MEIOSISMEIOSIS

Meiosis• The form of cell divisioncell division by which

gametesgametes, , 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 egg)(sperm or egg).

• GametesGametes have halfhalf the # of chromosomeschromosomes.

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

Three sexual life cycles differing in the timing of meiosis and fertilization

Moss with Sporophytes

Gametophytes

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

SpermatogenesisSpermatogenesis

2n=46

Spermato-cyte

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid(n)

meiosis II

OogenesisOogenesis

2n=46

Oocyte

diploid (2n)

n=23

n=23

meiosis I

n=23

ovum

haploid(n)

meiosis II

polar polar bodiesbodies

Interphase Interphase • 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 Interphase

• 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• Longest and most complex Longest and most complex

phase (90%).phase (90%).

• ChromosomesChromosomes condense.

• SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes come together to form a tetradtetrad.

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.

a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Prophase IProphase I

centriolesspindle fiber

asterfibers

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

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

occur between nonsister chromatidschromatids.

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

Crossing Over - variationCrossing Over - variation

nonsister chromatids

site of crossing over variation

Tetrad

Metaphase IMetaphase I

equator

OR

equator

Metaphase IMetaphase I• Shortest phaseShortest phase• TetradsTetrads align on the equatorequator

• INDEPENDENT ASSORTMENT OCCURSINDEPENDENT ASSORTMENT OCCURS: : genes for different traitsgenes for different traits

sort independently of one anotherinto gametes

Anaphase IAnaphase I

Anaphase IAnaphase I

• Homologous Homologous chromosomeschromosomes separate and move towards the poles.

• Sister chromatidsSister chromatids remain attached at their centromerescentromeres.

Telophase ITelophase I

Telophase ITelophase I

• Each pole now has haploidhaploid set of chromosomeschromosomes.

• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Meiosis IIMeiosis II

• No interphase No interphase (or very short - no more DNA DNA replicationreplication)

• Remember:Remember: Meiosis II Meiosis II is similar to mitosis but the cells are mitosis but the cells are different from each otherdifferent from each other

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

• same as metaphasemetaphase in mitosismitosis

equator equator

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

Telophase IITelophase II

Telophase IITelophase II• Same as telophasetelophase in mitosismitosis.

• Nuclei form. CytokinesisCytokinesis occurs.

• Remember:Remember: four haploid daughter cells four haploid daughter cells produced; 3 polar bodies and anproduced; 3 polar bodies and an

egg in females, 4 sperm in malesegg in females, 4 sperm in males

gametes = sperm or egggametes = sperm or egg

FertilizationFertilization• The fusion of a spermsperm and eggegg to

form a zygotezygote - a fertilized egg

n=23egg

sperm n=23

2n=46zygote

VariationVariation• Important to population as the Important to population as the

raw material for raw material for natural natural selectionselection..

• Question:Question:

What are the three sexual What are the three sexual sources of genetic variation? sources of genetic variation?

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

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

3. random fertilization3. random fertilization

Remember: variation is good !

The Results of Alternative Arrangements of Two Homologous Chromosome Pairs on the Metaphase

Plate in Meiosis I

A Comparison of Mitosis and Meiosis

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)

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

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

1) Three major differences 1) Three major differences between meiosis and the mitosis between meiosis and the mitosis are:are:

2) Compare the number of 2) Compare the number of chromatids present in human cells chromatids present in human cells before and after DNA replication before and after DNA replication occurs in interphase.occurs in interphase.

3) Compare and contrast oogenesis3) Compare and contrast oogenesiswith spermatogenesis.with spermatogenesis.

Determine the number of chromo-Determine the number of chromo-somes in a cell whose diploid (2n)somes in a cell whose diploid (2n)number is 10 after:number is 10 after:

a. meiosis Ia. meiosis Ib. meiosis IIb. meiosis IIc. mitosisc. mitosis

What is the primary difference What is the primary difference between sexual life cycles between sexual life cycles in animals when compared to fungiin animals when compared to fungiand plants?and plants?

Failure of chromosomes to Failure of chromosomes to separate properly during meiosisseparate properly during meiosismay result in a/an ____________may result in a/an ____________Disorder, such as Down Syndrome. Disorder, such as Down Syndrome.

Explain the evolutionary reason thatExplain the evolutionary reason thatSickle Cell Anemia is more commonSickle Cell Anemia is more commonamong African Americans. among African Americans.

If you understand the process of If you understand the process of meiosis then you know thatmeiosis then you know that::1)1) It occurs only to make sex cells/gametes.It occurs only to make sex cells/gametes.

2) Four different cells are produced.2) Four different cells are produced.

3) Crossing over (exchange among DNA of 3) Crossing over (exchange among DNA of tetrads) occurs in Prophase I to make thetetrads) occurs in Prophase I to make thecells genetically different. cells genetically different.

4) The amount of DNA is reduced by4) The amount of DNA is reduced by

half in Meiosis I.half in Meiosis I.