Meiotic cell division

Meiotic Cell division

1. MEIOTIC CELL DIVISION
Mr.BinuBabu
MBA, M.Sc. (N)
Asst. Professor
Mrs. JincyEalias
M.Sc. (N)
Asst. Professor

2. Mostcellsofthehumanbodyundergocell
division,theprocessbywhichcells
reproducethemselves.
•Thetwotypesofcelldivision—
–Somaticcelldivision(mitoticdivisionor
mitosis)
–Reproductivecelldivision(meioticdivisionor
meiosis)

3. Somatic cell division
•Acellundergoesanucleardivisioncalled
mitosis
•Acytoplasmicdivisioncalledcytokinesis
•Thistwoprocessproducetwogenetically
identicalcells,eachwiththesamenumberand
kindofchromosomesastheoriginalcell.
•Somaticcelldivisionreplacesdeadorinjured
cellsandaddsnewonesduringtissuegrowth.

4. Reproductive cell division
•Itisthemechanismthatproducesgametes,
thecellsneededtoformthenextgeneration
ofsexuallyreproducingorganisms.
•Thisprocessconsistsofaspecialtwostep
divisioncalledmeiosisinwhichthe
numberofchromosomesinthenucleusis
reducedbyhalf.

5. Meiosisisaformofcelldivisionbywhichgametes,
withhalfthenumberofchromosomes,are
produced.
•Diploid(2n) haploid(n)
•Meiosisissexualreproduction.Itisatwo
divisionsprocess(meiosisIandmeiosisII).

6. •Sex cells divide to produce gametes(sperm or
egg).
•Gameteshave halfof the chromosomes.
•Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
•Meiosisis similar to mitosiswith some
chromosomal differences.

7. Fertilization
•The fusion of a spermand eggto form a zygote.
•A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

8. Spermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II

9. Interphase
•DuringinterphasethecellreplicatesitsDNA,italso
producesadditionalorganellesandcytosoliccomponentsin
anticipationofcelldivision.
•Interphaseisastateofhighmetabolicactivitybutcelldoes
notgrow.
•Interphaseconsistsofthreephases
–G1:-itistheintervalbetweenthemitoticphaseandtheS
phase
–Sorsynthesis:-DNAreplicationoccurs
–G2istheintervalbetweentheSphaseandthemitotic
phase
G
0:-CellsthatremaininG1foraverylongtime,perhaps
destinednevertodivideagain.

10. Meiosis I
Cell division that reduces the chromosomenumber
by one-half.
•Four phases:
a.Prophase I
b.Metaphase I
c.Anaphase I
d.TelophaseI

11. Prophase I
It is the longest and most complex phase (90%)
in meiosis. In this phase
•The chromosomescondense.
•Synapsisoccurs:homologous chromosomes come
together to form a tetrad.
•Tetradis two chromosomesor four chromatids
(sister and nonsisterchromatids).

13. Prophase I-Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

14. Homologous Chromosomes
•Pair of chromosomes (maternal and paternal) that are
similar in shape and size.
•Homologous pairs (tetrads) carry genes controlling the
same inherited traits.
•Each locus (position of a gene) is in the same position on
homologues.
•Humans have 23 pairs of homologous chromosomes.
a.22 pairs of autosomes
b.01 pair of sex chromosomes
•Karyotype:-A method of organizing the chromosomes
of a cell in relation to number, size, and type.

15. Crossing Over
•Crossingover(variation)mayoccurbetweennon
sisterchromatidsatthechiasmata.
•Crossingover:segmentsofnonsisterchromatids
breakandreattachtotheotherchromatid.
•Chiasmata(chiasma)arethesitesofcrossing
over.

16. Crossing Over -variation
nonsister chromatids
chiasmata: site of
crossing over
variation
Tetrad

17. Sex Chromosomes
XX chromosome -female XY chromosome -male

18. Metaphase I
•It is the shortest phase in meiosis
•Tetradsalign on the metaphase plate.

19. Anaphase I
•Themembersofeachhomologouspairof
chromosomesseparateastheyarepulledto
oppositepolesofthecell
•Thepairedchromatids,heldbya
centromere,remaintogether.

21. Telophase I
•Each pole now has haploidset of chromosomes.
•Cytokinesisoccurs and two haploid daughter cells
are formed.

24. Meiosis II
•No interphase II
(or very short -no more DNA replication)
•Meiosis II is similar to mitosis

25. Prophase -II
•Duringearlyprophase-II,thechromatin
fiberscondenseandshorteninto
chromosomes.
•Thecondensationprocessmayprevent
entanglingofthelongDNAstrandsasthey
moveduringmitosis.

27. Metaphase -II
•Duringthisphase,themicrotubulesofthe
mitoticspindlealignthecentromeresofthe
chromatidpairsattheexactcenterofthe
mitoticspindle.
•Thismidpointregioniscalledthemetaphase
plate.

29. Anaphase -II
•Duringthisphase,thecentromeressplit,
separatingthetwomembersofeach
chromatidpair,whichmovetoward
oppositepolesofthecell.
•Onceseparated,thechromatidsaretermed
chromosomes.
•Asthechromosomesarepulledbythe
microtubulesofthemitoticspindleduring
anaphase,theyappearV-shaped.

31. Telophase -II
•ThefinalstageofmeosisII,telophaseII,
beginsafterchromosomalmovementstops.
•Theidenticalsetsofchromosomes,nowat
oppositepolesofthecell,uncoilandrevert
tothethreadlikechromatinform.
•Anuclearenvelopeformsaroundeach
chromatinmass,nucleolireappearinthe
identicalnuclei,andthemitoticspindle
breaksup.

33. •Afterthecompletionoftelophase-II
Cytokinesisoccurs.
•After completion of meiosis –II four
haploid daughter cells produced.
•Gametes = sperm or egg