A Reproduction ill
a s P ro tists, Fun 91i
Animals
_,c last section, you learned host important the cell cv elc is to hr man growth and
repair. Mitosis and cell division are important for another reason. They are the
^ for the asexual reproduction of many organ.i_smaas. Asexua l reproduction is the
t
I.
7iari on of a new individual that has identical genetic infori nation to its parent.
is section.
ou will explore the ways, in which bacteria, protists, fungi, and
aninmis reproduce asexually. Before on begin reading this section, refresh
knowledge of the live-Lingdorn classification system by studying Appendix A.
".^ xuat Reproduction in Bacteria
Icy ,hers of the kingdom Monera, such as bacteria shown if Figure 1.21, are 111.11organisers that do not contain a true nucleus. Bacteria reproduce asexually
P
t1,%,;.,rgh a process called binary fission. In this process, a parent cell divides so that
e= wk= new cell contains a single chromosome carrying a complete set of DNA iclen
tic.,,t ro that of the parent (see Figure 1.22 on page 330)
29
DidYouKnow?
Cells that lack a true
nucleus. such as bacteria. are known as
prokaryotes. All other
cells - those of animals, plants, fungi, and
protists - have a
nucleus and other membrane- bound organelles
and are known as
eukaryotes.
Pause
Reflect
Bacteria, such as
Clostridium botulinum,
can cause a serious form
of food poisoning. Other
bacteria, such as
Lactobacillus acidophilus,
help digest food and
destroy other harmful
bacteria in your intestinal
tract. In Your Science
L og, write a one-page
essay explaining why
bacteria can be both
harmful and beneficial to
humans. Search the
Internet for information
about "useful" bacteria,
such as those that are
used to break down
toxins in the environment.
swfr•en&A
C:cclr
aril 1^exc^.31 Repra^fscte ii
A. The cell prepares for replication. Do you see
where the cell wall has ruptured?
B. The cell makes a copy of its single
chromosome. Do you see the new
membrane that has formed? Why do
you think it is needed?
C. The original chromosome and
ts copy soon separate as the cell
i
grows larger, each moving to
opposite ends of the cell.
D. The cell membrane begins to
pinch inward near the Middle
of the cell, creating two
smaller parts, each with a
single chromosome carrying
identical genetic information.
E. A new cell wall forms
around each of the two
new cells.
Figure 1.22 Binary fission is one method of asexual reproduction used by bacteria. Under ideal conditions, the
entire process takes about 20 minutes. Why might such a short cell cycle be an advantage?
Suppose you get on a bus to go to school. At 8:20 A.M., you yawn, and a single Streptococcus
bacterium drifts into your open mouth and settles in your throat. Under ideal conditions, a
single bacterium can reproduce in 20 min. So, by the time the bus drops you off at school at
[ 8:40 A.M., there are two bacteria in your throat. When your homeroom period starts at 9:00 A.M.,
there are four.
1. Assume that the bacteria continue to reproduce asexually every 20 min. How many bacteria will
be in your throat by: (a) lunchtime (12.00 noon); (b) by the time you get the bus in the afternoon (3:20 P..); (c) suppertime (6:00 P.M.); (d) bedtime (10:00 P.M.)?
2. Make a graph showing the growth of the Streptococcus population in your throat.
3. Do you think the bacteria can keep multiplying in this way indefinitely?Explain.
4. At what time do you think your throat may feel sore? How many bacteria will be present in your
throat?
30 1 he flower of Reproduction
Asexual Reproduction in Protists
Unicellular organisms in the kingdom Protista, such as the amoeba, also rely
ramainly, on asexual reproduction to increase their numbers. Unlike bacteria,
the DNA of protists is contained within a true nucleus. Mitotic cell division
in these unicellular organisms results in the formation of two identical offspring. Protists are important because they form the basis of many food
chains and because they are responsible for many diseases that affect humans.
Figure 1 .23 Entanoeba
histcytrca is a famous amceba
that has its own disease named
after it-amoebic dysentery.
This disease causes diarrhea,
fever, and abdominal cramps.
Find out how E. htstoiytica is
M
transmitted.
Figure 1 .24 This simple sketch shows mitosis and cell division in an amoeba. How does this process differ from
binary fission in bacteria?
Asexual Reproduction in Fungi
Figure 1.25 This fungus grows by obtaining nutrients
From a. ne
Moulds, yeast, and mushrooms are members of the Fungi kingdom. The bodies of these organisms are composed of many thin
filaments called hyphae. The hyphae grow over the surface of
and into the bodies of other organisms to obtain food (see
Figure 1.25). Three methods that fungi use to reproduce asexually are fragmentation, budding, and spores.
Fungi can reproduce asexually by fragmen tation . A small piece,
or fragment, breaks away from the main mass of hyphae and grows
into a new individual. What must the fragment contain to enable it
to develop into a new organism identical to its parent?
ch
The word "hyphae"is
descended from the
Greek word "hyphos,
meaning web. (The singular form of hyphae is
hypha.) Why is hyphae a
good word to describe
the filaments that make
up the body of a
fungus?
Figure 1.26 Most fungi feed on dead and decaying organisms. How is this of benefit to the environment?
32 The Power of Reproduction
€re unicellular fungi often used to €nake bread or alcohol
As. When conditions are favourable for grov.%t:h, least
ce asexually by budding. First, a copy of the
is is n jade. Can you explain why is this .ne cessary?
a tiny bud begins to form on the cell wall. This
draining the new nucleus, continues to grow
It eventually breaks away to become a single,
.'Indent cell. Look carefully at the micrograph in
What are the small circles at the lower left
parent cell=
eproduce a sexually; moulds, such as Rhhizopu.v, produce
_^ spore is a reproductive cell that can grow into a new
idual through mitotic cell division.
cs are stored in a case called a .cpuWhen spores are mature, they
}n a characteristic colour, such as
r17n.
^, ^ :. = llow, blue, or red. Think of
so,,e places where you have seen mould
rc ' _ In ly. \'4 is the place moist or dry?
eorcold? You can test which conc i,3^r tavour mould growth in the next
When each sporangium
splits open, it releases
spores that float about
on air currents.
Airborne spores have
been found in the
atmosphere at altitudes
of more than 160 km.
Lli^
Figure 1 .28 This bread is covered by the common
mould Rhrzopos.
sporangium
of bread
Ot bread
pure 1.29 SbC aI qu cro
r. har':i. rs or qc e
[he tips of hvphae that h ave weal
!,d rcc
,C
..i
3_-toot'c ce' d ivas 0cn.
broad
p'r 0 `.h„ sir ace Lf
''.`hy `1o
yo u
±h 1
r
eawcrl .,Fa t
Is
c
apao
H
f
rill=ca G
i`h,^ (Xi!
t Cte :end
A.
yu:aI
Reproduc-Crnl
33
Science Igyr
Making.. a..
Mould
include fps to disuse of mat als properly
et the end of the experirrre t. I+Iow F tips
turn to
on "how experiments are desrgn
pagelS "
Like all living organisms, bread mould mires
suitable co ►ditions in which to reproduce." You
know that mould u ill grow on bread but" dries
any kind of bread provide the right food
source. Is there a difference E t Teen hi e
made breadand store-bught. ad` Wha#
other condtions must be present for bread.
mould to grow
Mat TO C €a
When you hays received approval from you
teacher, cone your experiment
What Dirt ;You t10CPv
►`
Propose a l yp thesis about the growth cif
bread mould that you would like to explore
your by th Pis;
In your design proposal, be sure to include
the materials you '
your Fr ate
and the steps you will follow tcr" € nd your
research; You should include the use to a Io
parer rriicroscope oriss on :ape #o
examine the bread for results Be sure to
2. Design AA .. "exp r rrrent to #
I Describe your results li ch its ding
far photographs ^e brief'' descriptions to
cco any your it ultra#rons
id your result sul
h or', v^y nab
your hyp a#hesis'
3. That addition al research or experiment
could ou carry out, based can your results9
4.' Suggest howthe spread of harmful moulds
could tae con rolled,
Asexual Reproduction i n Animals
In the kingdom Animalia, animals can be divided into two main groups vertebrates (those with backbones) and invertebrates (those without backbones).
Invertebrates, such as sponges, jellyfish, worms, shellfish, and insects, make up
about 97% of all animal species. Many invertebrates can reproduce asexually to
form one or more identical offspring from a single parent.
Figure 1.30 An injury to the
head of this planarian divided
its head into two sections. Two
complete heads are forming as
a result of regeneration.
Planaria are a type of flatworm that can reproduce
asexually by dividing in two and
regenerating the parts they are
missing. The part of the planarian that retains the head will
grow a new tail. The part with
the tail will grow a new head.
What happens in the process of
dividing that enables the two
new planaria to grow the body
parts they need? Planaria can
also regenerate injured body
parts, as shown in Figure 1.30.
34 The Power of Reproduction
Figure 1 .31 Sea stars reproduce asexually and can grow new
body parts through the process of regeneration.
animals, such as sponges and hydras, reproduce asexually by budding. A cell,
near the base of these organisms, undergoes mitosis and cell division repeatproduce a group of new cells, or a bud. Eventually, when the bud completes
cvelopnaent, it detaches and becomes independent.
1.32 Sponge buds may remain attached to the
shich resuit in a colony, What aavantage does the
by being attached to its parent?
Figure 1.33 Hydras are very smali organisms that live
in water. Based on what you have learned about
budding, describe what is happening in this picture,
K Your Understanding
s•Vhat is binary fission How does binary fission differ from rnitotic cell division?
Describe three ways fungi can reproduce asexually.
# Describe how an animal reproduces by budding.
y'il VNIat is one advantage of this type of reproduction?
-ply Explain why° covering and refrigerating food helps to reduce spoilage
from moulds.
Thinking Critically One arm of a sea star with. part of the central disk
attached can grow into a complete sea star. Sea stars are considered a nuisance
by people who raise oysters. A sea star can attach itself to an oyster's shell,
open it, and eat the contents. In the past, oyster fanners tried to destroy the
sea stars by cutting them into pieces and throwing there back into the ocean.
V 'hat do you think was the outcome of this action?
Thinking Critically Some invertebrates, such as crabs, have the ability to
regrow body parts that have been lost through injury. What cell part do you
think scientists study to learn more about the process of regeneration in these
animals=
Thy C=e?t Cyele ;nicl .^s^s^L1l R^prr3cluccicsr
35
1.5 Asexual Reproduction in Plants
The trees, bushes, and lawns you pass on your way to school are such a
familiar part of your landscape that you probably pay little attention to
them. The only time you night notice them is when a tree limb is
broken off or a bush has been pruned, or a lawn mowed. ,after a few
weeks, you can see the branches and twigs growing hack, and the grass
needs cutting again. ".I'he ability of plants to reproduce asexually and to
repair themselves is, as in all other kingdoms, the result of mitotic cell
division. Asexual reproduction in plants ensures that each new plant
will have the same DNA as its parent.
eristem: The Basis of Asexual Reproduction in Plants
Figure 1.34 It seems hard to believe that this
uprooted tree could survive, but this willow is able
to send up new growth Where do you think
mitosis and cell division are occurring here?
Unlike many animals, plants continue to grow throughout their lives.
The tips of their roots and stems contain growing areas called
eristem. Meristem is made tip of unspecialized cells that undergo
mitosis and cell division repeatedly, producing new cells. At a certain
point in a plant's growth, meristernatic cells specialize into the cells
that make up the roots, stems, and leaves of the plant. Once these
plant structures mature, their cells do not divide under normal condiLions. If any of these structures becomes damaged, meristematic cells
activate and repairs are made. Asexual reproduction can occur in plants
by activating the rneristematic cells in different plant structures.
For centuries, plant growers have been using their knowledge of the
ability of plants to reproduce asexually. One of the simplest methods of propagating,
or increasing the number of, a plant is through the use of cuttings. Cuttings are
lengths of a "parent" stein that are used to establish offspring that are exact copies of
the parent. This is called cloning, the process by which identical offspring are produced from a single cell or tissue. In the next investigation, you will clone a plant
using a cutting.
Figure 1 .35 A micrograph of the lip of a growing plant root Where
36 The Power of Reproduction
are
the meristematic cells?
Pause
-Reflect
Think back to Investigation
1-C on page 20. Study
the observations of cell
division in an onion root
cell that you made. In
your Science Log, write a
paragraph connecting
what you saw occurring
in the tips of the root
with what you now know
about meristematic cells.
Surveying Asexual Reproduction in Plants
Not all plants reproduce asexually in the same way. Asexual reproduction in the
plant world has several different forms. As our knowledge has improved, so has our
ability to use these natural processes in growing plants for commercial and nonconamercial purposes. As you read through this section, make sketches in your
notebook of the different ways in which new plants can be produced from the same
parent.
New Plants from Roots
Have you ever pulled dandelions out of a lawn?
" Vhat happens if you don't remove the entire root
from the ground? Why does this occur? In some
plants, meristern cells in the roots divide mitotically
to produce stems, leaves, and other roots. This is
what happens in dandelions and makes them so difficult to get rid of.
Because of the reliability of some plants to
reproduce asexually from their roots (asparagus, for
example), growers can sell just the roots, instead of
a whole plant. Can you think of any advantages to
developing and selling plants in this way? Are there
any disadvantages?
Figure 1.36 Asparagus plants die back
to the ground in the winter. In the spring,
new plants regrow from their roots.
What do you see
when you look at
this photograph? A forest, right? What
you actually see is a forest of one - a
single male aspen in the Wasatch
Mountains of Utah. Researchers estimate that this plant has been growing
for tens of thousands of years. The
continuous asexual reproduction of
this aspen has produced a root system
that connects 47 000 stems and occupies 43 ha. This plant could live forever as long as environmental conditions continue to favour asexual reproduction. What environmental threats
might this magnificent giant face in the future?
New Plants from Stems
In some plants, meristematic cells in the stern can divide to produce cells that will
become a new plant. Strawberries have special sterns called runners. New strawberry plants grow from the tips of these runners, and the new strawberry plants
can be removed and planted.
38 The Power of Reproduction
(,; owea s have developed a technique called lay-er•ing, which uses the ability of some
I la, ry to reproduce more easily from stems than from other plant parts (see Fi rre
Blackberry. raspbcrry, and rose bushes can be grow 3 from a parent plant
rirdais technique. A branch of the parent plant is bent down to the ground and
ri it is covered with soil. Roots will grow fronn the buried stern and the exposed
^srow into a new shoot. This new plant can be cut away and replanted.
Pause &
Reflect
Based on what you have
learned in this section.
=Trite a brief description in
your Science Log of how
a plant might be repro duced asexually from its
leaves. Research the
African violet and make a
sketch of a technique that
could be used to grow
several African violets
asexually from a leaf.
DidY0u Cinow?
The potatoes we eat
are actually part of an
underground stem
called a tuber. The
eyes of the potato are
the buds of a new
plant. The bulbs of
.re 1.37 A new plant is produced by layering. What advantage does the offspring plant have by
Sig attached to its parent while it develops?
daffodils and tulips
are other kinds of
underground stems.
is another technique plant growers have developed to use the ability of stems
to reproduce asexually. Stems from plants with desirable qualities can be attached, or
µr: feed, to the rooted stock of related or similar plants. This technique is commonly
raa:d with apples, grapes, and roses. For example, 'McIntosh apples are produced
rhrough grafting. Figure 1.38 shows two methods of grafting.
(.i;"'=fting
A
These plants repro duce asexually by
growing new shoots
from underground
stems,
B
stock
/ s cion
rooted stock
with T-shaped
slit in bark
rooted stock
Figure 1.38 `=fiat is ti
em Lei g hralfed cr.lc the ruo.ed stcc'k in A cal cd `Nh e-
ha cart in B?
The Cell C:ycic tid .Asexu l Repn; duetion
39