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DNA Replication
DNA replication occurs during S phase of both mitosis and meiosis.Replication begins at specific sites in the
DNA strand called origins of replication.○ Each strand has many origins.
DNA Replication cont’d
Helicase binds to the origin of replication.Unwinds and separates the two stands.Single strand binding proteins prevent the
strands from closing up too soon.
DNA Replication cont’d
DNA polymerase binds to each of the strands.Reads both strands simultaneously in the 3’
5’ direction.Guides incoming nucleotides to pair with the
original strand.○ Adenine pairs with thymine, guanine pairs with
cytosine.
DNA Replication cont’d
The new strand grows in the 5’ 3’ direction.DNA polymerase binds the incoming
nucleotides together.
DNA Replication cont’d
Leading strand-the new, complementary strand made continuously in the 3’ 5’ direction.
Lagging strand-the new, complementary strand made discontinuously.The lagging strand is made in chunks
(Okazaki fragments) that will eventually fused into a single strand.
DNA Replication cont’d
Replication ends when DNA polymerase finishes reading the DNA strand.DNA polymerase releases the DNA.Single strand binding proteins let go.Helicase binds the strands together and
rewinds the helix.Semiconservative replication-each new
DNA consists of one original and one new strand.
Proofreading
DNA polymerase proofreads the DNA as it works.If an incoming nucleotide is paired
incorrectly, DNA polymerase stops, and removes the wrong nucleotide.○ The correct one is inserted and replication
resumes.