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 and meiosis IImeiosis II).

MEIOSISMEIOSIS

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

Produces two diploid daughter cells

Produces daughter cells that ARE genetically identical

Produce four haploid daughter cells

Produces daughter cells that are NOT genetically identical

SPERMATOGENESIS - SPERMATOGENESIS - FORMATION OF SPERM FORMATION OF SPERM

CELLSCELLS

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

Longest and most complex phase (90%).Longest and most complex phase (90%).

ChromosomesChromosomes condense.

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

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

PROPHASE IPROPHASE I - - SYNAPSISSYNAPSIS

Homologous chromosomes

sister chromatids sister chromatidsTetrad

HOMOLOGOUS HOMOLOGOUS CHROMOSOMESCHROMOSOMES

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.

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

CROSSING OVERCROSSING OVER

Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.

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

Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing overcrossing over.

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:

1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2 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 = ~8 million combinationscombinations

ANAPHASE IANAPHASE I

Homologous 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

Figure 2.3

Interkinesis

A short interphase between the two meiotic divisions

Chromosomes unfold into very thin threads

Proteins are manufactured

However, DNA is NOT replicated a second time

MEIOSIS IIMEIOSIS II

No interphase II No interphase II (or very short - no more DNA replicationDNA replication)

Remember:Remember: Meiosis II 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 four haploid daughter cells 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 Important to population as the raw material for material for natural selectionnatural selection..

Question:Question:

What are the three sexual sources What are the three sexual sources ofof

genetic variation? 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!

Table 3.1

Comparison of Mitosis and Meiosis

Interphase

CytokinesisTelophase Anapha

se

Metaphase

Prophase

What looks similar between mitosis and meiosis?What looks different?

Figure 2.3

Meiosis vs. Mitosis Animation

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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)

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