A Reproduction illa s Protists, Fun i91

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 7iari on of a new individual that has identical genetic infori nation to its parent.

I . 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 111.11-

P

organisers that do not contain a true nucleus. Bacteria reproduce asexuallyt1,%,;.,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)

DidYouKnow?Cells that lack a truenucleus. such as bacte-ria. are known asprokaryotes. All othercells - those of ani-mals, plants, fungi, andprotists - have anucleus and other mem-brane-bound organellesand are known aseukaryotes.

PauseReflect

Bacteria, such asClostridium botulinum,

can cause a serious formof food poisoning. Otherbacteria, such asLactobacillus acidophilus,

help digest food anddestroy other harmful

harmful and beneficial to

bacteria in your intestinaltract. In Your ScienceL og, write a one-page

essay explaining whybacteria can be both

humans. Search theInternet for informationabout "useful" bacteria,such as those that areused to break downtoxins in the environment.

swfr•en&A

C:cclr aril 1^exc^.31 Repra^fscte ii29

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

its copy soon separate as the cellgrows 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, theentire 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 after-

noon (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 ProtistsUnicellular 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 divisionin these unicellular organisms results in the formation of two identical off-spring. 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

transmitted.

M

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 FungiMoulds, yeast, and mushrooms are members of the Fungi king-dom. The bodies of these organisms are composed of many thinfilaments called hyphae. The hyphae grow over the surface ofand into the bodies of other organisms to obtain food (seeFigure 1.25). Three methods that fungi use to reproduce asexu-ally 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 growsinto a new individual. What must the fragment contain to enable itto develop into a new organism identical to its parent?

Figure 1.25 This fungus grows by obtaining nutrientsFrom a. ne ch

The word "hyphae"is

descended from the

Greek word "hyphos,

meaning web. (The sin-

gular 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

.'Indent cell. Look carefully at the micrograph in

What are the small circles at the lower left

parent cell=

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,

r17n. When spores are mature, they

}n a characteristic colour, such as

idual through mitotic cell division.

cs are stored in a case called a .cpu-

€re unicellular fungi often used to €nake bread or alcohol

As. When conditions are favourable for grov.%t:h, least

eproduce a sexually; moulds, such as Rhhizopu.v, produce

_^ spore is a reproductive cell that can grow into a new

ce asexually by budding. First, a copy of the

^, ^ :. = 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

Lli^Figure 1 .28 This bread is covered by the common

mould Rhrzopos.

sporangium

When each sporangiumsplits open, it releasesspores that float about

on air currents.Airborne spores havebeen found in theatmosphere at altitudesof more than 160 km.

of bread

Ot bread

pure 1.29 SbC aI qu cro

[he tips of hvphae that h ave weal p'r 0 `.h„ sir ace Lf

broad

!,d r. har':i. rs or qc e,Crcc ..i 3_-toot'c ce' d ivas 0cn. ''.`hy `1o yo u ±h 1 eawcrl .,Fa r t Is c apao

rill=ca G

i`h,^ (Xi! t Cte :end A. yu:aI Reproduc-Crnl 33

H

f

Making.. a.. MouldLike all living organisms, bread mould mires

suitable co ►ditions in which to reproduce." You

know that mould u ill grow on bread but" driesany kind of bread provide the right foodsource. Is there a difference E t Teen himade breadand store-bught. ad` Wha#other condtions must be present for bread.mould to grow

Mat TO C €a ►̀

Propose a l yp thesis about the growth cifbread mould that you would like to explore

2. Design AA .. "exp r rrrent to # your by th Pis;In your design proposal, be sure to includeyour Fr ate the materials you 'and the steps you will follow tcr" € nd yourresearch; You should include the use to a Io

parer rriicroscope oriss on :ape #oexamine the bread for results Be sure to

Science Igyr

include fps to disuse of mat als properly.

tipset the end of the experirrre t I+Iow Fon "how experiments are desrgn turn topagelS "

When you hays received approval from youteacher, cone your experiment

What Dirt ;You t10CPv

I Describe your results li ch its dingfar photographs ^e brief'' descriptions tocco any your it ultra#rons

id your result sul

your hyp a#hesis'h or', v^y nab

3. That additional research or experiment

could ou carry out, based can your results9

4.' Suggest howthe spread of harmful mouldscould tae con rolled,

e

Figure 1.30 An injury to thehead of this planarian dividedits head into two sections. Twocomplete heads are forming asa result of regeneration.

Asexual Reproduction in 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.

Planaria are a type of flat-

worm that can reproduce

asexually by dividing in two and

regenerating the parts they are

missing. The part of the pla-

narian 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.

Figure 1 .31 Sea stars reproduce asexually and can grow new

body parts through the process of regeneration.

34 The Power of Reproduction

animals, such as sponges and hydras, reproduce asexually by budding. A cell,

near the base of these organisms, undergoes mitosis and cell division repeat-

produce 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 theshich 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 PlantsThe 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 fewweeks, 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 celldivision. Asexual reproduction in plants ensures that each new plant

will have the same DNA as its parent.

eristem: The Basis of Asexual Reproduction in PlantsUnlike 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 certainpoint in a plant's growth, meristernatic cells specialize into the cells

that make up the roots, stems, and leaves of the plant. Once theseplant structures mature, their cells do not divide under normal condi-

Figure 1.34 It seems hard to believe that this

Lions. If any of these structures becomes damaged, meristematic cellsuprooted tree could survive, but this willow is ableto send up new growth Where do you think

activate and repairs are made. Asexual reproduction can occur in plants

mitosis and cell division are occurring here?

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 pro-duced 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 are the meristematic cells?

36 The Power of Reproduction

Pause-Reflect

Think back to Investigation1-C on page 20. Study

the observations of celldivision in an onion rootcell that you made. Inyour Science Log, write aparagraph connectingwhat you saw occurringin the tips of the rootwith what you now knowabout meristematic cells.

Surveying Asexual Reproduction in PlantsNot 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 ourability to use these natural processes in growing plants for commercial and non-

conamercial purposes. As you read through this section, make sketches in yournotebook of the different ways in which new plants can be produced from the same

parent.

New Plants from RootsHave 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 iswhat happens in dandelions and makes them so dif-

ficult to get rid of.Because of the reliability of some plants to

reproduce asexually from their roots (asparagus, forexample), growers can sell just the roots, instead of

a whole plant. Can you think of any advantages todeveloping and selling plants in this way? Are there

any disadvantages?

Figure 1.36 Asparagus plants die backto the ground in the winter. In the spring,new plants regrow from their roots.

this photograph? A forest, right? Whatyou actually see is a forest of one - asingle male aspen in the WasatchMountains of Utah. Researchers esti-mate that this plant has been growingfor tens of thousands of years. Thecontinuous asexual reproduction ofthis aspen has produced a root system

What do you seewhen you look at

that connects 47 000 stems and occupies 43 ha. This plant could live forever as long as envi-ronmental conditions continue to favour asexual reproduction. What environmental threatsmight this magnificent giant face in the future?

New Plants from StemsIn 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 straw-berry 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 rreBlackberry. 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.

.re 1.37 A new plant is produced by layering. What advantage does the offspring plant have bySig attached to its parent while it develops?

(.i;"'=fting 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 producedrhrough grafting. Figure 1.38 shows two methods of grafting.

A B

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

daffodils and tulips

are other kinds of

underground stems.

These plants repro -

duce asexually by

growing new shoots

from underground

stems,

stock

/ scion

rooted stock

rooted stockwith T-shapedslit in bark

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