Chapter 11: Gene Expression

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Chapter 11 Gene Expression
11-1 Control of Gene Expression
11-2 Gene Expression and Development
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11-1 Control of Gene Expression
I. Role of Gene Expression (3 Key Points)

• Cells use different genes to build different
  proteins.

• NOT all proteins are required at same time ?
  REGULATING gene expression, cells are able to
  control WHEN each protein is made.

• Gene expression is thus the activation of a gene
  resulting in the synthesis of a protein.

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Critical Thinking
(1) A molecular biologist isolates mRNA from the
brain and liver of a mouse and finds that the two
types of mRNA are different. Can these results
be correct or has the biologist made an error?
Explain your answer.
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(1) Genome (e.g., Human Genome)

• Complete set of genes contained within an
  individuals cells.

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II. Gene Expression in Prokaryotes

• Francois Jacob and Jacques Monod were first to
  discover how genes control the metabolism of
  lactose in Escherichia coli (E. coli), a species
  of bacteria.

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(1) Structural Genes

• DNA segments (genes) that code for specific
  polypeptides (proteins).

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(2) Promoter

• DNA segment that recognizes RNA polymerase and
  promotes (initiates) transcription.

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(3) Operator

• DNA segment that serves as a binding site for an
  inhibitory protein that blocks transcription
  and prevents protein synthesis.

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(4) Operon (entire structure)

• The structural genes, the promoter, and the
  operator collectively form a series of genes
  called the operon.

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Critical Thinking
(2) What region of a prokaryotic gene is
analogous to the ENHANCER region of a eukaryotic
gene?
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(A) Repression (middle diagram)

• Occurs when a repressor protein physically
  blocks transcription (prevents protein synthesis).

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(1) Repressor Protein (purple structure below)

• A protein that inhibits a specific set of
  structural genes from being expressed.

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(2) Regulator Gene (yellow stretch of DNA below)

• DNA sequence codes for production of a repressor
  protein (i.e., it regulates whether or not
  transcription will be repressed)

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(B) Activation (bottom diagram)

• The initiation of transcription as a result of
  the removal of a repressor protein (leads to
  protein synthesis)

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(1) Inducer (see red molecule, middle diagram)

• A molecule that initiates gene expression by
  removing the repressor protein (e.g., lactose is
  an inducer)

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(2) Activation

• Activation (bottom) works with repression
  (middle) as a means to conserve NRG and produce
  only proteins immediately desired.

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II. Gene Expression in Eukaryotes

• DNAEuk is located in duplicated (X) chromosomes
  instead of a single circular chromosome (plasmid)
  like that of DNAPro

(A) Structure of a Eukaryotic Gene

• In eukaryotes, gene expression is partly related
  to the coiling and uncoiling of DNA within each
  chromosome.

• AFTER M phase of cell cycle, certain regions of
  the DNA coils relax, making transcription
  possible.

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(1) Euchromatin

• Uncoiled form of DNA the site of active
  transcription of DNA to RNA (the coiled portion
  remaining is unable to be transcribed)

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(2) Introns (mutation resistant)

• (Beyond the promoter), sections of a structural
  gene that do NOT code for amino acids (i.e., do
  not get translated to amino acids)

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(3) Exons (mutation vulnerable)

• (Beyond the promoter), sections of a structural
  gene that DO get expressed and will be translated
  into amino acids (proteins)

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Side Note Evolutionarily speaking, it is
believed that the intron-exon pattern could
facilitate the exchange of exons among homologous
chromosomes during crossing over in meiosis,
leading to an additional source of genetic
diversity essential for evolution.
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(B) Control After Transcription

• Unlike prokaryotes, eukaryotes can control gene
  expression by modifying RNA AFTER transcription.

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(1) Pre-mRNA

• Transcription results in both introns and exons
  being copied from DNA onto RNA the pre-RNA is
  the large, resulting molecule.

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• Final mRNA

• Formed when introns are removed and remaining
  exons are spliced to one another, resulting in
  mRNA with only the exons.

• Similar splicing occurs following the
  transcription of tRNA and rRNA (enzyme mediated)

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(C) Enhancer Control

• Eukaryotic genes also have non-coding control
  sequences that facilitate transcription.

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(1) Enhancer

• Non-coding control sequence of DNA that must be
  activated for its associated gene to be expressed.

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(2) Transcription Factors

• Additional proteins that bind to enhancers and
  RNA polymerase and regulate transcription.

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11-2 Gene Expression and Development
I. Cell Differentiation

• Although EACH cell in a developing organism
  contains the SAME genes, only a FRACTION of the
  genes are expressed. In order to become
  specialized, cells must FIRST differentiate.

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(1) Morphogenesis

• As cells differentiate and organisms grow,
  organs and tissues develop to produce a
  characteristic form.

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(A) Homeotic Genes

• Regulatory genes that determine where certain
  anatomical structures, such as appendages, will
  develop in an organism during morphogenesis.

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• I.E., Master genes of development that determine
  the overall body organization (See mutant fruit
  fly below with a second thorax).

Ex In fruit flies, each homeotic gene shares a
common DNA sequence of 180 b.p. this specific
sequence within a homeotic gene regulates
patterns of development (i.e., a homeobox)
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(1) Homeobox

• It is believed that all organisms may have
  similar homeoboxes that code their anatomy.

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II. Cancer

• A condition of abnormal proliferation of
  defective, unregulated cells (i.e., a tumor) that
  result from a specific alteration to the cells
  DNA.

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Critical Thinking
(3) Why might X rays be more dangerous to an
ovary or a testis than to muscle tissue?
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(1) Benign Tumor (non-cancerous growth)

• Cells remain within a mass and are unlikely to
  spread. Examples include fibroid cysts (breast
  tissue) and warts. (Surgery)

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(2) Malignant Tumor (cancerous growth)

• Cells invade and destroy healthy tissues
  elsewhere in the body Malignant cells tend to
  break away and form new tumors in other parts of
  the body, hijacking resources and out-competing
  healthy cells.

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(3) Metastasis

• Spread of cancer beyond its original site,
  results in cancerous growths in parts of body not
  originally affected. (Chemotherapy and/or
  Radiation Therapy)

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(A) Types of Cancer

• Malignant tumors are categorized according to
  the types of tissues they affect in the organism.

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(1) Carcinomas

• Grow in the skin and the tissues that line the
  organs of the body.

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(2) Sarcomas (progressive bone cancer shown below
in two x rays)

• Grow in the bone and muscle tissue.

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(3) Lymphomas

• Solid tumors that grow in the liquid tissues of
  the blood

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(4) Leukemia

• Malignant cancer of the white blood cells, a
  class of lymphoma

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(B) Cancer and the Cell Cycle

• Factors that normally govern the rate of cell
  division include

• Cell must receive adequate nutrition

• Cell must be attached to other cells, to a
  membrane, or to fibers between cells.

• Normal cells stop dividing when they become too
  crowded, usually after 20-50 divisions

• Cancer cells will continue dividing despite
  crowded conditions and despite a lack of
  attachment (can lead to metastasis)

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(C) Causes of Cancer

• Cancer results when a cell loses the ability to
  regulate cell growth and division due to a change
  in its regulatory genes.

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(1) Growth Factors (see blue squares below)

• Are proteins made by regulatory genes to ensure
  the events of cell division occur in proper
  sequence and at correct rate (mutations of these
  genes can lead to cancerous cells)

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Note Cells that produce tumor angiogenesis
factor (TAF) form some of the most malignant
tumors. TAF affects nearby blood vessels,
causing them to grow toward the tumor.
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(2) Carcinogen

• A class of molecules that are known to increase
  the risk of altering the regulatory genes in
  cells (i.e, have been known to lead to cancer)

• Ex tobacco smoke, air pollutants, asbestos, and
  radiation from X-rays or ultraviolet radiation
  (sun)

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(3) Mutagen

• A class of molecules that are known to cause
  mutations to occur within the cell (within
  sequences of DNA)

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Critical Thinking
(4) Mutations may occur in gametes or in body
cells. In which cell type is a mutation likely
to be a source of genetic variation for
evolution? Why?
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(D) Oncogenes

• A gene that causes cancer or other uncontrolled
  cell proliferation.

• Oncogenes begin as normal genes
  (proto-oncogenes) that control a cells growth
  and differentiation.

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(1) Proto-oncogenes

• Normal genes that code for proteins that
  regulate cell growth, cell division, and the
  ability to adhere to other cells.

• A mutation in these genes could cause a change
  in protein production, leading to an increased
  division rate and possibly cancer.

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(2) Tumor-suppressor genes

• Healthy genes that code for proteins that stop
  the uncontrolled rate of cell division if THESE
  GENES mutate, a change in protein production may
  occur, predisposing the cell to becoming
  cancerous.

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(E) Viruses and Cancer

• Many viral genes are oncogenes viruses can also
  initiate cancer in an infected cell by causing
  mutations in proto-oncogenes or tumor-suppressor
  genes, altering the rate of cell division.

• Leukemia has shown ties to certain viral
  infections.

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Extra Slides AND Answers for Critical Thinking
Questions
(1) The operator region of a prokaryote is
analogous to the enhancer region of a eukaryotic
gene. Both operators and enhancers act as a
switch that must be turned on to activate the
expression of a gene.
(2) The ovaries and testes contain rapidly
dividing cells that will become egg and sperm
cells, respectively. A mutation due to X-ray
exposure could thus be passed on to offspring.
(3) The results are probably correct because
different genes were expressed in the brain and
liver tissues, resulting in the production of two
types of mRNA.
(4) A mutation in the gametes is likely to be a
source of genetic variation because gametes pass
on the mutation to the next generation when
forming the zygote.