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DNA Replication
“Replication“ is the process in which each strand of the
original double-stranded DNA molecule serves as
template for the reproduction of the complementary
strand.
Two identical DNA molecules have been produced from
a single double-stranded DNA molecule
Characteristics
Semi-conservative
Bi-directional
Iniatiates at specifics origins
Catalyzed by DNA polymerases
DNA Replication-Correlate with Cell Cycle
DNA replication occurs in the nucleus during S phase of
eukaryotic cell cycle
The two identical sister chromatids separate during
mitosis when cell divide.
Prokaryotic and Eukaryotic Replication
Prokaryotic DNA is a closed circular double stranded
molecule with single origin of replication
Eukaryotic DNA is a long linear molecule with multiple
origins of replication
DNA POLYMERASE
Enzymes that synthesize nucleic acids by formingphosphodiester bonds
PROKARYOTIC DNA POLYMERASE: DNA Polymerase I, DNAPolymerase III
EUKARYOTIC DNA POLYMERASE:
DNA polymerase α synthesizes lagging strand
Origin of Replication
Recognition of Origin of Replication
Ori C is recognized and bound by dna A Protein
The parental strands of DNA are pulled apart
A replication bubble is created
Unwinding of double helix
Unwinding of parental strands is brought about by action of Helicase
Helicases break Hydrogen bonds holding the two strands of DNA usingenergy from ATP
The two strands begin unwinding and form two replication forks
Stabilization of single stranded DNA
Single stranded DNA binding proteins (SSB) bind to each of the singlestranded DNA molecule
SSB stabilize, preventing them from re-associating.
SSB prevents the single strands from degradation by nucleases
Two types of DNA strands are synthesized Leading strand Lagging strand
LEADING AND LAGGING STRANDS
Leading strand is synthesized continuously as one long, singlepiece
Lagging strand is synthesized discontinuously as series ofsmall fragments.
Each fragment on Lagging Strand is 1000 nucleotide long calledOkazaki fragments.
Each fragment is made in 5’-3’ direction
Leading and lagging strands
Removal of RNA
primers
DNA Polymerase I removes RNA primers by virtue of
5’-3’ exonuclease activity
The resulting gaps are filled by synthesis of DNA at the3’end of neighbouring Okazaki fragment by DNA
Polymerase III
Joining of Okazaki fragments
Small nicks between the Okazaki fragments are sealed
by DNA ligase
All Okazaki fragments are joined together to form one
continuous strand
Removal of positive super coiling
Telomers
Telomers are short sequences at the end of linear
eukaryotic molecule.
DNA polymerase can not complete synthesis at 5’ end of
each strand
With each round of replication telomers are shortened
Telomer shortening is thought to be responsible for
aging.
Telomerase
Eukaryotic enzyme used to maintain telomers.
Telomrase has:
Reverse transcriptase activity (hTRT) Short RNA template complementary to DNA telomer sequence
Telomerase activity is presents in germ cells and stem cells
Somatic cells do not have telomerase activity correlates with
aging.
