Cell Division

Meiosis Meiosis occurs in sexual reproduction when a diploid germ cell produces four haploid daughter cells that can mature to become gametes (sperm or egg).

Meiosis Goal: reduce genetic material by half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content Just right!

Meiosis: 2 part cell division Sister chromatids separate Haploid Diploid Meiosis I Meiosis II Homologous Chromosomes pairs separate 2n n n Result: one copy of each numbered chromosome in a gamete.

Meiosis I Prophase I (early) (diploid) (late) Metaphase I Anaphase I Telophase I Nucleus Spindle fibers Nuclear envelope

Nuclear envelope breaks apart Prophase I Early Early prophase Chromosomes condense Spindle forms Nuclear envelope breaks apart .

Chromatids pair (synapse) Prophase I Late Late prophase Chromatids pair (synapse) Crossing over occurs between chromatids of the homologous chromosome pair

Metaphase I Homologous chromosome pairs line up on the cell’s equator

Anaphase I Chromosomes separate and move to opposite poles. Sister chromatids remain Attached at their centromeres.

Telophase I Nuclear envelopes reforms Spindle disappears. Cytokinesis divides cell into two

Meiosis II Sister chromatids have carry different genetic information End product of meiosis is Each Sex cell /gamete has : 1 copy of each chromosome 1 copy of each gene segment Sister chromatids have carry different genetic information due to crossing over

Meiosis II Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

Prophase II Nuclear envelope fragments. Spindle forms.

Metaphase II Chromosomes align along equator of cell.

Anaphase II Sister chromatids separate and move to opposite poles.

Telophase II Nuclear envelope assembles. Chromosomes unravel. Spindle disappears. Cytokinesis divides cell into two.

Results of meiosis Four haploid cells One copy of each chromosome Not the same genes on each chromosome

“Putting It All Together” - Fertilization

What Meiosis is About Meiosis allows the creation of unique individuals through sexual reproduction. Meiosis Video Link

Meiosis Video Link #1 Meiosis Video Link #2 Meiosis Video Link#3

2 diploid 4 haploid Crossing over between nonsister chromatids Reduction divisiion Increases genetic variability vs. clones Meiosis 1 Homologous chrom separate Meiosis 2 Sister chromatids separate

Mitosis Meiosis Number of divisions 1 2 Number of daughter cells made through process 4 Are the new cells Genetically identical? Yes No # of Chromosomes compared to parent cell Same as parent Half of parent What cell go thru this division process Somatic cells Sex cells When do cells got thru this division process Throughout life At sexual maturity Role Growth and repair Sexual reproduction

MITOSIS MEIOSIS Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes) Produces cells for repair, maintenance, growth, asexual reproduction Only produces gametes (sex cells: egg/sperm) Results in identical diploid (2n) daughter cells Reduction division results in haploid (n) cells

Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.