VIII. DNA Function: Replication
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model: The original DNA is used as a template for the
formation of new strand, and then they reanneal:
Original DNA
new strand formation
Reannealing of
original strands and
synthesis of new
double helix.
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model: The original DNA is used as a template for the
formation of new strand, and then they reanneal :
Original DNA
new strand formation
Reannealing of
original strands and
synthesis of new
double helix.
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model: The original DNA is used as a template for the
formation of new strand, and then they reanneal :
Original DNA
new strand formation
Reannealing of
original strands and
synthesis of new
double helix.
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model: The original DNA is used as a template for the
formation of new strand, which then bind together:
Original DNA
new strand formation
Binding of old and
new strands
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model: The original DNA is used as a template for the
formation of new strand, which then bind together:
Original DNA
new strand formation
Binding of old and
new strands
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model: The original DNA is used as a template for the
formation of new strand, which then bind together:
Original DNA
new strand formation
Binding of old and
new strands
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model:
3. Dispersive: Old and new DNA distributed throughout double-helix
Original DNA
new strand formation
Dispersal of old and
new DNA throughout
both chromatids
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model:
3. Dispersive: Old and new DNA distributed throughout double-helix
Original DNA
new strand formation
Dispersal of old and
new DNA throughout
both chromatids
VIII. DNA Function: Replication
A. Recap
- occurs in the S-phase of Interphase
- unreplicated chromosomes, each consisting of a complementary doublehelix, is REPLICATED: produce a chromosomes with 2 identical chromatids.
- Once DNA replication has occurred, cells will proceed to division.
B. Hypotheses:
1. Conservative Model:
2. Semi-conservative Model:
3. Dispersive: Old and new DNA distributed throughout double-helix
Original DNA
new strand formation
Dispersal of old and
new DNA throughout
both chromatids
VIII. DNA Function: Replication
A. Recap
B. Hypotheses
C. The Meselson and Stahl Experiment (1958)
C. The Meselson and Stahl Experiment (1958)
- They grew E. coli for many generations on an agar where the available Nitrogen
was a heavy isotope. The heavy nitrogen was incorporated into NEW DNA that is
synthesized by the cell.
C. The Meselson and Stahl Experiment (1958)
- They grew E. coli for many generations on an agar where the available Nitrogen
was a heavy isotope. The heavy nitrogen was incorporated into NEW DNA that is
synthesized by the cell. When the DNA is spun in a centrifuge, it spins down ‘far’
C. The Meselson and Stahl Experiment (1958)
- Then, grew bacteria on an N15 medium for 1 generation and took samples.
What banding pattern would you expect to see under each of the three hypotheses?
C. The Meselson and Stahl Experiment (1958)
- Then, grew bacteria on an N15 medium for 1 generation and took samples.
What banding pattern would you expect to see under each of the three hypotheses?
- Results refuted the conservative hypothesis.
C. The Meselson and Stahl Experiment (1958)
- Then, grew bacteria on an N15 medium for 1 generation and took samples.
What banding pattern would you expect to see under each of the three hypotheses?
- Results refuted the conservative hypothesis.
- Grew bacteria for a second generation on N14 medium…. Predictions of 2 models?
C. The Meselson and Stahl Experiment (1958)
- Then, grew bacteria on an N15 medium for 1 generation and took samples.
What banding pattern would you expect to see under each of the three hypotheses?
- Results refuted the conservative hypothesis.
- Grew bacteria for a second generation on N14 medium…. Predictions of 2 models?
- Results refuted the dispersive model and confirmed the semi-conservative model.
D. Replication at the Molecular Level
D. Replication at the Molecular Level
1. Replication in E. coli
a. A specific sequence of bases is recognized as the binding site for the replication
complex – this is called the Replication Origin, and the DNA replicated from this site
is called a replicon.
D. Replication at the Molecular Level
1. Replication in E. coli
a. A specific sequence of bases is recognized as the binding site for the replication
complex – this is called the Replication Origin, and the DNA replicated from this site
is called a replicon.
- Bacteria have only a single replication origin, and the entire circular chromosome
is replicated from this point.
1. Replication in E. coli
a. Replication Origin has 9-mers and 13mers – homologous sequences…
b. Three enzymes (DNaA,B, C),
collectively called “helicases” bind to
the origin and break the hydrogen
bonds holding the helices together.
“Single-strand binding proteins”
stabilize the DNA.
1. Replication in E. coli
a. Replication Origin has 9-mers and 13mers – homologous sequences…
b. Three enzymes (DNaA,B, C),
collectively called “helicases” bind to
the origin and break the hydrogen
bonds holding the helices together.
“Single-strand binding proteins”
stabilize the DNA.
c. The enzyme gyrase works
downstream, cutting single or double
strands to relieved the torque on the
molecule. (topoisomerases affect
shape).