1928 Fred Griffith- Discovered that something from the heat killed wild type (smooth) bacteria was turning the mutant (rough) type bacteria into smooth. Called it the transforming factor Didn’t really know what it was
1944- Avery, McCarty and MacLeod Purified both protein, RNA, and DNA separately from Streptococcus pneumonia bacteria to determine which would transform the harmless bacteria- How? Injected protein into bacteria no effect Injected RNA into bacteria no effect Injected DNA into bacteria mice died Conclusion: DNA is the genetic material!!
Nobody believed Avery People thought that Avery had inaccurate results and contamination and still thought the transforming factor could be. Back to square 1! We know there is a “transforming factor” Is it protein or DNA? Lets ask Hershey and Chase!
Hershey and Chase Worked with viruses to determine what the “transforming factor” was. *Before we discuss their experiment we must understand what a virus is and how it works!*
Viruses- 6 Characteristics of Viruses 1. Have their own genome (genetic make up) made of either DNA or RNA 2. Does not have enzymes, ribosomes, or ATP 3. Have external protein shells (capsids)
Viruses- 6 Characteristics of Viruses 4. Infect only specific cells 5. 2 life cycles (lytic and lysogenic) – Lytic – breaks out and spreads – Lysogenic- hangs around 6. Smaller than bacteria
5 Stages of Attack!! 1) Attachment: virus attaches to it’s host 2) Injection: virus injects it’s DNA into the host cell. 3) Replication: virus copies it’s genetic material many times. 4) Assembly: virus assembles more viruses 5) Release: Viruses are release from the cell ready to attach to new cells!
Homework! Read pgs. 326-328, Summarize the experiments of Griffith and Avery
2/23 DO NOW Take out your homework!!! Which strain of bacteria killed the mice in Griffith’s experiment? What is the process that caused rough bacteria to become smooth? What did Avery discover? What are the 5 stages of attack for a virus?
Objectives Summarize the experiments of Hershey and Chase. Identify the stages of replication of a virus. Understand the use of viruses in discovering DNA.
Lets Review… Who are the scientists we’ve talked about so far? What are the 6 characteristics of a virus?
1952 Hershey and Chase Radioactive Labeling: – Used radioactive phosphorus ( 32 P) to identify DNA in the bacteriophages – Used radioactive sulfer ( 35 S) to identify proteins in the bacteriophagesTracking DNA Radioactive bacteriophages infected bacteria cells After review results found radioactive sulfur outside of bacteria cells and radioactive phosphorus inside bacteria cells
1952 Hershey and Chase Used Bacteriophages Two radioactively labeled viruses. – 1. Sulfur Protein coat – 2. Phosphorous DNA After infecting cells, only the DNA was still radioactively labeled DNA is genetic material
Watson and Crick Built a model of the double helix that conformed to the others’ research – two outside strands consist of alternating sugar and phosphate molecules – cytosine and guanine bases pair to each other by three hydrogen bonds – thymine and adenine bases pair to each other by two hydrogen bonds
DNA Structure DNA often is compared to a twisted ladder. Rails of the ladder are represented by the alternating deoxyribose and phosphate. The pairs of bases (cytosine– guanine or thymine–adenine) form the steps.
Base Pairs- Rungs of the ladder -2 Strands of DNA form a helix
Orientation of DNA- Anti Parallel On the top rail, the strand is said to be oriented 5′ to 3′. The strand on the bottom runs in the opposite direction and is oriented 3′ to 5′.
Chromosome Structure DNA coils around histones (proteins) to form nucleosomes, This coils to form chromatin fibers. The chromatin fibers supercoil to form chromosomes that are visible in the metaphase stage of mitosis.