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© Houghton Mifflin Harcourt Publishing Company Identifying DNA as the Genetic Material KEY CONCEPT DNA was identified as the genetic material through a series of experiments. DNA was identified as the genetic material relatively recently—in the 1950s. This section reviews three research projects that all added up to this discovery: 1) Griffith’s research, 2) Avery’s research, and 3) Hershey and Chase’s research. Together, these scientists’ findings led to the con- clusion that DNA is the genetic material. Griffith finds a “transforming principle.” In 1928 a British microbiologist named Frederick Griffith investigated two forms of the bacterium that causes pneumonia*. He injected the two different forms into mice. One form of the bacterium killed the mice, but the other form did not. Griffith used heat to kill a sample of the deadly disease-causing bacteria and then mixed the dead bacteria with a sample of live, harmless bacteria. He injected this mixture into mice. Even though the disease-causing bacteria that he injected were heat- killed, the mice still died. Griffith concluded that some material must have been transferred from the heat-killed bacteria to the harmless bacteria. Whatever that material was, it con- tained information that changed the harmless bacteria into disease-causing bacteria. Griffith called this mys- tery material the “transforming principle.” Transform means “to change.” Why do you think Griffith called the mystery material the “transforming principle”? SECTION 8.1 live S bacteria dead mouse heat-killed S bacteria heat-killed S bacteria + live R bacteria dead mouse live mouse live R bacteria live mouse The S form of the bacterium is deadly; the R form is not. GRIFFITH’S EXPERIMENTS * ACADEMIC VOCABULARY pneumonia a disease that affects the lungs Holt McDougal Biology 126 3F, 6A

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Page 1: Griffith finds a “transforming principle.”...principle, or genetic material. Some scientists questioned this conclu-sion. Scientists also wondered whether DNA was the genetic material

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Identifying DNA as the Genetic MaterialKEyConCEPT DNA was identified as the genetic material through a series of experiments.

DNA was identified as the genetic material relatively recently—in the 1950s. This section reviews three research projects that all added up to this discovery: 1) Griffith’s research, 2) Avery’s research, and 3) Hershey and Chase’s research. Together, these scientists’ findings led to the con-clusion that DNA is the genetic material.

Griffithfindsa“transformingprinciple.”In 1928 a British microbiologist named Frederick Griffith investigated two forms of the bacterium that causes pneumonia*. He injected the two different forms into mice. One form of the bacterium killed the mice, but the other form did not.

Griffith used heat to kill a sample of the deadly disease-causing bacteria and then mixed the dead bacteria with a sample of live, harmless bacteria. He injected this mixture into mice. Even though the disease-causing bacteria that he injected were heat-killed, the mice still died.

Griffith concluded that some material must have been transferred from the heat-killed bacteria to the harmless bacteria. Whatever that material was, it con-tained information that changed the harmless bacteria into disease-causing bacteria. Griffith called this mys-tery material the “transforming principle.”

Transform means “to change.” Why do you think Griffith called the mystery material the “transforming principle”?

SEcTIoN

8.1

live S bacteria

liveR bacteria

heat-killed S bacteria

heat-killed S bacteria + live R bacteria

dead mouse dead mouse live mouse live mouse

live S bacteria

liveR bacteria

heat-killed S bacteria

heat-killed S bacteria + live R bacteria

dead mouse dead mouse live mouse live mouse

live S bacteria

liveR bacteria

heat-killed S bacteria

heat-killed S bacteria + live R bacteria

dead mouse dead mouse live mouse live mouse

TheSformofthebacteriumisdeadly;theRformisnot.

GRiffiTH’SExPERimEnTS

* AcADemic VocAbUlARy

pneumonia a disease that affects the lungs

Holt McDougal Biology126

3F, 6A

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Page 2: Griffith finds a “transforming principle.”...principle, or genetic material. Some scientists questioned this conclu-sion. Scientists also wondered whether DNA was the genetic material

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DNA

protein coat

AveryidentifiesDnAasthetransformingprinciple.

Oswald Avery worked with other biologists for over ten years trying to figure out what Griffith’s transforming principle was. They thought the transforming principle could be protein or it could be DNA. Avery’s group found a way to separate this mystery material from samples of bacteria. They ran several tests on the material. Their tests and findings are shown in the chart below.

AvERy’SRESEARCH

TEST RESULT

1. Standard chemical test for protein and DNA found presence of DNA, but no protein

2. Tests to determine the amounts of nitrogen (N) and phosphorous (P)

the proportions of N and P matched the makeup of DNA, but not protein

3. Tests to see which enzymes could break down the substance

enzymes that break down DNA broke down this substance, but enzymes that break down protein did not break down the substance

Avery and his group concluded that DNA must be the transforming principle, or genetic material. Some scientists questioned this conclu-sion. Scientists also wondered whether DNA was the genetic material for all organisms or just for bacteria.

on the chart above, highlight three reasons Avery concluded that the mystery material was DNA, not protein.

HersheyandChaseconfirmthatDnAisthegeneticmaterial.

In 1952 the work of Alfred Hershey and Martha Chase provided evi-dence that DNA is indeed the genetic material. Hershey and Chase studied a type of virus that infects bacteria, called a bacteriophage (bak-TEER-ee-uh-fayj), or “phage” for short.

A phage infects a bacterium by inserting its genetic material into the bacterium. Hershey and Chase wanted to find out what material the phage inserted into a bacterium—was it protein or was it DNA? They conducted two experiments. In the first experiment, Hershey and Chase marked the phages’ proteins with radioactive labels. When the phages infected the bacteria, no significant radioactivity was found inside the bacteria. In the second experiment, Hershey and Chase marked the phages’ DNA with radioactive labels. When the phages infected the

Bacteriophages are viruses that infect bacteria.

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Page 3: Griffith finds a “transforming principle.”...principle, or genetic material. Some scientists questioned this conclu-sion. Scientists also wondered whether DNA was the genetic material

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bacteria, high levels of radioactivity were found inside the bacteria. These results finally convinced scientists that the genetic material is DNA and not protein.

What is the material that phages insert into bacteria?

bacteriophage

1. How did Hershey and Chase’s research with bacteriophages help con-firm that DNA is the genetic material?

2. Complete the following chart about the three main research projects that led to the identification of DNA as the genetic material.

RESEARCHERS’nAmES SUmmARyofRESEARCH SUmmARyofConCLUSionS

Griffith found a “transforming principle”

Avery

hersheyandchase identified the substance that phages inject into bacteria

3. Which of the three experiments gave evidence that protein was not the genetic material?

8.1 vocabularyCheck

Go back and highlight each sentence that has a vocabulary word in bold.

Mark It Up

8.1 TheBigPicture

128 Holt McDougal Biology

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