The molecular straucture of DNA
A nucliec acid consists of a chemically linkid sequence of
nucleotides. Each nucleotide contains heterocyclic ring of
carbon and nitrogen atoms ( the nitrogenous base), a five –
carbon sugar in ring form (a Pentose), and (a phosphate) group.
The nitrogenous bases fall into two types Pyrimidines and
purines. Pyrimidines have a six-member ring: purines have
fused five – and six member rings. Each nucleic acid is
synthesized from only four types of base . the same two purines,
adenine (A) and guanine (G), are present in both DNA and
RNA. The two pyrimidines in DNA are cytosine (C) and
thymine(T).The only differences between uracil and thymine is
the presence of methyl substituent at position C5.
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The sugar involved
The sugar found in DNA is a five carbon sugar, otherwise
known as a pentose sugar. These sugar can exist in two forms
either as a ring structure or as a long ,five carbon chain.
However , it is the ring form which is found here ,as depicted in
the figure . One of the important points to see here is that the 2 ′
carbon (pronounced TwoPrime carbon) has no OH group
attached to it, unlike the 1′ and 3 ′ carbons around the ring, and
hence this sugar is called 2 ′ -deoxyribose .In RNA , the 2 ′
carbon does attach to an OH group, and here we can see one of
the the major differences between DNA and RNA. r not
The phosphate
To complet the monomer structure requires the addition of a
phosphate unit to the nucleoside. One this happens the whole
monomer unit is reffered to as a nucleotide. In a nucleotide we
find three phosphate groups which have been joined in a long
chain, here we should note that the phosphate groups are
labelled α ,β, and γ.
DNA is a polymer of nucleotide units
We can see that the unitary structure of DNA is the nucleotide
monomer, made up of sugar, a phosphate chain and a base,
where the base is any one of four.G, T,Aand C.
Therefore, we have building blocks prepared and can now
polymerize these to form a long chain, or polynucleotid. Here,
the nucleotides are held together by the bonding of the 3′ carbon
of the deoxyribose sugar to the first or α phosphate group of the
next nucleotide. This bond is reffered to as phosphodiester
bond, and results in the β, and γ phosphate groups of bonding
phosphate chain being lost. Bis- phosphate by product is further
cleaved to yield inorganic phosphatet Pi with a release of
energy.
Several important points need to be noted :
 As hinted at the earlier, the resulting polynucleotide has a
direction to it, that is, the ends are not the same. At one
end there is still a triphosphate unit attached to 5′ carbon of
the sugar, and therefor, this end of the chain is called 5′
end ( pronounced five prime) . At the other end is an
unused 3′ ( three prime) carbon still attached to an
untouched OH group. This is the 3′ end of the chain.
Further polymerization could still take place but, even if it
did, we still end up with a 5′ end and a 3′ end.