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