BIO1022 Lecture Notes - Lecture 3: Telomerase, Telomere, Dna Ligase
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30 May 2018
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BIO1022 – Lecture 3 – Week 2 – DNA Replication
- DNA is replicated by semi-conservative replication
• hydrogen bonds broken - become single strands - act as a template - new
molecule forms acting on the template of the old molecule
• new molecules each contain one of the old and one of the new strands
- initiation of replication
• prokaryotes
•
o circular genomes
o one origin or replication
o double stranded DNA molecule - replication begins at the origin -
proceeds in both directions - unwinding of double stranded DNA -
old strand able to be used a template for the new strand to form
• eukaryotes
•
o linear genomes
o multiple origins of replication
• a replication fork at initiation of replication
•
o helicase - enzyme required to unwind and separate the parental
DNA strands
o topoisomerase - allows the DNA to spin around - releases the
tension that has been built up by the helicase
o single strand binding proteins stabilise the unwound parental
strands
o primase - synthesises RNA primers - act as a site for the RNA
polymerase to produce DNA template - 3 - 5 prime direction
- DNA synthesis
• new strand - 5 - 3 prime
• leading strand - primer added by the primase - allowed the recruitment of
DNA polymerase III - sliding clamp protein - binds the DNA -
• lagging strand
•
o short fragments of the new strand
o 5 - 3 prime
o primase joins RNA nucleotides into a primer
o DNA pol III adds DNA nucleotides to the primer - forming Okazaki
fragment I
o after reaching the next RNA primer to the right, DNA pol III
detaches
o fragment 2 is primed
o cant have RNA
o DNA pol I replaces RNA with DNA - adding nucleotides to the 3’
end of fragment 2
o DNA ligase forms a bond between the newest DNA and the DNA of
fragment 1
- telomeres - telomerase adds new DNA at the ends
- modification of an organism’s DNA
• to create a modified organism for use
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