The Revolution in Cell Technology

1
The Revolution in Cell Technology

• Chapter 16

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Proving That Reproductive Cloning is Possible

• Hans Spemann proposed in 1938 that cloning might
  be possible by removing the nucleus from an egg
  cell and replacing with a nucleus from another
  cell
• early attempts in cloning were unsuccessful
  unless the transplanted nucleus was from a very
  early stage of development
• the theory of irreversible determination
  suggested that animal cells become irreversibly
  committed after the first cell divisions

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A cloning experiment
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Proving That Reproductive Cloning is Possible

• Geneticists working in Scotland made a key
  breakthrough in animal cloning
• Keith Campbell suggested they conduct nuclear
  transfers between a donor and recipient cell that
  were at identical stages of the cell cycle
• the cells were starved so that they were at the
  beginning of the cell cycle at the G1 checkpoint
• Neil First in 1994 and Campbell and Ian Wilmut in
  1995 successfully cloned farm animals from
  advanced embryos using this method

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Proving That Reproductive Cloning is Possible

• Wilmut progressed with the method to transfer
  successfully a nucleus from an adult cell into an
  enucleated egg
• Wilmut used an adult sheeps mammary gland as the
  nuclear donor
• both the donor mammary cells and the enucleated
  eggs were first starved and a brief electrical
  shock allowed the contents to fuse together
• the resulting embryos developed into blastulae
  and were implanted into a surrogate mother
• Dolly the cloned lamb was born on July 5, 1996

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Wilmuts animal cloning experiment
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A parade of cloned critters
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Problems with Reproductive Cloning

• many problems have been encountered with
  reproductive cloning since Dolly
• most transplanted cloned embryos die late in
  pregnancy
• large offspring syndrome
• describes the oversized condition of many of
  these transplants
• among surviving cloned offspring, their
  development into adults goes unexpectedly haywire
• most do not survive to live a normal life span
• Dolly died prematurely in 2002

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Problems with Reproductive Cloning

• Reprogramming by parent males and females of the
  sperm and eggs may affect cloning success
• genomic imprinting
• Chemical changes to DNA that alters when genes
  are expressed without changing the sequences
• Genes can be locked on or off
• Normal animal development depends on precise
  genomic imprinting

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Problems with Reproductive Cloning

• genomic imprinting occurs at different stages
• gametic imprinting
• takes place in adult reproductive tissue
• requires months for sperm and years for eggs
• zygotic imprinting
• the egg cell cytoplasm acts to reprogram the DNA
  introduced by the sperm
• donor DNA to be cloned may be less efficient at
  doing this reprogramming

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Two forms of genomic imprinting
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Embryonic Stem Cells

• embryonic stem cells are totipotent
• Have the ability to form any body tissue, and
  even an adult animal
• later in development the embryonic stem cells
  differentiate into adult stem cells
• these cells produce only one kind of tissue
• the genes needed to produce other types of
  tissues are turned off

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Human embryonic stem cells (x20)
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Embryonic Stem Cells

• Offer the possibility of restoring damaged
  tissues
• Embryonic stem cells grown in culture could be
  induced to form any type of tissue in the body
• Healthy tissue can be injected into a patient
  where it will grow and replace damaged tissue

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Using embryonic stem cells to restore damaged
tissue
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Therapeutic Cloning

• What happens when immune rejection of
  transplanted stem cells occurs??
• DNA from adult cells are used to create an embryo
  from an individual
• stem cells are then harvested from the embryo,
  which is subsequently destroyed
• the tissue developed from these stem cells can be
  injected into the damaged host
• there is no issue of immunological tolerance
  because the donor and recipient of the stem cell
  therapy are the same individual

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How human embryos might be used for therapeutic
cloning
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Grappling with the Ethics of Stem Cell Research

• Use of embryonic stem cells raises a number of
  important ethical concerns, including
• should human embryos be destroyed?
• how can the potential for future abuse be
  avoided?
• are there alternative sources to using embryonic
  stem cells?

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Initial Attempts at Gene Therapy

• gene transfer therapy
• Transferring healthy versions of a gene into
  cells that lack them
• while the research is promising, problems remain
  in finding an appropriate vector for gene
  transfer
• adenovirus vector used to piggyback healthy genes
  
• subject to immune attack and prone to introduce
  mutations that lead to cancer

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Initial Attempts at Gene Therapy
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More Promising Vectors

• adeno-associated virus (AAV) is a more promising
  choice for vector
• this parvovirus needs adenovirus to replicate but
  is a good gene carrier once its two genes are
  removed
• it infects easily but, when the AAV genes are
  removed, does not produce an immune response or
  introduce cancerous mutations

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Using gene therapy to cure a retinal degenerative
disease in dogs
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Ethical Issues Raised by Gene Therapy

• ethicists prefer the term gene intervention in
  lieu of gene therapy to describe any procedure
  that deliberately alters a persons genes
• it is important to consider the permanence of
  these changes
• changes to somatic tissue are not inherited
• changes to germ-line tissues are inherited

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Ethical Issues Raised by Gene Therapy

• beneficence principle
• ethicists use this to weigh the risks versus the
  benefits when making decisions about potential
  therapy
• respect-for-persons principle
• ethicists respect the right of persons affected
  by the procedure to make their own informed
  decisions

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Inquiry Analysis

• Judging by visual similarity, which adult dog is
  the closer relative of Snuppy?
• What evidence would you accept that Snuppy is
  indeed a clone?

Photo of Snuppy, with his Clone Dad and Surrogate
Mom