Principles of Biochemistry

Nucleic acids are polynucleotides. There are two types of nucleic acids –DNA and RNA.

Deoxyribonucleic acids (DNA): DNA exists as a double-stranded structure, with both strands coiled together to form the characteristic double-helix. Each single strand of DNA is a chain of four types of nucleotides: adenine, cytosine, guanine, and thymine. The monomeric units are held in polymeric form by 3’, 5’ phosphodiester linkages.

The Waston-Crick model of DNA

1. Two polynucleotide chains wind in a right handed manner around a common axis to produce a 20Å -diameter double helix.

2.The two strands of DNA are antiparallel (run in opposite directions), but each forms at right-handed helix.

3. The bases occupy the core of the helix and sugar-phosphate chains run along the periphery, thereby minimizing the repulsions between charges phosphate groups. The surface of the double helix contains two grooves of unequal width; the major and minor grooves.

4. Each base is hydrogen bonded to a base in the opposite strand to form a plane base pair. The Waston-Crick structure can accommodate only two types of base pairs. Each adenine residue must pair with a thymine residue and vice versa, and each guanine residue must pair with a cytosine residue and vice versa. These hydrogen-bonding interactions, a phenomenon known as complementary base pairing, result in the specific association of the two chains of the double helix.

5. The hydrogen bonding between base pairs is an important factor in stabilizing the DNA double helix structure. The pairing of bases in DNA through hydrogen bonding means that the information contained within each strand is redundant.

6. The nucleotides on a single strand can be used to reconstruct nucleotides on a newly synthesized partner strand.DNA strands have a directionality, and the different ends of a single strand are called the 3' end and the 5' end. These terms refer to the carbon atom in ribose to which the next phosphate in the chain attaches.

7. In addition to being complementary, the two strands of DNA are antiparallel: they are orientated in opposite directions. This directionality has consequences in DNA synthesis, because DNA polymerase can only synthesize DNA in one direction by (5'-3') adding nucleotides to the 3' end of a DNA strand.Deoxyribonucleic acid contains the genetic instructions used in the development and functioning of all known living organisms.

8. The main role of DNA molecules is the long-term storage of information and it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes or cistrons, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.