2. Chapter Outline
Basic Features of DNA Replication In
Vivo
DNA Polymerases and DNA Synthesis
In Vitro
The Complex Replication Apparatus
Unique Aspects of Eukaryotic
Chromosome replication
3. Basic Features of DNA
Replication In Vivo
DNA replication occurs
semiconservatively, is initiated at
unique origins, and usually
proceeds bidirectionally from each
origin of replication.
4. DNA Replication is Semiconservative
Each strand serves
as a template
Complementary
base pairing
determines the
sequence of the
new strand
Each strand of the
parental helix is
conserved
18. Key Points
DNA replicates by a semiconservative mechanism:
as the two complementary strands of a parental
double helix unwind and separate, each serves as a
template for the synthesis of a new complementary
strand.
The hydrogen-bonding potentials of the bases in the
template strands specify complementary base
sequences in the nascent DNA strands.
Replication is initiated at unique origins and usually
proceeds bidirectionally from each origin.
19. DNA Polymerases and DNA
Synthesis In Vitro
Much of what we know about DNA
synthesis was deduced from in
vitro studies.
20. Requirements of DNA Polymerases
Primer DNA with
free 3'-OH
Template DNA to
specify the
sequence of the
new strand
Substrates: dNTPs
Mg2+
25. DNA Polymerases
Polymerases in E. coli
– DNA Replication: DNA Polymerases III and I
– DNA Repair: DNA Polymerases II, IV, and V
Polymerases in Eukaryotes
– Replication of Nuclear DNA: Polymerase
and/or
– Replication of Mitochondrial DNA: Polymerase
– DNA Repair: Polymerases and
All of these enzymes synthesize DNA 5' to 3'
and require a free 3'-OH at the end of a primer
30. Key Points
DNA synthesis is catalyzed by enzymes called DNA
polymerases.
All DNA polymerases require a primer strand, which
is extended, and a template strand, which is copied.
All DNA polymerases have an absolute requirement
for a free 3'-OH on the primer strand, and all DNA
synthesis occurs in the 5' to 3' direction.
31. Key Points
The 3'5' exonuclease activities of DNA
polymerases proofread nascent strands as
they are synthesized, removing any
mispaired nucleotides at the 3' termini of
primer strands.
34. DNA Replication
Synthesis of the leading strand is
continuous.
Synthesis of the lagging strand is
discontinuous. The new DNA is
synthesized in short segments (Okazaki
fragment) that are later joined together.
48. Key Points
DNA replication is complex, requiring the
participation of a large number of proteins.
DNA synthesis is continuous on the progeny
strand that is being extended in the overall
5'3' direction, but is discontinuous on the
strand growing in the overall 3'5' direction.
49. Key Points
New DNA chains are initiated by short RNA
primers synthesized by DNA primase.
The enzymes and DNA-binding proteins
involved in replication assembled into a
replisome at each replication fork and act in
concert as the fork moves along the parental
DNA molecule.
50. Unique Aspects of Eukaryotic
Chromosome Replication
Although the main features of
DNA replication are the same in
all organisms, some processes
occur only in eukaryotes.
51. DNA Replication in Eukaryotes
Shorter RNA primers and Okazaki
fragments
DNA replication only during S phase
Multiple origins of replication
Nucleosomes
Telomeres
60. Telomere Length and Aging
Most human somatic
cells lack telomerase
activity.
Shorter telomeres are
associated with cellular
senescence and death.
Diseases causing
premature aging are
associated with short
telomeres.
61. Key Points
The large DNA molecules in eukaryotic chromosome
replicate bidirectionally from multiple origins.
Two or three DNA polymerases (and/or ) are
present at each replication fork in eukaryotes.
Telomeres, the unique sequences at the ends of
chromosomes, are added to chromosome by a
unique enzyme called telomerase.