Meiosis

Lesson: MEIOSISBy

Dasari Sreekanth. M.Sc.,NET, TS/AP-SET,GATE, (Ph.D.)

Research ScholarDepartment of botany, O.U.

Email: [email protected]

Meiosis• Makes 4 cells genetically different from

parent cell & from each other• Production of gametes = sperm & egg• Contain half the number of chromosomes

= haploid (1N)• Called the “reduction” division =

– Diploid Haploid– 2N 1N

• Used in sexual reproduction

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Meiosis allows sex cells to join to form a complete set of chromosomes/instructions to make a new

organism.

Copyright Pearson Prentice Hall

Haploidgametes

MEIOSIS

Diploidzygote

MITOSIS

Diploidadult

If egg and sperm had same number of chromosomes as other body cells . . . baby would have too many chromosomes!

No Good!

The Problem!

The Solution!

Much Better!

If egg and sperm had half the number of chromosomes than other body cells . . . baby would have just the right amount!

You tell me!

• Drosophila (fruit flies)– Diploid: 2N = 8– Haploid: 1N = ?

• Lettuce– Diploid: 2N = 8– Haploid: 1N = ?

• Goldfish:– Diploid: 2N = ?– Haploid: 1N = 48

In which cells this

MeiosisTakes place….?

MeiosisMeiosis• Sex cells divide to produce gametes (sperm or

egg).

• Gametes have half the # of chromosomes.

• In animals Occurs only in gonads (testes or ovaries).

Male: spermatogenesisFemale: oogenesis

In plants meiosis takes place in : Microscporangium and megasporangium.

During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23

+ =

Sperm (23) + Egg (23) = Fertilized Cell (46)

Meiosis ensures that

all living organisms

will maintain both Genetic Diversity and

Genetic Integrity

SpermatogenesisSpermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

FertilizationFertilization

• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Interphase IInterphase I

• DNA replicates forming duplicate homologous chromosomes. (S phase).

• Centriole pairs also replicate.

• Nucleus & nucleolus visible.

Meiosis Meiosis

• Meiosis can be devided into 2 phases;

• Meiosis-I• Meiosis-II

Meiosis IMeiosis I• Cell division that reduces the

chromosome number by one-half.

• four phases:a. prophase Ib. metaphase Ic. anaphase Id. telophase I & cytokinesis

Prophase I

Metaphase I

Anaphase I

Telophase I &Cytokinesis

Prophase IProphase I

• Longest and most complex phase (90%).

• Homologous chromosomes come together to form a tetrad = Synapsis

• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

MEIOSIS I• Prophase I

• The The longestlongest phase. phase. This phase can be This phase can be divided into 5 divided into 5 stages;stages;

a) Leptoteneb) Zygotenec) Pachytened) Diplotenee) Diakinesis

1 .Leptotene

2.Zygotene

3.Pachytene 4.Diplotene 5.Diakinesis

Prophase I

LEPTOTENE(THIN- THREAD LEPTOTENE(THIN- THREAD STAGE)STAGE)

Leptonema.Chromosomes condense into long threads.Two sister chromatids are tightly bound.Flower bouquet phaseCentriole duplicate and earch daughter centriole migrates towards opposite poles of the cell.

Zygotene (yoked- thread Zygotene (yoked- thread stage)stage)

ZygonemaChromosomes become shorter and thicker.Pairing of homologous chromosomes = synapsis or syndesisA pair of homologous chromosomes lying together is called as Bivalent.Synoptonemal complex formed in between two homologous chromosomes.Pairing may occur in 3 ways

1. proterminal pairing2.procentric pairing3. random pairing.

Zygotene.Zygotene.

Homologous chromosomes

sister chromatidspaternal

sister chromatidsmaternalTetrad

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Pachytene (thick-thread Pachytene (thick-thread stage)stage)

PachynemaGenetic recombination stageTwo visible chromatids of each chromosomes are called = Dyad.A group of 4 homologous chromatids are called = Tetrad (2 Dyads).Crossing over (Genetic recombination ) takes place between non non sister chromatids of homologous chromosomes.Breakage and reunion of chromatid segments mediated by endonuclease and ligase.Pachytene responsible for the origin of new species and thus leads to evolution.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Diplotene (double stage)Diplotene (double stage)Homologous chromosomes start separating called disjunction.Separation will not takes place at chiasmata.Chiasmata are clearly visible as ‘X’ shape.Chiasmata are the sites where the crossing over occurred during pachytene.Chiasmata helps in holding homologous chromosomes.

DiakinesisDiakinesis

Chromosomes are highly condensed.Chiasmata begin to move towards the chromosomes end.Displacement of chiasmata towards the terminal position is called Terminalization.Nucleolus and nuclear membrane disappear.Chromosomes are released into cytoplasm.

Metaphase IMetaphase I• Shortest phase• Spindle fibres attached to the

centromeres of chromosomes.• Tetrads align on the metaphase

plate.• INDEPENDENT ASSORTMENT OCCURS:

– Orientation of homologous pair to poles is random.

– Leads to variation.– Congression .

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase ISeparation of homologous chromosomesTowards opposite pole with the help of spindle fibers.Actual reduction of chromosomes takes place.Paternal and maternal chromosomes of each homologous Pair segregate during anaphase I.This phase corresponds to mendel’s law of independent Assortment.Anaphase I introduces genetic variability.

Anaphase IAnaphase I

Telophase ITelophase IChromosomes arrived at the poles.Nuclear membrane and nucleolus reappear.Two daughter nuclei are organized each possessing a haploid number of chromosomes.Spindle fibers will disappear.The cytoplasm divides as its middle by cleavage (constriction) in animal cell.In plant cell by cell plate formation.2 daughter cells are produced.

Telophase ITelophase I

Metaphase I Anaphase I

Telophase IProphase II

Prophase I

Meiosis IIMeiosis II

• No interphase II • four phases:

a. prophase IIb. metaphase IIc. anaphase IId. telophase II & cytokinesis

• Similar to mitosis

Prophase IIProphase II• Centrioles separate and moved to polar

regions.• Nucleoli and Nuclear envelope disappears.• Prophase II is inversely proportional time

compared to telophase I.

Metaphase IIMetaphase II

• Sister chromatids are oriented on equitorial plate.

• Spindle fibers attach at centromere

Metaphase IIMetaphase II

metaphase platemetaphase plate

Anaphase IIAnaphase IICentromere is cleaved.Sister chromatids are separatedSister chromatids are pulled towards opposite poles.

Anaphase II

Telophase II• Nuclei and nuclear

membrane reform.• Cytokinesis occurs.• Four haploid daughter cells

produced.– gametes = sperm or egg.– Spindle fibers lost.

Telophase IITelophase II

Meiosis 2Meiosis 2ndnd division: division: like mitosislike mitosis with haploid set with haploid set

4 Haploid cells Gametes4 Haploid cells Gametes

VariationVariation Also known as Also known as GENETIC GENETIC

RECOMBINATIONRECOMBINATION Important to population as the raw Important to population as the raw

material for material for NATURAL SELECTION.NATURAL SELECTION. All organisms are All organisms are NOTNOT alike alike StrongestStrongest “ “most fit” most fit” survive to survive to

reproduce & pass on traitsreproduce & pass on traits

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Question:Question:

What are the What are the 33 sources of sources of genetic recombination genetic recombination or or variation? variation?

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Answer:Answer:

1.1. CROSSING OVER CROSSING OVER (prophase I)(prophase I)2.2. INDEPENDENT ASSORTMENT INDEPENDENT ASSORTMENT

(metaphase I)(metaphase I)3.3. RANDOM FERTILIZATIONRANDOM FERTILIZATION

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Question:Question:

A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

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Answer:Answer:

1010 chromosomes (haploid or chromosomes (haploid or 1n)1n)

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• Genetic Diversity through Crossing Over:• Prophase I • Genetic Diversity through Independent Assortment:• During Metaphase I

Disadvantages-Aneuploidy

The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in

Meiosis

Mitosis The first (and distinguishing) division of meiosis

END OF INTERPHASE

PROPHASE I METAPHASE I ANAPHASE I

MEIOSIS I

Genetic recombination results from crossing over during prophase I of meiosis

TELOPHASE IIANAPHASE II

METAPHASE IIPROPHASE IITELOPHASE I

MEIOSIS

Mitosis vs MeiosisMitosis vs Meiosis

MitosisMitosis MeiosisMeiosisResults inResults in 2 Diploid Cells 2 Diploid Cells

(2N)(2N)4 Haploid Cells 4 Haploid Cells (N)(N)

Cells areCells are Genetically Genetically IdenticalIdentical

Genetically Genetically DifferentDifferent

Occurs inOccurs in Somatic (Body) Somatic (Body) CellsCells

Sex CellsSex Cells

Mitosis vs. Meiosis

• Mitosis • Meiosis

Diploid (2n) Parent CellsDiploid (2n) Parent Cells

Replication of Replication of Chromosomes – double Chromosomes – double

the #the #End of Mitosis & Meiosis IEnd of Mitosis & Meiosis IResults in 2 diploid (2n) Results in 2 diploid (2n)

daughter cellsdaughter cells

End of Meiosis II End of Meiosis II Results in 4 haploid (n) Results in 4 haploid (n)

daughter cells daughter cells

•The end Meiosis

Thank you.