Mitosis Meiosis Teachers Guide Discovery Education

8/8/2019 Mitosis Meiosis Teachers Guide Discovery Education

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Mitosis and Meiosis

catalog #2142

Teachers Guide

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1

MITOSIS AND MEIOSIS

Runn ing Time: 23 minu tes

INTRODUCTION

The ability to reprodu ce is perhap s the most un ique pro-cess that characterizes living things. Given that all living

organ isms eventu ally die, it is essential that life be trans-

mitted to future gen erations. The incredible diversity of

life that surroun ds u s stands as testimony to the success of

the biological reprodu ctive pr ocesses that have continu ed

un abated for the three-and -one-half billion years that life

has existed on earth.

Reprod uction, w hen viewed at its most elementary level,assures that the DN A of one living cell will be passed on in

an u naltered form to its two dau ghter cells. This is what

happ ens when one-celled organisms reproduce, when new

cells are formed to replace dead cells in adult multicellular

organisms, and during embryological developm ent: DNA

instructions are passed on so that the new cells can function

normally and reprodu ce prop erly.

In order to rep rodu ce, all cells that m ake up the bodies oforganisms follow the sam e sequential steps: DNA replica-

tion, mitosis, cell d ivision, and cell grow th. These pro-

cesses char acterize the life cycle of the cell. The intricate

steps followed by the cell in d up licating its DNA, sorting it

into chromosomes, and then separating the chromosomes

into two equal and identical groups, forms the basis of the

first part of this program .

More biologically advanced organisms have two sexes andare capable of combining in their offspring the DN A from

the female parent w ith the DNA from th e male parent--a

process wh ich results in more d iverse and h ardy ind ividu -

als than is possible in the asexual reproductive processes

that involve just one parent. From a pu rely biological point

of view, the comp lex bodies of sexually rep rodu ctive or-

ganisms can be seen as fantastically intricate containers

that hav e developed primarily to protect the germ cells

that produ ce the sperm and eggs


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The cell reprodu ctive p rocess that m akes sexual rep rodu c-

tion possible is called meiosis or redu ction division. The

steps involved in d up licating the DNA and properly sort-

ing it out th at occurs d uring meiosis is the subject of the

second half of this program.

PROGRAM SUMMARY

This video program is designed for grades 10-12, but could

be very useful in introducing cell reprodu ction to students

in grad es 7-9 as well.

The p rogram consists of two p arts

The first par t, lasting about 15 minutes, is d evoted to th e

subjects of the interphase state, DNA rep lication, chromo-

some stru cture, and offers a detailed look at each stage of

mitosis. A short, optional video quiz follows the first part

of the program .

The second part is about 8 minutes in length. It looks at the

diploid and haploid states, at fertilization, and at the

various stages of meiosis. A short, optional video qu iz also

follows this portion of the program.

Throughou t this entire program , microscopic images are

interwoven w ith animated sequen ces to allow the pro-

cesses of mitosis and meiosis to be more easily un derstood .

STUDENT O BJECTIVES

After viewing this program and p articipating in the learn-

ing activities, studen ts should be able to

Describe the subcellular events that occur during Inter-

phase, Prophase, Metaphase, Anaph ase, and Telophase.

Describe the subcellular events that occur during each

major stage of m eiosis.

Contrast Mitosis and Meiosis and evaluate the impor-

tance of these two pr ocesses to living organisms


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Describe the life cycle of a typical cell.

Define and use in context vocabulary words appropri-

ate to their academ ic level.

SUGGESTED LESSON G UIDE

TEACHER PREPARATION

1. Read this guide and p review the video before showing

it to your class. Although th is lesson combines both video

parts as one learning session, you m ay choose to show the

video in two sessions or to stop the video at the end of the

first part an d review before going on to the second part.

There is a short interactive video qu iz at the end of each

par t of the video. These video qu izzes are also prov ided in

the form of blackline master 6.

2. Review the blackline masters and d up licate those you

choose to use.

3. Make arrangements to have a VCR and television set

available for use at the time and p lace selected for viewing.

4. Arrange to have microscopes available for student u se.

5. Purchase, or obtain from school collections, the follow-

ing microscope slides: allium (onion) root tip m itosis,

sperm atogenesis in rat testis, Drosophila salivary glan d

chromosome prep aration. Op tional materials: wh itefish

blastula mitosis and fertilized living eggs of snails, frogs, or

toads.

BLACKLINE MASTERS

Blacklin e M asters 1 & 2, Vocabu lary List, are words used

in the video and their definitions. This list can be d istrib-

uted to the class before the video pr esentation so stud ents

can better und erstand the terms presented . This list shou ld

be retained by stu dents for reference. The list has also been

includ ed in th is Teacher's Guide on pages 7-10.


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Blacklin e Master 3 is a diagram ofThe Stages of Mitosis .

Blacklin e Masters 4 and 4a are diagrams ofThe Stages of

Meiosis.

Blacklin e Master 5, Crossword Pu zzle, gives stud ents an

opp ortunity to use some of the terms introdu ced in the

video. The answers to the puzzle can be found on page 21

of this guide.

Blacklin e Master 6 is a copy of the questions posed in th e

Interactive Video Q uiz for both parts of the video. You can

either distribute the qu iz before the viewing of the video or

after, whichever app roach you've chosen to take. Answers

to the quiz are p rovided in th is Teacher's Guide on p age 16

for Part 1 and page 20 for Part 2.

INTRODUCINGTH E PROGRAM

Introduce this program by describing the mitotic processes

involved in cell reprod uction. Describe the role of mitosis

in the developmen t of an embryo, in replacing w orn ou t

cells, and in cancer where the rate of cell reproduction is

rapid and uncontrolled.

Contrast m itosis to meiosis and explain the importan ce of

sexual reproduction in creating biologically vigorous indi-

viduals.

Describe how meiosis makes it possible for sperm and egg

to join together so that the fertilized egg will have the same,

dip loid, nu mber of chrom osomes present in the somatic

cells of each pa rent.

Before starting the vid eo, distribute the b lackline masters

you've chosen to use du ring the video presentation, e.g.

Blacklin e M asters 1 & 2, Vocabu lary List, and Blackline

Master 6, Interactive Video Q uiz . If you 've chosen to use

the Interactive Video Qu iz , you should inform the stu-

den ts that they will be expected to answ er the questions

wh ich w ill appear on the screen following each part of thevideo program.


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VIDEO PRESENTATION

Viewing time: Part 1, 15 minu tes; Part 2, 8 minu tes

FOLLOW-UP ACTIVITIES

D ISCUSSION

The script of the video presentation has been provided on pages

10-21 for your reference in leading the discussion.

1. Discuss the "life cycle" of a typ ical cell.

2. Besides replicating its DNA, w hat other activities might

be expected to occur during interp hase in different types of

cells--for exam ple, w hite blood cells, nerve cells, and car-

diac muscle cells?

3. What reasons might account for the fact that some cells,

such as bacterial cells, cancer cells, emb ryon ic cells, and

disease fighting cells, repr od uce each day w hile others,

such as nerve cells, hardly ever reproduce?

4. Discuss the biological significance of the hap loid state,

the d iploid state, and the polyploid state.

PROJECTS

1. Microscopic Examination of Plant Mitosis

Have students examine stained and prepared slides of the

root tip cells of an onion. These pr epara tions provid e a

simple, inexpensive biological system for observ ing m i-

totic stages in a clear and un ambiguous w ay. Have stu-

dents locate each m itotic stage and r ecord the nu mber of

interphase, prophase, metaphase, anaphase, and telophasecells present in the field of view at med ium magn ification.

Tally num bers for the entire class and convert these nu m-

bers into p ercentages of total cells found in each stage--

these num bers will be prop ortional to the time spent in

each stage of the life cycle of these cells.

Have students locate newly-divided cells that are just

grow ing into full-sized cells and are in the interphase

stage Discuss cell grow th as part of interph ase


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2. Microscopic Examination of Animal Mitosis (Optional)

Have stud ents locate each mitotic stage in slides p repared

from wh itefish blastulas. (You w ill find that the m itotic

stages in these p reparations are mu ch less clear than in the

onion samples--but they are useful in that they allow

students to visually compare d ividing p lant and animal

cells.)

If you hav e been able to obtain r ecently fertilized snail,

frog, or toad eggs, observe them under a dissecting micro-

scope as they divide. Try to locate 2 cell, 4 cell, 8 cell, 16 cell

and the more ad vanced stages of embryological develop-

ment. Raise these embryos to adulthood in an aquarium

and observe the daily changes that occur d uring their

dev elopm ent. Discuss the differentiation of cells in the

context of embryological developm ent, or assign this sub -

ject as a topic for library research.

3. M icroscopic Examination of Chromosomes and Genes

Have stud ents observe the giant chromosomes found in

specially p repared slides of the salivary gland s of the fruit

fly Drosoph ila melanogaster. Because the DNA of these

cells has rep licated 9-10 times withou t sub sequent celldivision, a large num ber of DNA strand s will be present

side by side in a single chromosome. It is because of this

that these chrom osomes are very th ick and , hence, ex-

tremely useful for laboratory observation.

Each band of light or dark stain on a chromosom e may

indicate the location of an individu al gene. Chrom osome

"Puffing" in the r egions of sp ecific band s is believed to

indicate gene activity (messenger RNA syn thesis, inactiva-tion of pr otein rep ressors, etc.).

In the context of the fruit fly chrom osom es, discuss the

"Hu ma n Genom e Project." Describe the basic research

techniques emp loyed in mapp ing the exact location of all

hum an genes found on the 23 different hum an chromo-

somes, or assign this subject as a topic for library research.


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4. Microscopic Examination of Cells Undergoing Meiosis

Have stud ents observe the meiotic processes of sperm ato-

genesis in prepared slides obtained from the testes of a rat.

Althou gh it is difficult to identify specific meiotic stages, it

is possible to iden tify germ cells in the early, midd le, and

later meiotic stages simply by noting their proximity to the

sperm found in the centers of the tubules (peripheral cells

will be in early meiotic stages and those toward the center

will be in the later meiotic stages). Comp are the process of

spermatogenesis to the complementary female meiotic

process called oogen esis by w hich hap loid eggs a re pro-

duced, or assign this subject as a topic for library research.

VOCABULARY LIST

Anaphase: The mitotic stage that follows metaphase;

du plicated chromosomes separate at the centromere and

migrate toward the mitotic centers.

Asters: Microtubu les and fibers that radiate out from the

centrioles.

Asexual Reproduction: Reproduction involving only one

parent.

Blastula: A hollow ball of cells formed du ring the early

stages of embryological developmen t. Whitefish blastula

cells are used to dem onstrate anim al mitosis.

Centromere: The part of a chrom osome wh ere the chro-

matids are joined together.

Centriole: In anima l cells, a cytoplasm ic organ elle that

organ izes the mitotic spind le fibers du ring cell reprod uc-

tion.

Chromatid : One of the two strands that make up chromo-

somes seen in proph ase and m etaphase that have dup li-

cated their DNA du ring interphase. During anaphase,

chromatids separate to form d augh ter chromosomes.


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Chromatin: The coils of DNA and protein that condense to

form chromosomes. Chromatin can be though t of as

chromosomes with no distinct shape.

Chromosome: Distinct worm like structures formed from

chromatin du ring cell reprodu ction.

Crossing Over: An exchange of chromosomal material

between homologous pairs that occurs d uring Prophase

One o f meiosis.

Cytokinesis: Cytop lasmic d ivision that follows division

of the nucleus.

Diploid : Having two of each chromosome. Hu mans have

23 different chromosom es, yet in each bod y cell, these

chromosomes occur in tw os called hom ologous pairs. For

this reason, each bod y cell possesses a diploid num ber of 46

chromosomes.

DN A Rep lication: The process of dou bling the DNA tha t

occurs before mitosis.

Germ Cells: The only cells that can und ergo meiosis--

found in the ovar ies of females and th e testes of males.

Haploid : The actual num ber of different types of chromo-

somes a cell possesses.

Homologous Pairs: In dip loid cells, a pair of identical

chromosomes is called an homologous pair.

Interphase: The ph ase of a cell's life cycle between the

repr odu ctive stages of mitosis. DN A replication occurs

du ring interphase. Most cells spend about 95% of their life

cycles in interph ase.

Meiosis: The process that germ cells undergo by wh ich the

number of diploid chromosomes is reduced by half. Sperm

and egg cells are created by meiosis.


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Metaphase: The stage of mitosis where du plicated chro-

mosom es line up a long the center of the mitotic spind le.

Microtubules: Tiny tubes that m ake up most of a cell's

"cytoskeleton." Spind le fibers are mad e up of microtu -

bules.

Mitosis : The dup lication and division of the chromosomes

and nucleus du ring cell reprod uction.

Mitotic Centers: The centers of mitotic activity of a cell--

toward wh ich separated chromosomes migrate.

Oogenesis: The m eiotic process that results in the forma-

tion of eggs in a female.

Ova : Another word for eggs.

Ovum : One egg.

Polyploid : Having more than a diploid number of chro-

mosomes.

Prophase: The first stage of mitosis when chrom osomes

form from chrom atin and the nu clear membr ane is ab-

sorbed into the cell.

Reduction D ivision: Cell division such as occurs in meio-

sis that results in the pr odu ction of cells with ha lf the

num ber of chromosomes found in the original parent cells;

cell reprodu ction w ithout DN A rep lication.

Sexual Reproduction : Reprodu ction requiring two par-

ents.

Somatic Cells: Body cells. Cells other than germ cells.

Spermatogenesis: The meiotic process that results in the

formation of sperm cells in m ales.


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M ITOSISAN D MEIOSIS

SCRIPTOF VIDEO PRESENTATION

Nearly every organism tha t is mad e up of man y cellsbegins life as the single cell of a fertilized egg.

That single cell divid es over and ov er again un til eventu -

ally an embryo is formed that is mad e up of trillions of cells

of many different types.

Over time, the embryo develops into a baby, and even at a

very you ng age, some of the babys cells begin to wear out.

In fact, in a typ ical hum an being, every second of every day

witnesses the death of abou t 50 million cells.

Therefore, new cells must be constantly prod uced to re-

place old, dead and dam aged cells.

For cells to reproduce themselves, whether in a developing

embryo or in a fully grown adult organism, certain definite

steps mu st be followed to assure that the n ew cells willcontain exactly the same gen etic ma terials, or genes, tha t

were originally p resent in the p arent cell, and th e essential

process un derlying th e reprodu ction of cells is called mito-

sis.

Mitosis is defined as "the d up lication an d division of the

nu cleus of a cell and its chrom osomes d uring cell repro-

duction."

Spind le Fibers: Microtubules visible du ring cell division

that are involved in separating the chromosomes into two

separa te, yet identical grou ps.

Synapsis: The pairing of hom ologous chrom osomes dur -

ing meiosis. Synapsis does not occur du ring mitosis.

Telophase: The last stage of mitosis wh en the chromo-

somes return to the form called chroma tin and the nu clear

membran e reforms. Telophase usually happ ens simu lta-

neously with cytokinesis.


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Scientists recognize fou r d istinct stages of m itosis: first is

Prophase, second is Metaphase, third is Anaphase, and

fourth is Teloph ase. An average of abou t six percent of a

cells total lifespan is spent in these four stages of mitosis,

while the other 94% of its life is spent in a stage tha t is not

considered to be a part of mitosis called interp hase.

INTERPHASE: The Resting Stage

Because the events tha t occur in interphase allow mitosis to

take place, let us begin our examination of mitosis by

taking a closer look at interp hase cells.

Interp hase is d efined as "the per iod of a cells life cycle

between one mitosis and the next mitosis; the period of cell

growth."

As can be seen in th is microscopic image on an onion root,

some cells are d ividing, bu t the nuclei of most of the cells

are roun d an d intact...these are interph ase cells.

Average hu man cells, such as these that form connective

tissue, spend about 19 hours in interphase and only be-

tween 50 to 90 minutes in m itosis and cell division.

How ever, there is considerable variation among d ifferent

types of cells as to how mu ch of their life cycles are spent

in interphase and in mitosis.

For example, this human nerve cell very rarely reproduces;

instead, it can rem ain in interphase for up to sixty years.

Because nerve cells rarely reproduce, even to replace dead

or d amaged cells, nerve d amage, such as results from

strokes or spinal cord injuries, is usu ally very serious.

On th e opp osite end of the scale from nerve cells are cancer

cells. These purp le stained cancer cells, which stand out

clearly against a background of red blood cells, were taken

from a leukem ia victim.


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Cancer cells like these do not remain in the interphase state

very long; instead, mitosis and cell division continues at a

furious pace.

In fact, cancer can be thou ght of as cells that never rest, that

divide over and over again, never remaining very long in

interphase.

Even though it is often thou ght of as a resting state between

cell divisions, a lot is actually hap pening inside the nucleus

throu ghou t m uch of this stage of a cells life cycle.

During interphase, deoxyribonucleic acid, or DN A, the

enormous molecule that forms the cells genes and that

hold s all of the cells opera ting instru ctions, du plicates

itself.

Deep inside the nucleus of interp hase cells, the tw o DNA

strands un wind as new copies of the DNA are created.

Scientists refer to this process of DNA duplication as

replication because exact rep licas of the D NA are p ro-

duced.

The rep lication of a cells DNA m akes it possible for two

complete sets of biological instructions to be sent into th e

new dau ghter cells.

During m itosis, DNA is foun d w ithin chromosom es; but

chromosom es dont actually exist du ring interph ase, in-

stead, DNA and protein form the grainy threads w e see

here called chromatin.

The pr esence of chrom atin is one ind ication a cell is in

interph ase, for only with th e onset of mitosis does the

chromatin m old itself into the distinct shape of the chromo-

somes seen here.

Scientists still do not un derstand the exact roles that chro-

mosomes p lay within the cell, but i t seems likely that

chromosomes are the best way to package DNA, and


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consequently genes, for shipm ent into the new ly-forming

cells.

The chromosomes seen here, taken from the salivary gland

of a fruit fly, provide a convenient material for scientific

study because they are ten times longer and one hun dred

times thicker than normal chromosomes.

Working with these cells, scientists have discovered that

the dark and light bands we see here are the actual location

of genes on the chromosomes.

And their stud ies have also shown that wh en a particular

gene on one of these chromosom es becomes active, the

spot on the chromosome w here that gene is located takes

on a strange, puffed-out app earance.

During m itosis, the appearan ce, movem ent, separation

and disap peara nce of chromosom es is of critical imp or-

tance to the cell reprod uctive process. In fact, by recogniz-

ing what is happ ening to the chromosomes, we can iden-

tify each of the fou r stages of mitosis.

Now let u s follow th e events inside of a cell as it starts to

un der go the intricate process of mitosis.

PROPH ASE: The First Stage of Mitosis

The earliest sign tha t a cell is leaving interph ase and is

entering th e first stage of mitosis, called pr oph ase, is that

the chromatin begins to form itself into the definite shapes

of separa te chromosomes; and at the same time, the nu clear

membrane that separates the nu cleus from the cytoplasm

begins to be absorbed into the cell. As these things happen,

the tiny nu cleolus, found in the n ucleus, also disappears.

Thus, prop hase can be defined as "the stage of mitosis

wh en the chromosomes first app ear and the nu clear mem-

brane and nu cleolus d isappear from view."

As prop hase progresses, the chromosomes become m ore

distinct; and now that the chromosom es can be seen, a


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rather odd fact becomes apparent: that chromosomes are

always found in grou ps of two called h omologous chro-

mosomes. This means there are two copies of each chromo-

some present in every cell of the body with the exception of

certain sex cells. In our exam ple, there is one pair of tall

chromosomes and one pair of short chromosomes.

This state, where double chromosomes are present, is

called th e dip loid cond ition of a cell.

The diploid cond ition of a cell means that there w ill be two

complete sets of DNA instructions present inside its nucleus,

so that if something is wrong w ith one set of instructions,

the other set w ill still be able to p rovide the cell with th e

information it needs to function prop erly.

Und er close examination, we find that in early prop hase

each individu al chromosome--whether tall or short--has a

rather thickish app earance. This is because the DN A of

each chromosome has replicated itself during interphase.

On looking even m ore closely, we see that each chrom o-

some actually consists of two p arts called sister chromatids

that contain the duplicated DNA; and at first, the sister

chromatids are stuck very close together all along theirlengths.

I t can now be seen that du ring prophase, and p ar t of

interphase, there are actually four complete sets of DNA

instructions present because the DNA of each hom ologous

pair is doubled at this point.

The sister chromatids are held together at a specific region

called the centromere. Joined to geth er in this way , the

chromosom es begin to arrange themselves so they can

separate into the n ewly-forming cells; and as they d o so,

the sister chromatids become mu ch more obvious.

Besides the changes in chromatin and chromosomes, other

importan t events are also occurr ing inside the cell wh en

prop hase begins. In animal cells, a stru cture called a


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centriole or centrosome divides into two daughter centri-

oles that migrate to opposite end s of the cell.

Between the centrioles, a delicate arrangement of microtu-

bules, called th e spindle, is formed. The microtubules that

make u p the spind le are called spindle fibers.

Spindle fibers are critical to cell reproduction because they

help arrange th e chromosomes and later in mitosis sepa-

rate them into two equal groups.

In norm al cells, all the comp licated even ts that define

prop hase take between 30 and 60 minu tes to comp lete,

finally end ing when th e next stage of mitosis, called met-

aphase, is reached.

METAPHASE

Metaph ase, the second stage of m itosis, is defined as "the

stage of mitosis wh en all the chrom osomes are lined u p

along the center or equator of the cell."

Throughou t this short 5- to 10-minute stage, the chromo-

somes are attached to the sp indle fibers, and th e centro-

meres that bind th e sister chrom atids together split apart.

ANAPHASE

The splitting of the centromeres signals the start of the

third stage of mitosis called anap hase.

Anap hase is defined as "the stage of mitosis when the sister

chromatids separate and m ove toward op posite poles ofthe cell," and w hen th is happ ens, they are no longer called

sister chromatids--they are now called "daughter" chromo-

somes.

This movement happ ens fairly rapidly, in about five min-

utes, with the result that the spindle fibers disapp ear from

view and a full "diploid" set of chromosomes is now found

at each end of the cell.


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TELOPH ASE AND CYTOKINESIS

The fourth and final stage of mitosis is called telophase.

Teloph ase is defined as "the stage of mitosis when the new

dau ghter chromosomes change back into the threads of

chromatin and new nuclear membranes begin to form."

Also during telophase, new nucleoli appear in each newly-

forming nucleus.

As the final stage of mitosis conclud es, the cytoplasm

divides in half as cell mem branes close up aroun d the two

new dau ghter cells.

This final p rocess of cell reprod uction is called cytokinesis.Cytokinesis is defined simply as "the d ivision of the cyto-

plasm d uring mitosis."

Between ten and fifteen minutes are requ ired to finish both

telophase and cytokinesis.

When th e entire cell reprod uctive process is completed, the

two new cells are returned to the interphase state, and each

possesses the iden tical genes of the pa rent cell.

These two n ew half-size cells will then go on to grow

larger un til each achieves the fu ll size of the original parent

cell.

End of Part 1

Video Instructional Quiz, Part 1

1. The state of mitosis seen here is called

__________________.

A. metaphase

2. Chrom atin shortens and th ickens to form these distinct

structures called _________________.

A . chromosomes


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3. DN A is replicated d uring th e stage between cell divi-

sions called ___________________.

A. interphase

4. Tru e or False: The d ivision of the cytoplasm following

mitosis is called cytokinesis.

A. True

5. The first stage of mitosis, the stage wh en th e nu clear

mem bran e begins to be reabsorbed into the cell, is called

_____________________.

A. prophase

Part 2 MEIOSIS

In the first part of this program w e learned h ow bod y cellsreprod uce themselves by mitosis so that the n ew cells

contain the same diploid, or double, num ber of chromo-

somes foun d in the p arent cell.

Now we w ill learn how certain d iploid germ cells foun d

only in the ovaries and testis undergo the process similar to

mitosis called meiosis that results in the p rodu ction of sex

cells that have only one half the number of chromosom es

found in the parent cells.

In the case of human beings, the diploid bod y cells all have

46 chromosom es, whereas the sex cells, the sperm and egg,

have only 23 chromosomes.

This condition wh ere paired chrom osomes are absent is

called th e hap loid cond ition of a cell.

When tw o hap loid cells, sperm an d egg, join together atfertilization, a new d iploid cell is form ed t hat w ill then

un derg o mitosis over and over again, resulting in the

creation of a brand new individual.

Meiosis is similar to mitosis in man y w ays, but in oth er

ways, it is quite different. Now let us take a closer look at

some of the details of this fascinating p rocess.


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PROPHASE ONE

The earliest part of the first stage of meiosis, called prophase

one, begins just as it does in mitosis: chromosomes form

from chromatin, and these chromosomes contain DNA

replicated du ring the p receding interphase.

As proph ase one progresses, two things hap pen in the

germ cells that did nt occur d uring p roph ase in mitosis:

first, the iden tical, or hom ologous, chromosom es form

pairs w ith one another, so that the tall chromosomes find

their mates and the short chromosomes d o likewise.

This pairing of homologous chrom osomes du ring the first

proph ase of meiosis is defined as synapsis.

The second difference between p roph ase one of meiosis

and the one we saw in mitosis is that par ts of one paired

chromosome can actually be exchang ed w ith parts of its

opp osite pair. This process, called crossing over , is defined

as the exchange of chrom osomal material dur ing synapsis

and occurs only d uring th e first prop hase of meiosis.

METAPHASE ON E

During the next stage of meiosis called metap hase one, the

centromeres of the hom ologous chromosomes line u p on

the equ ator of the cell.

In contrast, during mitosis, the centromeres of the homolo-

gous chrom osomes align themselves in a completely inde-

pend ent fashion.

ANAPHASE ONE

During th e third stage of meiosis called anaph ase one, the

homologous chromosom es, each made u p of two sister

chromatids, move ap art, centromeres intact, one toward

each m itotic center.

In contrast, du ring mitosis the centromeres separate d ur-

ing anaphase and the sister chromotids move apart as new


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"daughter" chromosomes.

TELOPHASE ONE

Dur ing the fourth stage of meiosis called telophase one,

each new cell contains just one of each homologou s chro-

mosome, and each of these chromosomes still consists oftwo "sister" chromatid s.

PROPHASE TWO

The fifth stage of meiosis, called prop hase tw o, follows

right on the h eels of telophase one w ithout an interphase

stage in between them , as happens in mitosis. This means

the DNA of the chrom osomes in the prophase two cells has

not been replicatedand this explains wh y the chromo-somes of prop hase two cells have not sprou ted any new

sister chromatids.

From now on out, meiosis follow s the same stages as

mitosis.

METAPHASE TWO

During the sixth stage of meiosis, called metaphase tw o,the chrom osomes line up on the equ ator of the cell, and

now the centromeres split apart.

ANAPHASE TWO

And du ring the seventh stage of meiosis, called anaphase

two, the centromeres separate for the first time, and now

the sister chrom atids become d augh ter chrom osomes as

they m ove toward op posite poles of the cell.

TELOPH ASE TWO

Finally, dur ing the last stage of meiosis, called telophase

two, new nuclear mem branes form. And as the chrom o-

somes change back into chromatin, w e can see that four

new sex cells have been p rodu ced from the single original


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diploid germ cell.

But since the DNA w as only replicated once throughou t a

series of two cytop lasmic divisions, the final num ber of

chromosomes in the sex cells is only half the nu mber

present in the d iploid parent cell, in other word s, the sex

cells are hap loid.

Now wh en tw o hap loid sex cells, sperm and egg, join

together, the fertilized egg will contain th e same dip loid

num ber of chromosomes found in any bod y cell.

Meiosis is very imp ortant. It enables all organ isms that

reprod uce sexually, that is require both m ale and female

parents, to do so without continually multiplying the

amoun t of DNA in the nucleus. For without meiosis, thehu man d iploid nu mber of 46 chromosomes wou ld reach

368 in just th ree generations, thu s creating a very messy

state inside the third generation cells.

Meiosis also provides an effective mechanism whereby the

DNA of the two parents may be re-sorted and re-combined

in new w ays, and for this reason, no two hum an beings are

exactly the sam e.

Meiosis assures that each n ew gen eration will be unique

and will always have new possibilities.

The result is that each generation will always have its own

great ath letes, scientists, artists, musicians and inventors.

And , as strange as it seems, meiosis also provides a p hysi-

cal bridge between successive generations, as pa rents pass

along a tiny amou nt of their own sub stance--their ownDNA--to their children.

Video Instructional Quiz, Part 2

1. As a resu lt of meiosis, sex cells are formed that h ave

____________the number of chromosomes of body cells.

A. half


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2. True or False: Pairs of iden tical chrom osom es are also

called h omologous chromosomes.

A. True

3. True or False: No rm al body cells are d iploid because

they possess one of each different chrom osome.A. False

4. Tru e or False: Du ring m eiosis, the DN A is replicated

twice.

A. False

5. True or False: In p lant an d anim al cells, meiosis occurs

only in germ cells.

A. True

BLACKLILNE MASTER 5

CROSSWORD PUZZLE

ANSWERS


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Documents

Mitosis Meiosis Teachers Guide Discovery Education