MeiosisMeiosis

Sex cell (gamete) division= egg Sex cell (gamete) division= egg and spermand sperm

Products= 4 cells all genetically Products= 4 cells all genetically differentdifferent

How is this different from mitosis?How is this different from mitosis? Mitosis=2 identical cells! (clones)Mitosis=2 identical cells! (clones) Genes Genes are located on the are located on the

chromosomes.chromosomes.

Chromosome numberChromosome number

HomologousHomologous ChromosomeChromosome – The – The chromosomes that make up a pair, chromosomes that make up a pair, one chromosome from each parent.one chromosome from each parent.

DiploidDiploid= 2n, the # of chromosomes = 2n, the # of chromosomes contained in “body” cellscontained in “body” cells

GameteGamete – sex cells that have half the – sex cells that have half the number of chromosomes.number of chromosomes.

HaploidHaploid – n, the # of chromosomes – n, the # of chromosomes found in sex cells (gametes)found in sex cells (gametes)

Diploid Vs. HaploidDiploid Vs. Haploid

Why do sex cells have to be Why do sex cells have to be haploid?haploid?

They must have half the # of They must have half the # of chromosomes to produce a viable chromosomes to produce a viable gamete (23 mom+ 23 dad= gamete (23 mom+ 23 dad= zygote)zygote)

Quick Thinking…Quick Thinking…

If a haploid cell in a frog is 13, If a haploid cell in a frog is 13, what is the diploid cell?what is the diploid cell?

2626 If the muscle cell of a dog has 78 If the muscle cell of a dog has 78

chromosomes, what does it’s egg chromosomes, what does it’s egg cell have?cell have?

3939

Here’s Some More….Here’s Some More….

If Diploid= 8If Diploid= 8 Then Haploid= 4Then Haploid= 4 If Haploid=70If Haploid=70 Then Diploid= 140Then Diploid= 140 If Diploid= 46If Diploid= 46 Then Haploid= 23Then Haploid= 23 If Haploid = 56If Haploid = 56 Then Diploid= 112Then Diploid= 112

Phases of MeiosisPhases of Meiosis Meiosis Meiosis – – cell division that reduces cell division that reduces

the # chromosomes, referred to as the # chromosomes, referred to as “reduction division”.“reduction division”.

Meiosis I:Meiosis I:– Interphase – chromosomes replicate Interphase – chromosomes replicate

(DNA)(DNA)– Prophase IProphase I– Metapase IMetapase I– Anaphase IAnaphase I– Telophase ITelophase I

Prophase IProphase I

same as mitosis same as mitosis PLUS…PLUS…

tetrads -tetrads -2 homologous 2 homologous chromosomes pair up.chromosomes pair up.Crossing over occursCrossing over occurs =each tetrad =each tetrad “swaps” genes; provides for “swaps” genes; provides for genetic variationgenetic variation

Crossing Over!!!Crossing Over!!!

Metaphase IMetaphase I

Same as mitosis, but the Same as mitosis, but the chromosomes chromosomes line up as PAIRSline up as PAIRS (tetrads) at the plate(tetrads) at the plate

Anaphase IAnaphase I

Same as mitosis, except the Same as mitosis, except the pairs pairs are separatedare separated, not the , not the chromosome!!chromosome!!

Telophase ITelophase I

Same as mitosis, except…Same as mitosis, except… DNA DNA does not unwinddoes not unwind, it stays in , it stays in

chromosome form for the next set chromosome form for the next set of divisionof division

2 cells 2 cells not identical…not identical… why? why?

Do the 2 daughter cells undergo Do the 2 daughter cells undergo interphase II?interphase II?

NO INTERPHASE IINO INTERPHASE II

The DNA stays wound The DNA stays wound for round 2 of meiosis for round 2 of meiosis for one reason.for one reason.

We now have 2 We now have 2 diploid chromosomes, diploid chromosomes, we must divide again we must divide again to reach the haploid to reach the haploid state.state.

We We do not replicatedo not replicate the DNA the DNA againagain……

Can you guess the Can you guess the next stages?next stages? Prophase IIProphase II Metaphase IIMetaphase II Anaphase IIAnaphase II Telophase IITelophase II CytokinesisCytokinesis All look similar to mitosis, sisters All look similar to mitosis, sisters

get split in Anaphase II like get split in Anaphase II like mitosis!!!mitosis!!!

Prophase IIProphase II

Chromosomes condense.Chromosomes condense. Spindles form in each new cell.Spindles form in each new cell. Spindle fibers attach to Spindle fibers attach to

chromosomeschromosomes

Metaphase IIMetaphase II

Centromeres ofCentromeres of

chromosomeschromosomes

line up randomly atline up randomly at

the equator of the equator of

each celleach cell

Anaphase IIAnaphase II

Centromeres split Centromeres split Sister chromatids Sister chromatids

separate and moveseparate and move

to opposite to opposite

polespoles

Telophase IITelophase II

4 nuclei form around 4 nuclei form around chromosomes.chromosomes.

Spindles break down.Spindles break down. Cell dividesCell divides

End Result…Finally!End Result…Finally!

Sperm= 4 viable Sperm= 4 viable haploid (23) spermhaploid (23) sperm

Egg= 1 viable egg Egg= 1 viable egg (23), three polar (23), three polar bodiesbodies

Meiosis = Genetic Meiosis = Genetic VariationVariation

1)1) Independent AssortmentIndependent Assortment-all pairs separate independently -all pairs separate independently -gives approx. 8 million variations-gives approx. 8 million variations

2)2) Crossing Over (prophase I)Crossing Over (prophase I)-adds recombination-adds recombination-limitless variations-limitless variations

3)3) Random FertilizationRandom Fertilization- gametes from independent organisms - gametes from independent organisms squares the variations (64 million)squares the variations (64 million)