DNA REPLICATION
 DNA replication is a biological process that occurs in all living
organisms and copies their DNA; it is the basis for biological inheritance. The
process starts with one double-stranded DNA molecule and produces two
identical copies of the molecule. Each strand of the original double-stranded
DNA molecule serves as template for the production of the complementary
strand. Cellular proofreading and error toe-checking mechanisms
ensure near perfect fidelity for DNA replication.
 In a cell, DNA replication begins at specific locations in the genome, called
"origins". Unwinding of DNA at the origin, and synthesis of new strands,
forms a replication fork. In addition to DNA polymerase, the enzyme that
synthesizes the new DNA by adding nucleotides matched to the template
strand, a number of other proteins are associated with the fork and assist in
the initiation and continuation of DNA synthesis.
 DNA replication can also be performed in vitro (artificially, outside a
cell). DNA polymerases, isolated from cells, and artificial DNA primers are
used to initiate DNA synthesis at known sequences in a template molecule.
The polymerase chain reaction (PCR), a common laboratory technique,
employs such artificial synthesis in a cyclic manner to amplify a specific
target DNA fragment from a pool of DNA.
DNA STRUCTURE
 DNA usually exists as a double-stranded structure, with both strands
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coiled together to form the characteristic double-helix.
Each single strand of DNA is a chain of four types of nucleotides having
the bases: adenine, cytosine, guanine, and thymine.
A nucleotide is a mono-, di-, or triphosphate deoxyribonucleoside;
that is, a deoxyribose sugar is attached to one, two, or three
phosphates.
Chemical interaction of these nucleotides forms phosphodiester
linkages, creating the phosphate-deoxyribose backbone of the DNA
double helix with the bases pointing inward.
Nucleotides (bases) are matched between strands through hydrogen
bonds to form base pairs. Adenine pairs with thymine, and cytosine
pairs with guanine.
a)Key features of DNA
structure
b) Partial chemical structure
c) Space-filling model
 DNA strands have a directionality, and the different ends of a
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single strand are called the "3' (three-prime) end" and the "5'
(five-prime) end".
These terms refer to the carbon atom in deoxyribose to which
the next phosphate in the chain attaches. In addition to being
complementary, the two strands of DNA are antiparallel.
They are orientated in opposite directions. This directionality has
consequences in DNA synthesis, because DNA polymerase can
synthesize DNA in only one direction by adding nucleotides to
the 3' end of a DNA strand.
The pairing of bases in DNA through hydrogen bonding means
that the information contained within each strand is redundant.
The nucleotides on a single strand can be used to reconstruct
nucleotides on a newly synthesized partner strand
DNA REPLICATION AND REPAIR
 The relationship between structure and function is manifest in the
double helix. The idea that there is specific pairing of nitrogenous
bases in DNA was the flash of inspiration that led Watson and Crick
to the correct double helix.
 At the same time, they saw the functional significance of the basepairing rules.
 They ended their classic paper with this wry statement : “ It has not
escaped our notice that the specific pairing we have postulated
immediately suggests a possible copying mechanism for the genetic
mechanism for the genetic material.”
 The figure below is an illustrates by Watson n Crick’s basic idea. To
make it easier to follow, we show only a short section of double
helix in untwisted form.
 The two strands are complementary; each stores the information
necessary to reconstruct the other. When a cell copies a DNA
molecule, each strand serves as a template for ordering nucleotides
into a new strand.
 Nucleotides line up along the template strand according to the
base-pairing rules and are linked to form the new strand. Where
there was one double-stranded DNA molecule at the beginning of
the process, there are soon two, each an exact replica of the
‘parent’ molecule.
 The copying mechanism is analogous to using a photographic
negative to make a positive image, which can in turn be used to
make another positive, and so on.
 This model of DNA replication remained untested for several years
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following publication of the DNA structure. The requisite experiments
were simple in concept but difficult to perform.
Watson and Crick’s model predicts that when a double helix replicates,
each of the two daughter molecules will have one old strand, derived
from the parent molecules, and one newly made strand.
This semiconservative model can be distinguish from a conservative
model of replication, in which the two parent strands somehow come
back together after the process( that is, the parent molecule is
conserved).
In yet a third model, called the dispersive model, all four strands of
DNA following replication have a mixture of old and new DNA.
Although mechanisms for conservatives or dispersive DNA replication
are not easy devise, these models remained possibilities until they could
be ruled out.
STEPS OF REPLICATION
 The replication of DNA molecule begins at special sites called origin
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of replication.
Multiple replication bubbles form and eventually fuse, thus speeding up
the copying of the very long DNA molecules.
At each end of a replication bubble is a replication fork, a Y-shaped
region where the parental strands of DNA are being unwound.
Helicases are enzymes that untwist the double helix at the replication
forks, separating the two parental strands and making them available as
template strands.
After parental strands separation, single-strand binding proteins
bind to the unpaired DNA strands, stabilizing them. The untwisting of
the double helix causes tighter twisting and strain ahead of the
replication fork.
Topoisomerase helps releive this strain by breaking, swiveling, and
rejoining DNA strands.
 The unwound sections of parental DNA strands are now available to serve
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as templates for the synthesis of new complementary DNA strands.
However, the enzyme that synthesize DNA cannot initiate the synthesis of
a polynucleotide; they can only add nucleotides to the end of an already
existing chain that is base-paired with the template strand.
The initial nucleotide chin that is produced during DNA synthesis ia
actually a short stretch of RNA, not DNA.
This RNA chain is called primer and is synthesized by the enzyme
primase.
Primase starts an RNA chain from a single nucleotide, adding RNA
nucleotides one at a time, using the parental DNA strand as a template.
The completed primer, generally 5 to 10 nucleotides long, is thus basepaired to the template strand.
The new DNA strand will start from the 3’end of the RNA primer.
Synthesis of leading strand
Synthesis of lagging strand