Meiosis creates sex cells such as eggs and sperm. It results in 4 haploid cells that each have a single set of chromosomes. Meiosis involves two divisions, Meiosis I and Meiosis II, which separates the homologous chromosomes and then the sister chromatids. This leads to genetic variation between the resulting gametes and allows for genetic recombination during crossing over. Nondisjunction during meiosis can result in conditions like Down syndrome if an extra chromosome is present (trisomy 21) or Turner syndrome if a female is missing an X chromosome (monosomy X).

1. MeiosisMeiosis

2. Review of MitosisReview of Mitosis
 Why do cells go through mitosis?Why do cells go through mitosis?
 To make new cells for growth and repairTo make new cells for growth and repair
 Remember the cells are identical to the parent (cloned)Remember the cells are identical to the parent (cloned)
 They areThey are diploiddiploid- have two sets of- have two sets of homologoushomologous
(matching) chromosomes (46)(matching) chromosomes (46)
 Prophase, Metaphase, Anaphase, TelophaseProphase, Metaphase, Anaphase, Telophase

3. What is Meiosis?What is Meiosis?
 Meiosis creates sex cellsMeiosis creates sex cells
((gametesgametes))
 Each resulting cells isEach resulting cells is
genetically different from thegenetically different from the
parent.parent.
 Why would this beWhy would this be
necessary?necessary?
 Meiosis results in 4Meiosis results in 4 haploidhaploid
cells- only 1 set ofcells- only 1 set of
chromosomes or 23chromosomes or 23

5. Meiosis leads to genetic variationMeiosis leads to genetic variation
 Meiosis creates 4 cells that are a different mix of yourMeiosis creates 4 cells that are a different mix of your
parents’parents’ genes.genes.
 Why would this be beneficial?Why would this be beneficial?

6. Can we seeCan we see
Genes?Genes?
KaryotypeKaryotype--
Picture of organismPicture of organism’s’s
ChromosomesChromosomes
Human Has 23 pairsHuman Has 23 pairs
2323RDRD
Pair is thePair is the
sex chromosomesex chromosome
What is the sex of thisWhat is the sex of this
Individual??Individual??

7. Process of MeiosisProcess of Meiosis
Meiosis results in a egg or spermMeiosis results in a egg or sperm
 There are 2 parts:There are 2 parts:
 Meiosis IMeiosis I
 Meiosis IIMeiosis II

8. Meiosis IMeiosis I
• Prophase IProphase I
– Proteins causeProteins cause homologoushomologous chromosomes to stick togetherchromosomes to stick together
• 4 chromatids (tetrads)4 chromatids (tetrads)
– Tetrads attach to spindleTetrads attach to spindle
– Genetic info is exchanged (Genetic info is exchanged (crossing overcrossing over))

10. • Metaphase IMetaphase I
– Tetrads move to middle of cell and line upTetrads move to middle of cell and line up
• Anaphase IAnaphase I
– Homologous chromosomes separateHomologous chromosomes separate
– 2 chromosomes move toward opposite poles2 chromosomes move toward opposite poles

11.  Telophase ITelophase I
 Chromosomes arrive at polesChromosomes arrive at poles
 Cytokinesis occurs resulting in two haploid daughter cellsCytokinesis occurs resulting in two haploid daughter cells
But it doesnBut it doesn’t end here. . .’t end here. . .

13. Meiosis IIMeiosis II
 Prophase IIProphase II
 Spindle forms in haploid daughter cellSpindle forms in haploid daughter cell
 Moves chromosomes to middle of cellMoves chromosomes to middle of cell
 Metaphase IIMetaphase II
 Chromosomes line up in middle of cellChromosomes line up in middle of cell

14. • Anaphase IIAnaphase II
– Sister chromatids separateSister chromatids separate
– Move to opposite polesMove to opposite poles
• Telophase IITelophase II
– Chromatids (individualChromatids (individual
chromosomes) arrive at poleschromosomes) arrive at poles
– Cytokinesis splits cellsCytokinesis splits cells
End Result. .End Result. . .. 4 HAPLOID DAUGHTER CELLS4 HAPLOID DAUGHTER CELLS

16. ReviewReview
1. Define1. Define ASEXUAL REPRODUCTIONASEXUAL REPRODUCTION
– is the production of Offspring from ONE PARENT.is the production of Offspring from ONE PARENT.
2. What is the result of Asexual Reproduction?2. What is the result of Asexual Reproduction?
-Two genetically identical daughter cells-Two genetically identical daughter cells
3. Are the results of Mitosis3. Are the results of Mitosis HaploidHaploid oror diploid?diploid?
- Diploid- Diploid
4. What does4. What does diploiddiploid mean?mean?
-cell with two homologous sets of chromosomes-cell with two homologous sets of chromosomes
5. What is5. What is homologoushomologous??
-matching set of chromosomes in a diploid cell.-matching set of chromosomes in a diploid cell.
6. What does Asexual reproduction involve? Mitosis or Meiosis.6. What does Asexual reproduction involve? Mitosis or Meiosis.
-Mitosis-Mitosis

17. 7. In unicellular organisms, such as bacteria, how are new7. In unicellular organisms, such as bacteria, how are new
organisms created?organisms created?
-MITOSIS-MITOSIS..
8.8. SEXUAL REPRODUCTIONSEXUAL REPRODUCTION is the production of offspringis the production of offspring
through which method mitosis or meiosis?through which method mitosis or meiosis?
-Meiosis-Meiosis
9. What cells are involved in sexual reproduction?9. What cells are involved in sexual reproduction?
-sex cells (-sex cells (egg and sperm)egg and sperm)
10. MEIOSIS AND SEXUAL REPRODUCTION RESULTS IN How10. MEIOSIS AND SEXUAL REPRODUCTION RESULTS IN How
many cells?many cells?
--44 Haploid daughter cellsHaploid daughter cells
11. What is meant by11. What is meant by haploidhaploid??
- Cell with a single set of chromosomes- Cell with a single set of chromosomes

19. Law of IndependentLaw of Independent
AssortmentAssortment All genes are inheritedAll genes are inherited independentlyindependently
 Having one gene does not mean you willHaving one gene does not mean you will
automatically inherit another.automatically inherit another.
 Ex: If you have blond hair, what color will yourEx: If you have blond hair, what color will your
eyes be?eyes be?
 They could be brown, blue or green.They could be brown, blue or green.
 Hair color and eye color are inheritedHair color and eye color are inherited
independently of each other.independently of each other.

20. Meiosis IncreasesMeiosis Increases
VariationVariationRandom AssortmentRandom Assortment - different- different
chromosome combinations inchromosome combinations in
gametes contributes to geneticgametes contributes to genetic
variationvariation
Crossing OverCrossing Over – exchange of genetic– exchange of genetic
material between homologousmaterial between homologous
chromosomeschromosomes
 Segments of chromatids can beSegments of chromatids can be
exchanged at many sitesexchanged at many sites
 Single chromosome contains new comboSingle chromosome contains new combo
of genetic info from each parentof genetic info from each parent
 Result. .Result. . . Genetic Recombination. Genetic Recombination

21. But what happens when thisBut what happens when this
messes up?messes up?
 Sometimes homologous chromosomes fail to separateSometimes homologous chromosomes fail to separate
properly – RESULT…properly – RESULT…
 NondisjunctionNondisjunction
 What does this look like?What does this look like?
 Trisomy 21 – Down syndromeTrisomy 21 – Down syndrome
 Monosomy 23 – female with only one X chromosome =Monosomy 23 – female with only one X chromosome =
Turner syndrome= X0Turner syndrome= X0

24. TrisomyTrisomy 2121
 Down SyndromeDown Syndrome
 Affects 1 out of every 700 children in USAffects 1 out of every 700 children in US
 Abnormal facial featuresAbnormal facial features
 ShortnessShortness
 Heart defectsHeart defects
 Impaired immune systemImpaired immune system
 Mental disabilityMental disability

26. Monosomy 23Monosomy 23
Turner syndromeTurner syndrome
Affects 1 in 2500 girlsAffects 1 in 2500 girls
 Swollen hands and feetSwollen hands and feet
 Wide and webbed neckWide and webbed neck
 Broad, flat chest shaped like a shieldBroad, flat chest shaped like a shield
 Drooping eyelidsDrooping eyelids
 Dry eyesDry eyes
 InfertilityInfertility
 Short heightShort height