4 Science 1 RPK Meiosis

8/9/2019 4 Science 1 RPK Meiosis

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MEIOSISMEIOSIS


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MeiosisMeiosis The word "meiosis" comes from

the Greek verb meioun, meaning"to make small,

The form ofcell divisioncell divisionby

which gametes,gametes, with halfhalfthenumber of chromosomes (chromosomes (

homologous chromosome)homologous chromosome) are

produced.

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

MeiosisMeiosis is sexual reproduction.sexual reproduction.

Two divisionsTwo divisions (meiosis Imeiosis I and

meiosis IImeiosis II).


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MeiosisMeiosis

Sex cellsSex cells divide to produce gametesgametes (sperm(sperm

or egg)or egg).

GametesGametes have halfhalf ofchromosomeschromosomes.

Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).

Male: spermatogenesisMale: spermatogenesis

Female:Female: oogenesisoogenesis

MeiosisMeiosis is similar to mitosismitosis with some

chromosomal differences.


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SpermatogenesisSpermatogenesis

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


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meiosis ismade up oftwo phasesof celldivision

- meiosis I:-the reductiondivision

- meiosis II:-

separatesthe sisterchromatids


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InterphaseInterphase Similar to mitosismitosis

interphase.

ChromosomesChromosomes replicate

(S phase).(S phase).

Each duplicated

chromosomechromosome consist of

two identical sister

chromatidschromatids attached attheircentromerescentromeres.

CentrioleCentriole pairs also

replicate.


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InterphaseInterphase NucleusNucleus and nucleolusnucleolus visible.

nuclear

membrane

nucleolus

cell membrane

chromatin


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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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Homologous ChromosomesHomologous Chromosomes

Pair ofchromosomeschromosomes (maternalmaternal and paternalpaternal) that are

similar in shape and size.

Homologous pairs (tetrads)(tetrads) carry genes controlling

the same 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. 01 pair of sex chromosomessex chromosomes


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Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color

locus

eye color

locus

hair color

locushair color

locus


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Crossing OverCrossing Over

Crossing over (variation)Crossing over (variation)may occur between nonsister

chromatidschromatids at the chiasmatachiasmata.

Crossing overCrossing over: segments ofnonsisterchromatidschromatids break

and reattach to the other

chromatidchromatid.

ChiasmataChiasmata ((chiasmachiasma)) are the

sites ofcrossing overcrossing over.


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Crossing OverCrossing Over -- variationvariation

nonsister chromatids

chiasmata: siteof crossing over variation

Tetrad


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Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male


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Prophase IP

rophase I

centriolesspindle fiber

aster

fibers


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Metaphase IMetaphase I Shortest phaseShortest phase

TetradsTetrads align on the metaphase platemetaphase plate.

INDEP

ENDENT ASSORTMENT OCCURS:INDEP

ENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.

2. Variation

3. Formula: 2n

Example:Example: 2n = 42n = 4thenthen n = 2n = 2

thusthus 2222 = 4 combinations= 4 combinations


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Metaphase IMetaphase I

metaphase plate

OR

metaphase plate


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Question:

Question:

In terms ofIn terms ofIndependent AssortmentIndependent Assortment --

how many different combinations ofhow many different combinations of

sperm could asperm could a humanhuman malemale produce?produce?


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AnswerAnswer Formula: 2Formula: 2nn

Human chromosomes:Human chromosomes: 2n = 462n = 46

n = 23n = 23

222323

= ~8 million combinations= ~8 million combinations


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Anaphase IAnaphase I Homologous chromosomesHomologous chromosomes separate and

move towards the poles.

Sister chromatidsSister chromatids remain attached at their

centromerescentromeres.


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Anaphase IAnaphase I


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Telophase ITelophase I


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Meiosis IIMeiosis II No interphase IINo interphase II

(or very short - no more DNA replicationDNA replication)

Remember:Remember: Meiosis IIMeiosis II is similar to mitosismitosis


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Prophase IIP

rophase II same as prophaseprophase in 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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MeiosisMeiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

meiosis I

n=2

n=2

n=2

n=2

sperm

haploid (n)

meiosis II


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VariationV

ariation Important to population as the rawImportant to population as the raw

material formaterial fornatural selectionnatural selection..

Question:Question:

What are the three sexual sources ofWhat are the three sexual sources of

genetic variation?genetic variation?


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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 howmany chromosomeschromosomes?


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

A zygote is a fertilized egg

n=23

egg

sperm

n=23

2n=46

zygote


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Question:

Question:

A cell containing 40 chromatids40 chromatids at the

beginning of meiosis would, at its completion,

produce cells containing how manychromosomeschromosomes?


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Answer:Answer: 10 chromosomes10 chromosomes


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Comparing Meiosis and Mitosis

Chromosome behavior

Mitosis: Homologous chromosomes independent

Meiosis: Homologous chromosomes pair forming bivalents

until anaphase I

Chromosome number

Mitosis- daughter cells diploid

Meiosis- daughter cells haploid

Genetic identity of progeny: Mitosis: identical to parent cell

Meiosis: not identical to parents cell


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Appreciating the movement of

chromosomes during Mitosis and Meiosis

To ensure continuity of life on Earth

Mitosis and meiosis are regulated in precise

manner.

For example:

- If the sister chromatids do not separate during

anaphase, the number of chromosome in

each cell would not be the same as the

parent cell.

- The gametes produced would have an

abnormal number of chromosome


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Downs syndrome:

The result of an additional

chromosome, causing

each somatic cell to have

a total of 47 chromosomes

instead of normal 46

chromosomes.- Shanted eyes, short

stature, mental

retardation.


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Klinefelter

4 Science 1 RPK Meiosis

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