Meiosis

• The form of cell divisioncell division by which, , with halfhalf the number of chromosomes,chromosomes, are produced.

• Diploid (2n) Diploid (2n) haploid (n)haploid (n)

• MeiosisMeiosis is sexual reproduction.sexual reproduction.

• Two divisionsTwo divisions (meiosis Imeiosis I and meiosis IImeiosis II).

MeiosisMeiosis

• Sex cells Sex cells divide to produce gametesgametes (sperm or (sperm or egg)egg).

• GametesGametes have halfhalf the # of chromosomeschromosomes.

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

Male: spermatogenesisMale: spermatogenesis

Female: oogenesisFemale: oogenesis

• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

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

Interphase IInterphase I

• Similar to mitosismitosis interphase.

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

• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

• CentrioleCentriole pairs also replicate.

Interphase IInterphase I

• NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Meiosis I (four phases)Meiosis I (four phases)

• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

• four phasesfour phases:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Prophase IProphase I

• SynapsisSynapsis occurs: homologous homologous chromosomeschromosomes come together.

• Homologous chromosomes carry similar information. You get one from your mom and one from your dad.

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Homologous ChromosomesHomologous Chromosomes

• Pair of chromosomeschromosomes (from momfrom mom and daddad) that are similar in shape and size.

• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.

• Each position of a gene position of a gene is in the same position on homologues.

• Humans have 23 pairs of homologous chromosomeshomologous chromosomes..

a. 22 pairs of autosomesautosomes

b. 01 pair of sex chromosomessex chromosomes

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color gene

eye color gene

hair color gene

hair color gene

Crossing OverCrossing Over

• Crossing overCrossing over may occur between chromatids.chromatids.

• Crossing overCrossing over: segments of chromatidschromatids break and reattach to the other chromatidchromatid.

• Crossing overCrossing over adds variation.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align on the metaphase platemetaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:

INCREASES VARIATION.

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Question:Question:

• In terms of In terms of Independent AssortmentIndependent Assortment - -how many different combinations of how many different combinations of sperm could a sperm could a humanhuman malemale produce? produce?

AnswerAnswer

• Formula: 2Formula: 2nn

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

n = 23n = 23

• 222323 = ~8 million combinations = ~8 million combinations

Anaphase IAnaphase I

• Homologous chromosomesHomologous chromosomes separate and move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

Telophase ITelophase I

• Each pole now has haploidhaploid set of chromosomeschromosomes.

Telophase ITelophase I

Meiosis IIMeiosis II

• No interphase II No interphase II

(or very short - no more DNA replicationDNA replication)

• Remember: Meiosis IIRemember: Meiosis II is similar to mitosismitosis

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

• same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis.

• Nuclei form.

• CytokinesisCytokinesis occurs.

• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

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

VariationVariation

• Important to population as the raw material Important to population as the raw material for for natural selectionnatural selection..

• Question:Question:

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

genetic variation? genetic variation?

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!

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)

KaryotypeKaryotype• A method of organizing the A method of organizing the chromosomes chromosomes of a of a

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

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