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DNA Replication.pptx
1. DNA REPLICATION
Dr. Sangram Singh Pant
Department of Genetics and Plant Breeding
Madhesh Agricultural University
Saptari, Nepal
2. Replication of DNA
β’ Process of synthesis of new DNA molecule using the parent DNA as a
TEMPLATE.
Semiconservative Model of DNA replication-
MeselsonβStahl
3. MeselsonβStahl experiment
β’ E. Coli cells were grown on 15N-
medium for multiple
generations resulting in all DNA
to be 15N containing HEAVY
DNA.
β’ Next, cells were transferred to
14N-media and studied using
density gradient centrifugation
for multiple generations.
4. Process of DNA Replication
β’ INITIATION
β’ ELONGATION
β’ TERMINATION
5. INITIATION
β’ For the replication to begin there is a
particular region called the origin of
replication. This is the point where
the replication originates.
Replication begins with the spotting
of this origin followed by the
unwinding of the two DNA strands.
β’ Unzipping of DNA strands in their
entire length is not feasible due to
high energy input. Hence, first, a
replication fork is created catalysed
by the helicase enzyme, which
unzips the DNA strand.
Single-stranded
binding protein (ssb)
6. ELONGATION
β’ As the strands are separated, RNA primer is synthesized by enzyme
PRIMASE. The polymerase enzyme start synthesising the
complementary sequence in each of the strands. The parental strands
will act as a template for newly synthesising daughter strands.
β’ It is to be noted that elongation is unidirectional i.e. DNA is always
polymerised only in the 5β² to 3β² direction. Therefore, in one strand
(the template 3ββ5β) it is continuous, hence called continuous
replication. DNA synthesis is carried-out by Pol III in prokaryotes
and DNA Pol Ξ± and DNA Pol Ξ΅ in eukaryotes adds new nucleotides
β’ The other strand (the template 5ββ3β) it is discontinuous
replication. They occur as fragments called Okazaki fragments.
DNA synthesis is carried-out by DNA Pol I in prokaryotes and
DNA Pol Ξ± and Ξ΄ in eukaryotes.
β’ RNA PRIMERS are replaced with DNA by DNA Pol I in bacteria.
Primers are removed by enzyme RNaseH and FEN-1 (Flap
endonuclease-1) in eukaryotes. The enzyme called DNA ligase joins
the ends of Okazaki fragments later.
7. TERMINATION
β’ Termination of replication occurs in different ways in different organisms.
β’ In E.coli like organisms, chromosomes are circular. And this happens when the two replication forks
between the two terminals meet each other.
β’ In eukaryotes replication terminates when polymerase enzyme meet the newly synthesized 5β
nucleotide ends or two replication bubbles meet.
