DNA replication

DNA replication

Objectives

At the end of this
lecture, student will be able to

• Outline on the important components in DNA replication

• Discuss the mechanism involved in DNA replication

Content

DNA replication

• Important players in DNA replication

• Mechanism of DNA replication

• Sequence specific DNA binding factors

• Protein that binds to specific DNA sequence

• Control the flow of genetic information from DNA to mRNA

• They perform function alone or with other protein in
complex

• Act either by promoting or blocking the recruitment of RNA
polymerase to specific gene 

DNA
replication 

• Process of copying of DNA

• Parental DNA unwinds and each DNA acts as its own template
for replication

• Two new DNA molecules consists of one parental strand and
one newly made strand

• Known as semi conservative DNA replication

Important
players in DNA replication

Mechanism
of DNA replication

Getting replication
started

• Replication begins at special site – origins of replication

• Proteins recognize the special site, binds to sequences
and separate the two strands

• Opens a replication “bubble”

• Replication proceeds in both the direction

• Replication fork –
Y shaped region where parental strand of

Replication Fork and Replication bubble 

DNA unwinds

• Helicases –
untwist the DNA at replication fork and additional twisting at DNA ahead of
replication fork

• Topoisomerase –
relieves the strain by breaking, untwisting and rejoining the DNA strand

• SSBPs- binds to
parental strand to keep them from reannealing to each other

Formation of
replication fork and its progression

• Unwound DNA strand is ready to be copied

• RNA primase – adds a short sequence (primer) of RNA
complementary to template DNA

• DNA polymerase synthesize new DNA by adding nucleotide to
the primer

Making a new DNA
strand

• DNA polymerase catalyses the synthesis of new DNA by
adding nucleotides to the RNA primer laid down by RNA primer

• E. coli has many DNA polymerase, 2 plays major role – DNA pol III and DNA pol I

• DNA pol III adds DNA nucleotide to 3’ end of RNA primer

• Two ends of DNA strands are anti-parallel to each other

• DNA polymerase can only add nucleotides to 3’end of
growing DNA strand

• Do not add to 5’ end

• A new DNA strand can only be made in the 5’ to 3’
direction

Leading and Lagging
strands

• Leading strand – strand of nascent DNA which is being
synthesized in the same direction as the growing replication fork

• DNA polymerase III is responsible for the formation of
leading strand

• Elongates the other strand in 5’→3’ direction

• Lagging strand – strand of nascent DNA whose direction of
synthesis is opposite to the direction of growing replication fork

• RNA primer puts down a new primer for DNA Pol III to
elongate

• Lagging strand is completed in discontinuous manner

• Synthesized DNA fragments on lagging strand – Okazaki
fragments

• DNA polymerase I is responsible for removing the RNA
primer

• Replaces them with DNA

• DNA ligases seals the gaps between okazaki fragments to
complete newly synthesized lagging strand

Summary

• DNA replication is a process of copying of DNA

• Parental DNA unwinds and each DNA acts as its own template
for replication

• Two new DNA smolecule consists of one parental strand and
one newly made strand and this known as semi conservative DNA replication

• Replication begins at special site called origins of
replication

• DNA polymerase, DNA helicase, Topoisomerase, SSBPs,
primases and DNA ligases are the important components for DNA replication