PowerPoint for Biology 12, Chapter 4-1 and 4-2

2. 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

3. UNIT A Chapter 4: DNA Structure and Gene Expression
Chapter 4: DNA Structure and Gene
Expression
In this chapter you will learn about the expression of an organism’s
genes, a complex series of events involving genetic and
environmental factors.
How does DNA store
information that leads to the
development, structure, and
metabolic activities of
organisms?
How are genes expressed?
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4. UNIT A Chapter 4: DNA Structure and Gene Expression Section 4.1
4.1 DNA Structure
Determining 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.
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5. UNIT A Chapter 4: DNA Structure and Gene Expression
Section 4.1
The Nature of Genetic Material
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:
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6. UNIT A Chapter 4: DNA Structure and Gene Expression
Section 4.1
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
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Figure 4.2a Hershey-Chase experiments.

7. UNIT A Chapter 4: DNA Structure and Gene Expression
Section 4.1
• 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
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Figure 4.2b Hershey-Chase experiments.

8. UNIT A Chapter 4: DNA Structure and Gene Expression
Section 4.1
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)
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Figure 4.3c Overview of DNA structure.

9. UNIT A Chapter 4: DNA Structure and Gene Expression
Section 4.1
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)
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Figure 4.3a and b Overview of DNA structure.

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

12. UNIT A Section 4.2
4.2 DNA Replication
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)
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Chapter 4: DNA Structure and Gene Expression
Figure 4.4 Overview of DNA Replication

13. 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

14. UNIT A Section 4.2
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.

15. 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.

16. UNIT A Section 4.2
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Chapter 4: DNA Structure and Gene Expression