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NEL Cell Division 567

is commonly called cloning. Many commercially important plant species, including

orchids, are now produced from clones. Unlike plants that arise from sexual reproduc-

tion, cloned plants are identical to their parents. This allows production of strains of

plants with predictable characteristics. Not all plant species can be cloned, however.

Carrots, ferns, tobacco, petunias, and lettuce respond well to cloning, but the grass and

legume families do not. Scientists continue to investigate these differences.

Each cell in the cloned plant contains the complete complement of chromosomes

from the parent. As the new plant develops, it undergoes mitosis to increase in size.

Some cells then specialize (differentiate) and form roots, stems, or leaves, until a com-

plete plant is formed.

Animal Cloning Technology

While plant cloning experiments were being conducted, Robert Briggs and Thomas King

were busy investigating nuclear transplants in frogs. Working with the common grass frog,

the scientists extracted the nucleus from an unfertilized egg cell by inserting a fine glass tube,

or micropipette, into the cytoplasm and sucking out the nucleus (Figure 4). A cell without

a nucleus is referred to as enucleated.

Section17.

enucleatedthe condition where a

cell does not contain a nucleus

Figure 4
A small glass tube, called a micropipette, is used to remove the nucleus from a cell and
later introduce a new nucleus.

Figure 5

Cloning a common grass frog using

embryo splitting

unfertilized egg

sperm

mitotic division

blastula stage:

mitosis has occurred

nucleus is removed

nucleus

transferred

cell with the

transplanted

nucleus begins

to divide by mitosis

blastula

enucleated cell

separated cells

adult frog

Next, the nucleus of a cell from a frog embryo in the blastula stage of development was

removed and inserted into the enucleated cell (Figure 5). The egg cell with the transplanted

nucleus began to divide much like any normal fertilized egg cell. In later trials, the cell

with the transplanted nucleus occasionally grew into an adult frog. The adult frogs dis-

played the characteristics from the transplanted nucleus. Careful analysis proved that

the adults were clones of the frog that donated the nucleus.

However, different results were obtained when the nucleus was taken from cells at later

stages of development. For example, the nucleus from cells in a later stage, called the gas-

trula stage, did not bring the enucleated egg from the single-cell stage to the adult. If

mitosis occurred at all, it did not progress as far as it did in eggs that received a blastula

nucleus. The difference is that the nucleus of a cell in the gastrula stage of development,

unlike a cell in the earlier blastula stage, has specialized. As cells begin to specialize, they

become less able to undergo mitosis.