Sheep Thrills
How Dolly
Was Designed
Cloning,
Nature, and Nurture
The Week/The
Clones
(Something slightly
overlooked in the frenzy of speculation about Dolly, the sheep cloned by
Scottish scientists last week, are the details of the experiment that resulted
in her creation. We asked Andrew Berry, a geneticist, to explain how Dolly came
into being, and why, from a biologist's point of view, she matters.)
First, we have to look at
what we mean by "cloning." We've been cloning genes for some 25 years now. In
other words, we can propagate small strips of DNA outside their parent
organisms. (We put a piece of, say, sheep DNA into a bacterial cell, which
reproduces itself asexually--doubling its genetic material and splitting--and
each time, it replicates our sheep gene.) We've been cloning cells for longer.
Certain cells can be "cultured," reproducing themselves like bacteria in a
petri dish. Both forms of cloning constitute major technological
accomplishments, but neither comes close to reconstituting an organism. That is
a much bigger problem.
Not an
insurmountable one. As every gardener knows, you can regenerate entire plants
from the smallest cutting. But animals are not so straightforward. The only
kind of whole-organism cloning we've been able to perform up till now has
involved DNA taken from early embryonic tissue. In the 1950s, biologists were
able to generate entire frogs from DNA that came from embryos less than 80
hours old. Any older, and no new frog developed. That is because cells in very
early tissue are what is known as undifferentiated--not yet committed to a
particular developmental path. What's revolutionary about Dolly is that the DNA
that created her was taken from a grown-up, differentiated cell.
until now, the conventional wisdom in biology has had it
that while early embryonic cells are "totipotent," capable of becoming any kind
of tissue, once cells have differentiated into a particular tissue type they
must remain that type. Pre-Dolly, differentiation in animal tissue was regarded
as an irreversible process. If we cloned muscle cells, we got more muscle
cells. Muscle cells were not going to generate liver cells.
How does
differentiation work? We don't really know. We do know that every cell contains
an identical and complete set of genetic instructions, the genome. As cells
mature, however, they switch on some parts of those instructions, and switch
off others. The cells in your muscle tissue switch on muscle genes. The cells
in your liver switch on liver genes. But we can't explain this process--why
some cells express some genes, and not others. The process of development (or
differentiation) is one of the least understood in biology. At one end of the
black box you put in a simple cell, the fertilized egg, and out of the other
you get a sheep, an earthworm, a human--a mature animal of staggering
structural complexity.
Dolly is living proof that the process can go
backward. Dolly is derived from a single mammary cell. Conventional wisdom
would have dictated that mammary cells, which are highly differentiated and
specialized, could produce only more mammary cells. But the mammary DNA that
gave rise to Dolly became dedifferentiated , capable of generating the
full range of different cell types that make up a sheep.
Given the
high-tech world of modern biology, the method by which the Scottish team
managed to provoke this extraordinary dedifferentiation is almost old-fashioned
in its simplicity. In Dolly's case, they took a mammary cell and grew it on a
petri dish in cell culture so that it produced more copies of itself. They then
essentially starved the cells so that they shut down normal metabolic function,
entering a "quiescent" state, in which dedifferentiation apparently occurs. The
DNA from one of these cells was then transferred into an unfertilized sheep egg
cell from which they had carefully removed the DNA. (Unfertilized eggs contain
only the mother's half of genes; by adding the genes from an adult mammary
cell--which has both its mother's and father's genes--the egg cell then had the
full amount of DNA found in adult sheep cells.) The egg cell was biochemically
pre-programmed to commence development, and that, apparently, was enough to
jerk the mammary-cell DNA out of its quiescent state. The egg was put into a
surrogate sheep mother and, 21 weeks later, we got Dolly.
One question that inevitably comes up is whether there is
something peculiar about the way sheep mammary tissues differentiate. A second
result in the Scottish study that has been largely ignored by the media--but
which is, in biological terms, equally important--suggests that the answer is
no. Biologist Ian Wilmut's team also cloned lambs out of DNA derived from
sheep-fetus fibroblasts, cells found in connective tissue. Even in a fetus, a
fibroblast is as highly specialized and fully differentiated as a mammary cell.
Dolly is more of a media magnet than her unnamed fibroblast-derived cousins
because she came from adult tissue. Biologically, though, the two experiments
are comparable: In both cases differentiated DNA became sufficiently
dedifferentiated to generate a whole new sheep. The result is, therefore, not
mammary-specific, but more general.
Dolly and her
fibroblast-derived cousins have changed forever the way we think about animal
development. Cells in mature animal tissues are not, as we had thought,
irreversibly differentiated. Understanding differentiation is the key to
understanding development, and Dolly embodies the extraordinary possibility of
manipulating the process--of doing experiments to identify the basic
construction rules used in putting animals together.
