5.

4 Asexual Reproduction
KEY CONCEPT Many organisms reproduce by cell division.

MAIN IDEAS VOCABULARY

• Binary fission is similar in function asexual reproduction, p. 148
to mitosis. binary fission, p. 148
• Some eukaryotes reproduce
through mitosis.

Connect In this flashy world of ours, you may think that the humble bacterium
would have little chance of finding a mate. No dazzling smile, no fancy hair
products, no shiny car, and—if we are brutally honest—not even a brain. With
all of these limitatio\ns, it may seem that our bacteria friends would be destined
to die out. And yet, bacteria are found in abundance and live just about every-
where on Earth. How can there be so many bacteria?

MAIN IDEA
Binary fission is similar in function to mitosis.
Reproduction is a process that makes new organisms from one or more
parent organisms. It happens in two ways—sexually and asexually. Sexual
reproduction involves the joining of two specialized cells called gametes (eggs
and sperm cells), one from each of two parents. The offspring that result are
genetically unique; they have a mixture of genes from both parents. In con-
trast, asexual reproduction is the creation of offspring from a single parent
and does not involve the joining of gametes. The offspring that result are, for
the most part, genetically identical to each other and to the single parent.

Binary Fission and Mitosis

Connecting CONCEPTS Most prokaryotes reproduce through VISUAL VOCAB
Cell Structure Recall from binary fission. Binary fission (BY-nuh- Binary fission is the asexual reproduc-
Chapter 3 that many scientists ree FIHSH-uhn) is the asexual repro- tion of a single-celled organism by
hypothesize that mitochondria duction of a single-celled organism by division into two roughly equal parts.
and chloroplasts were originally
free-living prokaryotes. One division into two roughly equal parts.
parent cell
piece of evidence that supports Binary fission and mitosis have
this hypothesis is the fact similar results. That is, both processes
that these two organelles DNA duplicates
replicate much as bacteria
form two daughter cells that are
do, through fission. genetically identical to the parent cell.
cell begins to divide
However, the actual processes are
different in several important ways.
daughter cells
As you already learned, prokary-
otes such as bacteria do not have
nuclei. And although they do have DNA, they have much less of it than do
most eukaryotes. Also, most of a bacterium’s DNA is in the form of one
circular chromosome, and bacteria have no spindle fibers.

148 Unit 2: Cells

 FIGURE 5.13 Binary fission is
shown in this micrograph of three
2 individual bacteria, each at a
BIOLOGY different stage of binary fission.
See binary fission First, a cell elongates (1), and the
in action at DNA is replicated. Next, the cell
ClassZone.com. membrane pinches inward (2).
Finally, the membrane meets, and
a new cell wall is laid down, form-
3 ing two separate cells (3). (colored
TEM; magnification 60,000⫻)
1

Binary fission, shown in FIGURE 5.13, starts when the bacterial FIGURE 5.14 BACTERIA GROWTH
chromosome is copied. The two chromosomes are both attached to the
&'%%
cell membrane. As the cell grows and gets longer, the chromosomes
move away from each other. When the cell is about twice its original &%%%
size, it undergoes cytokinesis. The membrane pinches inward, and a

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new cell wall is laid down between the two chromosomes, which -%%
completes the separation into two daughter cells.
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Advantages and Disadvantages of Asexual Reproduction
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Very often, whether something is helpful or harmful depends on the
situation. In favorable environments that do not change much, asexual '%%
reproduction can be more efficient than sexual reproduction. Recall
that asexual reproduction results in genetically identical offspring. If %
& ' ( ) * + , - . &%
they are well suited to the environment, genetic variation could be 8nXaZhd[XZaaY^k^h^dc
more harmful than helpful. In other words, if it ain’t broke, don’t fix it.
One bacterium can result in a total
However, asexual reproduction may be a disadvantage in changing of 1024 cells after only 10 rounds of
conditions. Genetically identical offspring will respond to the environ- cell division.
ment in the same way. If population members lack traits that enable
them to reproduce, the entire population could die off. In contrast, sexual
reproduction increases genetic diversity, which raises the chance that at least a
few individuals will survive or even thrive in changing conditions.
Keep in mind, however, that the act of asexual reproduction itself is not
more efficient; rather, the associated costs of sexual reproduction are greater.
For example, all asexually reproducing organisms can potentially reproduce.
Suppose two organisms each have ten offspring. If one organism reproduces
Connecting CONCEPTS
Evolution As you will learn in
asexually, all ten offspring can have offspring of their own. If the other organ- Chapter 18, the misuse of anti-
ism reproduces sexually, having five females and five males, only the five biotics has resulted in multidrug-
females can bear offspring. In addition, sexually reproducing organisms must resistant bacteria. The bacteria
attract a mate. This effort involves not only the time and energy needed to not killed by antibiotics can
reproduce quickly, passing the
find a mate but also many structures, signals, and behaviors that have evolved genes for antibiotic resistance
to attract mates. Organisms that reproduce asexually do not have these costs. on to their offspring.
Summarize How is asexual reproduction an advantage in some conditions?

Chapter 5: Cell Growth and Division 149

 MAIN IDEA
Some eukaryotes reproduce through mitosis.
Some eukaryotes also reproduce asexually, through mitosis. Have you ever
FIGURE 5.15 Yeast and hydras grown a new plant from a stem cutting? Or seen a new sea star growing from
can reproduce by budding.
(hydra: LM, magnification 12; yeast:
the arm of another one? These new organisms are the result of mitotic repro-
colored SEM, magnification 3,200) duction and are therefore genetically the same as the parent organism. Mitotic
reproduction is especially common in simpler plants and animals. It occurs in
both multicellular and unicellular eukaryotes. It can take several forms,
including budding, fragmentation, and vegetative reproduction.
In budding, a small projection grows on the surface of the parent organ-
ism, forming a separate new individual. The new organism may live indepen-
dently or attached as part of a colony. For instance, hydras and some types of
Hydra yeast reproduce by budding. Examples are shown in FIGURE 5.15.
In fragmentation, a parent organism splits into pieces, each of which can
grow into a new organism. Flatworms and sea stars both reproduce by frag-
mentation. Many plants, including strawberries and potatoes, reproduce via
vegetative reproduction. In general, vegetative reproduction involves the
modification of a stem or underground structures of the parent organism.
The offspring often stay connected to the original organism, through struc-
tures called runners, for example.
Many organisms can reproduce both asexually and sexually. The form
of reproduction may depend on the current conditions. The sea anem-
one can reproduce in many ways. It can reproduce asexually by dividing
in half, by breaking off small pieces from its base, or by budding. It can
also reproduce sexually by making eggs and sperm. Some species of anemone
have males and females. In other anemone species, the same organism can
bud produce both eggs and sperm cells.
Synthesize How might the asexual reproduction of genetically identical plants
be useful to humans? How could it prove harmful to our food supply?
Yeast

ONLINE QUIZ
5.4 ASSESSMENT ClassZone.com

REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONCEPTS

1. Explain how mitosis differs from 3. Infer How does an organism benefit 5. Ecology Two populations live
binary fission. by being able to reproduce both in the same habitat and
2. Briefly explain why cutting a sexually and asexually? compete for food. The first
flatworm into pieces would not 4. Apply Yeasts are growing in two group is larger and uses
kill it. dishes. You treat one dish with a asexual reproduction; the
chemical that blocks DNA replica- second reproduces sexually.
tion but forget to label it. How can What could happen to cause
you identify the treated dish? the second group to outnum-
ber the first?

150 Unit 2: Cells