THE 100 MOST INFLUENTIAL INVENTORS OF ALL TIME

7 The 100 Most Influential Inventors of All Time 7

each cell progresses from one cell division to the next) at
the time of nuclear transfer determined the success or
failure of the experiment. They realized that the four
clones they had generated happened by chance.
In 1991 Wilmut hired English biologist Keith Campbell
(Smith had left the research centre in 1990), whose knowl-
edge of the cell cycle proved instrumental in advancing
the technique of nuclear transfer developed at Roslin.
Wilmut and Campbell’s first major success came in 1995,
with the generation of two cloned Welsh mountain sheep,
Megan and Morag. The following year Wilmut, Campbell,
and their team of scientists decided to test a new theory
based on the idea that the age or the stage of differentiation
of a donor cell did not matter in nuclear transfer. Prior to
this theory, nuclear transfer was believed to work only if
the nucleus used as the donor for nuclear transfer came
from a cell that was totipotent—i.e., having the ability
to differentiate into any type of cell in the body and
therefore possessing no characteristics of differentiation
itself. However, observations from laboratory experiments
and from Megan and Morag, who were produced using
nine-day-old embryonic cells, which are presumably less
totipotent than younger embryonic cells, indicated that
an enucleated host egg could somehow reverse the differ-
entiation of the donor cell nucleus, converting it back to
a state of totipotency or pluripotency (slightly more
differentiated than a totipotent cell). This led to the idea
of using the nucleus from an already differentiated adult
cell as a donor nucleus.

Dolly and Polly

During the winter of 1995– 96, Wilmut was involved in
three pivotal cloning experiments conducted at Roslin. In
the first, Wilmut and his team of scientists performed