results in the formation of narrow (minor) and wide (major) grooves.