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Gene Expression and Control

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Title: Gene Expression and Control


1
Gene Expression and Control
  • Chapter 7
  • Part 2

2
7.6 Mutated Genes and Their Products
  • Mutations are permanent changes in the nucleotide
    sequence of DNA, which may alter a gene product
  • A mutation that changes a genes product may have
    harmful effects
  • Example Mutations that affect the proteins in
    hemoglobin reduce bloods ability to carry oxygen

3
Types of Mutations
  • Deletion
  • Mutation in which one or more base pairs are lost
  • Insertion
  • Mutation in which one or more base pairs become
    inserted into DNA
  • Base-pair substitution
  • Type of mutation in which a single base-pair
    changes

4
Two Common Mutations in Hemoglobin
5
A Hemoglobin, an oxygen-transport protein in red
blood cells. This protein consists of four globin
chains two alpha chains (blue) and two beta
chains (green). Each globin chain folds up to
form a pocket that cradles a type of cofactor
called a heme (red). Oxygen binds to the iron
atom at the center of each heme group.
Fig. 7-9a, p. 125
6
part of DNA
mRNA transcribed from DNA
resulting amino acid sequence
threonine (thr)
proline (pro)
glutamic acid (glu)
glutamic acid (glu)
lysine (lys)
B Part of the DNA, mRNA, and amino acid sequence
of the beta chain of a normal hemoglobin molecule.
Fig. 7-9b, p. 125
7
deletion in DNA
altered mRNA
altered amino acid sequence
threonine (thr)
proline (pro)
glycine (gly)
arginine (arg)
threonine (thr)
C A single base-pair deletion causes the reading
frame for the rest of the mRNA to shift, so a
completely different protein product forms. This
mutation results in a defective globin chain. The
outcome is thalassemia, a genetic disorder in
which a person has an abnormally low amount of
hemoglobin.
Fig. 7-9c, p. 125
8
base-pair substitution in DNA
altered mRNA
altered amino acid sequence
threonine (thr)
proline (pro)
valine (val)
glutamic acid (glu)
lysine (lys)
D A base-pair substitution in DNA replaces a
thymine with an adenine. When the altered mRNA is
translated, valine replaces glutamate as the
sixth amino acid of the new polypeptide chain.
Hemoglobin with this chain is called HbS, or
sickle hemoglobin.
Fig. 7-9d, p. 125
9
Base-pair substitution
10
Sickle-Cell Anemia A Base-Pair Substitution
glutamic acid (glu)
valine (val)
histidine (his)
leucine (leu)
threonine (thr)
proline (pro)
glutamic acid (glu)
1 Normal amino acid sequence at the start of the
hemoglobin beta chain.
valine (val)
valine (val)
proline (pro)
histidine (his)
threonine (thr)
leucine (leu)
glutamic acid (glu)
2 One amino acid substitution results in the
abnormal beta chain of sickle hemoglobin (HbS).
The sixth amino acid in such chains is valine,
not glutamic acid.
sickled cell
3 Glutamic acid carries an overall negative
charge valine carries no charge. This difference
causes the protein to behave differently. At low
oxygen levels, HbS molecules stick together and
form rod-shaped clumps that distort normally
round red blood cells into sickle shapes. (A
sickle is a farm tool with a crescent-shaped
blade.)
normal cell
4 Tionne T-Boz Watkins of the music group TLC
is a celebrity spokesperson for the Sickle Cell
Disease Association of America. She was diagnosed
with sickle-cell anemia as a child.
Fig. 7-10a, p. 126
11
Fig. 7-10b, p. 126
12
What Causes Mutations?
  • Most mutations result from unrepaired DNA
    polymerase errors during DNA replication
  • Some result from transposable element activity,
    or from exposure to radiation or chemicals
  • Transposable element
  • Small segment of DNA that can spontaneously move
    to a new location in a chromosome

13
Ionizing Radiation Damage
  • Ionizing radiation (x-rays) breaks chromosomes
    and produces free radicals

14
Nonionizing Radiation Damage
  • Nonionizing radiation (UV light) results in
    thymine dimers, which lead to skin cancer

15
thymine dimer
Fig. 7-11b, p. 127
16
Environmental Damage
  • Some natural and synthetic chemicals cause
    mutations in DNA
  • Example Cigarette smoke transfers small
    hydrocarbon groups to bases in DNA, causing
    mispairing during replication

17
Frameshift mutation
18
Duplication
19
Deletion
20
Inversion
21
Translocation
22
Sickle-cell anemia
23
7.7 Examples of Eukaryotic Gene Controls
  • All cells in your body carry the same DNA
  • Some genes are transcribed by all cells, but most
    cells are specialized (differentiated) to use
    only certain genes
  • Which genes are expressed at a given time depends
    on the type of cell and conditions

24
Cell Differentiation
  • Cells differentiate when they start expressing a
    unique subset of their genes controls over gene
    expression are the basis of differentiation
  • Differentiation
  • The process by which cells become specialized
  • Occurs as different cell lineages begin to
    express different subsets of their genes

25
Controlling Gene Expression
  • Controlling gene expression is critical for
    normal development and function of a eukaryotic
    body
  • All steps between transcription and delivery of
    gene product are regulated
  • Transcription factor
  • Protein that influences transcription by binding
    to DNA

26
Homeotic Genes
  • Homeotic gene
  • Type of master gene that controls formation of
    specific body parts during development
  • Master gene
  • Gene encoding a product that affects the
    expression of many other genes
  • Controls an intricate task such as eye formation

27
Homeodomains
  • All homeotic genes encode transcription factors
    with a homeodomain a region of about 60 amino
    acids that can bind to a promoter or some other
    DNA sequence

28
Identifying Homeotic Genes and Their Functions
  • Researchers study the function of a homeotic gene
    by altering its expression by introducing a
    mutation or deleting it entirely
  • Examples eyeless, dunce, tinman, groucho
  • Gene knockout
  • A gene that has been inactivated in an organism

29
Gene Knockout Experiment Eyeless
30
Fig. 7-12a, p. 128
31
Fig. 7-12b, p. 128
32
Fig. 7-12c, p. 128
33
PAX6 Gene Function
  • Many master genes are interchangeable among
    species in humans and many other animals, the
    PAX6 gene affects eye formation

34
Sex Chromosome Genes
  • In mammals, males have only one X chromosome
    females have two, but one is tightly condensed
    into a Barr body and inactive
  • Dosage compensation
  • Theory that X chromosome inactivation equalizes
    gene expression between males and females

35
X Chromosome Inactivation
  • Female cells have Barr bodies, male cells do not

36
The Y Chromosome
  • The SRY gene, found on the Y chromosome, is the
    master gene for male sex determination
  • Triggers formation of testes
  • Testosterone produced by testes controls
    formation of male secondary traits
  • Absence of SRY gene in females triggers
    development of ovaries, female characteristics

37
Structures that will give rise to external
genitalia appear at seven weeks
Development of Human Reproductive Organs
SRY expressed
no SRY present
penis
vaginal opening
birth approaching
Fig. 7-14, p. 129
38
Cancer Gene Expression Out of Control
  • Many gene expression controls regulate cell
    growth and division mutations that disrupt
    normal controls can cause cancer
  • Cancer
  • Disease that occurs when a malignant neoplasm
    physically and metabolically disrupts body
    tissues

39
Tumors
  • Tumor
  • Abnormally growing and dividing mass of cells
  • Metastasis
  • A process of cancer in which tumor cells lose
    membrane recognition proteins, break free, and
    establish themselves in other parts of the body

40
Cancer and Mutations
  • Cancer begins with a mutation in a gene whose
    product controls cell growth and division
  • A mutation that causes cancer may be inherited or
    be caused by environmental agents
  • Tumors are more likely to occur when mutations
    occur in tumor suppressor genes, such as BRCA1
    and BRCA2

41
BRCA Genes and Cancer
42
normal cells in organized clusters
irregular clusters of cancer cells
Fig. 7-15b, p. 130
43
Controls of eukaryotic gene expression
44
Fate map
45
X-chromosome inactivation
46
Protein synthesis summary
47
7.8 Impacts/Issues Revisited
  • Ricin causes ribosomes to stop working protein
    synthesis stops, and the cell quickly dies
  • Researchers are trying to kill cancer cells
    without harming normal cells by attaching ricin
    to an antibody that can find cancer cells in the
    body

48
Digging Into Data BRCA Mutations in Women
Diagnosed with Breast Cancer
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