CellBio Sitzung9 Meiosis

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Chapter 7

Meiosis and germ cell

formation

Asexual reproduction

Mitosis division produces daughter cells whichare usually genetically identical

to each other

to the parental cell

Prokaryotes and most unicellular eukaryotesreproduce asexually: by binary fission, buddingor more exotic ways of dividing.

Asexual reproduction does not allow easyadaptation toward changing environmentalconditions.

Where does genetic variation comefrom?


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Where does geneticvariation come from?

Mutations

Sexual reproduction

Allows genetic material of two parental cellsto be mixed.

As a result, progeny is formed which isgenetically different: from parent cells and in

between various progeny organisms.

Diploidity: a problem insexual reproduction

A diploid genome:One copy of eachchromosome is from

the mother and oneis from the father.

Both carry the samegenes, but may have

different alleles.


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Diploidity: a problem insexual reproduction

Somatic cells are diploid (2n)

Example: an organism is 2n = 4chromosomes altogether.

Chromosomes 1 & 2 arehomologous chromosomes

Chromosomes 3 & 4 arehomologous chromosomes

Chromosomes 1 & 3 came from the

mother Chromosomes 2 & 4 came from the

father Fusion of two 2n-cells: 4n-cell.

for sexual reproductiondiploid cells are useless

For sexual reproduction,specialized cells are needed thatcarry half the set ofchromosomes: gametes.

In more complex organisms suchhaploid (=1n) organisms aregametes, typically ovum andspermatozoon.

Union of both gametes(fertilization) results in a diploidzygote.


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Sexualreproductionin higher andlowereukaryotes

While the way theymake use of theirhaploid cells differs

the generalmechanism to

generate haploidcells is the same:meiosis.

Stages of meiosis

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2n2c

2n4c

1n2c

1n1c


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Meiosis I

Segregates homologous chromosomes intodifferent daughter cells.

In meiosis I exchange of corresponding parts ofhomologous chromosomes can occur.

Both meiosis I and II can be divided insubphases similar to those of mitosis:

Prophase

MetaphaseAnaphase

Telophase

Prophase of meiosis I

The chromosomes condense and two centrosomesstart to migrate to opposite poles of the cell.

Each chromosome consist of two chromatids.

Homologous pairs form bivalents (=synapses ofhomologous chromosomes).


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Prophase of meiosis I consistsof five subphases

The events of prophase I can be furtherdivided into five stages:

Leptotene

Zygotene

Pachytene

Diplotene

Diakinesis

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Crossing over leads toexchange of genetic material


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What about sex-chromosomes?

Human males are the heterogametic sex withdifferent sex chromosomes, (XY)

Human females are the homogametic sex (XX)

In other species sex can be determined in manyways. For example, in birds and snakes

Males are homogametic ZZ

Females are heterogametic ZW

During mitotic division there is no specialchallenge for the distribution of sex-chromosomes.

X and Y Chromosomes arevery different

X chromosome contains more than 1,500 genes

larger than the Y chromosome

acts as a homolog to Ychromosome in males

Y chromosome contains 231 genes

many DNA segments are

palindromes and may destabilizeDNA

Figure 6.2


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Genes on the YChromosome

Genes shared with Xchromosome definethe pseudoautosomalregions (PAR1 andPAR2) - only herecan crossing overoccur.

Male specific (MSY)

region includes SRYgene

SRY gene isimportant indetermining sex

Figure 6.3

Metaphase ofmeiosis I Bivalents attach via

their kinetochores tospindle microtubulesand migrate to

spindle equator. Bivalents are

randomly orientatedat this point.

Homologouschromosomes areheld together bychiasmata.

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Anaphase of meiosis I

The members of each pairof homologouschromosomes separatefrom each other.

Start migrating towardsopposite spindle poles

Telophase I and cytokinesis

A haploid set ofchromosomes arrive ateach spindle pole.

Nuclear envelopes formaround thechromosomes,cytokinesis occurs.


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Meiosis II

Has the function to divide the sister chromatidesformed in the initial round of DNA-replicationinto two newly forming cells.

The phases of meiosis II (pro-II, meta-II, ana-IIand telophase II) resemble very much those of anormal mitotic division.

Prophase II Metaphase II Anaphase II Telophase II

Meiosis II

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Nondisjunction

Nondisjunction might occur inanaphase I when the two membersof homologous chromosomes donot separate.

Gametes resulting from suchnondisjunctions have an incorrectnumber of chromosomes.

Nondisjunction of chromosome21 can lead to trisomy 21.

The risk of trisomy 21 increaseswith age of parents.

Comparing mitosis andmeiosis

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Comparing mitosis andmeiosis

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Comparingmitosisandmeiosis

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Meiosis contributes togenetic variability

By random assortment of homologueschromosomes

Occurring in metaphase I of meiosis

By crossing over

Occurring in prophase I of meiosis

Exchange of DNA between homologouschromosomes

By fertilization

Occurring when two gametes meet and fuse

Random assortment


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Crossingover andrandom

assortment

Gametogenesis is more thanjust meiosis

In males, in meiosis a diploid spermatocyte isconverted into four haploid spermatids.

After meiosis, these differentiate into spermcells:

They loose most of their cytoplasma

Build out a flagellum-like tail etc.


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Sperm are continouslyproduced in most mammals

Primordial germ cellsmigrate into the testisearly in embryogenesis

Immature germ cells,(spermatogonia)proliferate continuouslyby mitosis.

Some of the daughtercells stop proliferatingand differentiate intoprimary spermatocytes.

Sperm are continouslyproduced in most mammals

Primary spermatocytescanenter meiosis toproduce four spermatids,each with a haploid

number of singlechromosomes. These haploid spermatids

then undergomorphologicaldifferentiation into sperm.

Differentiation occursAFTER Meiosis.


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Primordial germ cellsmigrate to theforming gonad:oogonia, whichproliferate by mitosisbefore differentiatinginto primary oocytes.

usually before birththe first meiotic

division begins andremains arrested inprophase I

Oogenesisoccurs in stages

Oogenesis occursin stages

Oocyte maturationoccurs followinghormone stimulation.

Cell resumes division I of

meiosis (and completelyor partly does division II) Divisions are

asymmetrical: onemature egg is formedand the smalleraccompanying polarbodies oocytesmaintain their large size;polar bodies degenerate.


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Fertilization

The 3. step in

increasing geneticvariability is fertilization

where two haploidgerm cells (of oppositetype) randomly fuse

with each other.

For this fusion to occurgerm cells must notonly be haploid but alsomorphologically altered.

Fertilization

Once fertilized, theegg is called azygote

Fertilization is notcomplete, however,until the two haploidnuclei (pronuclei)have come togetherto form a singlediploid nucleus.


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Fertilization

Entry of furthersperm cell isprevented byrearrangement of ZPas soon as the firstsperm nucleus hasentered.

Spermal flagellumand mitochondria donot enter the ovum.

The sperm provides thecentriole to the zygote


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Stages of Development

Early Development:Ovulation to Implantation

Figure 3.14

CellBio Sitzung9 Meiosis

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