DNA Structure and Replication - Section 4-1 and 4-2
1.
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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SLIDE
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
Editor's Notes
Presentation title slide
Chapter opener background notes
Monozygotic twins, also known as identical twins, develop from a single fertilized egg that splits into two separate embryos. Because they come from a single zygote, they are born with the same genes. Since 1875, researchers have been studying twins to gain insight on the degree to which genes and the environment interact. Studies comparing monozygotic twins reveal that differences between twins exist because of environmental factors and the individual experiences of each twin. Twin studies have helped us understand the extent to which genetic influence is dependent on the environment. Historically, twin studies have been used in the field of behavioural genetics to distinguish between the effects of two important factors in human development: nature (genes) and nurture (environment).
Today, epigenetics is identified as a third factor that influences individual differences. Previous twin studies assumed that monozygotic twins are genetically identical. However, recent studies have shown that although monozygotic twins are born with the same genetic makeup, individual differences surface and become more apparent as the twins get older, whether or not the twins grow up in different environments or are raised together. As twins age, the differences between them increase because of the cumulative effects of environmentally induced changes to their DNA. These changes are referred to as epigenetic processes that change gene expression patterns. Younger twins have few epigenetic differences, while older twins have significantly more epigenetic differences. These epigenetic differences are a result of environmental factors, such as stress and diet, which influence how genes are expressed and behave. Some of these epigenetic factors also determine expression in subsequent generations.
Caption text:
Figure 4.1 Griffith’s experiment. a. Encapsulated S strain is virulent and kills mice. b. Nonencapsulated R strain is not virulent and does not kill mice. c. Heat-killed S strain bacteria do not kill mice. d. If heat-killed S strain and R strain are both injected into mice, they die because the R strain bacteria have been transformed into the virulent S strain.
DNA (deoxyribonucleic acid): a nucleic acid that stores genetic information in the cell and organism
Caption text
Figure 4.2 Hershey-Chase experiments. The Hershey-Chase experiments concluded that viral DNA, not protein, was responsible for directing the production of new viruses.
Caption text
Figure 4.2 Hershey-Chase experiments. The Hershey-Chase experiments concluded that viral DNA, not protein, was responsible for directing the production of new viruses.
Figure 4.3 Caption Notes
c. Notice that 3’ and 5’ are part of the system for numbering the carbon atoms that make up
the sugar.
purines: nucleotides with a double-ring structure; e.g., adenine, guanine
pyrimidines: nucleotides with a single-ring structure; e.g., thymine, cytosine
Caption text
Figure 4.3 Overview of DNA structure. a. DNA double helix. b. Unwinding the helix reveals a ladder configuration in which the sides are composed of sugar and phosphate molecules and the rungs are complementary bases. The bases in DNA pair in such a way that the sugar-phosphate backbones are oriented in different directions.
double helix: the structure of DNA resulting from two strands twisting about each other
complementary base pairing: hydrogen bonding between purines and pyrimidines in DNA
Answers
1. Griffith found that something he called “the transformation principle” could be carried from one organism to another and cause a difference in the phenotype (types of proteins) expressed. Griffith’s experiments could not conclude whether it was DNA or proteins that were being moved from one organism to another, but at that time it was thought proteins were the likely cause of the change in phenotype because of their much greater structural diversity. Avery was able to show that it was DNA, not proteins, that caused the phenotype change or transformation.
2. The Hershey Chase experiment used a T phage, composed of radioactively labelled DNA and capsid coat proteins, to infect E. coli. The radioactive tracers for DNA, but not protein, ended up inside the bacterial cells, causing them to become transformed. Since only the genetic material could have caused this transformation, Hershey and Chase concluded that DNA must be the genetic material.
3. DNA is a two-stranded molecule with alternating sugar phosphates in the backbone strands. The monomer units are nucleotides consisting of a sugar, phosphate, and a base. The overall structure of the molecule is like a twisted ladder. The sugar and phosphates are covalently bonded to form the sides of the ladder. The steps or rungs of the ladder are made of the bases A, G, C, and T. Pairs of complementary bases form hydrogen bonds. Adenine pairs with thymine through two hydrogen bonds. Guanine pairs with cytosine through three hydrogen bonds. The two strands are antiparallel.
Caption text:
Figure 4.4 Overview of DNA replication. Replication is called semiconservative because each new double helix is composed of an old (parental) strand and a new (daughter) strand.
DNA replication: the process of copying one DNA double helix into two identical double helices
Answers
1. DNA replication is said to be semiconservative, because the original parent strand is retained in the next generation. The new cell that results following cell division contains one original parent strand and a newly formed strand. Therefore, half (semi is Latin for half) of the original DNA is passed to the next generation.
2. In DNA replication, first the DNA molecule unzips and hydrogen bonds are broken. Second, complementary bases join to both parent strand backbones, and this synthesis reaction is catalyzed by the enzyme DNA polymerase. Third, Okazaki fragments formed from the lagging strand are joined together. Proofreading of the nucleotides then occurs to ensure that the two double-helix molecules are identical to the original DNA molecule.
3. The key enzymes are first DNA helicase, which breaks the hydrogen bonds through hydrolysis reactions. Second, DNA polymerase provides a site for the new complementary bases to enter and hydrogen-bond together. Third, DNA ligase helps connect the short pieces of new DNA formed from the lagging strand. Finally, another DNA polymerase proofreads and corrects errors.