Transcript chapter 11

11.3 Differentiated cells may retain all of their
genetic potential
• Most differentiated (specialized) cells retain a
complete set of genes
– In general, all somatic cells of a multicellular
organism have the same genes whether it is a
liver cell, heart cell, muscle cell etc.
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Table 11.2
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CELLULAR DIFFERENTIATION AND THE
CLONING OF EUKARYOTES
11.2 Differentiation yields a variety of cell types,
each expressing a different combination of
genes
• In multicellular eukaryotes, cells become
specialized as a zygote develops into a mature
organism
– Different types of cells make different kinds of
proteins
– Different combinations of genes are active in
each type
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GENE REGULATION IN EUKARYOTES
11.6 DNA packing in eukaryotic chromosomes
helps regulate gene expression
• A chromosome contains a DNA double helix
wound around clusters of histone proteins
• DNA packing tends to block gene expression
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DNA
double
helix
(2-nm
diameter)
Histones
“Beads on
a string”
Nucleosome
(10-nm diameter)
Tight helical fiber
(30-nm diameter)
Supercoil
(200-nm diameter)
700
nm
Figure 11.6
Metaphase chromosome
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11.7 In female mammals, one X chromosome is
inactive in each cell
• An extreme example of DNA packing in
interphase cells is X chromosome inactivation
EARLY EMBRYO
TWO CELL POPULATIONS
IN ADULT
Cell division
and
X chromosome
inactivation
X chromosomes
Allele for
orange fur
Active X
Inactive X
Inactive X
Active X
Orange fur
Black fur
Allele for
black fur
Figure 11.7
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11.8 Complex assemblies of proteins control
eukaryotic transcription
• A variety of regulatory proteins interact with
DNA and each other
– These interactions
turn the
transcription
of eukaryotic
genes
on or off
Enhancers
Promoter
Gene
DNA
Transcription
factors
Activator
proteins
Other
proteins
RNA polymerase
Bending
of DNA
Figure 11.8
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Transcription
11.9 Eukaryotic RNA may be spliced in more than
one way
• After transcription, alternative splicing may
generate two or more types of mRNA from the
same transcript
Exons
DNA
RNA
transcript
RNA splicing
mRNA
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or
Figure 11.9
11.10 Translation and later stages of gene
expression are also subject to regulation
• The lifetime of an mRNA molecule helps
determine how much protein is made
– The protein may need to be activated in some
way
Folding of polypeptide and
formation of S–S linkages
Initial polypeptide
(inactive)
Folded polypeptide
(inactive)
Cleavage
Active form
of insulin
Figure 11.10
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11.11 Review: Multiple mechanisms regulate gene
expression in eukaryotes
• Each stage of eukaryotic expression offers an
opportunity for regulation
– The process can be turned on or off, speeded
up, or slowed down
• The most important control point is usually
the start of transcription
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Chromosome
DNA unpacking
Other changes
to DNA
GENE
GENE
TRANSCRIPTION
Exon
RNA transcript
Intron
Addition of
cap and tail
Splicing
Tail
Cap
mRNA in nucleus
NUCLEUS
Flow
through
nuclear envelope
mRNA in cytoplasm
CYTOPLASM
Breakdown of mRNA
Translation
Brokendown
mRNA
Polypeptide
Cleavage/modification/
activation
ACTIVE PROTEIN
Breakdown
of protein
Brokendown
protein
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Figure 11.11
– Can differentiated cells reverse become
dedifferentiated? This is common in plants
– So a carrot plant can be grown from a single
carrot cell
Root of
carrot plant
Plantlet
Cell division
in culture
Single cell
Root cells cultured in nutrient medium
Figure 11.3A
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Adult plant
• Early experiments in animal nuclear
transplantation were performed on frogs
– The cloning of tadpoles showed that the nuclei of
differentiated animal cells retain their full
genetic potential
Tadpole (frog larva)
Frog egg cell
Nucleus
UV
Intestinal cell
Nucleus
Transplantation
of nucleus
Nucleus
destroyed
Tadpole
Eight-cell
embryo
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Figure 11.3B
• In reproductive cloning, the embryo is
implanted in a surrogate mother
• In therapeutic cloning, the idea is to produce a
source of embryonic stem cells
– Stem cells can help patients with damaged
tissues
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Donor
cell
Nucleus from
donor cell
Remove
nucleus
from egg
cell
Add somatic
cell from
adult donor
Implant blastocyst
in surrogate mother
Clone of donor
is born
(REPRODUCTIVE
cloning)
Remove embryonic
stem cells from
blastocyst and
grow in culture
Induce stem
cells to form
specialized cells
for THERAPEUTIC
use
Grow in culture to produce
an early embryo (blastocyst)
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• The first mammalian
clone, a sheep named
Dolly, was produced in
1997
– Dolly provided further
evidence for the
developmental
potential of cell nuclei
Figure 11.3C
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11.4 Connection: Reproductive cloning of
nonhuman mammals has applications in basic
research, agriculture, and medicine
• Scientists clone farm
animals with specific
sets of desirable
traits
• Piglet clones might
someday provide a
source of organs for
human transplant
Figure 11.4
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11.5 Connection: Because stem cells can both
perpetuate themselves and give rise to
differentiated cells, they have great
therapeutic potential
• Adult stem cells can also perpetuate themselves
in culture and give rise to differentiated cells
– But they are harder to culture than embryonic
stem cells
– They generally give rise to only a limited range
of cell types, in contrast with embryonic stem
cells
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• Differentiation of embryonic stem cells in
culture
Liver cells
Cultured
embryonic
stem cells
Nerve cells
Heart muscle cells
Figure 11.5
Different culture
conditions
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Different types of
differentiated cells
GENE REGULATION IN PROKARYOTES
11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
• The process by which
genetic information flows
from genes to proteins is
called gene expression
– Our earliest
understanding of gene
control came from the
bacterium E. coli
Figure 11.1A
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• In prokaryotes, genes for related enzymes are
often controlled together by being grouped into
regulatory units called operons
• Regulatory proteins bind to control sequences
in the DNA and turn operons on or off in
response to environmental changes
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• The lac operon produces enzymes that break
down lactose only when lactose is present
OPERON
Regulatory
gene
Promoter Operator
Lactose-utilization genes
DNA
mRNA
RNA polymerase
cannot attach to
promoter
Active
repressor
Protein
OPERON TURNED OFF (lactose absent)
DNA
RNA polymerase
bound to promoter
mRNA
Protein
Lactose
Inactive
repressor
Enzymes for lactose utilization
OPERON TURNED ON (lactose inactivates repressor)
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Figure 11.1B
• Two types of repressor-controlled operons
Promoter
Operator
Genes
DNA
Active
repressor
Active
repressor
Tryptophan
Inactive
repressor
Inactive
repressor
Lactose
lac OPERON
Figure 11.1C
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trp OPERON