meiosis rev2

8/8/2019 MEIOSIS rev2

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MeiosisMeiosis

The form ofcell divisioncell division by which gametes,gametes, with

halfhalfthe number ofchromosomes,chromosomes, are

produced.

Diploid (2n)Diploid (2n) pp haploid (n)haploid (n)

MeiosisMeiosis is required forsexual reproduction.sexual reproduction.

Two divisionsTwo divisions

meiosis Imeiosis I and meiosis IImeiosis II.


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MeiosisMeiosis

Sex cells (also known germ cells) divide toproduce gametes (sperm or egg).

Gametes have halfthe # ofchromosomes.

Occurs only in gonads (testes or ovaries).

Male: spermatogenesis

Female: oogenesis

Meiosis is similar to mitosis with some key

chromosomal differences.


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MeiosisMeiosis

2n=4

germ cell

diploid (2n)diploid (2n)

n=2

n=2

meiosis I

n=2

n=2

n=2

n=2

sex cells

(sperm,eggs, ovules,

pollen)

haploid (n)haploid (n)

meiosis II


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InterphaseInterphase II Similar to mitosismitosis interphase.

ChromosomesChromosomes replicate (S phase).(S phase).

Each duplicated chromosomechromosome consists of

two identical sisterchromatidschromatids attached at

theircentromerescentromeres.

CentrioleCentriole pairs also replicate.


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Interphase IInterphase I NucleusNucleus and nucleolusnucleolus visible.

nuclear

membrane

nucleolus

cell membrane

chromatin


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Homologous ChromosomesHomologous Chromosomes

Pair ofchromosomeschromosomes (maternalmaternal and paternalpaternal) that are

similar in shapeshape andand sizesize.

Homologous pairs (tetrads)(tetrads) carry genes controlling

the same inherited traitssame inherited traits.

Each locuslocus (position of a gene)(position of a gene) is in the same

position on homologues.

Humans have 23 pairs ofhomologous chromosomes.homologous chromosomes.a. 22 pairs ofautosomesautosomes

b. 1 pair ofsex chromosomessex chromosomes


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Homologous ChromosomesH

omologous Chromosomes

Paternal Maternal

eye color

locus

eye color

locus

hair color

locushair color

locus


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Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male


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Meiosis I (four phases)Meiosis I (four phases) Cell divisionCell division that reduces the chromosomechromosome

number by oneone--half.half.

four phasesfour phases:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I


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Prophase IProphase I Longest and most complex phase (90%).Longest and most complex phase (90%).

ChromosomesChromosomes condense. SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes

come together to form a tetradtetrad.

TetradTetrad is two chromosomeschromosomes or fourchromatidschromatids (sister and nonsister chromatids).


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Prophase IProphase I -- SynapsisSynapsisHomologous chromosomes

sister chromatids sister chromatids

Tetrad


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Crossing OverCrossing Over

segments of nonsisterchromatidschromatids break and

reattach to the otherchromatidchromatid.

This is an important source ofgenetic variation.


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Crossing OverCrossing Over -- variationvariation

nonsister chromatids

chiasmata: siteof crossing over variation

Tetrad


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Crossing over

Now, chromatidsPaternal AA and

Maternal aa are

changed from their

original form.

This is a source of

genetic variationgenetic variation.

The DNA is a

mixture of genes

that has neverexisted before.


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Prophase IProphase I

centriolesspindle fiber


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Metaphase IMetaphase I

OR

Shortest phaseShortest phase

TetradsTetrads align on the metaphase platemetaphase plate.

Independent

assortment


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Anaphase IAnaphase I

Homologous chromosomesHomologous chromosomes separate andmove towards the poles.

SisterSister chromatidschromatids remain attached at their

centromerescentromeres.


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Telophase ITelophase I Each pole now has haploidhaploid set of

chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter

cells are formed.


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Telophase ITelophase I


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Meiosis IIMeiosis II

NoNo interphaseinterphase IIII(or very short - no more DNA replicationDNA replication)

Prophase IIProphase II

Metaphase IIMetaphase II

Anaphase IIAnaphase II

TelophaseTelophase IIIIRemember:Remember:

Meiosis IIMeiosis II is very similar to mitosismitosis


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Metaphase IIMetaphase II same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate


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Anaphase IIAnaphase II same as anaphaseanaphase in mitosismitosis

sister chromatids separatesister chromatids separate


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Telophase IITelophase II Same as telophasetelophase in mitosismitosis.

Nuclei form. CytokinesisCytokinesis occurs.

Remember:Remember: four haploid daughter cellsfour haploid daughter cells

produced.produced.

gametes = sperm or egggametes = sperm or egg


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Telophase IITelophase II


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SpermatogenesisSpermatogenesis

2n=46

Male germ cell


n=23

n=23

meiosis I

n=23

n=23

n=23

n=23


meiosis II

sperm


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OogenesisOogenesis

2n=46

Female germ cell


n=23

n=23

meiosis I

n=23

n=23 egg


meiosis II

n=23

n=23polar

bodies


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FertilizationFertilization

The fusion of a spermsperm and eggegg to form a zygotezygote.

A zygote is a fertilized egg

n=23

egg

spermn=23

2n=46zygote


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Genetic VariationGenetic Variation Important to population as the rawImportant to population as the raw

material formaterial fornatural selectionnatural selection..

Question:Question:

What are the three sources ofWhat are the three sources of

genetic variation during sexualgenetic variation during sexualreproduction?reproduction?


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Answer:Answer:1. crossing over (prophase I)1. crossing over (prophase I)

2. independent assortment (metaphase I)2. independent assortment (metaphase I)

3. random fertilization3. random fertilization

Remember: variation is good!


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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?

Answer:Answer:10 chromosomes (haploid)10 chromosomes (haploid)


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KaryotypeKaryotype A method of organizing theA method of organizing the chromosomeschromosomes of aof a

cell in relation to number, size, and type.cell in relation to number, size, and type.


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KaryotypeKaryotype

Question

What is theWhat is the

sex of thissex of this

person?person?

Answer

MALEMALE


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Question:Question:

A cell containing 4040 chromatidschromatids atthe beginning of meiosis would, at its

completion, produce cells containinghow many chromosomeschromosomes?

Answer:Answer:

10 chromosomes10 chromosomes

meiosis rev2

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