Key Concepts ~?~‘ESSENTIAL QUESTIONSO What are the phases of the

cell cycle?O Why is the result of the cell

cycle important?

Vocabtdarycell cycle p. 85

interphase p. 86

sister chromatid

centromere p. 88

mitosis p. 89cytokinesis p. 89

daughter cell p.89

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Unicellular organisms such as these reproduce whenone cell divides into two new cells. The two cells areidentical to each other. What do you think happened tothe contents of the original cell before it divided?

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The Cell CycleNo matter where you live, you have prob

ably noticed that the weather changes in aregular pattern each year Some areas experience four seasons—winter, spring, summer,and fall. In other parts of the world, thereare only two seasons—rainy and dry As seasons change, temperature, precipitation, andthe number of hours of sunlight vary in aregular cycle.

These changes can affect the life cycles oforganisms such as trees. Notice how the trçein Figure 1 changes with the seasons Likechanging seasons or the growth of trees, cellsgo through cycles Most cells in an organismgo through a cycle ofgrowth, development, and

~Jdivision called the cell cycle. Through the cell~cycle, organisms grow, develop, replace old¶or damaged cells, and produce iiew cells.

~Visual Check List the seasonal changes ofthis maple tree.

Lesson 1

EXPLORE

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Read and complete a lab safety form

Why isn’t your cell like mine?All living things are made of cells. Some are made of onlyone cell, while others are made of trillions of cells. Wheredo all those cells come from?

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Ask your team members to face away from you. Drawan animal cell on a sheet of paper. Include as manyorganelles as you can.Use scissors to cut the cell drawing into equal halves.Fold each sheet of paper in half so the drawing capnotbe seen.Ask your team members to face you. Give each teammember half of the cell drawing.

0 Compare the two new cells to your original cell.

Think About This

Have team members sit facing away from each other. Each person should use a glue stick toattach the cell half to one side of a sheet of pape~~ Then, each person should draw themissing cell half

1. How did the new cells compare to the original cell?

2. tiIj~a~’ Key Concept What are some things that might be done in the early steps to produce

L two new cells that are more like the original cell?

Figure 1 This maple tree changes in response toa seasonal cycle.

Make a folded bookfrom a sheet of paper.Label the front The CellCycle, and label theinside of the book asshown. Open the bookcompletely and use thefull sheet to illustratethe cell cycle.

Phases of the CeN CydeThere are two main phases in the cell cycle—interphase and

the mitotic (ml TAH tihk) phase. Interphase is the period duringthe cell cycle of a cell’s growth and development A cell spendsmost of its life in interphase, as shown in Figure 2. Duringinterphase, most cells go through three stages:

o rapid growth and replication, or copying, of the membrane-bound structures called organelles;

o copying of DNA, the genetic information in a cell; and

o preparation for cell division.

Interphase is followed by a shorter period of the cell cycleknown as the mitotic phase. A cell reproduces during thisphase. The mitotic phase has two stages, as illustrated inFigure 2. The nucleus divides in the first stage, and the cell’sfluid, called the cytoplasm, divides in the second stage. Themitotic phase creates two new identical cells. At the end of thisphase, the original cell no longer exists.

@~2~ KeyConceptCheck Whatarethetwo main phasesofthe

Preparation forcell division ‘¼ Mitotic

~phaseMitosisC~okinesis

Cs

cell cycle?

Figure 2 A cell spends most of its life growing and developing during interphase.

S

S Visual Check Which stage of interphase is the longest?

D 86 Chapter 3EXPLAIN

c

Length of ~ Cell CydeThe time it takes a cell to complete the cell cycle depends

on the type of cell that is dividing. Recall that a enkaryoticcell has membrane-bound organelles, including a nucleus. Forsome eukaryotic cells, the cell cycle might last only eight minutes. For other cells, the cycle might take as long as one year.Most dividing human cells normally complete th~ cell cycle inabout 24 hours. As illustrated in Figure 3, the cells of someorganisms divide very quickly.

~nterphaseAs you have read, interphase makes up most of the cell

cycle. Newly produced cells begin interphase with a period ofrapid growth—the cell gets bigger. This is followed by cellularactivities such as making proteins. Next, actively dividing cellsmake copies of their DNA and prepare for cell division. Duringinterphase, the DNA is called chromatin (KROH muh tun).Chromatin is long, thin strands of DNA, as shown in Figure 4.When scientists dye a cell in interphase, the nucleus looks likea plate of spaghetti. This is because the nucleus contains manystrands of chromatin tangled together.

2-cell stageSEM Magnification: 160x

4-cell stageSEM MngnifKalfon: 155x

32-cell stageSEM MagnificatMn: iSOx

256-cell stageSEM Megnification: 130x

A Figure 3 The fertilizedegg of a zebra fish dividesinto 256 cells in 2.5 hours.

RnilEw VOCABULARYeukaryotica cell with membrane-boundstructures

Figure 4 Duringinterphase, the nuclei of ananimal cell and a plant cellcontain long, thin strands ofDNA called chromatin. V

Plant Cell Animal CellLesson 1

I EXPLAINI (I)Dr. flEchard Kernel & Dr. Gone ShilWiaualr Unlimited.

growth and cellularfunctions; organelle replication

growth and chromosome replication; organelle replication

growth and cellularfunctions; organelle replication

mitosis division of nucleus

Mitotic phase —________________________

cytokinesis division of cytoplasm

FigureS The coiled DNAforms a duplicatedchromosome made oftwo sister chromatidsconnected atthecentromere. V

SisterChromatid

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Centromere—~I I I

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Duplicated chromosome II4

‘I.1

Interphase s

A Table 1 The two phasesof the cell cycle can eachbe divided into differentstages.

Phases of k~terphaseScientists divide interphase into three stages, as shown in

Table 1. Interphase begins with a period of rapid growth—theG1 stage. This stage lasts longer than other stages of the cellcycle. During G1, a cell grows and carries out its normal cellfunctions. For example, during G1, cells that line your stomachmake enzymes that help digest your food. Although most cellscontinue the cell cycle, some cells stop the cell cycle at thispoint. For example, mature nerve cells in your brain remain inG1 and do not divide again.

During the second stage of interphase—the S stage—a cellcontinues to grow and copies its DNA. There are now identicalstrands of QNA. These identical strands of DNA ensure thateach new cell gets a copy of the original cell’s genetic information. Each strand of DNA coils up and forms a chromosome.Identical chromosomes join together. The cell’s DNA is nowarranged as pairs of identical chromosomes. Each pair is calleda duplicated chromosome. Two identical chromosomes, calledsister chromatids, make up a duplicated chromosome, as shownin Figure 5. Notice that the sister chmmatids are held together bya structure called the centromere.

SisterChromatid

Theperiodphase.Duringmitotic

final stage of interphase—the G2 stage—is anotherof growth and the final preparation for the mitoticA cell uses energy copying DNA during the S stage.G2, the cell stores energy that will be used during thephase of the cell cycle.

Chapter 3p

EXPLAIN

• Reading Check Describe what happens in the C2 phase.

- I

)rga~ieHe RepHcat~onDuring cell division, the organelles in a cell are distubuted

etween the two new cells Before a cell divides, it makes aopy of each organelle This enables the two new cells to funcLon properly Some organelles, such as the energy-processing-itochondria and chloroplasts, have their own DNA Theseirganelles can make copies of themselves on their own, as‘~own in Figure 6 A cell produces other organelles from mateLals such as proteins and lipids A cell makes these materialsising the information contained in the DNA inside the nucleus)rganelles are copied during all stages of mterphase

(he Mftotk PhaseThe mitotic phase of the cell cycle follows interphase It

~.nnsists of two stages mitosis (mi TOH sus) and cytokinesis(si toh kuh NEE sus) In mitosis, the nucleus and its êontents(wide In cytokmesis, the cytoplasm and its contents divideItiughter cells are the two new cells that result from mitosis and

cytokinesis

During mitosis, the contents of the nucleus divide, formingtwo identical nuclei The sister chromatids of the duplicatedchromosomes separate from each other This gives each daughter cell the same genetic information For example, a cell thathas ten duplicated chromosomes actually has 20 chromatids.When the cell divides, each daughter cell will have ten different chromatids Chromatids are now called chromosomes

In cytokinesis, the cytoplasm divides and forms the twonew daughter cells. Organelles that were made during inter-phase are divided between the daughter cells

L~

‘L....~TEM Magn~ITcaticn: Unavaflable

Figure 6 Thismitochondrion is in thefinal stage of dividing.

WORD ORlGaimitosisfrom Greek mitos, means“warp thread”; and Latin—osis, means “process”

Lesson 1• 89

EXPLAIN

Phases of MftosñsLike interphase, mitosis is a continuous

process that scientists divide into differentphases, as shown in Figure 7.

Prophase During the first phase of mitosis,called prophase, the copied chromatin coilstogether tightly. The coils form visible duplicated chromosomes. The nucleolus disappears, and the nuclear membrane breaksdown. Structures called spindle fibers formin the cytoplasm.

Metaphase During metaphase, the spindlefibers pull and push the duplicated chromosomes to the middle of the cell. Notice inFigure 7 that the chromosomes line up alongthe middle of the cell. This arrangementensures that each new cell will receive onecopy of each chromoson~e. Metaphase is theshortest phase in mitosis, but it must becompleted successfully for the new cells tobe identical.

I

Animation

Prophase• Copied DNA condenses into

chromosomes.• The nucleolus disappears, and the

nuclear membrane breaks down.a Spindle fibers begin to form.

IM Magnirication: 250x

MetaphaseChromosomes line up in singlefile at the middle of the cell.

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Figure 7 Mitosis begins when replicated chromatin coilstogether and ends when two identical nuclei are formed.

chapter 3

EXPLAIN

IM Magnification; 250X

• ~naphase In anaphase, the third stage offljtOsiS, the two sister chromatids in each:hromosome separate from each other. The;pindle fibers pull them in opposite direcjons. Once separated, the chromatids areww two identical single-stranded chromo

soines. As they move to opposite sides of acell, the cell begins to get longer. Anaphaseis complete when the two identical sets ofchromosomes are at opposite ends of a cell.

Telophase During telophase, the spindlefibers begin to disappear. Also, the chromosomes begin to uncoil. A nuclear membraneforms around each set of chromosomes ateither end of the cell. This forms two newidentical nuclei. Telophase is the final stageof mitosis. It is often described as the reverseof prophase because many of the processesthat occur during prophase are reversed during telophase.

IM Ma9nitkalion: 250x

Lesson 1

EXPLAIN

a Reading Check What are the phases of“~~‘ mitosis?

IM Magnificallon: 250x

Anaphase• Sister chromatids separate.• Spindle fibers begin to shorten,

pulling chromatids towardopposite sides of the cell.

• The cell begins to lengthen.

N

Telophase° A nuclear membrane forms around

the chromatin.o Chromosomes begin to unwind.° Spindlefibers begin to break down.• Two identical nuclei form.

91a

Dividing the CeWs ComponentsFollowing the last phase of mitosis, a cell’s cytoplasm divides

in a process called cytokinesis. The specific steps of cytokinesisdiffer depending on the type of cell that is dividing. In animalcells, the cell membrane contracts, or squeezes together, aroundthe middle of the cell. Fibers around the center of the cell pulltogether. This forms a crease, called a furrow, in the middle ofthe cell. The furrow gets deeper and deeper until the cellmembrane comes together and divides the cell. An animal cellundergoing cytokinesis is shown in Figure 8.

Cytokinesis in plants happens in a different way. As shownin Figure 8, a new cell wall forms in the middle of a plant cell.First, organefles called vesicles join together to form a membrane-bound disk called a cell plate. Then the cell plate growsoutward toward the cell wall until two new cells form.

• Reading Check Compare cytokinesis in plant and animal cells.

Resuilts of Cell DJvk~onRecall that the cell cycle results in two new cells. These

daughter cells are genetically identical to each other and tothe original cell that no longer exists. For example, a humancell has 46 chromosomes. When that cell divides, it will produce two new cells with 46 chromosomes each. The cell cycleis important for reproduction in some organisms, growth inmulticellular organisms, replacement of worn out or damagedcells, and repair of damaged tissues.

~ ~

Animal Cell Color-Enhanced SEM Magnification: lSOtx Plant Cell IM Magnification: 400x

Figure 8 Cytokinesisdiffers in animal cells andplant cells.

NUse PercentagesA percentage is a ratio thatcompares a number to 100.If the length of the entirecell cycle is 24 hours,24 hours equals 100%. Ifpart of the cycle takes6.0 hours, it can beexpressed as 6.0 hours!24 hours. To calculate percentage, divide and multiplyby 100. Add a percent sign.

j*=0.25XlOO=25%

PracticeInterphase in human cellstakes about 23 hours. If thecell cycle is 24 hours, whatpercentage is interphase?

o Math Practiceo Personal Tutor

92 • Chapter 3EXPLAIN

~~eproducdonIn some unicellular organisms, cell divi

sion is a form of reproduction. For example,an organism called a paramecium oftenreproduces by dividing into two new daughter cells or two new paramecia Cell divisionis also impoitant in other methods of ieproductiOn in which the offspring are identicalto the parent organism.

GrowthCell division allows multicellular organ

isms, such as humans, to grow and developfrom one cell (a fertilized egg). In humans,cell division begins about 24 hours after fertilization and continues rapidly during thefirst few years of life. It is likely that duringthe next few years you will go throughanother period of rapid growth and development. This happens because cells divideand increase in number as you growand develop.

RepbcementEven after an organism is fully grown,

cell division continues. It replaces cells thatwear out or are damaged. The outermostlayer of your skin is always rubbing or flaking off. A layer of cells below the skin’s surface is constantly dividing. This producesmillions of new cells daily to replace theones that are rubbed off.

Repa rCell division is also critical for repairing

damage. When a bone breaks, cell divisionproduces new bone cells that patch the broken pieces back together.

Not all damage can be repaired, however,because not all cells continue to divide.Recall that mature nerve cells stop the cellcycle in interphase. For this reason, injuriesto nerve cells often cause permanentdamage.

IJ$n Key Concept Check Why is the result ofthe cell cycle important?

2. Integrate What would arñodel of yourcell look like during the stage immediatelyfollowing mitosis? What is this stage?

3. f Key Concept During mitosis, a cellforms two new, identical nuclei. Use yourmodels to explain why, in order todo this, mitosis must occur after eventsin interphase.

Lesson

EXPLAIN93~

iñqufryI

How does mitosis work?The dolix is a mythical animal whose cellscontain just two chromosomes. What happensto a dolix cell nucleus during mitosis?

CC

Read and complete a lab safety form.Form four 60-cm lengths of yarn intolarge circles on four separate sheets ofpaper. Each piece of paper represents onephase of mitosis, and the yarn representsthe cell membrane.

C On each sheet of paper, model one phaseof mitosis using different colors of yarn torepresent the nuclear membrane, thespindles, and the chromosomes. Use twistties to represent centromeres. Tape theyarn in place.

C Label your models, or develop a key toindicate which color is used for whichpart.

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Analyze and Conclude1. Identify If you were to model a dolix cell’s

nucleus before mitosis began, what wouldyour model look like? Would you be ableto see the individual chromosomes?

Online Quiz

Use your lesson Foldable to review the lesson.Save your Foldable for the project at the endof the chapter.

You first read the statements below at thebeginning of the chapter.

1. Cell division produces two identical cells.

2. Cell division is important for growth.

3, At the end of the cell cycle, the original cellno longer exists.

Did you change your mind about whether youagree or disagree with the statements? Rewriteany false statements to make them true.

-4,

Use Vocabullary

0 Distinguish between mitosis andcytokinesis.

0 A duplicated chromosome is made oftwo ________

Ouse the term interphase in a sentence.

Understand Key Concepts LI$TJW0 Which is NOT part of mitosis?

0 Construct a table to show the differentphases of mitosis and what happensduring each.

0 Give three examples of why the resultof the cell cycle is important.

interpret GraphksIdentify The animalcell on the right is inwhat phase ofmitosis? Explain youranswer.

o Organize Copy and fill in the graphicorganizer below to show the results ofcell division.

Crütk& Thinking

O Predict what might happen to a cell ifwere unable to divide by mitosis.

it

~_~_L_J__ ~__“~~ Math Practice

O The mitotic phase of the human cellcycle takes approximately 1 hour. Whatpercentage of the 24-hour cell cycle isthe mitotic phase?

I

The nucleus andits contents divide.during mitosis.

A.

B.anaphaseinterphase

During interphase,most cells gothrough periods ofrapid growth andreplication of organelles, copying DNA,and preparation forcell division.

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____ ____ V/i

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C. prophase0. telophase

The cytoplasm andits contents divideduring cytokinesis.

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Chapter 3

EVALUATE94

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~ very cell in your body has the same DNA in its nucleus. Unless you are an.~identical twin, your DNA is entirely unique. Identical t’~

DNA because they begin as one cell that dividesbegin mitosis, they copy their DNA. Every new c~original cells. That is why DNA can be used to iderpeople have the same fingerprints, your DNA heir

Using scientific methods to solvcfingerprinting is now a basic tool in!scene can be compared to miHt~tnt

ndexed in a comp

Every day,:ells. It mighcomm~Investi

DNAnvestic,cenc;uilt~

V The FederalL_Investigation (PBlihasnationwide index of DNAsamples called CODIS (Cornbined DNA Index System).

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