Download Control of Gene Expression

Control of Gene Expression
S0matic cell nuclear transfer
Cloning
Somatic Cell Nuclear Transfer
 Researchers clone animals by nuclear
transplantation


SOMATIC CELL NUCLEAR TRANSFER: A nucleus of an egg cell is
replaced with the nucleus of a somatic cell from an adult
Thus far, attempts at human cloning have not
succeeded in producing an embryo of more than six
cells

Embryonic development depends on the control of gene
expression (aka protein synthesis)
Reproductive vs. Therapeutic Cloning
 In reproductive cloning, the embryo is implanted in a
surrogate mother. The goal is to produce a cloned
offspring.
 In therapeutic cloning, the idea is to produce a source
of embryonic stem cells.
 Stem cells can help patients with damaged tissues.
 Stem cells are NOT specialized in structure and function,
therefore they can take on the role of damaged cells in
damaged tissues.
Dolly the Sheep
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)
Reproductive vs. Therapeutic Cloning
Genetic Regulation in
Prokaryotes
Proteins turn genes on or off
 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 (Reminder: Bacteria are prokaryotes.)
Cellular Differentiation & the
Cloning of Eukaryotes
Cellular Differentiation
 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.
Retain Genetic Potential
 Differentiated cells may retain all of their genetic
potential. (Even if turned off, the gene is still present.)
 Most differentiated cells retain a complete set of genes
 In general, all somatic cells of a multicellular organism have
the same genes.
 So a carrot plant can be grown from a single carrot cell.
Early Nuclear Transfer
 The cloning of tadpoles showed that the nuclei of
differentiated animal cells retain their full genetic
Tadpole (frog larva)
Frog egg cell
potential
Nucleus
UV
Intestinal cell
Nucleus
Transplantation
of nucleus
Nucleus
destroyed
Tadpole
Eight-cell
embryo
Dolly again
 The first mammalian clone, a sheep named Dolly, was
produced in 1997
 Dolly provided further evidence for the developmental
potential of cell nuclei.
Applications of Nonhuman
Mammalian Cloning
 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.
Therapeutic Cloning
 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.
 Differentiation of embryonic stem cells in culture
Liver cells
Cultured
embryonic
stem cells
Nerve cells
Heart muscle cells
Different culture
conditions
Different types of
differentiated cells
Gene Regulation in Eukaryotes
 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.
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
Metaphase chromosome
Female Mammals
 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
Allele for
black fur
Active X
Inactive X
Inactive X
Active X
Orange fur
Black fur
Genetic Biotechnology
Selective Breeding
 Since ancient times, humans have bred animals and plants to increase
the likelihood of certain desirable traits.
 Example: Hunting dogs or larger fruits
 Two methods are used:
 Hybridization: crossing different parent organisms with different forms
of a trait to produce an offspring with a specific trait
 Example: hybrid rice that produces greater yield and another hybrid rice
that contains greater nutritional properties
 Disadvantage: Time-consuming and expensive
 Inbreeding: crossing closely related parent organisms who have been
bred to have desired traits to ensure that the traits are passed on
 Example: German shepherd dogs
 Disadvantage: Undesirable recessive traits are often passed down with
the desirable traits.
Genetic Engineering
 A process by which an organism’s DNA is manipulated in order
to insert the DNA of another organism (creates recombinant
DNA)
 Purpose: Incorporate the desirable traits of one organism into
another organism
 Example: Bioluminescent trait – A type of jellyfish contains a
protein (GFP: green fluorescent protein) that causes it to glow.
Scientists insert the DNA that codes for GFP into the DNA of
mosquito larvae so that they will glow. Mosquito populations can
be controlled as the larvae are more easily located.
 Produces “genetically modified organisms”
DNA Tools
 Selective breeding and genetic engineering require scientists
to use special tools or processes to manipulate DNA.
 Restriction Enzymes: cut DNA into smaller fragments with
“sticky ends” that allow it to connect to other fragments of DNA
 Gel Electrophoresis: electrical currents separate DNA fragments
based on size allowing fragments to be sorted and studied
Restriction
Enzymes
Useful in genetic engineering
where DNA of one organism is
inserted into the DNA of another
organism (recombinant DNA)
Example: EcoRI restriction
enzyme cuts a GAATTC
sequence
Restriction enzymes are found naturally
in bacteria cells. The bacteria developed
the enzymes to fight against viruses.
They chop up the viral DNA that gets
inserted into their cells.
Gel
Electrophoresis
Electrical currents run through
the DNA samples that have been
cut into fragments. Smaller
fragments travel more quickly
from the – electrode to the +
electrode.
Separated fragments can be studied or
combined with other fragments to create
recombinant DNA.
Gene Cloning
 When a particular new DNA sequence has been developed in
recombinant DNA, bacteria cells are used to make multiple
copies.
 The bacteria cells are heated which causes pores to open in their cell walls.
 The recombinant DNA moves through the pores into the bacteria cell.
 As the bacteria cell replicates, the recombinant DNA is replicated, too.
 Purpose: to create many copies of the desirable sequence
that can be used in genetically-modified organisms
DNA Sequencing
 Learning the sequence of DNA fragments can allow scientists
to understand the function of certain sequences of bases.
 Restriction enzymes are used to cut large DNA strands into
shorter fragments.
 Dyes are used to color known bases, and the known colored
known bases bind to the unknown sequence. The unknown
sequence can be read by following complementary basepairing rules.
Polymerase Chain Reaction
 Purpose: create copies of selected DNA segments when the
sample size of DNA is too small to run all of the needed tests
 Uses a thermal cycler to separate the DNA double helix and
DNA polymerase enzymes to create copies of the selected
segments.
 Extremely useful in forensic science and medicine
Gene Therapy
 Purpose: Insert beneficial genes into a needy organism
 How?
 A mutated gene is located on a chromosome.
 A “normal” gene is inserted into a chromosome to replace the
dysfunctional one using a “viral vector.”
 The virus infects a cell and injects its genetic material including the
“normal” gene. The cell begins replicated the “normal” gene, and
the mutation is corrected.