Chapter 12 DNA and RNA Digital Illustration DNA Structure

12-1 DNA

Discuss w/ Partner: You may use your textbook or notes. What organelle is considered the “control center” of the cell? What structures are found in the nucleus? What are chromosomes composed of? How do genes & chromosomes control the activity of the cell?

Answers: What organelle is considered the “control center” of the cell? The nucleus What structures are found in the nucleus? Chromosomes What are located on chromosomes? Genes What are chromosomes composed of? DNA wrapped around proteins called histones. How do genes & chromosomes control the activity of the cell? By producing proteins that regulate cellular functions or become part of the cell structure.

I. Griffith and Transformation Fredrick Griffith in 1936

In the 1920’s Griffith was a biologist trying to figure out how bacteria produced pneumonia (a deadly disease). 2 different stains (breeds) of pneumonia bacteria were isolated (separated) from mice The pneumonia was grown in the lab

Griffith’s Hypothesis: Material in dead bacterial cells can genetically change living bacterial cells

5. Griffith’s Experiments Griffith set up four individual experiments. Experiment 1: Mice were injected with the disease-causing strain of bacteria. Results: The mice developed pneumonia and died.

Experiment 2: Mice were injected with the harmless strain of bacteria. Results: These mice didn’t get sick

Experiment 3: Griffith heated the disease-causing bacteria Experiment 3: Griffith heated the disease-causing bacteria. He then injected the heat-killed bacteria into the mice. Results: The mice survived

Results: The mice developed pneumonia and died. Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. Results: The mice developed pneumonia and died.

Steps of Griffith’s transformation experiment which help scientist discover DNA as genetic material

Conclusion: the heat-killed bacteria passed their disease-causing ability to the harmless strain

Transformation- One strain of bacteria is changed by a gene or genes from another strain of bacteria.

Discuss w/ Partner: Look at fig. 12-1 on pg. 288 What was Griffith trying to learn when he set up this experiment? How did Griffith show that the disease-causing bacteria were killed by heat? What result was Griffith expecting when he injected the mixture of live harmless bacteria & heat-killed disease bacteria?

Answers: What was Griffith trying to learn when he set up this experiment? How bacteria caused pneumonia. How did Griffith show that the disease-causing bacteria were killed by heat? He tried to grow them in a petri dish. If bacteria grew, then he knew that he had not killed them. What result was Griffith expecting when he injected the mixture of live harmless bacteria & heat-killed disease bacteria? He expected the mice to live.

After his experiment Griffith hypothesized that: something must have information that could change harmless bacteria into disease-causing ones

Discuss w/ Partner: Who remembers getting immunizations ? Why do people get immunizations? How do they work?

Ethical Question: Some people believe autism is caused by preservatives in the vaccine. As a result some people have chosen not to immunize their children. What is your opinion?

II. Avery and DNA Oswald Avery in 1937

In the 1940’s a scientist named Avery repeated Griffith’s experiment The research question: what molecule in the heat-killed bacteria was the most important part of transformation. That molecule is probably a part of the gene that allows transformation to take place.

3 . The Experiment: Avery & other scientists made a bacterial extract (juice) from the heat-killed bacteria and used enzymes to destroy all of the molecules in the extract

The enzymes destroyed Proteins Lipids Carbohydrates RNA transformation still occurred, therefore those molecules are not responsible for transformation

The experiment was repeated using enzymes that destroy DNA. Transformation did not occur Conclusion: DNA stores & transmits genetic information Avery’s transformation experiment

III. The Hershey-Chase Experiment Martha Chase and Alfred Hershey in 1953

Virus- nonliving particle that is much smaller than a cell, that can infect living organisms S.E.M. of a T4 bacteriophage virus. (Reproduced by permission of Photo Researchers, Inc.)

Bacteriophage- a type of virus that infects bacteria. Are made of a DNA or RNA core surrounded by a protein coat S.E.M. of bacteriophages attacking a bacterium

When bacteriophages goes inside a bacterium: The virus attaches to the cell surface The virus then injects its DNA or RNA into the bacteria. The genes of the virus make the bacterium produce more viruses Result: the bacterium “pops” & is destroyed & hundreds of new viruses burst out. The lytic cycle considered the main method of viral replication, because it results in the destruction of the infected cell

Radioactive Markers- used to track a substance inside an organism. Hershey & Chase were trying to figure out if genes were made of protein or DNA. They wanted to figure out which part of the virus (the protein coat or DNA core) went inside the bacterium

Hershey & Chase used two radioactive markers to find out: Sulfur-35 (35S) tracks or marks proteins Phosporous-32 (32P) tracks or marks DNA.

If 35S was found in the bacteria, it would mean that the viruses’ protein had been injected into the bacteria.

If 32P was found in the bacteria, then it was the DNA that had been injected.

Result: Nearly all the radioactivity in the bacteria was from phosphorus (32P). Conclusion: The genetic material of the bacteriophage was DNA not protein.

IV. The Components & Structure of DNA Basic DNA structure

DNA is : Made of units called nucleotides A long molecule The nucleotides have 3 parts: deoxyribose, a phosphate group, & a nitrogenous base (has nitrogen in it).

There are four kinds of nitrogenous bases in DNA: Adenine (A) Guanine (G) Cytosine (C) Thymine (T)

Conclusion: Chargaff’s Rules: A biochemist named Erwin Chargaff studied the amount of each base in DNA. Conclusion: Chargaff’s Rules: The % of Guanine (G) is always equal to the % of Cytosine (C) The % of Adenine (A) is always equal to the % of Thymine (T) This is the same in all organisms Edwin Chargoff c. 1930

A scientist named Rosalind Franklin studied DNA X-ray diffraction- a technique she used to learn about the structure of DNA in 1952 Franklin was the first to see the structure of DNA Rosalind Franklin This x-ray image lead to the discovery of the structure of DNA

A double helix in which 2 strands are wound around each other In1953 two scientist named Watson & Crick studied the structure of DNA using x-ray diffraction data collected by Rosalind Franklin Based on Franklin’s x-ray they built a 3-D model out of wire & cardboard A double helix in which 2 strands are wound around each other James Watson & Francis Crick would not have been able to build their double helix DNA model for which they were awarded a Nobel Prize in 1962 w/o Franklin’s x-ray image of DNA structure

They discovered that hydrogen bonds form b/w the nitrogen bases This is what holds the 2 strands together (H bonds) Base Pairing- H bonds only form b/w adenine (A) & thymine (T); and cytosine (C) and guanine (G) Base Pairing explains Chargaff’s Rules

Discuss w/ Partner Pg. 292-293 Look at the timeline answer the following questions: Why did no DNA discoveries b/w the 1920’s and 50’s? Why did discovery speed up during the 1950’s?