Edited by Hasimah Azit

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 meiosis IIII).

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).

Male: spermatogenesisMale: spermatogenesisFemale: oogenesisFemale: oogenesis

MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

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

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.

NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Cell division Cell division that 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

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

Homologous chromosomes

sister chromatids sister chromatidsTetrad

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

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

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.

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

XX chromosome - female XY chromosome - male

centriolesspindle fiber

asterfibers

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 = 4 combinationscombinations

metaphase plate

OR

metaphase plate

Homologous chromosomes Homologous chromosomes separate and move towards the poles.

Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Each pole now has haploidhaploid set of chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

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

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

same as prophaseprophase in mitosismitosis

same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

same as anaphaseanaphase in mitosismitosis sister chromatids separatesister chromatids separate

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

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

MEIOSIS

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 What are the three sexual

sources ofsources ofgenetic variation? genetic variation?

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 fusion of a spermsperm and eggegg to form a zygotezygote.

A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

mitosis meiosis