Cambridge International A Level Biology Revision Guide

Chapter 5: The mitotic cell cycle
About 60–70% of the spindle microtubules that extend
from the centrosome are not attached to kinetochores, but
have free ends. Some of these are microtubules that have
not found kinetochores to attach to, since this is a random
process. Some may be involved in chromatid movement in
ways other than that described.
Biological significance of mitosis
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Growth. The two daughter cells formed have the
same number of chromosomes as the parent cell and
are genetically identical (clones). This allows growth
of multicellular organisms from unicellular zygotes.
Growth may occur over the entire body, as in animals,
or be confined to certain regions, as in the meristems
(growing points) of plants.
Replacement of cells and repair of tissues. This is
possible using mitosis followed by cell division. Cells
are constantly dying and being replaced by identical
cells. In the human body, for example, cell replacement
is particularly rapid in the skin and in the lining of the
gut. Some animals are able to regenerate whole parts
of the body; for example, starfish can regenerate
new arms.
Asexual reproduction. Mitosis is the basis of asexual
reproduction, the production of new individuals of
a species by a single parent organism. The offspring
are genetically identical to the parents. Asexual
reproduction can take many forms. For a unicellular
organism such as Amoeba, cell division inevitably
results in reproduction. For multicellular organisms,
new individuals may be produced which bud off from
the parent in various ways (Figure 5.11). Budding is
particularly common in plants; it is most commonly a
form of vegetative propagation in which a bud on part
of the stem simply grows a new plant. The new plant
eventually becomes detached from the parent and lives
independently. The bud may be part of the stem of an
overwintering structure such as a bulb or tuber. The
ability to generate whole organisms from single cells or
small groups of cells is important in biotechnology
and genetic engineering, and is the basis of cloning.
Immune response. The cloning of B- and
T-lymphocytes during the immune response is
dependent on mitosis (Chapter 11).
a
b
Figure 5.11 a Asexual reproduction by budding in Hydra
(× 60). Hydra lives in fresh water, catching its prey with the aid
of its tentacles. The bud growing from its side is genetically
identical to the parent and will eventually break free and live
independently. b Asexual reproduction in Kalanchoe pinnata.
The plant produces genetically identical new individuals along
the edges of its leaves.
QUESTION
5.2 In the mitotic cell cycle of a human cell:
a how many chromatids are present as the cell
enters mitosis?
b how many DNA molecules are present?
c how many kinetochores are present?
d how many chromatids are present in the nucleus
of each daughter cell after mitosis and cell
division?
e how many chromatids are present in the nucleus
of a cell after replication of DNA?
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