MeiosisMeiosisGamete ProductionGamete Production

Mr. YoungMr. Young

Cells do not increase in size. They increase in number.

• Meiosis = process of nuclear division that reduces the chromosome number by half in new cells (gametes)

• cells undergo interphase, grow, copy DNA just like mitosis, but in meiosis the cells divide twice

• two Diploid (2n) cells divide twice to produce four Haploid (1n) cells

• 2 stages = Meiosis I and Meiosis II

Meiosis

• Interphase• Meiosis I

– Prophase I– Metaphase I– Anaphase I– Telophase I & Cytokinesis (results in 2 haploid cells)

• Goes directly into Meiosis II (no interphase, no dna copying)– Prophase II– Metaphase II– Anaphase II– Telophase II & Cytokinesis (results in 4 haploid cells)

Meiosis

• Creates four haploid cells (gametes)

• Two nuclear divisions occur without DNA being copied in between

Crossing Over

• Crossing over - portions of chromatids break off and attach to adjacent chromatids on homologous chromosome

• Crossing over allows for the random mixing of genes (genetic recombination) which adds genetic variety to a species so that no two individuals are exactly the same.

Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing over

Crossing Over

Anaphase I Anaphase I

Telophase II Telophase II

Metaphase I Metaphase I

Telophase ITelophase IBecause of crossing over, every gamete receives a unique set of genetic information.

Crossing over animation

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__unique_features_of_meiosis.html

Stages Of Meiosis

• Meiosis resembles mitosis except that it is actually two divisions not one

• These divisions are called Meiosis I and Meiosis II

• Meiosis I results in haploid cells with chromosomes made up of two chromatids

• Meiosis II is essentially mitosis on haploid cells

Stages Of Meiosis - Meiosis I• Prophase I - The beginning phase -

– DNA which was unraveled and spread all over the nucleus is condensed and packaged

– Homologous chromosomes (each made of two identical chromatids) come together and form tetrads (4 chromatids)

– Crossing over, in which chromatids within tetrads exchange genetic material, occurs

• Metaphase I - Middle stage - Tetrads line up along the equator of the cell

Stages Of Meiosis - Meiosis I• Anaphase I - One copy of each

chromosome still composed of two chromatids moves to each pole of the cell

• Telophase I - End stage - New nuclear membranes are formed around the chromosomes and cytokinesis (cytoplasm division) occurs resulting in two haploid daughter cells

• NOTE: There is NO interphase between meiosis I and meiosis II.

• Why is this important?

• So that each new cell will only have half (haploid) the number of chromosomes as the original parent cell.

• Meiosis II is not preceded by the copying of DNA

Stages Of Meiosis - Meiosis II

• Prophase II - Cells do not go into interphase between meiosis I and II, thus chromosomes are already condensed

• Metaphase II - Chromosomes line up at the equator of the two haploid cells produced in meiosis I

• Anaphase II - Chromosomes made up of two chromatids split to make chromosomes with one chromatid which migrate to the poles of the cells

• Telophase II - Cytokinesis and reformation of the nuclear membrane in four haploid cells each with one set of chromosomes made of one chromatid

InterphaseInterphase

Mother cell Stages Of Meiosis: Meiosis I

Meiosis IIMeiosis II

Prophase I:Tetrad formation/

crossing over

Prophase I:Tetrad formation/

crossing overMetaphase I Metaphase I

Telophase ITelophase I

Prophase I:Condensing

Chromosomes

Prophase I:Condensing

Chromosomes

Anaphase I Anaphase I

Telophase ITelophase I

Stages Of Meiosis: Meiosis II

Metaphase II Metaphase II

Anaphase II Anaphase II

Telophase II Telophase II

The products of mitosis are 2 diploid cells with identical chromosomes.

The products of meiosis are 4 haploid cells each with a unique set of chromosomes.

Prophase IIProphase II

Let’s look at Meiosis Animations!!

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__stages_of_meiosis.html

Gametes Are Haploid• Gametes = haploid reproductive cells (sperm, egg cells) one

set of chromosomes (1n)• Gametes must have half the genetic material of a normal cell• If the genetic material in the gametes was not halved, when

they combined the zygote would have more genetic material than the parents

• Meiosis is specialized cell division resulting in cells with half the genetic material of the parents

“Putting It All Together” - Fertilization

Spermatogenesis

• Spermatogenesis - diploid reproductive cell divides through meiosis to form 4 sperm cells

Oogenesis

• Oogenesis - Production of mature egg cells through meiosis.

• Only one of the four develops into a mature egg cell, the other 3 are called ‘polar bodies’ and degenerate

What Meiosis is About

Meiosis allows the creation of unique individuals through sexual reproduction.