CHAPTER 11 DNA Replication
11.1 Structural Overview of DNA Replication
11.2 Bacterial DNA Replication: The Formation of Two Replication Forks at the Origin of
Replication
11.3 Bacterial DNA Replication: Synthesis of New DNA Strands
11.4 Bacterial DNA Replication: Chemistry and Accuracy
11.5 Eukaryotic DNA Replication
11.1 Structural Overview of DNA Replication
The double helix is composed of two DNA strands. The double-banded structure is held
together by base stacking and by hydrogen bonding (AT = 2, CG = 3) between the bases is
opposite strands.
During the replication process, the two complementary strands of DNA come apart and serve
as template strands or parental strands for the synthesis of two new strands of DNA. The
two newly made strands are referred to as daughter strands.
DNA replication follows the semiconservative model: the double-stranded DNA is half
conserved following the replication process; the newly made double-stranded DNA contains
one paternal strand and one daughter strand.
, 11.2 Bacterial DNA Replication: The Formation of Two Replication Forks at the Origin
of Replication
Origin of replication: the site on the chromosomes where DNA synthesis begins. Bacterial
chromosomes have a single origin of replication. The synthesis of new daughter strands is
initiated within the origin and proceeds bidirectionally, around the bacterial chromosome. Two
replication forks (the region where the parental DNA strands have separated and new daughter
strands are being made) move in opposite directions outward from the origin. The replication
forks meet each other on the opposite side of the bacterial chromosome to complete the
replication process.
The origin of replication of E.coli is oriC (origin of chromosomal replication).
Three types of DNA sequences are found within oriC:
1. An AT-rich region,
2. DnaA box sequences,
3. GATC methylation sites.
DNA replication begins with the binding of DnaA proteins to DnaA box sequences. The DnaA
proteins (in their ATP-bound form) bind to 5 DnaA boxes in oriC to initiate DNA replication.
HU and IHF (DNA-binding proteins) cause the DNA to bend around a complex of DnaA
proteins, which results in the separation of the strands at the AT-rich region.
The DnaA proteins and DnaC proteins recruit DNA helicase to the initiation site. DNA helicase
breaks the hydrogen bonds between the two strands of DNA and begins within the oriC region.
* these proteins use energy from ATP hydrolysis to catalyze the separation.
DNA helicases bind to single-stranded DNA and travel along the DNA in a 5’ to 3’ direction
to keep the replication fork moving; bacterial chromosome replication is bidirectional
replication.
The GATC methylation sites within oriC are involved with regulation DNA replication. Prior
to DNA replication, the sites are methylated (by Dam: DNA adenine methyltransferase) in both
strands. This facilitates the initiation of DNA replication at the origin.
* because it takes several minutes for Dam to methylate the GATC sites within the oriC region,
DNA replication does not occur again until after that methylation is completed.
11.1 Structural Overview of DNA Replication
11.2 Bacterial DNA Replication: The Formation of Two Replication Forks at the Origin of
Replication
11.3 Bacterial DNA Replication: Synthesis of New DNA Strands
11.4 Bacterial DNA Replication: Chemistry and Accuracy
11.5 Eukaryotic DNA Replication
11.1 Structural Overview of DNA Replication
The double helix is composed of two DNA strands. The double-banded structure is held
together by base stacking and by hydrogen bonding (AT = 2, CG = 3) between the bases is
opposite strands.
During the replication process, the two complementary strands of DNA come apart and serve
as template strands or parental strands for the synthesis of two new strands of DNA. The
two newly made strands are referred to as daughter strands.
DNA replication follows the semiconservative model: the double-stranded DNA is half
conserved following the replication process; the newly made double-stranded DNA contains
one paternal strand and one daughter strand.
, 11.2 Bacterial DNA Replication: The Formation of Two Replication Forks at the Origin
of Replication
Origin of replication: the site on the chromosomes where DNA synthesis begins. Bacterial
chromosomes have a single origin of replication. The synthesis of new daughter strands is
initiated within the origin and proceeds bidirectionally, around the bacterial chromosome. Two
replication forks (the region where the parental DNA strands have separated and new daughter
strands are being made) move in opposite directions outward from the origin. The replication
forks meet each other on the opposite side of the bacterial chromosome to complete the
replication process.
The origin of replication of E.coli is oriC (origin of chromosomal replication).
Three types of DNA sequences are found within oriC:
1. An AT-rich region,
2. DnaA box sequences,
3. GATC methylation sites.
DNA replication begins with the binding of DnaA proteins to DnaA box sequences. The DnaA
proteins (in their ATP-bound form) bind to 5 DnaA boxes in oriC to initiate DNA replication.
HU and IHF (DNA-binding proteins) cause the DNA to bend around a complex of DnaA
proteins, which results in the separation of the strands at the AT-rich region.
The DnaA proteins and DnaC proteins recruit DNA helicase to the initiation site. DNA helicase
breaks the hydrogen bonds between the two strands of DNA and begins within the oriC region.
* these proteins use energy from ATP hydrolysis to catalyze the separation.
DNA helicases bind to single-stranded DNA and travel along the DNA in a 5’ to 3’ direction
to keep the replication fork moving; bacterial chromosome replication is bidirectional
replication.
The GATC methylation sites within oriC are involved with regulation DNA replication. Prior
to DNA replication, the sites are methylated (by Dam: DNA adenine methyltransferase) in both
strands. This facilitates the initiation of DNA replication at the origin.
* because it takes several minutes for Dam to methylate the GATC sites within the oriC region,
DNA replication does not occur again until after that methylation is completed.
