The structure of a DNA stand

The double helix

Evidence that DNA is genetic material

• Griffith – bacterial transformation

• Avery – discovered that the transforming agent is DNA

• Hershey & Chase – used bacteriophages and radioactive isotopes to demonstrate that DNA, not protein is genetic material

• Chargraff – found that A = T and C = G

Bacterial Transformation

•Study by Griffith•Assimilation of external DNA by a cell•Inherited by future generations

The Hershey-Chase experiment: phages

The Hershey-Chase experiment

A model for DNA replication: the basic concept

Three alternative models of DNA replication

The Meselson-Stahl experiment tested three models of DNA replication

Origins of replication in eukaryotes

Antiparallel Strands

DNA is elongated in the 5’ to 3’ direction

DNA is elongated in the 5’ to 3’ directionLeading strand

Lagging strandOther strand – elongated in piecesOkazaki fragments

Priming DNA for Replication

Summary of DNA Replication

5’ to 3’ directionRNA primerLeading/ lagging strandsEnzymes

DNA Damage & Repair

Thymine Dimer – covalent bond between thymines distorts DNA

Nuclease cuts out damaged DNA

DNA Polymerase fixes error

DNA Ligase seals DNA

Telomeres & Telomerases

Ends of linear DNA get shorter and shorter

In comes Telomerase-Lengthens the end of a shortened DNA strand-Uses RNA template

Telomeres -non-coding DNA at end of chromosomes-repeated sequence-gives DNA room to be shortened

Figure 16.19b Telomeres and telomerase