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How Genes Work
Chapter 8
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Outline
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Genes Are DNA
DNA Structure
DNA Replication
Transcription
Genetic Code
Translation
Gene Architecture
Regulating Gene Expression
Mutations
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Genes Are DNA
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Very early it was discovered chromosomes
are composed of proteins and DNA. But it
took several experiments to conclusively
determine specifically which substance made
up genes.
 Griffith Experiment
- Documented movement of genes from
one organism to another
(transformation).
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Genes Are DNA
Avery Experiment
- Removed almost all protein from
bacteria, and found no reduction in
transforming activity.
 Hershey-Chase
- Used radioactive isotopes to label DNA
and protein. Found genes used to
specify new generations of viruses were
made of DNA.

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Genes Are DNA
Fraenkel-Conrat Experiment
- Heredity properties of viruses
determined by nucleic acid in core, not
by protein in coat.
Retroviruses - Viruses that contain RNA,
causing information to flow from RNA to
DNA.
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Discovering DNA Structure
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DNA made up of nucleotides.
 Central sugar, phosphate group, and an
organic base.
- Purines - Large bases
 Adenine and Guanine
- Pyrimidines - Small bases
 Cytosine and Thymine
 Chargaff’s Rule
- A =T and G=C
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Discovering DNA Structure
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Watson and Crick deduced structure of DNA
as a double helix.
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DNA Replication
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Weak hydrogen bonds between base pairs
hold DNA strands together.
Each chain in the helix is a complimentary
mirror image of the other.
 Double helix unzips and undergoes semiconservative replication.
- Confirmed by Meselson-Stahl
Experiment.
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DNA Replication
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Helicase enzyme unwinds double helix.
 Replication fork
DNA polymerase reads each strand and
adds nucleotides as it moves, creating a new
complementary strand.
DNA ligase joins the ends of new
synthesized segments.
 Many mechanisms exist to proofread
strands and repair DNA mistakes.
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Genes to Proteins
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Producing proteins from genes is known as
(Central Dogma).
 DNA - RNA - Protein
- Gene Expression - Use of information in
DNA to direct production of particular
proteins.
 Transcription - mRNA molecule is
synthesized from gene within DNA.
 Translation - mRNA used to direct
protein production.
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Transcription
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The RNA copy of a gene used to produce a
protein is called messenger RNA (mRNA).
 RNA polymerase binds to one strand of
DNA double helix at promoter site and
moves along pairing nucleotides A-U, G-C.
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Genetic Code
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Essence of gene expression is reading
information encoded within DNA and
directing protein production.
 Each gene is read from a fixed promoter
site where RNA polymerase binds to DNA.
 RNA polymerase moves down DNA in
three-nucleotide steps (Codons).
 Each codon corresponds to a particular
amino acid.
- 64 possible codons
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The Genetic Code
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Translation
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Ribosomes use mRNA produced by
transcription to direct protein synthesis.
 Ribosomes composed of two subunits.
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Translation
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Transfer (tRNA) molecules bring amino acids
to the ribosome to use in making proteins.
 Anticodon is the complimentary sequence
to 1 of the 64 codons on the genetic code.
Protein Production
 Complete ribosome composed of small
and large subunit bound together.
- mRNA threads through producing
polypeptide chain.
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Translation
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Gene Architecture
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In complex genes, DNA sequences encoding
amino acid sequences (exons) are
interrupted by extraneous nucleotides
(introns).
Eukaryotic cells transcribe a gene and
produce a primary RNA transcript.
 Enzyme complexes excise out introns and
join exons to form shorter, mature, RNA
that is translated into proteins.
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Gene Architecture
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Gene Architecture
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Most eukaryotic genes exist in multiple
copies; clusters of almost identical
sequences (multigene families).
Transposable sequences (transposons) are
repeated hundreds of thousands of times,
and move from one chromosomal location to
another.
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Regulating Gene Expression
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Many genes are turned off until needed.
 Regulatory site located between promoter
site and beginning edge of the gene.
- When repressor is bound to regulatory
site (operator), it blocks the movement of
polymerase toward the gene.
 Gene turned on by removing repressor.
 Enhancers are located far from the gene,
and bind specific regulatory proteins that
help RNA polymerase find and attach to its
binding site.
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Regulating Gene Expression
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Activators bind to DNA and help unwind
strands.
 Cells regulate genes by binding signal
molecules to activator protein.
Enhancers are located far from the gene, and
bind specific regulatory proteins to help RNA
polymerase attach to its binding site.
 Positioning at a distance permits a large
number of different regulatory sequences to
influence that particular gene.
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Regulating Gene Expression
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Mutations
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A mutation is any change in the content of
the genetic message.
 Recombination refers to a change in
position of a portion of the genetic material.
- Evolution can be viewed as selection of
allele combinations from a pool of
alternatives. The rate of evolution is
ultimately limited by the rate at which
alternatives are generated.
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Mutations
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Only mutations in the germ line can be
inherited. (Somatic mutations can be passed
from one cell to all its descendents).
 Point Mutation - Alteration of one or a few
bases.
- Base substitution, insertion, or deletion.
 Frame-shift mutation - Occurs when
insertion or deletion throws off reading
frame.
Mutagen - Any substance damaging DNA.
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Mutations
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Mutagen - Any substance damaging DNA.
Transposition occurs when a particular gene
moves to a different location.
 May alter expression, or expression of
nearby genes.
- Chromosomal Rearrangement
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Cancer and Mutations
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Carcinogens - Agents believed to cause
cancer.
 Investigations over the last 50 years have
determined many human synthetic
chemicals are capable of causing cancer in
laboratory animals.
 Carcinogens are also mutagens.
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Review
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Genes Are DNA
DNA Structure
DNA Replication
Transcription
Genetic Code
Translation
Gene Architecture
Regulating Gene Expression
Mutations
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies