NEL Cell Division 565
Applications of the Cell Cycle 17.217.
Scientists continue to study the cell cycle and to gain a deeper understanding of the
mechanisms and the role of the process. As more is learned about the cell cycle, we have
been able to apply this knowledge to many human needs. There are various perspec-
tives on the costs and benefits of these new technologies, and when they are appropriate
to use.
Cloning
Cloning is the process of forming identical offspring from a single cell or tissue in the
parent organ. A clone originates from a single parent cell, and both the clone and parent
have identical (or nearly identical) nuclear DNA. Although some clones show accidental
changes in genetic information, cloning does not result in the variation of traits that
would occur with the combination of male and female sex cells. Cloning is therefore con-
sidered a form of asexual reproduction. In fact, clones occur naturally. Some species, such
as hydra (Figure 1 (a)) reproduce by undergoing mitosis to produce buds with identical
DNA to the larger parent cell. The smaller plantlets on a runner of a strawberry plant
are identical clones of the larger parent plant (Figure 1 (b)). In animals, offspring with an
identical genetic makeup are sometimes produced when a single fertilized egg under-
goes mitosis and the resulting early embryo (called a zygote) then splits in two (Figure 1
(c)). This results in identical twins. They are also called monozygotic twins, since they
formed from a single zygote. Fraternal twins are formed when two different eggs are fer-
tilized separately. They are also known as dizygotic twins. Fraternal twins, therefore, are
no more genetically similar than are non-twin siblings (Figure 1 (d)).
Figure 1
(a)Hydra reproduce asexually by budding. The buds break off to form separate, genetically
identical organisms.
(b)The strawberry plant can reproduce asexually by forming genetically identical plantlets on
runners.
(c)Identical twins originate from a single fertilized egg that undergoes mitosis to produce an
early embryo which then splits into two, producing two genetically identical individuals.
(d)Development of fraternal twins does not involve the splitting of a fertilized egg. Instead,
fraternal twins develop from two independent fertilization events, such as occurs when a
mother has two eggs in her uterus that are fertilized by two different sperm cells. Each
fertilized egg then develops independently.
(a) (b) (c) (d)
Multiple Births
It has been estimated that 1 in 85
births will produce twins, 1 in 7500
will produce triplets, 1 in 650 000
will produce quadruplets, and 1 in
57 000 000 will produce
quintuplets.DID YOU KNOW??