Introduction
DNA (deoxyribonucleic acid) is a biological
polymer that stores hereditary information
in almost all living organisms. Each strand
of DNA is a pattern for duplicating new
strands of DNA, so, when cells divide,
each new cell gets an exact copy of
the DNA from the previous cell. DNA is
composed of two strands of repeating
units called nucleotides which are
entwined in the shape of a double helix.
Each DNA strand is 2.2 to 2.6 nanometres
wide. Individual DNA molecules can
contain hundreds of millions of nucleotides
and can be several mm long. Each
nucleotide is composed of a nucleobase,
a sugar residue and a phosphate group.
The nucleobases, adenine (A), cytosine
(C), guanine (G) or thymine (T), interact
(base pair) with a particular nucleobase
in an opposing strand of the double helix
by hydrogen bonding interactions; A
pairs with T and C pairs with G. The sugar
residues are joined together by negatively
charged phosphate groups; which form
phosphodiester bonds between the 5’
and 3’ hydroxyl groups of each sugar
residue. The DNA double helix is stabilized
primarily by electronic interactions
between the nucleobases and Watson-
Crick base pairs (hydrogen bonds) between
the nucleobases in the opposing strands.