ap biology ch. 16 embryonic development and cloning
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The Genetic Basis of DevelopmentCh. 16
Objective – What you should get from this lecture:
Be able to define genetic equivalence
Be able to describe the experiments that proved it
Identify the 3 processes that turn a single cell zygote into a complex, multicellular organism
BackgroundScientists study how organisms
develop from single celled zygote to complex multicellular organism
Model organisms studied include: Drosophila melanogaster (fruit fly) Caenorhabditis elegans (nematode) Mus musculus (mouse) Danio rerio (zebrafish) Arabidopsis thaliana (common wall cress)
Embryonic developmentMulticellular organisms have great
variety in cell types (cell differentiation) Those cells are further organized into
tissues, organs, organ systemsDuring development of an embryo, the
cells need to not only become different from each other, they need to also be organized in a highly structured manner
Transformation of a zygote to a multicellular organism
3 processes: Cell division
get a ball of undifferentiated cells
Cell differentiation these cells become
specialized in structure & function and are organized into tissues
Morphogenesis the processes that give an
organism it’s shape
All 3 events overlap each otherEarly morphogenesis
Establishes dorsal – ventral ; anterior – posterior
Later morphogenesis Establishes more specific locations –
where limbs will be, etc
The development of plants and animals is different
Animals: cells must move into position in the early embryo to transform it into its characteristic 3-D shape
Plants: morphogenesis and growth occur throughout the life of the plant, not just during embryonic and juvenile development
Plants keep growing…Apical
meristems – the structures on a plant that allow for it’s continual growth and development They are like
regions of the plant that are always embryonic
Found in roots and shoots
Quick ThinkWhat are the 3 processes that occur during embryonic development and how are they different between plants and animals?
Cell DifferentiationDifferences in cells
due to Differences in gene
expression Regulatory
mechanisms that turn genes on or off throughout development
Genomic equivalence Nearly all the cells in Nearly all the cells in
an organism have the an organism have the same genessame genes
Genes unnecessary Genes unnecessary to that cell are turned to that cell are turned offoff Are the genes
permanently disabled, or could they be activated again?
3 experiments helped prove the idea of genomic equivalence
Experiment 1Experiment 1 Can a differentiated plant Can a differentiated plant cell develop into a whole new plant?cell develop into a whole new plant?
Took cells from the root of a carrot plant
Put those cells in nutrient mediumThey grew into a whole new plantThe new plant was a clone of the
original parent plantProved that a differentiated cell
has all the genes necessary to make a new plant
When a cell can reverse its differentiation and generate all the cell types in a new individual, it is said to be totipotent
Experiment 2a: Can animal cells be totipotent?
Took the nucleus from an early embryo cell Not much differentiation in
these cellsRemoved the nucleus from an
egg cellPut donor nucleus into egg
cellMost of these developed into
new organisms
Experiment 2b: Can an animal cell be totipotent?Took the nucleus from a
fully differentiated cell (mature cell)
Removed the nucleus from an egg cell
Inserted the mature nucleus into the egg
Less than 2% of these developed into new organisms Many died prematurely
Experiment 2a & 2b ConclusionThe nucleus from an
animal cell can direct the development of a new individual
Less of an ability to do this as the cell becomes more differentiated The DNA is all the same Chromatin structure is
probably to blameMore methylation
(inactivation of genes) This can be reversible
Experiment 3: can a fully differentiated cell be reprogrammed to become totipotent?
Took mammary cells from sheep 1 “starved” them to stop
their cell cycles Removed the nucleus
from an ovary cell from sheep 2
Fused the 2 cells Grew this new cell in
culture to get an early embryo
Implanted the embryo in sheep 3 It grew and developed
into Dolly
Quick Think
How was the idea of genomic equivalence proven?
DollyGenetically identical to the
donor cell nucleus except for mitochondrial DNA that came from the donated egg cell
Seemed to age more quickly than normal
Died at about 1/2 the normal life expectancy Were her cells somehow
“older” than normal cells?
Reproductive Cloning What happened with Dolly is
called reproductive cloning - creating new individuals
Since then cows, sheep, pigs, cats, mice, & horses have been cloned
Not all clones are exactly alike in appearance or behavior
Copy Cat has a different coat pattern than her mother clone
What about cloning humans?
Random inactivation of the X chromosomes
Problems with CloningDolly was a rare success; the only
survivor out of several hundred triesMost cloned embryos don’t develop
fully to birthThose that do often have birth defects
Why? In normal, differentiated
cells, only certain genes are active
The rest are repressed This has to be reversed in
the donor nucleus to allow all the genes to be transcribed
Misplaced methyl groups (which help regulate gene expression) in the donor nucleus may lead to the problems we see in clones
Quick Think
What do you know or what have you heard about stem cells?
Embryonic Stem Cells A relatively unspecialized cell
that can reproduce itself and differentiate into different kinds of specialized cells
Can be harvested from the blastocyst stage of embryo development
Cloning embryos to the blastocyst stage as a source of stem cells is called therapeutic cloning.
Embryos come from donated embryos from fertility clinics
Adult Stem CellsSome stem cells found in
adult bodiesThey are pluirpotent - can
become several different kinds of cells, but not all Stem cells in the bone
marrow become different kinds of blood cells
Work is being done to try to coax adult stem cells to be more like embryonic stem cells
Why are stem cells valuable?Ultimate goal is to use stem cells to
repair damaged or diseased tissues and organs Brain cells for people with Parkinson’s
disease Pancreatic cells for people with diabetes
https://www.youtube.com/watch?v=5x8e2qsAVGc http://ed.ted.com/lessons/what-are-stem-cells-craig-a-kohn
Quick ThinkSome believe it is wrong to create or
use embryos for stem cell research that will be destroyed. Others, like the scientists who cloned Dolly, believe that “cloning promises such great benefits that it would be immoral not to do it.”
How do you feel about all this? (you don’t have to share if you don’t want
to)
Cell DeterminationDetermination
the development of an embryonic cell into its differentiated state
Irreversible (as far as we know now) Cell expresses genes specific
to the tissue it is a part of If moved to a new tissue, it will
still develop into that kind of cell
Determination starts to occur very early in embryonic development
Determined eye cells moved to a new part of the body - still become eye cells
Differentiated CellsBecome specialized
in producing particular gene products Cells in the eye
produce crystalline proteins
Cells in the liver produce albumin
Muscle cells produce actin & myosin
How cells become different in an embryo The cytoplasm of an
egg cell is not homogeneous mRNA, proteins,
organelles, etc are distributed unevenly
As the zygote divides, the cytoplasm of each new cell contains a different mix of cell components These components
influence the development of the cell - called cytoplasmic determinants
How cells become different in an embryo
The environment around each cell determines cell development Contact with signal
molecules from neighbor cells can cause changes in a cell - called induction
Quick ThinkWould a single stem cell be able to become an embryo all by itself? Why or why not?
So…Quick Write after discussionWhat are the events and processes
that contribute to the development of a single celled zygote into a complex, multicellular organism?
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