Edgey about BioTecknowledgey: A Case Study of Cloning
Commentary by Tom Zinnen
On Sunday, February 23, 1997, the news hit the news: researchers had
cloned an adult sheep. The weekend reporters didn't all get the
science, but they got the story: cloning of humans is plausible.
The story grew. On Monday, February 24, the president of the US gave
his ethics advisory board 90 days to produce a report assessing the
ethics of the new technology and providing ways to prevent is abuse.
The story peaked and receded on Tuesday and Wednesday--yet the
research paper describing Dolly was not available until Thursday,
February 27. Before the science could be scrutinzed, the first wave
of the story had run its first course. First impressions were pressed
without benefit of really knowing what had been done.
The following Monday a new wave rose. Cloning made the cover of Time
and Newsweek (both were dated March 10, 1997, but actually were
released a week earlier). And the president of the US issued an
executive order banning any federal money from being spent on research
into cloning humans. And in an unprecedented move, he asked for a
moratorium on such research by private individuals and institutions.
In this case study we'll analyze a paper that, for scientific and
social impact, will be a unique classic. It is available at
www.nature.com, and you will need a paper or electronic copy of the
paper to make best use of this case study.
Nature, Volume 385, 810 - 813, February 27, 1997
Viable offspring derived from fetal and adult mammalian cells
I. Wilmut, A. E. Schnieke*, J. McWhir, A. J. Kind* & K. H. S. Campbell
Roslin Institute (Edinburgh), Roslin, Midlothian EH25 9PS, UK
* PPL Therapeutics, Roslin, Midlothian EH25 9PP, UK
Overview of what some people thought happened (as of February 25,
Researchers took a cell from an adult sheep,transferred some DNA, grew
it somehow and somewhere (in a test tube, in a surrogate ewe??) and
produced a lamb genetically identical to the original adult sheep.
Furthermore, this technique will likely work on humans, and so the age
of human cloning is upon us.
Analyzing the title.
"Viable offspring derived from fetal and adult mammalian cells"
"What terms do the authors use in their report?"
Notice the language: "viable offspring" rather than "clone";
"derived from" rather than "cloned from" or "born" or "generated."
- What cells do the authors use in their work?
Note that both fetal cells and adult cells of sheep were the source of
the offspring. Dolly was the only offspring derived from an adult
cell's nucleus, however.
The Opening Sentence.
- What terms are used to describe the various cells and processes
involved in sexual reproduction and development and cell
differentiation and gene expression? Keep in mind the progression of
cells and processes: oocytes and spermatocytes undergo meiosis to produce gametes
(egg, sperm), cell fusion '(plasmogamy)' and nuclear fusion (karyogamy) leading to a
zygote (diploid),which divides by mitosis and cytokinesis to form an
embryo, which develops into a fetus, with differentiation of tissues
and generation of organs; the fetus is born and the offspring develops
into an adult.
The Second Sentence: "Transfer of a single nucleus at a specific
stage of development, to an enucleated unfertilized egg, provided an
opportunity to investigate whether cellular differentiation to that
stage involved irreversible genetic modification."
- Before February 23, 1997, what was the dominant view or paradigm
regarding the possibility of using a cell from an adult mammal to
generate a new animal?
"A basic idea of cell biology is that diffentiated cells
contain all the genetic information as the zygote from which they were
derived." If so, then
how to explain that a single zygote cell can divide, differentiate and
develop into a fetus, but single cells from adults apparently cannot?
The key question: are the genes of cells of adults, cells that have
differentiated, changed in some irreversible way that the cells cannot
develop into adults?
- To this question there are two possible answers: the genes of the
cells are changed irreversibly, or they are not.
But testing these two possiblities poses a dilemma. This is like
asking whether a seed is viable: if you plant it and it grows, you
know it was viable. If you plant it and it doesn't, you're don't know
if the seed is dead or if it's dormant and you just don't know how to
wake it up.
- In this experiment,there are two possible results: either the cell
isolated from an adult does grow into another adult, or it does not.
If it does, the answer is clear--and the implications are resounding:
this would disprove the idea of "irreversible genetic modifications"
in adult cells.
If it does not, there are two possible explanations: the cell didn't
because it can't; or the cell didn't because it was not cultivated
under the appropriate conditions.
Technical Ingenuities: Nuclear Swapping Through Cell Fusion, and
Using Quiescent Cells.
- In the case of Dolly, did the researchers use only one cell
isolated from an adult? Or did they actually build a combination cell
from two cells?
In fact, although the authors use the term "nuclear transfer" in the
methods section it becomes clear that they fused two cells together.
One cell had a nucleus, and came from a population of cells grown "in
vitro" (in test tubes) after originally being taken from the udder of
an adult sheep. The second cell was an unfertilized egg cell from
which the researchers had removed the nucleus. While the researchers
called it "nuclear transfer" and some wags called it "nuclear
swapping", could it not also be considered "cytoplasmic swapping"?
Could there be something in the cytoplasm of the egg cell that
activates the genes of the nucleus from the cell derived from the
cells from the udder?
- The authors point out they intentionally used "quiescent" cells.
What are quiescent cells?
How are they made?
How would you vigorously test the hunch or speculation that quiescence
is significant and that quiescence synchronizes the two cells?
Remember that the cells from the udder were grown "in vitro" for many
days. At first the researchers grew the cells in a liquid that
included 10% fetal calf serum--a rich diet. Then they switched the
cells to a diet with only 0.5% fetal calf serum for five days. That
diet was not enough to support cell division--but the cells continued
to live in a "resting phase" called G0 (in contrast to G1 and G2) by
cell biologists. Such cells can be called quiescent.
- The researchers report one animal (now known to the world as
Dolly), was derived from a combination cell with a nucleus from a
mammary cell (a differentiated cell) from an adult sheep.
The authors write that "the fact that a lamb was derived from an adult
cell confirms that differentiation of that cell did not involve the
irreversible modification of genetic material required for development
to term." Dolly is the evidence that disproves the idea of
irreversible modifications of genetic material in differentiated cells
- Question: What ideas could you test by putting an egg's nucleus
in a cytoplasm of a cell taken from an adult?
- Of the two animals that supplied cells for the combination cell,
which supplied the mitochondria?
This was not clear to this writer from the paper. There are three
possibilities for the origin of Dolly's mitochondria. 1) The
mitochondria came only from the egg cell. 2) The mitochondria came
only from the cell derived from udder cells. 3) Dolly's cells have a
mixture: some mitochondria originated from the egg cell, and some
from the cell derived from the udder cells.
- If Dolly's nucleus came from one animal and her mitochondria from
another, is she really genetically identical to the donor of the
Note that while the focus of the popular coverage of the story is the
cloning technology, the emphasis of the research report is a disproof
of the hypothesis that cell's genes are irreversibly modified by
- The researchers report cloning from three different sources of
cells, representing three different stages in the development from a
fertilized egg to a mature adult. Why would the researchers try a
series of cells of increasing age as sources of nuclei?
This is the skiing analogy. At ski hills, there are hills color-coded
by difficulty. Green, blue and black are easy, intermediate and
difficult. If you start out on the most difficult hill, you're likely
to fail before you get a chance to succeed. So most new skiers start
easy, and gradually go to the more challenging hills.
The researchers anticipated increasing difficulty of cloning with
increasing age of the donor cells--so they tried some young cells to
increase the liklihood of success. This also serves as a kind of
positive control, to show that their technique and tools could work.
In the event that none of the attempts to clone from adult cells
succeeded, they could refute the possibility that their technique or
tools were flawed.
- After making the "reconstructed embryos," did the researchers
grow them in a test tube or implant them into a surrogate ewe?
- How was pregnancy monitored? What were the two explanations
suggested for the observed decreases in detected pregnancies?
- How does the rate of fetal death compare between conventional
pregnancy and pregnancy from reconstructed embryos?
- What four kinds of evidence do the authors cite to conclude that
the lambs were not born from inadverdent matings?
Note that morphological evidence as well as DNA analysis are used
Comments on comparing this paper to the coverage of this paper in the
How often does the word "clone" appear in the paper by Wilmut and
By definition, a clone is genetically identical to its progenitor. Is
Dolly genetically identical to its nuclear donor? Is Dolly a true
clone? Do the authors make that claim?
- How often do the authors talk about "transfer of DNA" in contrast
to "transfer of a nucleus"? On February 24 and 25, many of the
stories in the press stated that Dolly was cloned by taking "the DNA"
from the cell of an adult sheep and transfering it to another cell.
The authors use terms such as "the nucleus" and "the genetic material"
but do not specifically refer to DNA transfer at all. In fact, of the
national media, USA Today got the story right in talking about
transfer of the nucleus.
Actually, it was a cell fusion, rather than solely the transfer of a
nucleus, but that did not become clear until the paper was published
on February 27.
Is the distinction between "DNA transfer" and "nuclear transfer"
Sure.The nucleus is an organized organelle, and it has organizational
capacity that its DNA does not. This was a transfer of organelles,
not just a transfer of DNA. Here, the distinction made ten years ago
by Sidney Brenner is important: chemistry-as-information compared to
chemistry-as-organization. Transferring an intact nucleus transfers
only one copy of all the DNA--but it's a well-organized copy!
Remember to consider this story not just from the "nuclear transfer"
point of view but also from the "cytoplasmic transfer" point of view.
What is it about the cytoplasm of the egg cell that can activate the
expression of genes in the received nucleus?
- Finally, compare this "cell fusion" to the cell fusion in
ordinary fertilization: sperm and egg fuse. The fusion involves two
steps: fusing of the cytoplasms (plasmogamy) and later, fusion of the
two nuclei (karyogamy). The cell fusion described in this paper would
emulate plasmogamy, but not karyogamy, of standard sperm and egg
fertilization. The results, if any, of the omission of this step
remain to be found.