BABS1201 Lecture Notes - Lecture 9: Telomerase, Dna Polymerase Iii Holoenzyme, Nucleic Acid Double Helix
28 May 2018
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DNA replication
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• Parental DNA molecule - each base is paired with its specific partner
• Separate the two DNA strands
• Each parental strand serves as a template for the order of nucleotides along the new
complementary strand
• Nucleotides are connected to form the sugar phosphate backbones of the new strands
• New nucleotide strand = daughter strand
▪ Replication process is semi-conservative - the two parental strands are not lost,
however are mixed with daughter strands
Origin of replication
• Replication begins in AT rich regions on the DNA
o AT - contains two hydrogen bonds, which are easier to break than GC. Hence is more
favourable for separation
Multiple origins are needed, for time efficiency during replication
Replication bubble expands along the strand until the entire molecule has been replicated
o Human cells contain 6 billion bases which are copied within a few hours
DNA strands are antiparallel - strands run in opposite directions with respect to the sugar-phosphate
backbone
Enzymes can only synthesise DNA in one direction - in the 3' to 5' direction
DNA polymerase - catalyses the addition of new nucleotides
Recognises the correct nucleotide to put in
Catalyses the addition of the nucleotide
find more resources at oneclass.com
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Incoming nucleotide contains a triphosphate group
o Nucleoside triphosphate - contains energy rich phosphate bonds.
▪ 2 phosphates (pyrophosphate) break off, releasing energy that fuels the bonding
between nucleotides
o Phosphate group binds to 3' hydroxyl group
Requirements of DNA polymerase
Must have a 3' OH group to add on to
o Can only elongate in the 5' to 3' direction
o Cannot initiate DNA synthesis on its own
▪ Must have a primer (short piece of RNA) that contains a 3'OH group
DNA replicated from the 5' to the 3' end
o Nucleoside triphosphate - contains phosphate group that attaches to the hydroxyl group on
the 3' end
▪ Contains energy to drive synthesis
o 5' end - only has one phosphate group - no energy to drive synthesis
o Pyrophosphate ion (2P) is released and broken down to phosphate ions
Leading strand synthesis
• Helicase - unwinds the DNA helix
• Single strands binding proteins - binds to single stranded DNA, stopping the DNA from
rewinding
• Primase - produces RNA (primer), attaching it to the start of the sequence.
o RNA doesn't not need to attach to a 3' OH group - allows polymerase to synthesise
onto it
DNA polymerase III starts the synthesise the leading strand
o Sliding clamp - attached to DNA pol III to secure the polymer onto the DNA strand
Leading strand is elongated continuously in the 5' -> 3' direction as the fork progresses
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Dna replication: parental dna molecule - each base is paired with its specific partner. Origin of replication: replication begins in at rich regions on the dna, at - contains two hydrogen bonds, which are easier to break than gc. Multiple origins are needed, for time efficiency during replication. Replication bubble expands along the strand until the entire molecule has been replicated: human cells contain 6 billion bases which are copied within a few hours. Dna strands are antiparallel - strands run in opposite directions with respect to the sugar-phosphate backbone. Enzymes can only synthesise dna in one direction - in the 3" to 5" direction. Dna polymerase - catalyses the addition of new nucleotides. Incoming nucleotide contains a triphosphate group: nucleoside triphosphate - contains energy rich phosphate bonds, 2 phosphates (pyrophosphate) break off, releasing energy that fuels the bonding between nucleotides, phosphate group binds to 3" hydroxyl group. Leading strand synthesis: helicase - unwinds the dna helix.
