DNA is the genetic material in all organisms
In
certain viruses like the AIDS virus and the tobacco mosaic virus RNA is genetic
material. Nucleic acid was first isolated in 1868 by a Swiss physician
Friedrich Miescher from the nucleus of pus cells. He called it nuclein. Nuclein
was renamed nucleic acid by R Altman in 1889. He discovered the existence of
two types of nucleic acids. Kornberg synthesized DNA in vitro in 1961.
In
the prokaryotic cell DNA is circular and Occurs in the cytoplasm as a component
of the pro chromosome and plasmids .In the eukaryotic cell DNA is linear and
largely confined to the nucleus as a component of the chromosomes. It is called
nuclear DNA .
A
small quantity of DNA also occurs in the mitochondria and plastids. This is
called extra nuclear or organellar DNA. It is circular like the prokaryotic DNA.
It is the largest macromolecule which consists of two complementary
strands of deoxyribonucleotides that Run antiparallel and are held Together by
hydrogen bonds between their complementary nitrogen bases. DNA is composed of
small monomeric units called nucleotides.
The
length of DNA is defined as the number of nucleotides present in it ,which is
the characteristics of an organism.
Escherichia
coli - 4.6 ×106 bp
human
beings - 3.3×109 bp (haploid)
Lambda
phage- 48502 bp
Structure of a polynucleotide chain of
DNA
Each
nucleotide has three components, a nitrogen base, a deoxyribose (pentose) sugar
and a phosphate group.
Nitrogenous
bases are of two types:
Purines
(adenine and guanine) and Pyrimidines (cytosine and thymine).
Purines
- heterocyclic nitrogenous compound( 9
membered double rings ). A purine has for nitrogen atoms 1,3 ,7 and 9
positions.
Pyrimidines
- Single ring nitrogenous compounds( six membered single ring) a pyrimidine has
two nitrogen atoms at one and three positions.
A
nitrogenous base is linked to the pentose sugar through a N-glycosidic bond to
form a nucleoside. When a phosphate group is attached to 5'’OH
of a nucleoside through phospho-ester linkage, a corresponding nucleotide is
formed. Two nucleotides are linked through 3'’-5'’ phospho-diester linkage to form a
di-nucleotide and when many nucleotides are linked in this manner, a
polynucleotide is formed.
The
polynucleotide chain has at the 5'’ end of the sugar a free phosphate
moiety (it is called 5'’-end)
and at the 3'’-end
an OH group (it is called 3'’-end).
The
backbone of the polynucleotide is formed by the sugar and the phosphates, while
the nitrogen bases project from the back bone.
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