Cell Division

Meiosis: Important Vocabulary Diploid- contains 2 sets of chromosomes (2n = 46) Example: somatic cells Haploid- contains 1 set of chromosomes (n = 23) Example: gametes

Meiosis: Important Vocabulary Fertilization- fusion of two haploid gametes forms a diploid zygote Zygote- fertilized egg cell The first cell of a new individual What would happen if gametes were diploid?

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: cell division in two parts Sister chromatids separate Haploid Diploid Meiosis I (reduction division) Meiosis II Chromatids separate (equational division) 2n n n Result: one copy of each chromosome in a gamete.

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

Nuclear envelope disappears. Prophase I Early prophase Chromatids pair. Crossing over occurs. Late prophase Chromosomes condense. Spindle forms. Nuclear envelope disappears.

Metaphase I Chromatid pairs align along the equator of the cell.

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

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

Meiosis II Gene X Sister chromatids carry identical genetic Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information.

Meiosis II : the equational division Prophase II (haploid) Metaphase II Anaphase II Telophase II Four Non-identical haploid daughter cells

Prophase II Nuclear envelope disappears. 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 uncoil. Spindle disappears. Cytokinesis divides cell into two.

Results of meiosis Four haploid cells One copy of each chromosome

“Putting It All Together” - Fertilization

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

Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Sex cells When 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