Nucleotides are the building blocks of DNA, DNA is a polymer of these units. We had to understand the chemical structure of this polynucleotide chain(DNA).
The components of a nucleotide
A nucleotide is made up of :
- a sugar (deoxyribose)
- a phosphate group
- one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C).
There are two types of nitrogenous bases –
- Purines(Adenine and Guanine)
- Pyrimidines (Cytosine, Uracil and Thymine).
Nitrogenous bases with one rings are called Pyrimidines (C, T & U) while those with two rings are called Purines (A & G).
Uracil is present in RNA at the place of Thymine.
Linking of Nucleotides
DNA nucleotides assemble in chains linked by covalent bonds, which form between the deoxyribose sugar of one nucleotide and the phosphate group of the next. This arrangement makes an alternating chain of deoxyribose sugar and phosphate groups in the DNA polymer, a structure known as the sugar-phosphate backbone.
Hydrogen bonding between nitrogenous bases on opposite strands make two stands of DNA double helix stable and held them together.
Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Guanine forms three H-bonds with Cytosine and vice-versa.
It concludes, a purine always comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix. These A-T & G-C associations are known as complementary base pairs.
If you are interested in knowing the packaging of DNA in eukaryotes, go to this post(DNA Packaging).