Centromere

Sister chromatids

� Figure 10–3 This is a human chromosome shown as itappears through an electron microscope. Each chromosome hastwo sister chromatids attached at the centromere. Inferring Whyis it important that the sister chromatids are identical?

Key Concepts• What are the main events of

the cell cycle?• What are the four phases of

mitosis?

Vocabularymitosiscytokinesischromatidcentromereinterphasecell cycleprophasecentriolespindlemetaphaseanaphasetelophase

Reading Strategy:Outlining As you read thissection, outline the major eventsof the cell cycle. Write a fewsentences to describe theactivity of chromosomes as theyprogress through each part ofthe cell cycle.

(magnification: 20,000�)

What do you think would happen if a cell were simply to

split into two, without any advance preparation? Would

each daughter cell have everything it needed to survive?

Because each cell has only one set of genetic information, the

answer is no. Every cell must first copy its genetic information

before cell division begins. Each daughter cell then gets a

complete copy of that information.

In most prokaryotes, the rest of the process of cell division is

a simple matter of separating the contents of the cell into two

parts. In eukaryotes, cell division is more complex and occurs in

two main stages. The first stage, division of the cell nucleus, is

called (my-TOH-sis). The second stage, division of the

cytoplasm, is called (sy-toh-kih-NEE-sis).

Many organisms, especially unicellular ones, reproduce by

means of mitosis and cytokinesis. Reproduction by mitosis is

classified as asexual, since the cells produced by mitosis are

genetically identical to the parent cell. Mitosis is also the source

of new cells when a multicellular organism grows and develops.

In humans, for example, mitosis begins shortly after the egg is

fertilized, producing the vast numbers of cells needed for the

embryo to take form.

ChromosomesIn eukaryotic cells, the genetic information that is passed on from

one generation of cells to the next is carried by chromosomes.

Chromosomes are made up of DNA—which carries the cell’s coded

genetic information—and proteins. The cells of every organism

have a specific number of chromosomes. The cells of fruit flies, for

example, have 8 chromosomes; human cells have 46 chromosomes;

and carrot cells have 18 chromosomes.

Chromosomes are not visible in most cells except during cell

division. This is because the DNA and protein molecules that

make up the chromosomes are spread throughout the nucleus. At

the beginning of cell division, however, the chromosomes condense

into compact, visible structures that can be seen through a light

microscope.

Well before cell division, each chromosome is replicated, or

copied. Because of this, each chromosome consists of two identical

“sister” (KROH-muh-tidz), as shown in Figure 10–3.When the cell divides, the “sister” chromatids separate from each

other. One chromatid goes to each of the two new cells.

chromatids

cytokinesismitosis

10–2 Cell Division

244 Chapter 10

Section 10–2

1 FOCUSObjectives10.2.1 Name the main events of

the cell cycle.10.2.2 Describe what happens dur-

ing the four phases of mitosis.

Vocabulary PreviewAsk students at random to pronouncethe Vocabulary words in the order in which they appear. Correct anymispronunciations.

Reading StrategySuggest that students write a sum-mary of the information in Figure10–5. Then, have them revise theirsummaries after reading the section.

2 INSTRUCT

ChromosomesAddress MisconceptionsStudents may think that there is achemical or structural differencebetween chromosomes and chro-matids. Explain that there is really nodifference between chromosomesand chromatids except that the chro-matids are always double structures(DNA replicas) fastened together attheir centromeres. Biologists avoidsaying that an organism has “dou-ble” the number of “chromosomes,”because each kind of organism has aspecific number of chromosomes. Achromatid becomes a chromosomewhen the sister chromatids separateduring anaphase.

DemonstrationReinforce students’ understanding ofchromatids and centromeres byusing two pipe cleaners and a pin.Show students a pipe cleaner andexplain that it represents a chromo-some. As you explain the process ofduplication of chromosomes duringinterphase, pick up another pipecleaner of the same size and wind itaround the first one. Then, push a pinthrough the contact joint of the twopipe cleaners and explain that the pinrepresents a centromere.

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 10 Lab• Laboratory Manual B, Chapter 10 Lab• Teaching Resources, Lesson Plan 10–2,

Adapted Section Summary 10–2, AdaptedWorksheets 10–2, Section Summary 10–2,Worksheets 10–2, Section Review 10–2,Enrichment

• Reading and Study Workbook A, Section 10–2• Adapted Reading and Study Workbook B,

Section 10–2

• Biotechnology Manual, Lab 3• Lab Worksheets, Chapter 10 Exploration

Technology:

• iText, Section 10–2• Animated Biological Concepts DVD,

16 Animal Cell Mitosis and Cytokinesis• Transparencies Plus, Section 10–2• Lab Simulations CD-ROM, Mitosis• Virtual Labs, Lab 9, Lab 10

Tim

eSaver

0240_0252_bi_c07_te 3/7/06 11:05 PM Page 244

Each pair of chromatids is attached at an area called the

centromere (SEN-troh-meer). are usually located

near the middle of the chromatids, although some lie near the

ends. A human body cell entering cell division contains 46

chromosomes, each of which consists of two chromatids.

The Cell CycleAt one time, biologists described the life of a cell as one cell

division after another separated by an “in-between” period of

growth called We now appreciate that a great

deal happens in the time between cell divisions, and use a

concept known as the cell cycle to represent recurring events in

the life of the cell. The cell cycle is the series of events that

cells go through as they grow and divide. During the cellcycle, a cell grows, prepares for division, and divides toform two daughter cells, each of which then begins thecycle again. The cell cycle is shown in Figure 10–4.

The cell cycle consists of four phases. Mitosis and cytokine-

sis take place during the M phase. Chromosome replication, or

synthesis, takes place during the S phase. When the cell copies

the chromosomes, it makes a duplicate set of DNA. Between the

M and S phases are G1 and G2. The G in the names of these

phases stands for “gap,” but the G1 and G2 are definitely not

periods when nothing takes place. They are actually periods of

intense growth and activity.

Events of the Cell CycleDuring the normal cell cycle, interphase can be quite long,

whereas the process of cell division takes place quickly.

Interphase is divided into three phases: G1, S, and G2.

The G1 phase is a period of activity in which cells do most

of their growing. During this phase, cells increase in size

and synthesize new proteins and organelles.

G1 is followed by the S phase, in which

chromosomes are replicated and the synthesis

of DNA molecules takes place. Key proteins

associated with the chromosomes are also

synthesized during the S phase. Usually,

once a cell enters the S phase and begins

the replication of its chromosomes, it

completes the rest of the cell cycle.

When the DNA replication is com-

pleted, the cell enters the G2 phase. G2 is

usually the shortest of the three phases of

interphase. During the G2 phase, many of

the organelles and molecules required for cell

division are produced. When the events of the

G2 phase are completed, the cell is ready to enter

the M phase and begin the process of cell division.

What happens during the G1 phase?

interphase.

Centromeres

Inte

rph

ase

Mitosis

Cell growth

Preparation for mitosis

DN

Are

plic

atio

n

G1 phase

S phase

G2 phase

CytokinesisTelophaseAnaphase

Metaphase

Prophase

M phase Celldivision

� Figure 10–4 During thecell cycle, the cell grows, repli-cates its DNA, and divides intotwo daughter cells. DNA synthesistakes place during the S phase. Celldivision takes place during the Mphase. G1 and G2 are gap phases.

For: Links on the cell cycle

Visit: www.SciLinks.orgWeb Code: cbn-3103

NSTA

The Cell CycleUse VisualsFigure 10–4 After students haveexamined the figure, ask: What arethe four phases of the cell cycle?(G1 phase, S phase, G2 phase, and Mphase) If you were to divide the cellcycle into two parts, what wouldthey be? (Interphase and cell division)Explain that this is called a cyclebecause the process is continuousthrough generations of cells, and onephase leads into the next. Then, ask:For each individual cell, when doesthe cell cycle begin? (When thedaughter cells form, at the end of theM phase, or after cytokinesis hasoccurred)

Events of the Cell CycleBuild Science SkillsUsing Models Divide the class intoeight groups, making sure eachgroup contains a mix of students ofvarying abilities. Assign one groupthe G1 phase, a second group the Sphase, a third group the G2 phase,four groups one of the four phases ofmitosis, and the eighth group cytoki-nesis. Explain that together, thegroups will make a wall-length car-toon strip that shows the events in thecell cycle. Give each group fourframes—four large sheets of paper—for its part of the total cartoonsequence. Advise group members towork together to plan what should beshown in the four frames, whichshould contain cartoon figures thatcreatively tell the story of that part ofthe cell cycle. When all groups havecompleted their work, tape all car-toons in sequence across the side ofthe classroom.

Answers to . . . Cells increase in size and

synthesize new proteins and organelles.

Figure 10–3 Each cell must receivethe same genetic information.

Comprehension: Link to VisualBeginning Use Figure 10–4 (page 245) to helpstudents understand the cell cycle. In the figure,point out the labels cell growth, DNA replication,preparation for mitosis, and cell division. Then,hand out a two-column graphic organizer titled“The Cell Cycle.” The left column should belabeled Phase, and the rows of the left columnshould be blank. The right column should belabeled Activity. Each row of the right column

should be filled in with a description of the activi-ty associated with one phase of the cell cycle (forexample, cell growth). Working with an English-proficient partner, the students can use theinformation in Figure 10–4 to complete the leftcolumn of the graphic organizer.Intermediate Modify the activity for beginningstudents by requiring the students to fill in bothcolumns of the graphic organizer using theinformation found in Figure 10–4.

SUPPORT FOR ENGLISH LANGUAGE LEARNERS

NSTA

Download a worksheeton the cell cycle for students tocomplete, and find additionalteacher support from NSTASciLinks.

Cell Growth and Division 245

0240_0252_bi_c07_te 3/7/06 11:05 PM Page 245

Nuclear

envelope

Chromatin

Centrioles

Nuclear

envelope

reforming

Interphase

The cell grows and

replicates its DNA

and centrioles.Cytokinesis

The cytoplasm

pinches in half.

Each daughter

cell has an identical

set of duplicate

chromosomes.

Telophase

The chromosomes

gather at opposite ends

of the cell and lose their

distinct shapes.

Two new nuclear

envelopes will form.

� Figure 10–5 Most eukaryotic cells gothrough a regular cycle of interphase, mitosis,and cytokinesis. Mitosis has four phases:prophase, metaphase, anaphase, andtelophase. The events shown here are typicalof animal cells. The photographs shown are from a developing whitefish embryo(magnification: 625�).

MitosisBiologists divide the events of mitosis into four

phases: prophase, metaphase, anaphase, and telophase.Depending on the type of cell, the four phases of mitosis may

last anywhere from a few minutes to several days. As you read

about each phase of mitosis, look at Figure 10–5.

Prophase The first and longest phase of mitosis,

can take as much as 50 to 60 percent of the total time required

to complete mitosis. During prophase, the chromosomes become

visible. The (SEN-tree-ohlz), two tiny structures

located in the cytoplasm near the nuclear envelope, separate

and take up positions on opposite sides of the nucleus.

centrioles

prophase,

For: Links on cell division

Visit: www.SciLinks.orgWeb Code: cbn-3102

NSTA

246 Chapter 10

MitosisBuild Science SkillsPredicting Divide the class intosmall groups, and give each group apacket of pictures, each of which rep-resents a phase of mitosis. To makeeach packet, copy photos or illustra-tions from a college textbook,eliminating any labels. Challengeeach group to make a predictionabout how mitosis proceeds by plac-ing the pictures in the correctsequence. Then, have groups presenttheir predictions to the class.

Use VisualsFigure 10–5 Explain that this figureshows each event in the cell cycle intwo ways, as a photomicrograph andas a labeled illustration. Ask: Whenare the cell’s chromosomes repli-cated during the cell cycle? (Duringthe S phase of interphase) What func-tion does the spindle serve duringmitosis? (The spindle helps separatethe chromosomes.) Have students lookback at Figure 10–4, and ask: Doescytokinesis start when telophaseends? (No. The figure shows thatcytokinesis ends after telophase, but itbegins during mitosis.) Explain thatcytokinesis overlaps mitosis and usu-ally begins during telophase. Finally,have students use the figure to maketheir own drawings of each phase ofmitosis.

Write the numbers 1 to 23 on two sets of indexcards. Distribute the cards to students, andhave them stand in a group. Tell them that theysymbolize a human body cell and each cardrepresents a chromosome. Then, have studentsillustrate replication of chromosomes duringinterphase by finding their matching numberand holding the cards out in front so that eachstudent is holding a card. To illustratemetaphase, have student pairs stand side by

side in a line while holding each other’s cards.Students should let go of their partner’s cardwhile still holding their own and walk to theopposite side of the room, illustratinganaphase. Now each of the two groups repre-sents a new cell.

—Michael LopatkaBiology TeacherEdgewater High SchoolOrlando, FL

TEACHER TO TEACHER

10–2 (continued)

NSTA

Download a worksheeton cell division for students tocomplete, and find additionalteacher support from NSTASciLinks.

0240_0252_bi_c07_te 3/7/06 11:05 PM Page 246

Spindle

forming

Centromere Chromosomes

(paired chromatids)

Spindle

Centriole

Centriole

Individual

chromosomes

AnaphaseThe sister chromatids

separate into individual

chromosomes and are

moved apart.

MetaphaseThe chromosomes

line up across the

center of the cell.

Each chromosome

is connected to a

spindle fiber at its

centromere.

ProphaseThe chromatin condenses

into chromosomes. The

centrioles separate, and

a spindle begins to form.

The nuclear envelope

breaks down.

The centrioles lie in a region called the centrosome that

helps to organize the a fanlike microtubule structure

that helps separate the chromosomes. During prophase, the

condensed chromosomes become attached to fibers in the spin-

dle at a point near the centromere of each chromatid. Inter-

estingly, plant cells do not have centrioles, but still organize

their mitotic spindles from similar regions.

Near the end of prophase, the chromosomes coil more

tightly. In addition, the nucleolus disappears, and the nuclear

envelope breaks down.

What is the function of the spindle?

spindle,

For: Cell Cycle activityVisit: PHSchool.comWeb Code: cbp-3102

Build Science SkillsObserving Set up microscope sta-tions around the room, and provide aparamecium culture. Ask each stu-dent to make a slide from theculture. Have students examine theirslides, using low power, to find anyparamecia that are pinched in themiddle or that look like “doublecells.” When such an example isfound, the student should switch tohigh power and make a sketch of theorganism. Students should also writea short description of what they thinkis occurring.

Build Science SkillsUsing Models Divide the class intosmall groups, and give each grouppieces of pipe cleaner and string.Have each group form a cell with thestring and use the pipe cleaners forchromosomes. Then, call on studentvolunteers to explain what occursduring prophase, metaphase,anaphase, and telophase. As a stu-dent explains each event, groupmembers should manipulate theirmaterials to model that phase ofmitosis. Circulate among the groupsto correct any misconceptions.

Cell Growth and Division 247

To reinforce students’ understanding of theprocesses of mitosis, I have them manipulatecommon materials as they mirror the sequenceof phases. I supply them with a paper towel torepresent the cell, gummy worms (which are bicolored) to represent chromosomes, andtoothpicks to represent spindle fibers. They useplastic knives to cut the gummy worms, makingsister chromatids. For mitosis, they organize aspindle with toothpicks on opposite sides of the

towel (prophase), line up the chromosomes(metaphase), separate the chromosomes intotwo groups (anaphase), form the chromosomesinto clusters (telophase), and split the papertowel in two (cytokinesis).

—Sheila SmithBiology TeacherTerry High SchoolTerry, MS

TEACHER TO TEACHER

Answer to . . . The spindle helps

separate the chromosomes.

For: Cell Cycle activityVisit: PHSchool.comWeb Code: cbe-3102Students can interact with theart of mitosis online.

0240_0252_bi_c07_te 3/7/06 11:05 PM Page 247

Metaphase The second phase of mitosis, often

lasts only a few minutes. During metaphase, the chromosomes

line up across the center of the cell. Microtubules connect the

centromere of each chromosome to the two poles of the spindle.

Anaphase is the third phase of mitosis. During

anaphase, the centromeres that join the sister chromatids split,

allowing the sister chromatids to separate and become individual

chromosomes. The chromosomes continue to move until they

have separated into two groups near the poles of the spindle.

Anaphase ends when the chromosomes stop moving.

Telophase Following anaphase is the fourth and

final phase of mitosis. In telophase, the chromosomes, which

were distinct and condensed, begin to disperse into a tangle of

dense material. A nuclear envelope re-forms around each cluster

of chromosomes. The spindle begins to break apart, and a

nucleolus becomes visible in each daughter nucleus. Mitosis is

complete. However, the process of cell division is not complete.

What happens during anaphase?

CytokinesisAs a result of mitosis, two nuclei—each with a duplicate set of

chromosomes—are formed, usually within the cytoplasm of a

single cell. All that remains to complete the M phase of the cycle is

cytokinesis, the division of the cytoplasm itself. Cytokinesis usually

occurs at the same time as telophase.

Cytokinesis can take place in a number of ways. In most

animal cells, the cell membrane is drawn inward until the

cytoplasm is pinched into two nearly equal parts. Each part

contains its own nucleus and cytoplasmic organelles. In plants,

a structure known as the cell plate forms midway between the

divided nuclei, as shown in Figure 10–6. The cell plate gradually

develops into a separating membrane. A cell wall then begins to

appear in the cell plate.

telophase,

Anaphase

metaphase,

� Figure 10–6 During cytokinesis in plant cells, thecytoplasm is divided by a cellplate. The thin line you cansee between the two darknuclei in this electron micro-graph of onion cells dividingis the cell plate forming.Interpreting GraphicsWhat structure forms betweenthe divided nuclei?

Cell plate Cell wall

Cytokinesis comes from theGreek words kytos, meaning“hollow vessel,” and kinesis,meaning “motion.” The prefix cyto- refers to cells, socytokinesis means movementwithin the cell. What do you think the term cytotoxic means?

(magnification: 2200�)

248 Chapter 10

Cytokinesis

The word cytotoxic means “related tosomething poisonous to a cell.”

How long does a cell cycle take?The time it takes for a cell to complete a cell cycle,called the generation time, varies widely amongcells. The minimum time for a complete cell cycleis about 10 minutes. It takes about 2 hours forcells to divide in a newly forming sea urchin. Inanimal and plant cells that are actively growing,generation time is often between 8 and 10 hours.The generation time for a bean cell is 19 hours;the G1 phase lasts about 5 hours, the S phase lasts

about 7 hours, the G2 phase lasts about 5 hours,and the M phase lasts about 2 hours. The genera-tion time for some mouse cells is about 22 hours;for these cells, the G1 phase lasts about 9 hours,the S phase lasts about 10 hours, the G2 phaselasts about 2 hours, and the M phase lasts about1 hour. Many mature cells, such as nerve and redblood cells, never divide; they are said to be in theG0 phase, which is much like the G1 phase.

FACTS AND FIGURES

10–2 (continued)

Many students have the misconcep-tion that mitosis occurs regularly inall cells. The information in the datatable will help address that mistakennotion.Answers1. Most white blood cells are neededby the body only for a short time tofight infection, so they do not haveto be long-lived.2. Because cardiac muscle cells andneurons cannot divide, injuries to theheart or spinal cord cannot healthrough the production of new heartor nerve cells. In contrast, becausethe cells of smooth muscle candivide, an injury to smooth musclemay be able to heal through cell division.3. A typical hypothesis might suggestthat cells lining the digestive system,where chemical and mechanicaldigestion occur, are more apt to bedestroyed or damaged by theseprocesses.4. A typical prediction will correctlysuggest that cancer cells (in a cellculture) are long-lived and divisioncan occur a seemingly unlimitednumber of times.

0240_0252_bi_c07_te 3/7/06 11:05 PM Page 248

Life Spans of Human CellsLike all organisms, cells have a given life span frombirth to death. In multicellular organisms, such ashumans, the health of the organism depends on cellsnot exceeding their life span. This is especially true of cells that tend to divide rapidly. If these cells didnot die on schedule, overcrowding of cells wouldoccur, causing uncontrolled growth that would belife-threatening.

The data table shows the life spans of varioushuman cells. It also contains information about theability of the cells to multiply through cell division.

1. Inferring White blood cells help protect the bodyfrom infection and disease-producing organisms.How might their function relate to their life span?

2. Comparing and Contrasting Based on thedata, how are the consequences of injuries to theheart and spinal cord similar to each other? Howare they different from the consequences of injuriesto smooth muscle?

3. Formulating Hypotheses Propose a hypothesisto account for the data related to the cell life spansof the lining of the esophagus, small intestine, andlarge intestine.

4. Going Further Cancer is a disease related to celllife span and cell division. If cancer cells wereadded to the data table, predict what would bewritten under the columns headed “Life Span” and“Cell Division.” Explain the reasoning underlyingyour predictions.

Cell Type Life Span Cell Division

Lining of esophagus

Lining of small intestine

Lining of large intestine

Red blood cells

White blood cells

Smooth muscle

Cardiac (heart) muscle

Skeletal muscle

Neuron (nerve cell)

2–3 days

1–2 days

6 days

Less than120 days

10 hoursto decades

Long-lived

Long-lived

Long-lived

Long-lived

Can divide

Can divide

Can divide

Cannot divide

Many do notdivide

Can divide

Cannot divide

Cannot divide

Most do notdivide

Life Spans of Various Human Cells

1. Key Concept Name themain events of the cell cycle.

2. Key Concept Describewhat happens during each of thefour phases of mitosis.

3. Describe what happens duringinterphase.

4. What are chromosomes made of?

5. How do prokaryotic cells divide?

6. Critical Thinking Comparingand Contrasting How iscytokinesis in plant cells similar tocytokinesis in animal cells? How isit different?

Creative WritingSuppose you were smallenough to hitch a ride on achromosome located in aplant cell that goes throughmitosis and cytokinesis.Describe what you would seehappening during each phaseof the process.

10–2 Section Assessment

3 ASSESSEvaluate UnderstandingAsk students to look at the pie chartin Figure 10–4. Then, call on volun-teers to describe the events in eachphase of interphase and each phaseof mitosis.

ReteachHave students make a flowchart ofthe cell division, including whatoccurs in each of the four phases ofmitosis as well as in cytokinesis.

Cell Growth and Division 249

Answers may vary. An excellentresponse will be a well-written,engaging “travelogue” throughthe M phase of the cell cycle.Students should include importantdetails from the subsectionsMitosis and Cytokinesis in Section10–2, including events presentedin the cycle diagram in Figure10–5.

10–2 Section Assessment1. A cell grows, prepares for division, and divides

to form two daughter cells.2. Students should describe what happens dur-

ing prophase, metaphase, anaphase, andtelophase, as in Figure 10–5.

3. Students should describe what happens dur-ing the G1 phase, S phase, and G2 phase.

4. DNA, which carries the cell’s coded geneticinformation, and proteins

5. A prokaryotic cell first replicates its geneticinformation before cell division begins. Inmost prokaryotes, the rest of the process ofcell division is a simple matter of separatingthe contents of the cell into two parts.

6. Cytokinesis is the division of the cytoplasm inboth types of cells. The difference is that inplant cells a cell plate forms midway betweenthe divided nuclei.

If your class subscribes to the iText,use it to review the Key Concepts inSection 10–2.

Answers to . . . The centromeres that

join the sister chromatids split, allowingthe sister chromatids to separate. Thechromosomes continue to move untilthey separate into two groups near thepoles of the spindle.

Figure 10–6 A cell wall forms in thecell plate.

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