DNA replicates in a semi-conservative manner, as proven by Meselson and Stahl's experiment in 1958. Replication begins with initiation, where helicase unwinds the DNA double helix and primase lays down RNA primers. During elongation, DNA polymerase adds nucleotides to the 3' end of the primers on the leading and lagging strands. Okazaki fragments are formed and ligated on the lagging strand. Replication terminates when DNA polymerase reaches the telomeres at the end of the DNA strands.

1. DNA REPLICATION

2. University of the Punjab
Lahore

3. DNA STRUCTURE
 In early 1900s scientist knew that
chromosomes are made up of DNA and
proteins containing genetic information.
 However, they didn’t know whether protein or
DNA was actual genetic material.

4. DNA STRUCTURE
 In 1940s various researches showed that
DNA was the genetic material.
 In early 1950s structure of DNA was
determined.

5. STRUCTURE OF DNA
 James Watson &
Francis Crick
determined the
structure of DNA in
1953.

6. STRUCTURE OF DNA
 DNA is polynucleotide; nucleotides are
composed of a phosphate, a sugar and a
nitrogen containing base.

7. STRUCTURE OF DNA
 Sugar in DNA is deoxyribose.
 Four nitrogen bases in DNA
i. Adenine
ii. Guanine
iii. Thymine
iv. cytosine

8. STRUCTURE OF DNA
 Watson and Crick showed that DNA is
double helix in which
A is paired with T
G is paired with C
 This is called complementary base pairing
because a purine is always paired with
pyrimidine.

9. STRUCTURE OF DNA

10. DOUBLE HELIX
 Each side of the double helix runs in
opposite (anti-parallel) directions.
 The beauty of this structure is that it can
unzip down the middle and each side can
serve as a pattern or template for the other
side.

12. REPLICATION
 Replica “copy”.
 DNA making copies of itself, we call it DNA
replication .

13. WHY DNA REPLICATE ITSELF?
 To reproduce, a cell must copy and transmit
its genetic information (DNA) to all of its
progeny. To do so, DNA replicates.
 DNA carries information for making all of the
cell’s protein.

14. REPLICATION IN DIFFERENT CELLS
 Different types of cells replicated their DNA
at different rates.
 Hair cells, finger nails, bone marrow cells.
constantly devide.
 Cells of brain, heart and muscles. cells go
through several rounds of cell division and
stop.
 Skin cells and liver cells. stop dividing, but
can be induced to divide to repair injury.

15. WHERE REPLICATION OCCUR?
 In prokaryotes, DNA replication occurs in the
cytoplasm.
 In eukaryotes, in the nucleus.

16. CLASSICAL MODELS FOR DNA
REPLICATION
 Conservative
 Semi conservative
 Dispersive

17. CONSERVATIVE MODEL
 Conservative Model
 In this model the two parental DNA strands
are back together after replication has
occurred. That is, one daughter molecule
contains both parental DNA strands, and the
other daughter molecule contains DNA
strands of all newly-synthesized material.

18. SEMI CONSERVATIVE MODEL
 Semi conservative Model
 In this model the two parental DNA strands
separate and each of those strands then
serves as a template for the synthesis of a
new DNA strand. The result is two DNA
double helices, both of which consist of one
parental and one new strand.

19. DISPERSIVE MODEL
 Dispersive Model
 In this model the parental double helix is broken
into double-stranded DNA segments that, as for
the Conservative Model, act as templates for
the synthesis of new double helix molecules.
The segments then reassemble into complete
DNA double helices, each with parental and
progeny DNA segments interspersed.

20. CLASSICAL MODELS OF DNA REPLICATION

21. MESELSON AND STAHL EXPERIMENT
 Nobody knew for sure how DNA replication
really worked until two scientists named
Matthew Meselson and Franklin Stahl
devised an ingenious experiment in 1958.
 Show that DNA follows semi conservative
model to replicate itself.

22. MESELSON AND STAHL EXPERIMENT
 Hypothesis
 Experimental procedure
 Result

23. HYPOTHESIS
 Three hypotheses had been previously
proposed for the method of replication of
DNA.
 Semiconservative hypothesis, proposed
by Watson and Crick.
 Conservative hypothesis proposed that the
entire DNA molecule acted as a template.
 Dispersive hypothesis is exemplified by a
model proposed by Max Delbruck.

24. EXPERIMENT DIAGRAM

25. RESULTS
1. Light DNA
2. Heavy DNA
3. Intermediate DNA
4. Light DNA &
intermediate DNA

26. RESULTS
 Disproved conservative replication.
 Disproved dispersive replication.
 Proved that DNA replicates in
semiconservative manner.

27. SEMI CONSERVATIVE REPLICATION
 Semiconservative replication describes the
mechanism by which DNA is replicated in all
known cells. This mechanism of replication
was one of three models originally proposed
for DNA replication.

28. REQUIREMENTS OF REPLICATION
 DNA template.
 Free 3’-OH group.
 Proteins of DNA replication

29. DNA TEMPLATE
 Template strand, that is to be copied.
 Each old strand act as a template.

30. FREE 3’-OH GROUP

31. PROTEINS OF REPLICATION
1. DNA Helicases
2. DNA single-stranded binding proteins
3. DNA Gyrase
4. DNA Polymerase
5. Primase
6. DNA Ligase

32. HELICASE
 DNA Helicases - These proteins bind to the
double stranded DNA and stimulate the
separation of the two strands.

33. DNA SINGLE-STRANDED BINDING
PROTEINS
 DNA single-stranded binding proteins -
These proteins bind to the DNA as a tetramer
and stabilize the single-stranded structure
that is generated by the action of the
helicases. Replication is 100 times faster
when these proteins are attached to the
single-stranded DNA.

34. DNA GYRASE
 DNA Gyrase - This enzyme catalyzes the
formation of negative supercoils that is
thought to aid with the unwinding process.

35. DNA POLYMERASE
 DNA Polymerase - DNA Polymerase I (Pol I)
was the first enzyme discovered with
polymerase activity, and it is the best
characterized enzyme. The DNA
polymerases travel up the DNA molecule
from an initiation site which is a region along
the DNA that the enzyme complex can
recognize.
 adds 5' C to 3' C in a phosphodiester linkage.

36. PRIMASE
 Primase - The requirement for a free 3'
hydroxyl group is fulfilled by the RNA primers
that are synthesized at the initiation sites by
these enzymes.

37. DNA LIGASE
 DNA ligase- forms a covalent
phosphodiester linkage between 3'-hydroxyl
and 5'-phosphate groups.

38. DNA POLYMERASE FUNCTION
 Requires an RNA or DNA primer (RNA primer in
eukaryotes).
 Reads DNA template in a 3'-->5- direction only
 Synthesizes new strand in 5'-->3' direction only -
adds 5' phosphate to 3' hydroxyl group.

39. DIRECTION OF REPLICATION
 It replicates from 3’ to 5’ of the template
strand.
 From 5’ to 3’ of the newly growing strand.

40. STEPS OF REPLICATION
 Initiation
 Elongation
 Termination

41. INITIATION
1. The first major step
for the DNA
Replication to take
place is the breaking
of hydrogen bonds
between bases of
the two antiparallel
strands.
2. Helicase is the
enzyme that splits
the two strands

42. INITIATION
1. One of the most
important steps of
DNA Replication is
the binding of RNA
Primase in the
initiation point of the
3'-5' parent chain.
2. RNA nucleotides are
the primers (starters)
for the binding of
DNA nucleotides.

43. ELONGATION
 RNA primase lays down primers.
 Replication starts at primer and lays down
nucleotides 5’ to 3’.
 Leading strand goes continuously, lagging
strand goes discontinuously.

44. ELONGATION
 The elongation process is
different for the 5'-3' and
3'-5' template.
 a)5'-3' Template: The 3'-5'
proceeding daughter
strand -that uses a 5'-3'
template- is
called leading
strand because DNA
Polymerase ä can "read"
the template and
continuously adds
nucleotides
(complementary to the
nucleotides of the
template, for example
Adenine opposite to

45. LEADING STRAND
 Leading strand synthesis is continuous.
 From 3’ to 5’ of the template.

46. ELONGATION
 5'-3'Template: The 5'-3'
template cannot be "read"
by DNA Polymerase ä.
The replication of this
template is complicated
and the new strand is
called lagging strand. In
the lagging strand the RNA
Primase adds more RNA
Primers. DNA polymerase
å reads the template and
lengthens the bursts. The
gap between two RNA
primers is called "Okazaki
Fragments".

47. LAGGING STRAND
 Lagging strand synthesis is discontinuous.
 Okazaki fragments.
 Ligase joins discontinuous fragments.

48. ELONGATION
 In the lagging strand
the DNA Pol I -
exonuclease- reads the
fragments and removes
the RNA Primers. The
gaps are closed with the
action of DNA Polymerase
(adds complementary
nucleotides to the gaps)
and DNA Ligase (adds
phosphate in the
remaining gaps of the
phosphate - sugar
backbone).

49. TERMINATION
 The last step of DNA Replication is
the Termination.
 RNA primer is removed. Replaced with DNA
nucleotides.
 DNA ligase joins okazaki fragments with
phosphodiester bonds.
 Helicase rewinds DNA together.

50. TERMINATION
 This process happens when the DNA
Polymerase reaches to an end of the
strands.
 These ends of linear (chromosomal) DNA
consists of noncoding DNA that contains
repeat sequences and are called telomeres.
 A part of the telomere is removed in every
cycle of DNA Replication.

51. TERMINATION
 The DNA Replication is
not completed before
a mechanism of
repair fixes possible
errors caused during
the replication.
Enzymes
like nucleases remove
the wrong nucleotides
and the DNA
Polymerase fills the
gaps.

52. TERMINATION
 Protein which binds to this sequence to
physically stop DNA replication proceeding.
 This is named the DNA replication terminus
site-binding protein or in other words, Ter-
protein.

54. ,