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 and meiosis IImeiosis II).
3. Meiosis
• Reduces the chromosome number such
that each daughter
• Cell has a haploid set of chromosomes
• Ensures that the next generation will
have:
– Diploid number of chromosome
– Exchange of genetic information
(combination of traits
– that differs from that of either parent)
4. MeiosisMeiosis
• Sex cellsSex cells divide to produce gametesgametes (sperm(sperm
or egg)or egg).
• GametesGametes have halfhalf the # of chromosomeschromosomes.
• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).
Male: spermatogenesisMale: spermatogenesis
Female: oogenesisFemale: oogenesis
• MeiosisMeiosis is similar to mitosismitosis with some
chromosomal differences.
5. Meiosis
• Only diploid cells
can divide by
meiosis.
• Prior to meiosis I,
DNA replication
occurs.
• During meiosis,
there will be two
nuclear divisions,
and the result will be
four haploid nuclei.
• No replication of
DNA occurs
between meiosis I
and meiosis II.
7. Interphase IInterphase I
• 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.
8. Interphase IInterphase I
• NucleusNucleus and nucleolusnucleolus visible.
nuclear
membrane
nucleolus
cell membrane
chromatin
9. Meiosis I (four phases)Meiosis I (four phases)
• Cell divisionCell division that 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
10. Prophase IProphase I
• Longest and most complex phase (90%).Longest and most complex phase (90%).
• ChromosomesChromosomes condense.
• SynapsisSynapsis occurs: homologous chromosomeshomologous chromosomes
come together to form a tetradtetrad.
• TetradTetrad is two chromosomeschromosomes or four
chromatidschromatids (sister and nonsister chromatids).
12. 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 homologous chromosomes.homologous chromosomes.
a. 22 pairs of autosomesautosomes
b. 01 pair of sex chromosomessex chromosomes
14. Crossing OverCrossing Over
• 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 crossingcrossing
overover.
15. Crossing Over - variationCrossing Over - variation
nonsister chromatids
chiasmata: site
of crossing over
variation
Tetrad
21. Anaphase IAnaphase I
• Homologous chromosomesHomologous chromosomes separate and
move towards the poles.
• Sister chromatidsSister chromatids remain attached at their
centromerescentromeres.
23. Telophase ITelophase I
• Each pole now has haploidhaploid set of
chromosomeschromosomes.
• CytokinesisCytokinesis occurs and two haploid daughter
cells are formed.
25. Meiosis IIMeiosis II
• No interphase IINo interphase II
(or very short - no more DNA replicationDNA replication)
• Remember:Remember: Meiosis IIMeiosis II is similar to mitosismitosis
28. Anaphase IIAnaphase II
• same as anaphaseanaphase in mitosismitosis
• sister chromatids separatesister chromatids separate
29. Telophase IITelophase II
• Same as telophasetelophase in mitosismitosis.
• Nuclei form.
• CytokinesisCytokinesis occurs.
• Remember:Remember: four haploid daughter cellsfour haploid daughter cells
produced.produced.
gametes = sperm or egggametes = sperm or egg
31. 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.
32. A Comparison of Mitosis and
Meiosis
• Mitosis conserves the number of
chromosome sets, producing cells that are
genetically identical to the parent cell
• Meiosis reduces the number of chromosomes
sets from two (diploid) to one (haploid),
producing cells that differ genetically from
each other and from the parent cell
• The mechanism for separating sister
chromatids is virtually identical in meiosis II
and mitosis
33. • Three events are unique to meiosis, and all three
occur in meiosis l:
– Synapsis and crossing over in prophase I: Homologous
chromosomes physically connect and exchange genetic
information
– At the metaphase plate, there are paired homologous
chromosomes (tetrads), instead of individual replicated
chromosomes
– At anaphase I of meiosis, homologous pairs move
toward opposite poles of the cell. In anaphase II of
meiosis, the sister chromatids separate
