Chapter 15
Controls over Genes
When DNA Can’t Be Fixed?
• Changes in DNA are triggers for skin
cancer, like the most deadly type–
malignant melanoma
• Cancers: are malignant forms of tumors
– Neoplasm (tumors) are tissue masses that
arise through mutations in the genes that
govern growth and division
– Malignant tumors grow rapidly causing
destructive effects on surrounding cells
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• Gene controls are molecular mechanisms
that govern when an how fast specific
genes will be transcribed and translated
Types of Control Mechanisms
• Because all cells in your body have the
same genetic instructions, only a relatively
small amount number of genes are active
at any given time in any given tissue
– Which genes are expressed depends on the
type of cell, its responses to chemical signals
and built-in control systems
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• Regulatory Proteins intervene before,
during or after gene transcription or
translation
– Regulatory proteins interact with DNA, RNA,
or actual gene products
Several controls system are used
by cells:
• Negative Control- a regulatory protein
binds to the DNA to block transcription; it
can be removed by an inducer
– Slows down or curtail gene activity
• Positive Control- a regulatory protein binds
to the DNA and promotes initiation of
transcription
– Regulatory proteins promote or enhance gene
activity
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• In chemical modification, regions of newly
replicated DNA can be shut down by
methylation and access to genes can be
controlled by acetylation of histones
proteins that organize DNA
Negative Control of the Lactose
Operon
• E. coli bacteria (commonly found in the
human digestive system) can metabolize
lactose because of a series of genes that
code for lactose-digesting enzymes
– Three genes are preceded by a promoter and
an operator – all together called an operon
– Operator is a binding site for a repressor
protein that can prevent gene transcription
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• A regulator gene nearby codes for a
repressor protein that binds to the operator
when lactose concentration are low and
effectively block RNA polymerase’s
access to the promoter
• When milk is consumed, the lactose binds
to the repressor changing its shape and
effectively removing its blockage of the
promoter; thus the RNA polymerase’s
access can now initiate transcription of the
genes
Gene controls in eukaryotic cells
• Much less is known about gene controls in
multicelled eukaryotes because patterns of
gene expression vary within and between
body tissue
Cell Differentiation and Selective
Gene Expression
• All body cells have the same genes, but
the cells of different tissues are
differentiated (specialized) because of
selective gene expression
Controls related to transcription
include: (Brown Box p. 242)
• Gene Amplication:DNA replicates
hundreds or thousands of copies prior to
transcription
• DNA Rearrangements: Some genes have
base sequences in the DNA, and they are
put together in different ways to make
different product molecules
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• Chemical Modification: histones and other
proteins interact with eukaryotic DNA in
organized ways. The DNA-protein
packaging, plus chemical modifications to
particular sequences
Post-transcriptional controls
• Transcript Processing: exons and introns
• Transport Controls: dictates which mature
transcripts will be shipped to the
cytoplasm for translation
• Post translational controls: govern the
modifications to polypeptides
• All found on p. 243 Figure 15-4 (b-d)
Types of Control Mechanisms
Homeotic Genes and Body Plans
• Homeotic gene interact with one another
and with control elements to brin about the
formation of tissues and organs in
accordance with the basic body plan.
• These genes code for regulatory proteins
that can bind to promoters and enhancers
to control transcription
X Chromosome Inactivation
• In mammalian females, the gene products
of only one X chromosome are needed the
other is condensed and inactive ---called
the Barr Body.
• Because in some cells the parental X
chromosome is inactivated while in other
cells the maternal X chromosome is
inactivated, each adult female is a mosaic
of X-linked called mosaic tissue effect
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• This mosaic effect is seen in human
females affected by anhidrotic ectodermal
dysplasia in which a mutant gene on one
X chromosome results in patches of skin
with no sweat glands
– Figure 15.6b and 15.7 -- page 245
Hormones Signals
• Hormones are major signaling molecules
that can stimulate or inhibit gene activity in
target cells
– Some hormones bind to membrane receptors
on cells surfaces
– Other enter cells to bind with regulatory
proteins to initiate transcription, often with the
aid of enhancer sequences
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• In the salivary glands of insect larvae, the
polytene chromosomes responds to the
hormones ecdysone by puffing out during
transcription
• In vertebrates, some hormones such as
somatotropin have widespread effects
because most of the body’s cells have
receptors for it; whereas prolactin affects
only the mammary glands
Sunlight as a Signal
• Plant seedlings will respond to a single
burst of light by making chlorophyll
• Phytochrome is a blue-green pigment that
helps plants adapt to the changing light
conditions of day/night and seasons by
signaling genes responsible for
germination, stem elongation, branching,
leaf expansion, and formation of flowers,
fruits, and seeds