Gene Expression lecture

1. 25-1
Chapter 25
DNA Structure
and Gene
Expression
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25.1 DNA Structure (1)
In the mid twentieth century, geneticists were
determining that DNA (deoxyribonucleic acid)
is the genetic material.
Biochemists were determining the structure of
DNA.
These research efforts led to our knowledge of
modern molecular biology.

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25.1 DNA Structure (2)
Prior to these efforts, it was known that
genetic material had certain qualities.
• It was able to store information for
development, structure, and metabolism of a
cell or organism.
• It was stable, so it could be replicated with
high accuracy and transmitted from
generation to generation.

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The Nature of the Genetic
Material (1)
In 1931, discovery of the genetic role of
DNA began with research by Frederick
Griffith.
He worked with Streptococcus
pneumoniae, bacteria that causes
pneumonia in animals.
• He noticed that one strain appeared smooth
because it had a capsule (S strain).
• Another strain appeared rough and lacked the
capsule (R strain).

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The Nature of the Genetic
Material (2)
When mice were injected with S strain
bacteria, the mice died.
When mice were injected with R strain
bacteria, the mice did not die.
To determine if the capsule alone caused
virulence of the S strain, he injected mice
with heat-killed S strain bacteria.
• The mice did not die.

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The Nature of the Genetic
Material (3)
In a final experiment, Griffith injected mice with a
mixture of heat-killed S strain and live R strain
bacteria.
• The mice died.
Living S strain bacteria were recovered from the
bodies.
Griffith concluded the following:
• Some substance must have passed from the dead S
strain to the live R strain, transforming the R strain.
• That substance was necessary for bacteria to
produce the capsule and be virulent.

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The Nature of the Genetic
Material (4)
The change in the phenotype of the R
strain must be due to a change in their
genotype.
The transforming substance was
potentially the genetic material.
Scientists began looking for the
transforming principle to determine the
chemical nature of the genetic material.

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Griffith’s Experiment
Figure 25.1
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The Nature of the Genetic
Material (5)
By the 1940’s, it was discovered that genes
existed on chromosomes, and that
chromosomes contain both proteins and nucleic
acids.
A debate arose about whether DNA or protein
was the genetic material.
In 1944, Avery, MacLeod, and McCarty reported
that DNA was the transforming substance.
• It allowed S. pneumoniae to produce a capsule and
be virulent.

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The Nature of the Genetic
Material (6)
Results of Avery, MacLeod, McCarthy:
• DNA from S strain bacteria causes R strain
bacteria to be transformed to produce a
capsule and be virulent.
• The addition of DNase (an enzyme that
digests DNA) prevents transformation from
occurring. This supports the hypothesis
that DNA is the genetic material.

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The Nature of the Genetic
Material (7)
Results of Avery, MacLeod, McCarthy
• The molecular weight of the transforming
substance is large, suggesting genetic
variability.
• Use of protein digesting enzymes has no
effect on the transforming substance nor
does RNase. This suggests that neither
protein nor RNA is the genetic material.

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The Nature of the Genetic
Material (8)
Alfred Hershey and Martha Chase (1952)
• Demonstrated that DNA is the genetic
material, not proteins
• Two experiments
• Viral DNA labeled with 32P was found in the
bacteria and not the medium—it had entered the
bacteria.
• Viral protein in capsids labeled with 35S was found
in medium and not in the bacterium—it never
entered the bacteria.
• Only DNA entered the bacteria.

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Hershey-Chase Experiments
Figure 25.2
a.Viral DNA is labeled (yellow).
b.Viral capsid is labeled (yellow).
Jump to Hershey-Chase Experiments Long Description

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Structure of DNA (1)
Structure of DNA
• James Watson and Francis Crick determined
the structure of DNA in 1953.
• DNA is a chain of nucleotides.
• Each nucleotide is a complex of three
subunits.
• Phosphoric acid (phosphate)
• A pentose sugar (deoxyribose)
• A nitrogen-containing base

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Structure of DNA (2)
Four possible bases in DNA
• Two purines with a double ring
• Adenine (A)
• Guanine (G)
• Two pyrimidines with a single ring
• Thymine (T)
• Cytosine (C)
DNA is a polynucleotide strand with a backbone
of alternating phosphate and sugar groups.
• The bases are attached to the sugar and project to
the side.

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Structure of DNA (3)
Two polynucleotide strands make up a
DNA double helix.
The strands are held together by hydrogen
bonding between the bases.
• Complimentary Base Pairing
• Adenine (A) always pairs with Thymine (T).
• Connected by two hydrogen bonds
• Guanine (G) always pairs with Cytosine (C).
• Connected by three hydrogen bonds
• A purine is always bonded to a pyrimidine.

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Structure of DNA (4)
Unwound DNA helix resembles a ladder.
• Sides of the ladder are sugar-phosphate
backbones.
• Rungs of the ladder are complementary base
pairs.
The two DNA strands are antiparallel-
oriented in opposite directions.
• The sugars are oriented differently.
• The 5 prime carbon atom is the uppermost on
one strand, and the 3 prime carbon atom is the
uppermost in the other strand.

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Overview of DNA Structure (1)
Figure 25.3
a.Double helix b.Ladder structure c.One pair of bases
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Overview of DNA Structure (2)
Figure 25.3(a,b)
a.Double helix b.Ladder structure
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Overview of DNA Structure (3)
Figure 25.3(c) c. One pair of bases
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