Meiosis & Mitosis

number of gametes

2^n rule

Prometaphase 1

Homologous chromosomes are attached to spindle microtubules at the fused kinetochore shared by sister chromatids. Chromosomes continue to condense, and the nuclear envelope completely disappears.

Prophase 1

Leptonene Zygotene Pachytene Diplotene Diakinese

Oogenesis

Primary oocytes- suspended in prophase of meiosis until puberty Matures & completed meiosis 1 - forming a 2 degree oocyte & 1st polar body Ovulation- initiates meiosis 2 to metaphase stage Fertilization- triggers completion of meiosis 2 & forms 2nd polar body & diploid zygote

Mitosis summary

Prophase: chromosomes condense Prometaphase: chromosomes attach Metaphase: chromosomes align Anaphase: chromosomes separate Telophase: chromosomes relax

Prophase 2

Sister chromatids condense. A new spindle begins to form. The nuclear envelope starts to fragment.

Anaphase 1

Spindle microtubules pull the homologous chromosomes apart. The sister chromatids are still attached at the centromere.

Mitosis prometaphase

centrioles arrive at poles, nuclear membrane breaks down, spindles attach to kinetochores

anaphase

centromeres split, daughter chromosomes move to opposite poles

diplonema

chiasmata visible between non-sister chromatids, pairs of sister chromatids begin to separate

diakinesis

chromatids continue to separate, non-sister chromatids attached via chiasmata moving towards ends of tetrad terminalization, and nuclear envelope starts to break down

Leptonema

chromatin condenses, chromosomes visible, homology search begins, homologous chromosomes start to align

Mitosis Prophase

chromosome condenses, nuclear envelope disappears, centrioles divide and migrate

Metaphase

chromosomes align at the midline, spindle microtubules visible

S phase

chromosomes are duplicated. The resulting sister chromatids are held together at the centromere. The centrosomes are also duplicated.

Telophase 2 and cytokinesis

chromosomes arrive at the poles of the cell and decondense. Nuclear envelopes surround the four nuclei. Cleavage furrows divide the two cells into four haploid cells.

zygonema

condensation continues, initial alignment, lateral elements visible, synaptonemal complex forms-bivalents

Pachynema

condensation continues, synapsis & homologous recombination, 4 chromatids/bivalent= tetrad

additional shuffling

critical to ensure normal chromosome disjunction

Meiosis provides

differing combinations of genes or alleles from the previous generation

down syndrome

extra 21st chromosome

Purpose of meiosis

form haploid gametes from a diploid parent cell

Metaphase 1

homologous chromosomes randomly assemble at the metaphase plate, where they have been maneuvered into place by the microtubules.

most nondisjunction occure

in the ovum or metaphase 1

telophase

nuclear membranes reform, chromosomes decondense, cell furrow/plate

chiasmata

points at which crossover occurs

consequences of meiosis

reduction of the chromosome number segregation of alleles shuffling of the genetic material by random assortment of the homologues (metaphase 1) segregation, reducing, shuffling

sister chromatids

separate during anaphase 2 of meiosis

homologous chromosomes

separated during anaphase 1 of meiosis

anaphase 2

sister chromatids are pulled apart by the shortening of the kinetochore microtubules. Non-kinetochore microtubules lengthen the cell

Telophase 1

sister chromatids arrive at the poles of the cell and begin to decondense. A nuclear envelope forms around each nucleus and the cytoplasm is divided by a cleavage furrow. The result is two haploid cells. Each cell contains one duplicated copy of each homologous chromosome pair.

Metaphase 2

sister chromatids line up at the metaphase plate

crossing over

takes place during meiosis 1- prophase 1

Independent assortment

takes place during metaphase 1

independent assortment

takes place during metaphase 1

Prometaphase 2

the nuclear envelope disappears, and the spindle fibers engage the individual kinetochores on the sister chromatids

during oogenesis

there can be long gaps between one stage and the next

purpose of meiosis

to form unique haploid gametes from a diploid parent cell