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Mitosis and MeiosisCell Division

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Why Do Cells Divide?

For growth, repair, and

reproduction

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Mitosis

Organisms grow by theadditionof cells

In multicellular organismsome of these cells

perform functions dierentfrom other cells.

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he process of a cellbecoming dierent is

diferentiation. !nder normal conditionsonce an animal cell becomes

speciali"ed it can nolongerform an entire organism,however plant cells are

totipotent and any cell canform an entire plant.

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When do cells divide?

Most limiting factor in si"e isthe si"e of the cell membrane.Cells must obtain nutrients

as volume increases, cellsurface area does not increaseas greatly

larger cells re#uire a largersurface area for survival

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Cell Division vs. $uclearDivision

Cyto%inesis& he actual division of thecell into two new cells.

Mitosis& he division of the nucleus ofthe cell into two new nuclei.

$ote& 'ometimes cells go throughmitosis without going throughcyto%inesis. Describe a cell that did this.

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erminology

Chromatin( thin )brousform of D$* and proteins

Sister chromatids(identical structures that

result from chromosomereplication, formed during 'phase

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*natomy of a

Chromosome Centromere(

point where sisterchromatids are

+oined together

Pshort arm-upward

Qlong arm-

downward

Telomere(tips ofchromosome

p -arm

centromere

q-arm

telomerechromatids

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ow Do Cells Divide/

Cell cycle( se#uence ofphases in the life cycle of

the cell

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Getting ready to split

Cell cycle has two parts:

growth and preparation

0interphase1cell division

mitosis 0nuclear division1cyto%inesis 0cytoplasm division1

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Interphase

Occurs betweendivisions

2ongest part of cycle 3 stages

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Interphase

45 or 4ap 5

he cell +ust )nisheddividing so in 4ap 5the cell is recovering

from mitosis

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Interphase

' or 'ynthesis

stageD$* replicates

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Interphase

4 6 or 4ap 6

his is preparationfor mitosis

Organelles are

replicated.More growthoccurs.

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MITOSIS

Mitosis begins after G 2 and ends

before G 1

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7rophase

Chromosome

condense Microtubles form

he nuclear

envelope brea%sdown

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Metaphase

Chromosomes

are pulled tocenter of cell

2ine up along8metaphaseplate9

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*naphase

Centromeres divide 'pindle )bers pullone set of

chromosomes toeach pole

7recise alignmentis critical todivision

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elophase

$uclear envelope form aroundchromosomes

Chromosomes uncoil

Cyto%inesisanimals ( pinching of plasma

membrane

plants( elongates and the cell plateforms0 future cellwall and cellmembrane1

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Meiosis

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What is Meiosis?

A division of the nucleus that reduces

chromosome number by half.

Important in sexual reproduction

Involves combining the genetic

information of one parent with that of

the the other parent to produce a

genetically distinct individual

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erminology

Diploid( two sets ofchromosomes 06n1, in

humans 63 pairs or :; total Haploid( one set of

chromosomes 0n1 ( gametesor se< cells, in humans 63chromosomes

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Chromosome 7airing

Homologous pair

each chromosome in pair

are identical to the other0 carry genes for same trait1

only one pair diers ( se

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7hases of Meiosis

* diploid cell replicates itschromosomes

wo stages of meiosis

Meiosis I and Meiosis II

Only 5 replication

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Synapsis( pairing ofhomologous chromosomes

forming a tetrad.Crossing over( chromatidsof tetrad e

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Meiosis I

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7rophase I

Chromosomes condense

omologous chromosomes pair

w? each other @ach pair contains four sister

chromatids ( tetrad

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Metaphase I

etrads or

homologouschromosomes move to

center ofcell

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*naphase I

omologou

schromosomes pulled to

oppositepoles

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elophase I

Daughter

nucleiformed

hese arehaploid 05n1

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Meiosis II

Daughter cells undergo a

second division- muchli%e mitosis

NO ADDITIONA!"#ICATION OCC$!S

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7rophase II

'pindle

)bers formagain

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Metaphase II

'isterchromatidsmove to thecenter

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*naphase II

Centromeressplit

Individualchromosome

s are pulledto poles

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elophase II A

Cyto%inesis Four haploid

daughtercells resultsfrom one

originaldiploid cell

ev ew os s

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ev ew os sMeiosis

oth are forms of nucleardivision

oth involve replication

oth involve disappearance ofthe nucleus, and nucleolus,

nuclear membrane oth involve formation ofspindle )bers

DI%%"!"NC"S

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DI%%"!"NC"S &eiosisproduces daughter cells that

have 1/2the number of chromosomesas the parent. 4o from 6n to 5n.

Daughter cellsproduced by meiosis

are notgenetically identicalto oneanother.

In meiosiscell division ta%es placetwice but replication occurs onlyonce.

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alue of ariation

'ariation( dierencesbetween members of apopulation.

Meiosis results in randomseparationof chromosomes ingametes.

Causes diverse populationsthat over time can be strongerfor survival.