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

Components

Functions

DNA helicase                       

Unwinds DNA

Topoisomerase

Alleviates positive supercoiling (twisting of DNA) ahead of
replication fork

Single stranded binding proteins (SSBPs)

Keep the parental strand apart

Primase          

Synthesizes RNA primer

DNA polymerase III            

Synthesizes daughter strands of DNA

DNA polymerase I              

Excises the RNA primer and fills in the DNA

DNA ligase                            

Covalently links the okazaki fragments together

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

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