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 |
|
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