(deoxyribonucleic acid) is a large molecule essential for life and cell
replication and is another example of a natural polymer.
It is the
natural polymer found
in all living organisms and viruses too.
DNA encodes genetic instructions for the development and
functioning of living organisms and viruses e.g. every protein molecule is
synthesised by other molecules reading the genetic code and combining the right
amino acids in the right order.
Most DNA molecules consist of two polymer chains, made
from four different monomers called nucleotides,
connected in the
form of a double helix (right diagram).
RNA (ribonucleic acid) is mainly single stranded.
Nucleotides form the building blocks of DNA or RNA
and an individual nucleotide consists of three molecular bits combined
together - (i) a phosphate group, (ii) an organic base (adenine, cytosine, guanine, thymine
and uracil replaces thymine in RNA), and a cyclo-pentose sugar (simplified
diagram on left).
In DNA the sugar unit is based the pentose molecule,
deoxyribose, and in RNA the sugar unit is on the pentose molecule, ribose
A DNA molecule consists of two 'molecular' strands coiled together to form a double helix,
but how is this helix held together?
The strands are linked by a series of
complementary base pairs joined together by weak hydrogen bonds (base-pairing H
bonds shown here as
There four bases in DNA holding the
structure together (the two molecular strands) always form the same pairing.
(i) adenine (A)
with thymine (T) i.e. AT,
and (ii) cytosine (C) with guanine (G) i.e. CG whererepresents
the weak (but crucial) intermolecular attractive force between pairs of
bases, called the hydrogen bond.
It is these
hydrogen bonds that holds the two DNA strands together as a double helix.
Part of the double helix structure is shown in the diagram
below, illustrating how the DNA is held together by the
llll hydrogen bonds.