this presentation covers structure of purines and pyrimidines and features of double helix DNA.

1. ASSIGNMENT – 1
SUBJECT : MICROBIAL BIOCHEMISTRY
(MMIC-EL-522)
TOPIC - STRUCTURE OF PURINES AND PYRIMIDINES
- FEATURES OF DNA DOUBLE HELIX
SUBMITTED BY-
JAHNVI ARORA
(11228108)
MSC MICROBIOLOGY 1ST YEAR
SUBMITTED TO-
DR. SONIYA GOYAL MA’AM

2. DISCOVERY OF DNA DOUBLE HELIX
STRUCTURE
o James Watson and
Francis Crick has
made a major
contribution in 1953
to the development of
the structure of DNA.

3. ABOUT DNA DOUBLE HELIX STRUCTURE
oThey combined the physical and chemical
data and proposed dna as double helical
twisted molecules connected together by
hydrogen bonds.
oDna is a backbone for all life and this model
has helped to unlock the genetic code of all
living organisms.
oDNA is a group of molecules, which carry and
mediate the genetic information from the
parents to the offspring.

4. COMPONENTS OF DNA
o DNA molecule comprises two individual strands of
polynucleotide molecule which are linked together by a
hydrogen bond. Each base of nucleotide pairs up with
one another i.e, purine bases always pair up with
pyrimidine bases.
o Most double helix DNA is right-handed and completes
one turn at a distance of 34A°, One of the exceptions of
left-handed DNA is the Z-DNA.
o The DNA strands are anti-parallel to each other i.e, if
the starting point of one strand is 5’ then the starting
point of another strand is 3’ and vice versa where the
nucleotide links together with the sugar group of 3’ and
5’ end which is due to the polarity of phosphodiester
linkage.
o Hydrogen bond provides easy access to DNA
replication and expression.

5. GROOVES IN DNA
It is the cut or depression present in the composition
of DNA.
It originates due to the antiparallel alignment of the
DNA strands.
The minor and the major grooves run spontaneously
along the DNA molecules’ entire length.
They run opposite each other.
Major and minor grooves:
Two distinct grooves known as the minor and the
major grooves form on the opposite sides of the base
pairs.
The formation of minor grooves takes place where
the sugar phosphate backbones are far apart.
While the formation of major grooves takes place
where the sugar phosphate backbones are close
together.

6. NUCLEOTIDES IN DNA
o DNA molecules are
composed of a sequence of four
nucleotide monomers namely
Adenine (A), Cytosine (C),
Thymine (T), and Guanine (G).
oEach nucleotide monomers
are made up of three molecular
parts:
1. Nitrogen bases
2. Deoxyribose sugar
3. A phosphate group

7. 1. NITROGEN BASE
oNitrogen-containing base functions in bonding nucleic acids and are also termed nucleobases
due to their role in nucleic acid.
oDNA is composed of two major nitrogen bases: Purine and Pyrimidine.
oTwo nucleotides of purine are Adenine (A) and Guanine (G) and pyrimidine are Cytosine
(C) and Thymine (T).
oTwo bases of a nucleotide pair each other with a hydrogen bond and forms two strands of double
helix structure.
oHydrogen atoms of amino groups act as a donor while carbonyl oxygen and ring nitrogen act as
an acceptor.
oAdenine always pairs up with Thymine by two hydrogen bonds (A=T) similarly, Guanine pairs up
with Cytosine forming triple hydrogen bonds (G≡C).
oChargaff’s rule states that purine and pyrimidine bases should have a 1:1 stoichiometric ratio i.e,
the amount of adenine must be equal to the amount of thymine and that similar to guanine and
cytosine.

8. DEOXYRIBOSE SUGAR
oDeoxyribose sugar is a five-carbon atom sugar molecules that lack one
hydroxyl group on the 2’ carbon i.e, one oxygen molecule is absent thus its
name deoxyribose sugar.
oIt is a cyclic molecule with five carbon atoms, among which four are
carbon molecules and one oxygen molecules which are arranged in a
cyclic manner.
oDue to its flexible structure, it can be twisted into various conformations
like in canonical B-DNA, the sugar configuration is C2’ endo.
oThe deoxyribose sugar molecule for all nucleobase is the same. When a
nucleobase is attached to a deoxyribose sugar it becomes a nucleoside.
oThe 1’ carbon of the pentose sugar bonds with the nitrogenous base and
the 5’ end carbon atom bonds to the phosphate group.
oThen, the phosphate group bonds to the 5’ end of the nucleotide. All
these bonding are followed by hydrogen carbon.
oDuring replication, several enzymes are involved in the breakdown of
hydrogen bonding which later forms a newly stranded DNA molecule.
oThese newly formed ribose molecules attach with nitrogenous bases and
a phosphate group before being deoxygenated and later become
deoxyribose and become an independent DNA molecule.

9. PHOSPHATE GROUP
oA phosphate group is a backbone for every
single strand of the DNA molecule.
oThe chemical structure of the phosphate group
comprises a functional group with a phosphorous
atom bonded with four oxygen atoms in which 3
are singly bonded and one with a double bond.
oThe phosphate group remains exterior of the
DNA and are attached to the 5’ of each sugar
atom and forms phosphodiester bond.
oThe phosphate group functions as an energy
donor when DNA synthesizes.
oPhosphate groups when attaches to the
nucleoside form nucleotide molecules.

10. STRUCTURE OF PURINES AND PYRIMIDINES

12. THANK YOU