VIII. DNA Function: Replication

A. Recap - occurs in the S-phase of Interphase - unreplicated chromosomes, each consisting of a complementary double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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 double- helix, 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)

- 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

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 13- mers – 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 13- mers – 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).