11 DNA replication
DNA replication= process in which original DNA strands are used as template for the synthesis of new DNA strands.
11.1 Structural overview of DNA replication
Existing DNA strands act as templates for the synthesis of new strands
During replication: 2 complementary DNA strands come apart+ serve as template strands (parental strands)= DNA
strand used to synthesize a complementary strand of DNA/ RNA; synthesis of 2 new DNA strands.
To complete replication: covalent bond formed between the phosphate of 1 nucleotide+ the sugar of the
previous nucleotide-> resulting in 2 daughter strands= 2 newly made DNA strands (DNA replication).
Three different models were proposed to describe the net result of DNA
replication (fig 11.2)
3 different mechanisms proposed for DNA replication:
Conservative model (incorrect)= both parental strands stay together after DNA replication.
Original arrangement parental strands conserved+ 2 newly made daughter strands also remain together.
Semiconservative model (correct)= newly made dsDNA containing 1 parental strand+ 1 daughter strand
after replication.
Dispersive model (incorrect)= segments of parental+ daughter DNA are interspersed in both strands
following replication process.
Meselson+ Stahl: devised a method to experimentally distinguish newly made daughter strands from the original
parental strands (fig 11.3):
1. Grow E. coli in the presence of 15N (15NH4+; heavy isotope of nitrogen) for many generations.
Population of cells had heavy-labeled DNA.
2. Switch E. coli to medium containing only 14N (light isotope of nitrogen).
3. Collect sample of cells after various times.
4. Analyze the density of the DNA by centrifugation using a Cesium Chloride (CsCl) density gradient.
INTERPRETING THE DATA
After 1.0 generation, DNA is half-heavy; consistent with both semiconservative+ dispersive models.
After ~2 generations (1.9), DNA is light+ half-heavy; consistent with only the semi-conservative model.
Replication is semiconservative-> if it was dispersed, the heavy nitrogen would be even dispersed among
4 strands; each strand containing ¼ heavy+ ¾ light nitrogen.
11.2 Bacterial DNA replication: The formation of 2
replication forks at the origin of replication
Bacterial chromosomes contain a single origin of replication (fig 11.4)
Origin on replication= site on a chromosome; initiation site for DNA synthesis; begins within the origin.
Each bacterial chromosome has 1 origin of replication.
DNA synthesis proceeds bidirectionally= 2 replication forks move in opposite direction outward from the ori.
Replication forks eventually meet at the opposite side of the bacterial chromosome; ends replication.
Topoisomerase II= cleaves 2 strands; separates the 2 interconnected circles.
Replication Is initiated by the binding of DnaA protein to the origin of replication
(fig 11.5+11.6)
oriC (origin of Chromosomal replication)= origin of replication in E. coli-> 3 types of DNA sequences in oriC:
AT-rich region: 2 hydrogen bonds; easy to separate the double helix.
DnaA box= DNA sequence that serves as a recognition site for the binding of a DnaA protein.
GATC methylation sites: sequences that are methylated at the adenine.
Begin DNA replication:
1. DnaA proteins bind to DnaA boxes+ to each other.
DnaA protein= binds to a DnaA box at the origin of replication+ initiates DNA replication.
DNA replication= process in which original DNA strands are used as template for the synthesis of new DNA strands.
11.1 Structural overview of DNA replication
Existing DNA strands act as templates for the synthesis of new strands
During replication: 2 complementary DNA strands come apart+ serve as template strands (parental strands)= DNA
strand used to synthesize a complementary strand of DNA/ RNA; synthesis of 2 new DNA strands.
To complete replication: covalent bond formed between the phosphate of 1 nucleotide+ the sugar of the
previous nucleotide-> resulting in 2 daughter strands= 2 newly made DNA strands (DNA replication).
Three different models were proposed to describe the net result of DNA
replication (fig 11.2)
3 different mechanisms proposed for DNA replication:
Conservative model (incorrect)= both parental strands stay together after DNA replication.
Original arrangement parental strands conserved+ 2 newly made daughter strands also remain together.
Semiconservative model (correct)= newly made dsDNA containing 1 parental strand+ 1 daughter strand
after replication.
Dispersive model (incorrect)= segments of parental+ daughter DNA are interspersed in both strands
following replication process.
Meselson+ Stahl: devised a method to experimentally distinguish newly made daughter strands from the original
parental strands (fig 11.3):
1. Grow E. coli in the presence of 15N (15NH4+; heavy isotope of nitrogen) for many generations.
Population of cells had heavy-labeled DNA.
2. Switch E. coli to medium containing only 14N (light isotope of nitrogen).
3. Collect sample of cells after various times.
4. Analyze the density of the DNA by centrifugation using a Cesium Chloride (CsCl) density gradient.
INTERPRETING THE DATA
After 1.0 generation, DNA is half-heavy; consistent with both semiconservative+ dispersive models.
After ~2 generations (1.9), DNA is light+ half-heavy; consistent with only the semi-conservative model.
Replication is semiconservative-> if it was dispersed, the heavy nitrogen would be even dispersed among
4 strands; each strand containing ¼ heavy+ ¾ light nitrogen.
11.2 Bacterial DNA replication: The formation of 2
replication forks at the origin of replication
Bacterial chromosomes contain a single origin of replication (fig 11.4)
Origin on replication= site on a chromosome; initiation site for DNA synthesis; begins within the origin.
Each bacterial chromosome has 1 origin of replication.
DNA synthesis proceeds bidirectionally= 2 replication forks move in opposite direction outward from the ori.
Replication forks eventually meet at the opposite side of the bacterial chromosome; ends replication.
Topoisomerase II= cleaves 2 strands; separates the 2 interconnected circles.
Replication Is initiated by the binding of DnaA protein to the origin of replication
(fig 11.5+11.6)
oriC (origin of Chromosomal replication)= origin of replication in E. coli-> 3 types of DNA sequences in oriC:
AT-rich region: 2 hydrogen bonds; easy to separate the double helix.
DnaA box= DNA sequence that serves as a recognition site for the binding of a DnaA protein.
GATC methylation sites: sequences that are methylated at the adenine.
Begin DNA replication:
1. DnaA proteins bind to DnaA boxes+ to each other.
DnaA protein= binds to a DnaA box at the origin of replication+ initiates DNA replication.
