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.

Griffith’s ExperimentsGriffith 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

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

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

TRANSFORMATIONOne 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 ExperimentAvery & other

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

The enzymes destroyed ProteinsLipidsCarbohydrates RNA

transformation still occurred, therefore those molecules are not responsible for transformation

The experiment was repeated using enzymes that destroy DNA.

Transformation did not occurThey 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 ofPhoto 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 surfaceThe virus then injects its DNA or RNA

into the bacteria.The genes of the virus make the

bacterium produce more virusesThe 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 outSulfur-35 (35S) tracks or marks proteinsPhosporous-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.

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

X-Ray EvidenceA 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