IB Biology adopted by Mrs. Selimovic-Milo
DNA replication
Metro High School
7.2.1 State that DNA replication occurs in a 5' - 3' direction
 DNA
replication is semi-conservative, meaning that a new strand is synthesised from an
original template strand
 DNA replication occurs in a 5' - 3' direction, in that new nucleotides are added to the C3
hydroxyl group such that the strand grows from the 3' end
 This means that the DNA polymerase enzyme responsible for adding new nucleotides moves
along the original template strand in a 3' - 5' direction
Direction of DNA Replication
7.2.2 Explain the process of DNA replication in prokaryotes, including the role of enzymes (helicase,
DNA polymerase, RNA primase and DNA ligase), Okazaki fragments and deoxynucleoside
triphosphates
 DNA replication is semi-conservative
 Helicase unwinds and separates the
and occurs during the S phase of interphase
double stranded DNA by breaking the hydrogen bonds
between base pairs
 This occurs at specific regions (replication origins), creating a replication fork of two
polynucleotide strands in antiparallel directions
 RNA primase synthesises a short RNA primer on each template strand to provide an
attachment and initiation point for DNA polymerase III
 DNA polymerase III adds deoxynucleoside triphosphates (dNTPs) to the 3' end of the
polynucleotide chain, synthesising in a 5' - 3' direction
 The dNTPs pair up opposite their complementary base partner (adenine pairs with
thymine ; guanine pairs with cytosine)
 As the dNTPs join with the DNA chain, two phosphates are broken off, releasing the
energy needed to form a phosphodiester bond
 Synthesis is continuous on the strand moving towards the replication fork (leading
strand)
 Synthesis is discontinuous on the strand moving away from the replication fork (lagging
strand) leading to the formation of Okazaki fragments
IB Biology adopted by Mrs. Selimovic-Milo
DNA replication
Metro High School
 DNA polymerase I removes the RNA primers and replaces them with DNA
 DNA ligase joins the Okazaki fragments together to create a continuous strand
Overview of DNA Replication
7.2.3 State that DNA replication is initiated at many points in eukaryotic chromosomes
 Because
eukaryotic genomes are (typically) much larger than prokaryotic genomes, DNA
replication is initiated at many points simultaneously in order to limit the time required for
DNA replication to occur
 The specific sites at which DNA unwinding and initiation of replication occurs are called origins
of replication and form replication bubbles
 As replication bubbles expand in both directions, they eventually fuse together, two generate
two separate semi-conservative double strands of DNA