Chapter 12 DNA and RNA

12-1 DNA

1. Griffith and Transformation

Explain DNA’s role in your existence.

 Discuss with your partner.

1. Griffith & Transformation

 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  He grew the pneumonia in the lab

Griffith made two observations:

 (1) The disease-causing strain of bacteria grew into smooth colonies on culture plates.  (2) The harmless strain grew into colonies with rough edges.

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Griffith’s Experiments

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

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

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Experiment 3:

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

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Experiment 4:

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

 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.

 What was Griffith trying to learn when he set up this experiment? Discuss w/ your partner.

 Griffith hypothesized that something must have information that could change harmless bacteria into disease-causing ones

Discuss:

 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?

2. Avery and DNA

 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.

The Experiment

 Avery & other scientists made a bacterial extract (juice) from the heat-killed bacteria and destroyed all of the molecules w/ enzymes

 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  They concluded that DNA stores & transmits genetic information

3. The Hershey-Chase Experiment

 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.)

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Bacteriophage- a type of virus that infects bacteria.

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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  The result is the bacterium “pops” & is destroyed & hundreds of new viruses burst out.

 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 ( 35 S) tracks or marks proteins  Phosporous-32 ( 32 P) tracks or marks DNA.

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

If 32 P 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 ( 32 P).

 Conclusion: The genetic material of the bacteriophage was DNA not protein.

4. The Components & Structure of DNA

 DNA is  A long molecule  Made of units called nucleotides  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)

Chargaff’s Rules:

 A biochemist named Erwin Chargaff studied the amount of each base in DNA.  He concluded the following:  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

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X-Ray Evidence

A scientist named Rosalind Franklin studied DNA  X-ray diffraction; a technique she used to learn about the structure of DNA

The Double Helix

   2 scientist named Watson & Crick studied the structure of DNA using 3-D models They used R. Franklin’s picture & built a 3-D model out of wire & cardboard A double helix in which 2 strands are wound around each other

 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?

 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