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Both sexual and asexual
BEFORE, you learned
NOW, you will learn
• Cells go through a cycle of
growth and division
• Mitosis produces two genetically identical cells
• About cell division and
• How sexual reproduction and
asexual reproduction compare
binary fission p. 89
regeneration p. 90
How does cell division
In multicellular organisms, cell
division functions in growth, repair,
and development. But in unicellular
organisms, each cell is itself an organism.
Unicellular organisms, like this paramecium, also undergo
cell division. What are some possible results of cell division
in unicellular organisms? How might they compare with
the results of cell division in multicellular organisms?
Asexual reproduction involves one parent.
Begin taking notes on
the main idea: Asexual
reproduction involves one
parent. Be sure to include
sketches of each method of
Mitosis and cytokinesis are the processes by which cells divide.
In multicellular organisms, the daughter cells that arise from these
processes are separate cells but do not live independent lives.
For example, new skin cells are part of skin tissue and cannot live
independently. In multicellular organisms, mitosis and cytokinesis
are not considered methods by which an organism reproduces.
Most unicellular organisms, and a few multicellular organisms,
use cell division to reproduce, in a process called asexual reproduction.
In asexual reproduction, one organism produces one or more new
organisms that are identical to itself and that live independently of
it. The organism that produces the new organism or organisms is the
parent. Each new organism is an offspring. The offspring produced
by asexual reproduction are genetically identical to their parents.
88 Unit: Cells and Heredity
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Cell Division in Unicellular Organisms
Cell division and reproduction are the same thing in all
single-celled organisms. However, the process of cell division
in prokaryotes and in single-celled eukaryotes differs.
is the form of asexual reproduction
occurring in prokaryotes. Binary fission occurs when the
parent organism splits in two, producing two completely
independent daughter cells. Genetically, the daughter
cells are exactly like the parent cell. Since all prokaryotic
organisms are single-celled, cell division and reproduction
by binary fission are the same process for them.
In single-celled eukaryotic organisms, however, reproduction by
cell division involves mitosis and cytokinesis. The unicellular organism undergoes mitosis, duplicating and separating its chromosomes.
Then its cytoplasm is divided through cytokinesis. The result is two
separate, independent, and genetically identical offspring. Examples
of single-celled eukaryotic organisms that reproduce by cell division
include algae, some yeasts, and protozoans, such as paramecium.
Both unicellular and multicellular organisms can reproduce
by budding. Budding is a process in which an organism
develops tiny buds on its body. Each bud forms from the
parents’ cells, so the bud’s genetic material is the same as
the parents’. The bud grows until it forms a complete or
nearly complete new organism that is genetically identical
to the parent.
In some budding organisms, buds can form from any
part of the body. In other organisms, buds can be produced
only by specialized cells in particular parts of the body. A new
organism produced by budding may remain attached to its
parent. Most often, when a bud reaches a certain size, it breaks
free of the parent and becomes a separate, independent organism.
Some yeast and single-celled organisms reproduce asexually by
budding. But budding is most notable in multicellular organisms.
Hydras are freshwater animals that are famous for reproducing by
budding. Among plants, the kalanchoe (KAL-uhn-KOH-ee) produces
tiny buds from the tips of its leaves. Each kalanchoe bud that lands on
a suitable growing surface will develop into a mature kalanchoe plant
that is genetically identical to the parent plant.
Check Your Reading
Binary fission results
in two nearly equal, independent cells, as shown
in these bacteria.
Budding Hydras reproduce by pinching off
How is budding different in unicellular and
Chapter 3: Cell Division 89
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This starfish is
legs that were lost.
In certain multicellular organisms, specialized cells at the site of a
wound or lost limb are able to become different types of tissues.
The process of new tissue growth at these sites is called regeneration.
Although one function of regeneration is the
regrowth of damaged or missing body parts, in some
organisms asexual reproduction is another function
Regeneration can be observed in many animals
called starfish. If a starfish is cut in half, each half can
regenerate its missing body parts from its own cells.
The result is two complete, independent, and genetically identical starfish. Sometimes a starfish will drop
off one of its limbs. The animal will eventually form
a new limb. In these cases, regeneration is considered
a form of asexual reproduction.
The growth of plants from cuttings is also a kind
of asexual reproduction through regeneration. Cells
near a cut made in a plant’s stem begin to produce
the missing part of the plant. Once the missing part
is grown, the cutting can be planted in soil. The cutting will grow into
a new, independent plant that is genetically identical to the plant from
which the cutting was taken.
Check Your Reading
Learn more about
Describe the process of regeneration in starfish.
Asexual Reproduction and Health.
You have probably had the following experience. In the morning you
feel fine. By afternoon, you have a strange feeling that something is
not quite right, but you are well enough to function normally. You
may even continue to feel well at dinner, and you eat heartily. Then,
later that evening, it hits you. You’re sick. That tickle in your throat
has become a sore throat requiring a visit to the doctor and antibiotics. How did you get so sick so fast?
You could have picked up bacteria in school that morning. Perhaps
another student coughed, spreading the bacteria that causes strep
throat. A population of bacteria, like populations of other organisms
that reproduce asexually through binary fission, increases in number
geometrically. Two cells become 4, which become 8, which become 16,
and so on.
90 Unit: Cells and Heredity
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The reason you get sick so fast is
that for many bacteria the generation
time is very short. Generation time is
the time it takes for one generation to
produce offspring—the next generation. In fact, some types of bacteria
can produce a new generation of cells
in less than 30 minutes. In about an
hour the number of bacteria can
increase to four times the starting number.
Although all offspring are genetically identical, the rare genetic
random change does occasionally occur during cell division. The
rapid reproduction rate makes it more likely that some offspring will
have a random genetic change that may be beneficial.
These bacteria are quickly
multiplying through asexual reproduction.
Which parts of plants can reproduce?
Some organisms can regenerate offspring from any part of their body.
Others can regenerate offspring from only one specialized body part.
In this activity, you will discover if a houseplant regenerates from various parts.
Obtain a plant part (leaf, stem, stem with leaf, or root) from your teacher.
Also get one flowerpot filled with potting soil.
2 Dip the plant part in water and set it into the soil, about 1 in. deep,
but make sure that most of the plant part is above the level of the soil.
Water the soil lightly.
3 Place all the class’s pots on the same window sill.
Observe your plant part every day for two to three
weeks. Record your observations.
WHAT DO YOU THINK?
• Which plant parts, if any, were able to regenerate
a new plant?
• What can you conclude about the ability of different
plant parts to grow into new plants?
CHALLENGE How does the plant in the experiment compare
with the kalanchoe plant you read about in the text? What
accounts for plants’ different abilities to produce viable offspring?
Chapter 3: Cell Division 91
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Sexual reproduction involves two
Reproduction of multicellular organisms often involves sexual
reproduction as well as asexual cell division. The table shows
some differences between asexual and sexual reproduction.
Comparing Asexual and Sexual Reproduction
Cell division and other processes
One parent organism
Two parent organisms
Rate of reproduction is rapid
Rate of reproduction is slower than rate for
Offspring identical to parents
Offspring have genetic information from two
If you grow a plant from a cutting, the new plant will be identical
to the parent. However, plants that grow from seeds contain genetic
material from two parents. Plants growing from seeds and animals
growing from eggs are examples of organisms that reproduce through
Cell division is part of both sexual and asexual reproduction.
The process of mitosis produces cells identical to the parent cells.
The diversity of life on Earth is possible because of the combining
of genetic materials from two parents in sexual reproduction.
In the next chapter, you will read about cell processes involved in
Check Your Reading
1. How does binary fission relate
to cell division?
4. Predict Do you think prokaryotes undergo regeneration?
Why or why not?
2. What is a bud, and where
does it form on an organism
that reproduces asexually?
3. Compare sexual and asexual
List two major differences between asexual and sexual
92 Unit: Cells and Heredity
5. Compare and Contrast
How is the process of binary
fission similar and different in
prokaryotic and eukaryotic
6. Synthesize Some bacteria
can exchange genetic material
with one another through a
process called conjugation.
How do you think offspring
produced during conjugation
would compare with offspring
produced in cell division?