10–2 cell divisionmrjohns2.weebly.com/uploads/9/8/5/0/98501120/031002.pdfwell before cell...
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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
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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
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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.
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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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