Unit 14: Quest Number 2: DNA Replication, Mutations and Cancer
Honors Biology 2011
The process of DNA replication is
fundamentally similar for prokaryotes and
DNA replication is a semi-conservative
process – this means that when the double
helix is unwound, each original strand serves
as a template or pattern for the newly
The replisome is a Major Components
machine that ◦ DNA Helicase
◦ DNA Primase
replication of DNA.
The replisome ◦ DNA Polymerases
contains all the
necessary enzymes ◦ DNA ligases
for replication. ◦ DNA Binding Proteins
The process of Write these steps down:
replication can be INITIATION: formation of
thought of as having a
beginning where the
process starts; a ELONGATION: a number of
middle where the enzymes work together to
complementary accomplish the task of
nucleotides are added
assembling a new strand
to the exposed
parental strands; and TERMINATION: An end
an end where the point to the replication
process is terminated
unwind and unzip
the DNA molecule
A replication fork is
a Y-shaped region
where the new
strands of DNA are
The elongation process is semidiscontinuous.
The leading strand of the DNA molecule
(exposed 3’ end) is synthesized continuously
in the direction of the opening replication
The lagging strand of the DNA molecule
(exposed 5’ end) is synthesized
discontinuously in the direction opposite to
the opening of the replication fork.
Remember - The
two strands of the
DNA double helix
This means that
their sugar –
backbones run in
DNA polymerases add nucleotides
only to the FREE 3’ end of a
growing DNA strand, NEVER to
the 5’ end.
ONLY in the direction 5’ to 3’.
DNA Primase synthesizes a short
complementary segment to the exposed 3’
end of the DNA strand.
DNA polymerases extend the complementary
segment toward the replication fork as it
The ability of the DNA polymerase to remain
attached to the template (leading strand) is
known as PROCESSIVITY.
The synthesis of the lagging strand is
described as discontinuous since it is
synthesized in pieces that get glued together.
DNA primase synthesizes a short segment of
complementary DNA to the exposed 5’ end.
DNA polymerases add complementary
nucleotides in the direction away from the
opening replication fork.
DNA ligases glue the Okazaki Fragments
together forming the new strand.
Exposed 3’ end of the Exposed 5’ end of the
original DNA molecule original DNA molecule
continuously toward discontinuously away
the opening replication from the opening
The ability of a DNA The Okazaki fragments
polymerase to remain
attached to the leading are glued together by
strand is known as DNA Ligases.
Leading Strand Lagging Strand
Prokaryotes have circular DNA so replication
ends when the process comes around to the
In eukaryotes, end points for each
chromosome are indicated by telomeres.
Telomeres are specialized structures on the
ends of eukaryotic chromosomes that are
composed of specific repeated DNA
Telomeres are special nucleotide sequences
on the ends of the linear eukaryotic
The typical repeating unit in human
telomeres is TTAGGG. The number of
repetitions in a telomere varies between 100
The telometric DNA protects an organism’s
genes from being eroded through successive
rounds of DNA replications.
Remember that DNA polymerases can only
add nucleotides in the 5’ to 3’ direction.
The exposed 5’ end represents the lagging
The replisome does not contain the
machinery to complete the 5’ ends of the
daughter strands of the DNA. As a result,
repeated replication produces shorter and
shorter DNA molecules.
Briefly describe the functions of the following
enzymes during DNA replication: DNA
helicases, DNA polymerases, and DNA
Briefly provide an overview of DNA replication
using the terms: initiation, elongation,
Distinguish between the leading strand and
lagging strand during DNA replication?