DNA Replication
Learning Objectives
Explain the role of DNA polymerase in copying DNA.
Compare DNA replication in prokaryotic cells and in
eukaryotic cells.
Review of DNA Structure
nitrogenous
bases
sugar-phosphate
backbone
double helix
Copying DNA
replication fork
DNA polymerase
Direction Direction
of replication of replication
new nucleotides
being added
DNA polymerase joins nucleotides to synthesize a new, complementary strand of
DNA.
the role of DNA polymerase: Besides producing the sugar-phosphate bonds that
join nucleotides together, DNA polymerase also “proofreads” each new DNA strand
so that each molecule is a near-perfect copy of the original.
How is DNA unzipped at the replication forks? Hydrogen bonds are broken.
What are the two roles of DNA polymerase in replication?
DNA polymerase joins individual nucleotides to produce a new strand of DNA and
proofreads the new strand.
DNA Replication
The blue strand represents the
original DNA strand.
The orange strand represents the
new DNA strand.
Is a new strand identical to its original
template strand?
No, they are complementary, not identical.
DNA Replication
• Since the two strands of DNA are complementary each strand acts as a template for building
a new strand in replication.
• In DNA replication
The parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
DNA replication is semiconservative
Each of the two new daughter molecules will have one old strand, derived from the
parent molecule, and one newly made strand
Okazaki
Leading & Lagging strands
Limits of DNA polymerase III
can only build onto 3 end of
an existing DNA strand 5
rag ments
ki f
Okaza 5
3 5 5 3
3
5 Lagging strand
3
ligase
growing 3
replication fork
5
Leading strand
Lagging strand
3 5
3
DNA polymerase III
Okazaki fragments
joined by ligase Leading strand
“spot welder” enzyme continuous synthesis
Summary
Initiation: DNA replication begins at specific locations called origins of
replication. Helicase unwinds the double helix, exposing the individual strands.
Single-strand binding proteins (SSBs) prevent the separated strands from
rejoining. Primase synthesizes short RNA primers that provide a starting point for
DNA polymerase to add complementary DNA nucleotides
Elongation: Two DNA polymerases—one for each strand—extend the nascent
DNA chains. Leading-strand synthesis is continuous, following the template
strand in the 5' to 3' direction. Lagging-strand synthesis occurs discontinuously
due to the antiparallel nature of the DNA strands, resulting in Okazaki fragments.
Each Okazaki fragment is extended in the 5' to 3' direction and requires post-
synthetic processing
Termination: Replication continues until the two replication forks meet and join
the newly synthesized DNA strands. Enzymes such as DNA ligase seal the gaps
between Okazaki fragments, creating full-length DNA strands
Telomeres
• Telomeres: the tips of eukaryotic chromosomes
• The enzyme telomerase adds short, repeated DNA
sequences to telomeres as the chromosomes are replicated.
Telomeres
The tips, or telomeres, of a DNA molecule are difficult to replicate. Cells use a
special enzyme called telomerase to add short repeated DNA sequences to
telomeres as chromosomes are replicated.
In the micrograph, the red structures are human chromosomes and the telomeres
are stained white, and appear in photograph as yellowish orange.
Prokaryotic DNA Replication
new DNA
replication fork
replication fork
unreplicated DNA
Unlike eukaryotic DNA, the DNA of prokaryotic organisms
exists as a single loop. Explain that replication in most
prokaryotic cells begins at a single starting point and
proceeds in two directions until the entire chromosome is
copied.
Eukaryotic DNA Replication
Unreplicated DNA
Replication
forks
New DNA
The process students looked at earlier in the presentation
was for replication eukaryotic DNA replication. Remind
students that eukaryotic chromosomes are linear.
Where is the origin of replication?
in the center of each newly forming strand
In eukaryotic cells replication may begin at dozens or
even hundreds of places on the DNA molecule,
proceeding in both directions until each chromosome is
completely copied.
