Read and review the notes and answer Meiosis Quiz

1. MEIOSISMEIOSIS
Answer the QUIZAnswer the QUIZ
after reviewing thisafter reviewing this
PowerPointPowerPoint

2. MeiosisMeiosis
• 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 Imeiosis I & meiosis IImeiosis II)

3. MeiosisMeiosis
• Sex cellsSex cells undergo meiosis to produce
gametesgametes
– GametesGametes have halfhalf the # of chromosomeschromosomes.
• Male: spermMale: sperm
• Female: eggFemale: egg
– Meiosis only occurs inMeiosis only occurs in gonadsgonads
• Male Testes: spermatogenesisMale Testes: spermatogenesis
• Female Ovaries: oogenesisFemale Ovaries: oogenesis
• MeiosisMeiosis is similar to mitosismitosis with some
chromosomal differences.

4. SpermatogenesisSpermatogenesis
2n=46
human
sex cell
diploid (2n)
n=23
n=23
meiosis I
n=23
n=23
n=23
n=23
sperm
haploid (n)
meiosis II
Head
Mid piece
Tail

5. Interphase IInterphase I
• Similar to mitosismitosis interphase
– ChromosomesChromosomes replicate (S phase)(S phase)
– Each duplicated chromosomechromosome consists of two
identical sister chromatidschromatids attached at their
centromerescentromeres
– CentrioleCentriole pairs also replicate

6. Interphase IInterphase I
• NucleusNucleus and nucleolusnucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin

7. Meiosis I (four phases)Meiosis I (four phases)
• Cell divisionCell division 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

8. Prophase IProphase I
Longest and most complex phaseLongest and most complex phase
(90%)(90%)
–ChromosomesChromosomes condense
–SynapsisSynapsis occurs
• Homologous chromosomesHomologous chromosomes come
together to form a TetradTetrad
–Two homologoushomologous chromosomeschromosomes which is
four chromatidschromatids (sister and nonsister
chromatids)

9. Prophase IProphase I - Synapsis- Synapsis
Homologous chromosomes
sister chromatids sister chromatids
Tetrad

10. Homologous ChromosomesHomologous Chromosomes
• 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 homologoushomologous
chromosomes.chromosomes.
– 22 pairs of autosomesautosomes
– 1 pair of sex chromosomessex chromosomes

11. Homologous ChromosomesHomologous Chromosomes
Paternal Maternal
eye color
locus
eye color
locus
hair color
locus
hair color
locus

12. Sex ChromosomesSex Chromosomes
XX chromosome - female XY chromosome - male

13. Crossing OverCrossing Over
• Crossing over (variation)Crossing over (variation) may
occur between nonsister
chromatidschromatids at the chiasmatachiasmata.
–Segments of nonsister chromatidschromatids
break and reattach to the other
chromatidchromatid.
• Chiasmata (chiasma)Chiasmata (chiasma) are the
sites of crossing overcrossing over.

14. Crossing Over - VariationCrossing Over - Variation
nonsister chromatids
chiasmata: site
of crossing over
VARIATIONVARIATION
Tetrad

15. Prophase IProphase I
centrioles
spindle fiber
aster
fibers

16. 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.Variation
3.Formula: 2n
1.1. Example:Example: 2n = 42n = 4
2.2. ThenThen n = 2n = 2
3.3. ThusThus 2222
= 4 combinations= 4 combinations

17. Metaphase IMetaphase I
metaphase plate
OR
metaphase plate

18. Question:Question:
In terms ofIn terms of IndependentIndependent
AssortmentAssortment
How many differentHow many different
combinations of sperm could acombinations of sperm could a
humanhuman malemale produce?produce?

19. AnswerAnswer
Formula: 2Formula: 2nn
Human chromosomes:Human chromosomes: 2n = 462n = 46
n = 23n = 23
222323
~8 million combinations~8 million combinations

20. Anaphase IAnaphase I
Homologous chromosomesHomologous chromosomes
separate and move towards the
poles
Sister chromatidsSister chromatids remain
attached at their centromerescentromeres

21. Anaphase IAnaphase I

22. Telophase ITelophase I
• Each pole now has haploidhaploid set
of chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two
haploid daughter cells are
formed.

23. Telophase ITelophase I

24. Meiosis IIMeiosis II
• No interphase IINo interphase II
(or very short - no more DNADNA
replicationreplication)
• Remember:Remember: Meiosis IIMeiosis II is similar to
mitosismitosis

25. Prophase IIProphase II
• Same as prophaseprophase in mitosismitosis

26. Metaphase IIMetaphase II
Same as metaphasemetaphase in mitosismitosis
metaphase platemetaphase plate

27. Anaphase IIAnaphase II
• Same as anaphaseanaphase in mitosismitosis
• Sister chromatids separateSister chromatids separate

28. Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis.
• Nuclei form.
• CytokinesisCytokinesis occurs.
• Remember: four haploid daughterRemember: four haploid daughter
cells produced.cells produced.
gametes = sperm or egggametes = sperm or egg

29. Telophase IITelophase II

30. 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

31. VariationVariation
• Important to population as the rawImportant to population as the raw
material formaterial for Natural SelectionNatural Selection..

32. KaryotypeKaryotype
• A method of organizing theA method of organizing the chromosomeschromosomes of aof a
cell in relation to number, size, and type.cell in relation to number, size, and type.

33. FertilizationFertilization
• The fusion of a spermsperm and eggegg to form a
zygotezygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote