What is DNA? Describe its structure and explain Watson and Crick
DNA is
Deoxyribonucleic acid. It is present in the nuclear reticulum (chromosome) DNA
has one less oxygen atom in its molecule. DNA is the controlling centre of all
the vital activities of the cell. DNA is the sole genetic (hereditary) material
migrating intact from generation to generation through the reproductive units
or gametes and is responsible for the development of specific characters of a
plant. It also controls the biosynthetic process of the cell including protein
synthesis.
DNA molecule is
large unbranched long chain polymer (polynucleotide). It consists of a number
of monomers (units) called nucleotides. Each nucleotide has 3 parts, a 5 carbon
deoxyribose sugar, phosphoric acid and one of the four complex bases of which
two are long parine (adenine and guanine) and two are short pyridines (thyamine
and cytosine). The deoxyribose sugar molecule together with one of the bases at
its No: 1 position from a nucleoside.
STRUCTURE OF DNA
The structure of
DNA was given by Watson and Crick in 1953 for which they were given Nobel Prize
in 1962.
According to
them it has double helices with two polynucleotides running in opposite
directions and connected with each other by hydrogen bonds.
In fact two
polynucleotides are coiled about the same axis and could be separated only by
uncoiling. The bases are set at right angles to the long axis. In two
nucleotides running opposite each other adenine always pairs with thyamine and
cytosine with guanine. Thus the number of A and T, or C and G are always
constant in a DNA molecule.
DUPLICATION OF
DNA
DNA has the
power of self duplication. The two strands separate from one another. Each of
these then synthesize its complimentary strand from the pool of nucleotides.
Thus in each of the newly formed DNA molecule there will be one parental and
the other newly formed strand.
In 1950 it was
pointed out by charge off in DNA the amount of adenine equals the amount of
cytosine. Physical measurements on DNA solutions of such properties as
viscosity and light scattering indicated that the DNA molecule is very large
and has the shape of very long stiff roads.
Watson and Crick
while collaborating at the Cavendish laboratory in Cambridge England proposed in
1953 a structure for DNA that satisfied the above observations. Using known
information about bond distances and bond angles in organic molecules they
built a scale made of DNA molecule. It occurred to them that 28 to 34. A repeat
pattern might be satisfied by a helix. Thus in fact the molecule was a double
helix of two separate strands of DNA twisted around each other must have
suggested itself to them when they considered the A = T, G = C complementarily discouraged
by charge off. The double stranded helix scale model that emerged proud to have
interesting properties.
First Watson and
Crick pointed out that in order to have maximum symmetry the two strands must
run in opposite directions. The strands of DNA in respective of the order of
the bases are not the same if read from the left and from the right. Second
they found that the double stranded structure is stabilized by the nitrogen
bases which point toward each other and are capable of forming hydrogen bonds.
It turned out
that these bridges that hold together two strands of the double helix can only
be formed between purines and pyramidines since there is not enough room for
two purines and too much room for two pyramidines.
Thus A = T, C =
G equally suggests a double helix, a double helix is constructed and one finds
that it has the stereo chemical properties uniquely determining A = T, C = G
equally.
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