Meiosis Notes

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!

Fertilization

Chromosomes

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

Prophase I Chomatids pair and condense. Crossing over occurs Spindle forms. Nuclear envelope fragments.

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 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

Meiosis allows the creation of unique individuals through sexual reproduction. Meiosis keeps the number of chromosomes the same for each generation.

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

Mistakes in Meiosis Nondisjunction = The failure of homologous chromosomes to separate properly during meiosis

Trisomy: When a gamete with extra chromosomes is fertilized by a normal gamete, the zygote will have an extra chromosome Ex. In human, if a gamete with an extra chromosome number 21 is fertilized by a normal gamete, the resulting zygote has 47 chromosomes instead of 46. This will develop into a baby with Down syndrome.

Monosomy = When a gamete that is missing a chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome. Most often this is lethal. One non-lethal example is Turner syndrome, in which human females have only a single X chromosome instead of two.

Polyploids = organisms with more than the usual number of chromosomes sets Often results in death of the zygote. However, polyploids often occur in plants