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MITOSIS


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Another of the characteristics that all livingAnother of the characteristics that all living

things possess is the ability to reproducethings possess is the ability to reproduce ---- truetrue

for organisms consisting of a single cell throughfor organisms consisting of a single cell through

those made up of billions of cells.those made up of billions of cells.

T

he biological characteristics of all organismsT

he biological characteristics of all organismsare determined by molecules ofare determined by molecules of DNADNA that residethat reside

in the nuclei of the cellsin the nuclei of the cells ---- DNA moleculesDNA molecules

contain "instructions" which determine cellcontain "instructions" which determine cell

function and all other characteristics.function and all other characteristics.


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ReproductionReproduction: the production of new organisms or cells from pre: the production of new organisms or cells from pre--

existing onesexisting ones ---- involves the transmission of DNA from parents toinvolves the transmission of DNA from parents to

offspringoffspring

Cells and organisms display two types of reproduction:Cells and organisms display two types of reproduction:

1.1. Asexual reproductionAsexual reproduction ---- only one parent and offspring areonly one parent and offspring are

genetically identical ("clones") of the parentgenetically identical ("clones") of the parent ---- offspring receiveoffspring receive

exactlyexactly the same DNA instructions as parent has.the same DNA instructions as parent has.

2.2. Sexual reproductionSexual reproduction ---- two parents and offspring have slightlytwo parents and offspring have slightly

different DNA message from those of the parents and from otherdifferent DNA message from those of the parents and from other

offspring.offspring.

Underlying each of these reproductive modes is a specific type ofUnderlying each of these reproductive modes is a specific type of

cell division:cell division: mitosismitosis for asexual reproduction andfor asexual reproduction and meiosismeiosis forforsexual reproduction. We will consider these two processes in asexual reproduction. We will consider these two processes in a

moment.moment.


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Lets first take a superficial look at the nature of the DNALets first take a superficial look at the nature of the DNA

instructions present in all cells.instructions present in all cells.

Molecules of DNA are found associated with proteins inMolecules of DNA are found associated with proteins in

structures calledstructures called chromosomeschromosomes ---- can think of these as long,can think of these as long,

threadthread--like fiberslike fibers ---- chromosomes are found in thechromosomes are found in the cell nucleuscell nucleus..

All species have a characteristic number of chromosomes in allAll species have a characteristic number of chromosomes in alltheir cellstheir cells ---- this is called thethis is called the diploid numberdiploid number and is symbolized asand is symbolized as

thethe 2N number2N number ---- e.g. for humans, 2n = 46; domestic dog, 2N =e.g. for humans, 2n = 46; domestic dog, 2N =

78; fruit fly, 2N = 14.78; fruit fly, 2N = 14.

In order for cells to reproduce, chromosomes must undergoIn order for cells to reproduce, chromosomes must undergo

replicationreplication so that there will be copies of the chromosomes to beso that there will be copies of the chromosomes to be

passed on to offspring cells.passed on to offspring cells.

A chromosome that has replicated then consists of two "A chromosome that has replicated then consists of two "sister"sister"

chromatidschromatids held together by a structure called aheld together by a structure called a centromerecentromere..


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Cells that are not dividing are said to be inCells that are not dividing are said to be in interphaseinterphase ---- majority of themajority of the

cell's life is spent incell's life is spent in interphaseinterphase ---- time during which cells growtime during which cells grow ----chromosomes appear as an amorphous mass calledchromosomes appear as an amorphous mass called chromatinchromatin ---- justjust

prior to the beginning of mitosis, chromosomeprior to the beginning of mitosis, chromosome replicationreplication occurs andoccurs and

chromosomes consist of pairs of duplicate sisterchromosomes consist of pairs of duplicate sister chromatidschromatids..


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(Prophase):(Prophase):

1.1. ProphaseProphase ---- first phase of mitosis.first phase of mitosis.

a. chromatin condenses into thick, short chromosomes.a. chromatin condenses into thick, short chromosomes. b.b. centriolescentrioles begin moving towards opposite ends ofbegin moving towards opposite ends of

nucleus.nucleus.

c. spindle fibers (c. spindle fibers (microtubulesmicrotubules

) begin to appear.) begin to appear.

d. nuclear envelope begins to disappears.d. nuclear envelope begins to disappears.


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2.2. MetaphaseMetaphase ---- second phase of mitosis.second phase of mitosis.

a. chromosomes align along equator ofa. chromosomes align along equator ofspindle apparatus.spindle apparatus.

b. attached to spindle fibers atb. attached to spindle fibers at centromerecentromere..


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3.3. AnaphaseAnaphase ---- third phase.third phase.

a.a. centromerescentromeres of each chromosome divide andof each chromosome divide and

chromatidschromatids now separatenow separate ---- sistersister chromatidschromatids are noware now

chromosomeschromosomes.. b. spindle fibers begin new chromosomes towardsb. spindle fibers begin new chromosomes towards

opposite poles.opposite poles.


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4.4. TelophaseTelophase ---- final phase of mitosis.final phase of mitosis.

a. nuclear envelope reappears around two newa. nuclear envelope reappears around two new

daughter nuclei.daughter nuclei.

b. chromosomes revert to the diffuse chromatinb. chromosomes revert to the diffuse chromatin

condition.condition.

New daughter cells now enterNew daughter cells now enter interphaseinterphase, where, where

they will spend the majority of their lives.they will spend the majority of their lives.


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Cytokinesis division of cytoplasmdivision of cytoplasm ---- occurringoccurring

concurrently with mitosisconcurrently with mitosis ---- takes placetakes place

differently in plants and animals.differently in plants and animals.


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Importance of mitosis each new daughter cell receives the same number and kind of chromosomes aseach new daughter cell receives the same number and kind of chromosomes as

the mother cell; daughter cell arethe mother cell; daughter cell are genetically identicalgenetically identical to their mother cell andto their mother cell and

to each other.to each other.

Mitosis serves two major functions:Mitosis serves two major functions:

-- process ofprocess of growth and repairgrowth and repair ofof multicellularmulticellular organismsorganisms ---- all of us began lifeall of us began life

as a single cell that has undergone and continues to undergo mitotic cellas a single cell that has undergone and continues to undergo mitotic celldivision.division.

-- process by which organismsprocess by which organisms reproducereproduce asexuallyasexually ---- offspring are "clones" ofoffspring are "clones" of

their parent.their parent.

a.a. binary fissionbinary fission ---- singlesingle--celled organisms (e.g. ameba) reproduce simplycelled organisms (e.g. ameba) reproduce simply

by splitting into two new cells.by splitting into two new cells.

b.b. buddingbudding ---- bud on parent organism (e.g. yeast) develops into a newbud on parent organism (e.g. yeast) develops into a new

organism through repeated cell divisions.organism through repeated cell divisions.

c.c. vegetative reproductionvegetative reproduction ---- plants send outplants send out stolonsstolons (above ground) or(above ground) or

rhizomesrhizomes (underground) which become new individuals(underground) which become new individuals ---- e.g.e.g.

strawberries, Bermuda grass, ferns.strawberries, Bermuda grass, ferns.


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Now, let's look at meiosis -- the

process underlying sexualreproduction.


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Here's a overview of sexual

reproduction: new organism begins its life as a singlenew organism begins its life as a single--cell called acell called a zygotezygote..

a zygote formed is from the fusion of specialized sex cells calleda zygote formed is from the fusion of specialized sex cells called gametesgametes

(e.g. sperm and egg).(e.g. sperm and egg).

each ofeach of two parentstwo parents contributes one gamete to the zygote.contributes one gamete to the zygote.

since the zygote issince the zygote is diploiddiploid ((2N2N), the gametes that fuse to form it must have), the gametes that fuse to form it must have

1/2 the diploid number1/2 the diploid number ---- gametes, therefore, are said to begametes, therefore, are said to be haploidhaploid ((1N1N)) ----

e.g. human zygotes, 2N = 46; human sperm cell (or ova) 1N = 23e.g. human zygotes, 2N = 46; human sperm cell (or ova) 1N = 23..

haploid gametes are produced inhaploid gametes are produced in germ tissuegerm tissue ((gonadsgonads) by a special type of) by a special type of

cell division that results in the daughter cells (gametes) having only 1/2 thecell division that results in the daughter cells (gametes) having only 1/2 thenumber of chromosomes of the mother cellnumber of chromosomes of the mother cell ---- this type of cell division isthis type of cell division is

calledcalled meiosismeiosis..


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In sexually reproducing organisms, chromosomes exist asIn sexually reproducing organisms, chromosomes exist as

homologous pairshomologous pairs ---- each chromosome has a "mate" oreach chromosome has a "mate" orhomologuehomologue that carries the same kind of geneticthat carries the same kind of genetic

informationinformation ---- one member of the homologous pair isone member of the homologous pair is

maternallymaternally inherited and the other isinherited and the other is paternallypaternally inheritedinherited ----

e.g. humans have a diploid number of 46 chromosomese.g. humans have a diploid number of 46 chromosomesarranged as 23arranged as 23 homologous pairshomologous pairs..

Let's look at meiosisLet's look at meiosis ---- consists of two nuclear divisionconsists of two nuclear division

(meiosis I and II)(meiosis I and II) ---- in the first division, the chromosomein the first division, the chromosome

number is reduced by 1/2 (diploid to haploid)number is reduced by 1/2 (diploid to haploid) ---- the secondthe second

division is actually nothing more than mitosis.division is actually nothing more than mitosis.

End result of meiosis isEnd result of meiosis is 4 haploid daughter ce4 haploid daughter cells.lls.


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Prior to meiosis, mother cells are in interphase -- like

mitosis, DNA replication occurs during interphasejust prior to meiosis.

Meiosis IMeiosis I: the reduction division: the reduction division ---- diploiddiploid

mother cell produces 2 haploid cells.mother cell produces 2 haploid cells.

Meiosis IIMeiosis II: mitotic division of 2 haploid: mitotic division of 2 haploid

cells to produce 4 haploid daughter cells.cells to produce 4 haploid daughter cells.

The important aspects of meiosis occur inThe important aspects of meiosis occur in

Meiosis I.Meiosis I.


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A twoA two--stage cell division found onlystage cell division found onlyin sexually reproducing organismsin sexually reproducing organisms

that result in gametes with half ofthat result in gametes with half of

the chromosome number of thethe chromosome number of theoriginal cell (1N)original cell (1N)

MEIOSIS:


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MEIOSIS I :

a). Prophase I:- many processes but theimportant one is a process called synapsis

or the formation of TETRAD of two homologouschromosomes that enable crossing over

between these two homologouschromosomes.Crossing over increases variability of

gametes and individuals.


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MEIOSIS I: Contd.

b). Metaphase I:- Chromosomes (still in

homologous pairs or tetrad) are arrangedon the metaphase plate.


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MEIOSIS I: Contd.

c). Anaphase I: - Chromosomes (still inpairs=sister chromatids) separate from theirhomologue and go towards the opposite poles.


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MEIOSIS I: Contd.

d). Telophase I and Cytokinesis: - Chromosomes(still in pairs=sister chromatids) are in each of

the opposite poles- no nuclear envelope formed- Cytokinesis produces two daughter cells that

have half of parent chromosomes and NONUCLEAR ENVELOPE -> as preparation tomeiosis II


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MEIOSIS II :

a). Prophase II: - similar to prophase in mitosis

b). Metaphase II: - similar to metaphase inmitosis

c). Anaphase II: - similar to anaphase in mitosis

d). Telophase II and Cytokinesis:- similar to telophase and cytokinesis in

mitosis- each starting daughter cell (resulted

from meiosis I) produces two daughter; so, inthe final, four daughter cells areproduced from meiosis.


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In our species, meiosis occurs in

spermatogenesis (sperm production) andoogenesis (egg production.

1. spermatogenesis -- 4 daughter cells that can

develop into 4 sperms -- tiny, flagellated cells

adapted for locomotion.

2. oogenesis -- only one of the daughter cells

becomes an ova -- other three are small,

nonfunctional polar bodies.


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Importance ofMeiosis:

1. Produces haploid gametes so that the diploid

number of the species remains constantgeneration after generation.

2. Source of genetic variation because crossing

over brings together new gene combinations

on chromosomes.

http://www.sci.sdsu.edu/classes/bio

100/Lectures/Lect05/lect05.html


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http://www.sci.sdsu.edu/classes/bio

100/Lectures/Lect05/lect05.html


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