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Mitosis and Meiosis
catalog #2142
Teachers Guide
Produced by ...
Chariot Productions
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E-Mail: info@agcu nited .com
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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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2
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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17
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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