Bioloc

^ Unit 6—Gene Expression

P Topics 1-6

For project ideas from ScientificAmerican, visit go.hrw.com and type in the keyword HM6SAB.

For project ideas from ScientificAmerican, visit go.hrw.com and type in the keyword HM6SAB.

Discovery of dna

From his studies with pea plants, Mendel concluded that hereditary factors determine many of an organism's traits. But what were these hereditary factors? How did these molecules store hereditary information? Scientists believed that if they could answer these questions, they could understand how cells pass on characteristics to their descendants. The answers to these questions began to emerge during an epidemic of pneumonia in London in the 1920s.

GRIFFITH'S EXPERIMENTS

In 1928, British medical officer Frederick Griffith was studying a bacterium called Streptococcus pneumoniae (abbreviated S. pneu-moniae). Some types, or strains, of this bacterium can cause the lung disease pneumonia in mammals. Griffith was trying to develop a vaccine against a disease-causing, or virulent (VIR-yoo-luhnt) strain of the bacterium.

As shown in Figure 10-1, each virulent bacterium is surrounded by a capsule made of polysaccharides that protects it from a body's defense systems. The bacteria in a virulent strain grow as smooth-edged colonies when grown in a Petri dish and are called the S strain. In contrast, a second strain of S. pneumoniae does not cause pneumonia and lacks a capsule. The second strain is called the R strain because it grows into rough colonies. The R strain is also shown in Figure 10-1.

figure 10-1

Colonies of the harmful (s) strain

Colonies of the harmless (r) strain objectives

• Relate how Griffith's bacterial experiments showed that a hereditary factor was involved in transformation.

• Summarize how Avery's experiments led his group to conclude that DNA is responsible for transformation in bacteria.

• Describe how Hershey and Chase's experiment led to the conclusion that DNA, not protein, is the hereditary molecule in viruses.

vocabulary virulent transformation bacteriophage figure 10-1

Griffith studied S. pneumoniae bacteria. The Sstrain can cause pneumonia. The R strain does not cause pneumonia.

Colonies of the harmful (s) strain

Colonies of the harmless (r) strain

Result

Conclusion

Experiment 1 _^

Experiment 2

Inject mouse with live R cells.

Inject mouse with live R cells.

Result

^ R cells do not kill the mouse.

^ R cells do not kill the mouse.

Experiment 3

Experiment 4

Inject mouse with live S cells.

Kill S cells with heat.

Inject mouse with heat-killed S cells.

Kill S cells with heat. Mix with live R cells.

Inject mouse with mixture.

Frederick Griffith used virulent (S) and nonvirulent (R) bacterial cells to show that the hereditary material can pass from cell to cell.

Inject mouse with live S cells.

S cells kill the mouse.

S cells kill the mouse.

Kill S cells with heat.

Experiment 3

Inject mouse with heat-killed S cells.

Heat-killed S cells do not kill the mouse.

Heat-killed S cells do not kill the mouse.

Experiment 4

Kill S cells with heat. Mix with live R cells.

Inject mouse with mixture.

Hereditary material from the heat-killed S cells transforms R cells. The transformed R cells kill the mouse.

Hereditary material from the heat-killed S cells transforms R cells. The transformed R cells kill the mouse.

figure 10-2

Frederick Griffith used virulent (S) and nonvirulent (R) bacterial cells to show that the hereditary material can pass from cell to cell.

Word Roots and Origins transformation from the Latin trans, meaning "across," and forma, meaning "a form": to change the condition, character, or function of something

Griffith used the two strains of S. pneumoniae bacteria in a series of four experiments, shown in Figure 10-2. These experiments provide insight about the nature of the hereditary material. In Experiments 1 and 2, Griffith injected either live R or live S cells into mice. He found that only S cells killed the mice. In Experiment 3, he injected heat-killed S bacteria into mice and found that the mice survived. In his fourth experiment, he injected mice with both heat-killed S cells and live R cells. He found that the mice died.

Griffith concluded from his four experiments that heat-killed virulent bacterial cells release a hereditary factor that transfers the disease-causing ability to the live harmless cells. This type of transfer of genetic material from one cell to another cell or from one organism to another organism is called transformation.

AVERY'S EXPERIMENTS

Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment