DNA Replication
DNA Structure Review
● DNA twists around histone proteins to
keep it compact in the nucleus
● Chromatin- condensed DNA wrapped
around histones
● DNA’s shape is a double helix
● Each strand of DNA in the helix is
complementary to the other
DNA Replication
● Before mitosis can happen,
DNA has to be copied
○ Occurs in S stage of the
cell cycle
● Both complementary
strands are used as a
template for a DNA copy
DNA Replication Steps
● The enzyme DNA helicase “unzips” the
DNA by breaking the hydrogen bonds
between nitrogenous bases
○ One DNA strand is the leading strand
○ The other is the lagging strand
DNA Replication Steps
● The leading strand is the easy strand
○ Complementary free-floating bases
pair up with the bases on the DNA
strand
○ Enzyme DNA polymerase bonds the
nucleotides together
DNA Replication Steps
DNA Replication Steps
● The lagging strand is more difficult
● New DNA strand is created in segments
○ segments of RNA are assembled by the
enzyme primase
○ DNA polymerase replaces the RNA
with DNA
○ Enzyme ligase “glues” the pieces
DNA Replication Steps
DNA Duplication Steps
● DNA polymerase also proofreads the DNA
to prevent errors and mutations
● Reduces error rate to 1 error per 10 billion
base pairs
DNA Duplication Steps
● The two new copies of DNA are semiconservative
○ semi-conservative because each new
DNA contains an original DNA strand
and a new one
Prokaryotic DNA Replication
● have one chromosome, circular DNA
● starts replication at one point and goes in
two directions
Eukaryotic DNA Replication
● have multiple chromosomes of DNA
● replicates at many points on chromosome