Download Control of Gene Expression

How do Cells Become Specialized?
• Cell Differentiation: a process where a generalized
cell changes in form and function to a
specialized cell (ex. neurons, RBCs)
– Often triggered chemically by neighbor cells (induction)
• Cell Fate: specialized function that cell acquires
• Cell Potency: range of cell types that cell could
acquire if exposed to different inductive environments; potency always includes fate
– Totipotent cells: unlimited potency
– Pluripotent cells: high, but not unlimited potency
• Cell Determination: when potency becomes
restricted to fate; timing can vary
• Heterotopic transplantation: method for testing
potency and timing of cell determination
Fig. 18.14
(a) Fertilized eggs of a frog
(b) Newly hatched tadpole
Fig. 18.15
Unfertilized egg cell
Sperm
Fertilization
Nucleus
Two different
cytoplasmic
determinants
Zygote
Mitotic
cell division
Two-celled
embryo
(a) Cytoplasmic determinants in the egg
Early embryo
(32 cells)
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells
NUCLEUS
What are In Vitro Fertilization and
Embryo Screening?
• In Vitro Fertilization (IVF)
–
Sperm and egg united in a petri dish, followed by implantation
of embryo(s) into mother’s uterus; pioneered by Robert
Edwards as a treatment for infertility (Nobel Prize, 2010);
roughly 20-30% success rate with a single embryo
– First “test-tube baby” (Louise Brown) born 25 July, 1978;
more than 4 million babies born via IVF; controversies
include multiple births (ex. octuplets) and un-used embryos
• Embryo Screening
– Cells from embryo harvested in lab (vs. amniocentesis with a
fetus), and subjected to karyotyping and DNA fingerprinting
– Pre-implantation genetic diagnosis (PGD): testing for genetic
conditions before an embryo is chosen to be used; allows
bone-marrow match for other sibling (e.g. My Sister’s Keeper)
Figure 14.18
How are Clones Produced?
• Cloning Techniques
– Enucleated egg cell implanted with donor
nucleus (frogs and mice) or fused with 8-cell
blastocyst (lamb, cow, pig, and monkey)
• Success rate is typically low, but improving (ex. Gaur,
early 2001: 44 embryos in 32 surrogate cows led
to eight pregnancies, only one successful)
• Questions over health of clones: lung problems; early
aging suspected
• Success rate using adult donor cells varies by cell
type (“Dolly” was from mammary gland cells)
• Success rate increased by incubation of donor nucleii
in oocyte cytoplasm
Figure 20.18
What Are the Potential Applications
of Cloning?
• Therapeutic Cloning
– Affected individual (ex. paralysis) cloned, resulting embryo
used as a source of embryonic stem cells (overcomes
rejection problem common with organ donations)
• Cloning Organisms (Reproductive Cloning)
– Conservation of endangered species (to increase
population from critically low population)
• Resurrection of extinct species (?)
– Cloning deceased pets
– Increased efficiency of biotechnology
• Transgenic animals can be cloned
• Production of genetically engineered organs that reduce rejections
– Human reproduction when other techniques have failed
(child would be genetically related to only one parent)
What are Stem Cells?
• Embryonic and Adult Stem Cells
– Embryonic: totipotent, can be cultured  stem line; can
be obtained from fertility clinics (un-used embryos), or
from umbilical tissue (companies store cord blood)
– Adult: success varies by cell type; typically not totipotent,
and potency may not last; not easily cultured, but techniques improving (cells re-programmed to embryonic
state)
• Potential Uses of Stem Cells
– Possible treatments of diseases involving dead or
damaged cells (spinal damage, Alzheimer’s, and
Parkinson’s); success in several cases with mice;
concerns regarding possibility of implanted cells
becoming cancerous
– From 2000-2008 in U.S., publicly funded research allowed
only on existing stem-cell lines (some leading specialists
relocated to places like Singapore); recent changes under
Obama Administration challenged in the courts
Figure 20.20