UNIT A: Cell Biology

Chapter 2: The Molecules of Cells

Chapter 3: Cell Structure and Function

Chapter 4: DNA Structure and Gene Expression: Sections 4.1, 4.2

Chapter 5: Metabolism: Energy and Enzymes

Chapter 6: Cellular Respiration

Chapter 7: Photosynthesis

In this chapter you will learn about the expression of an organism’s genes, a complex series of events involving genetic and environmental factors.

UNIT A Chapter 4: DNA Structure and Gene Expression

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Chapter 4: DNA Structure and Gene Expression

How does DNA store information that leads to the development, structure, and metabolic activities of organisms?

How are genes expressed?

4.1 DNA StructureDetermining that DNA is the genetic material was accomplished through decades of research by many scientists. •In the late 1920s, Frederick Griffith’s studies identified a transforming substance that could change nonlethal bacteria to lethal bacteria.

Figure 4.1 Griffith’s experiment.

UNIT A Chapter 4: DNA Structure and Gene Expression Section 4.1

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In 1944, Oswald Avery and his research colleagues showed that Griffith’s transforming substance was DNA and that this was the genetic material. Their findings were:

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

The Nature of Genetic Material

In the early 1950s, Hershey and Chase firmly established DNA as the genetic material. They used a virus (T phage) that infects bacteria, where it makes new copies of itself.•In one experiment, they used virus with radioactive DNA and identified where the radioactivity went after infection

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

Figure 4.2a Hershey-Chase experiments.

The Nature of Genetic Material

• In another experiment, they used a virus with radioactive protein and identified where the radioactivity went after infection.

• They discovered that radioactivity entered the bacterial cells when virus with radioactive DNA was added, but not virus with radioactive protein.

• Therefore, the hereditary material is DNA

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

Figure 4.2b Hershey-Chase experiments.

Structure of DNA

• DNA is a chain of nucleotides

• Each nucleotide consists of a phosphate group, a deoxyribose sugar, and a nitrogen-containing base

• There are four bases: two purines, adenine (A) and guanine(G), and two pyrimidines, cytosine (C) and thymine (T)

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

Figure 4.3c Overview of DNA structure.

Structure of DNA• A DNA strand has a

backbone of alternating phosphate and sugar molecules

• Two DNA strands twist about each other, forming a double helix

• Purines and pyrimidines on opposite strands form hydrogen bonds in complementary base pairing (A-T, G-C)

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

Figure 4.3a and b Overview of DNA structure.

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

Check Your Progress

1. Summarize the significance of the Griffith and Avery experiments.

2. How did results from the Hershey-Chase experiment suggest that DNA was the genetic material?

3. Describe the structure of the DNA molecule.

UNIT A Section 4.1

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Chapter 4: DNA Structure and Gene Expression

When cells divide, each new cell requires a copy of the DNA.DNA replication•Is the copying of one double helix into two identical double helices, which are also identical to the original•Is semiconservative (each new double helix has one original strand and one newly synthesized strand)

4.2 DNA Replication

UNIT A Section 4.2

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Chapter 4: DNA Structure and Gene Expression

Figure 4.4 Overview of DNA Replication

UNIT A

1. DNA helicase enzyme separates the DNA strands by breaking the hydrogen bonds between bases.

2. DNA polymerase enzyme catalyzes incorporation of new nucleotides by complementary base pairing.

3. DNA polymerase can only add nucleotides to one end of the growing chain. Therefore, replication is different for each strand. Leading strand synthesis follows the helicase enzyme. Lagging strand synthesis results in formation of Okazaki fragments.

4. DNA ligase connects the Okazaki fragments and seals any breaks in the sugar-phosphate backbone.

Section 4.2

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Chapter 4: DNA Structure and Gene Expression

Replication at the Molecular Level

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Chapter 4: DNA Structure and Gene Expression

Figure 4.5 Molecular mechanisms of DNA replication. The major enzymes involved in DNA replication. Note that the synthesis of the new DNA molecules occurs in opposite directions due to the orientation of the original DNA strands.

Replication at the Molecular Level

UNIT A Section 4.2

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Chapter 4: DNA Structure and Gene Expression

Check Your Progress

1. Explain why DNA replication is said to be semiconservative.

2. Summarize the sequence of events that occur during DNA replication.

3. Describe the key enzymes involved in DNA replication.

UNIT A Section 4.2

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Chapter 4: DNA Structure and Gene Expression