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Lecture 5

BIO130H1 Lecture Notes - Lecture 5: Pyrophosphate, Okazaki Fragments, Exonuclease


Department
Biology
Course Code
BIO130H1
Professor
Melody Neumann
Lecture
5

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Introduction to DNA Replication
Lecture Outline:
1) Overview of DNA replication
2) DNA replication in bacteria
3) PCR
Readings:
Pg. 546-559
Pg. 769-770
Initial Questions
1) Is DNA replication conservative or semiconservative?
2) What is the direction of DNA replication?
3) Where does DNA replication start?
Is DNA replication conservative or semiconservative?
1) DNA synthesis is conservative
once the DNA strands are replicated, one daughter cells get both of the parental
strands and the other get both of the newly synthesized strand
2) DNA synthesis is semiconservative -- YES
each get one parental and one newly synthesized
So If there is an error, you have a parental strand to use as a template to correct these
errors
What is the direction of DNA replication?
Nucleotides can only be added onto the 3’ end!
a new strand of DNA can only grow in the 5’ to 3’ direction!
Three possible models:

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Where does DNA replication start? (origin of replication)
Two possibilities:
1) Always start from the same location on DNA
What are some of the characteristics of the sequences at replication origins?
Easy to open, A-T rich
only 2 H bonds between AT -- takes less energy to open up the
double helix -- replication machinery can get in there more
easily
Recognized by and bound by initiator proteins
able to find where to perform
2) Random start -- NO
How many origins of replication?
1) Single
e.g. bacteria
can replicate the whole chromosome
quite efficiently even though
there's only one origin
2) Multiple
e.g. eukaryotes
two direction for every replication
factory -- very quickly
How does DNA replication proceed in bacteria?
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This style of replication only applies to: circular genomes
eg. bacteria, mitochondria, chloroplast genomes
consequence of replicating from one origin and going to 2 directions -- replication fork
What happens at the DNA replication forks?
replication fork is asymmetrical because:
dna only replicated in 5’ 3’ direction!
new DNA strands are formed in an antiparallel, complementary fashion
Leading strand can be replicated continuously, lagging strand replicated
discontinuously
lagging: making Ozakazi fragments
Overview of DNA replication
Ingredients for synthesis:
1)Origin
unique sequences
2)Primers
DNA polymerase cannot start a new strand from nothing
a primer needed to provide 3’OH group
3)dNTPs
4)ATP as an energy source
5) DNA polymerase
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