MEIOSISMEIOSISModified with permission from Modified with permission from

Robert GoodmanRobert Goodman, , Biology Department,Citrus Community CollegeBiology Department,Citrus Community College

Meiosis• The form of cell divisioncell division by which

gametes, gametes, 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 I meiosis I & meiosis IImeiosis II)

MeiosisMeiosis• Sex cells Sex cells undergo meiosis to produce

gametesgametes – GametesGametes have halfhalf the # of chromosomeschromosomes.

• Male: spermMale: sperm • Female: eggFemale: egg

– Meiosis only occurs in Meiosis only occurs in gonadsgonads • Male Testes: spermatogenesisMale Testes: spermatogenesis• Female Ovaries: oogenesisFemale Ovaries: 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

Head

Mid piece

Tail

Interphase IInterphase I

• Similar to mitosismitosis interphase

– ChromosomesChromosomes replicate (S phase)(S phase)

– Each duplicated chromosomechromosome consists 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 reduces the chromosomechromosome number by one-half.one-half.

• Four phasesFour phases:a.a. prophase Iprophase Ib.b. metaphase Imetaphase Ic.c. anaphase Ianaphase Id.d. telophase Itelophase I

Prophase IProphase ILongest and most complex phase Longest and most complex phase

(90%)(90%)–ChromosomesChromosomes condense–SynapsisSynapsis occurs

• Homologous chromosomes Homologous chromosomes come together to form a TetradTetrad

–Two homologoushomologous chromosomeschromosomes which is four chromatidschromatids (sister and nonsister chromatids)

Prophase IProphase I - Synapsis - Synapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Homologous ChromosomesHomologous Chromosomes• Pair of chromosomeschromosomes (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 of homologous homologous chromosomes.chromosomes.– 22 pairs of autosomesautosomes– 1 pair of sex chromosomessex chromosomes

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Crossing OverCrossing Over• Crossing over (variation) Crossing over (variation) may

occur between nonsister chromatidschromatids at the certain areas.–Segments of nonsister chromatidschromatids

break and reattach to the other chromatidchromatid.

Crossing Over - VariationCrossing Over - Variation

nonsister chromatids

chiasmata: site of crossing over

VARIATIONVARIATION

Tetrad

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:

1.Orientation of homologous pair to poles is random

2.Variation3.Formula: 2n

1.1. Example:Example: 2n = 42n = 42.2. Then Then n = 2 n = 23.3. ThusThus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Question:Question:In terms of In terms of Independent Independent

AssortmentAssortment

How many different How many different combinations of sperm could a combinations of 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 IHomologous chromosomes Homologous 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.

• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase ITelophase I

Meiosis IIMeiosis II• No interphase II No interphase II

(or very short - no more DNA DNA replicationreplication)

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

Prophase IIProphase II• Same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase IISame 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: four haploid daughter Remember: four haploid daughter cells produced.cells 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 Important to population as the raw

material for material for Natural SelectionNatural Selection..

• Question:Question:

What are the three sexual sources of What 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

Question:Question:A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at

its completion, produce cells containing how many

chromosomeschromosomes?

Answer:Answer:10 chromosomes10 chromosomes