3.2 Flashcards
what are the tree hallmark events that occur in meiosis I
1) homologous chromosome pairing
2) Crossing over between homologous chromosomes
3) Segregation (seperation) of the homologous chromosomes that reduces chromosomes to the haploid number
When do homologous chromosome pairing and recombination take place in meiosis?
prophase I
what are the 5 substages of prophase I
Leptotene Zygotene pachytene diplotene diakinesis
Leptotene stage
Cells entering the first substage of meiotic prophase I have passed through interphase and have had chromosomes duplicated. Progressive chromosome condensation begins in leptotene, but the chromosomes remain diffuse at this stage. Centrosomes begin to migrate toward opposite poles of the cell, and asters of microtubule spindle fibers are produced from each centrosome
Zygotene
second stage of prophase I
Chromosomes continue to condense and homologous chromosome enter synapsis. The synaptonemal complex forms between homologs. Centrosome migration toward opposite poles continues as microtubule polymerization progresses. Meiotic spindle forms
Pachytene
third stage of prophase 1
Chromosome condensation is partially complete, and synapsed homologous chromosomes are seen as bivalent structures. Crossing over occurs between nonsister chromatids of homologous chromosomes. Kinetochore microtubules attach to kinetochores, and nonkinetochore and astral microtubules emanate from centrosomes that are nearly at opposite poles in the cell. Nuclear envolope breakdown begins.
Diplotene
fourth stage of Prophase I
Crossing over is complete, and the synaptonemal complex dissolves, leaving chiasmata that hold nonsister chromatids together. Chromosome condensation has progressed, and tetrads comprised of the four chromatids of homologous pairs of chromosomes are visible. Nuclear envelope breakdown continues.
Prophase I summary
the meiotic spindle is well established, with bundles of kinetochore microtubues tethering homologous chromosomes of tetrads to opposite poles. the nuclear envelope is fully degraded. tetrads are moved toward the middle of the cell
Metaphase I
tetrads are aligned along the metaphase plate, with each chromosome of a homologous pair tethered to kinetochore microtubules emanating from centrosomes at opposite poles of the cell. The kinetochores of sister chromatids are attached to the same centrosome, and sister chromatids are joined by cohesin to prevent their premature seperation. Chiasmata linking nonsister chromatids are broken.
Anaphase I
Depolymerization of kinetochore microtubules begins the disjunction of homologous chromosomes, which start moving toward opposite poles. Sister chromatids remain joined by cohesin.
Telophase I and Cytokinesis
nuclear membranes re-form around the chromosomes clustered at each pole. Each newly formed nucleus contains a haploid set of chromosomes . Chromosomes may partially decondense. Cytokinesis divides the cytoplasmic material of the cell by seperating the nuclei. The cytoplasmic division may be unequal.
Prophase II
The nuclear envelope breaks down, and centosomes duplicate and begin migrating to opposite poles of the cell. Microtubules emanate from the centrosomes, producing kinetochore, nonkinetochore, and astral microtubules. Chromosome recondensation takes place.
Metaphase II
Sister chromatids are attached to kinetochore microtubules from opposite poles of the cell. The force of microtubule pull and the resistance created by cohesin leads to chromosome alignment along the metaphase plate.
Anaphase II
Sister chromatid seperation begins with the breakdown of cohesin by seperase and the depolymerization of kinetochore microtubules. As the sister chromatids move toward opposite poles, polymerization of nonkinetochore microtubules elongates the cell.
Telophase II and cytokinesis
Chromosome migration is completed, and the chromosomes begin to decondense. The nuclear envelope re-forms around chromosomes. Cytokinesis seperates the newly formed nuclei and divides the cytoplasmic material, perhaps unevenly.
homologous chromosome Synapsis
the alignment of homologous chromosome pairs.
initiates formation of synaptonemal complex
synaptonemal complex
a tri layer protein structure that maintains synapsis by tightly binding nonsister chromatids of homologous chromosomes to one another
initiated by synapse
“protein bridge”
Function:
properly align homologous chromosomes before their separation, and then facilitate recombination between homologous chromosomes.
nonsister chromatids
Chromatids belonging to different members of a homologous pair of chromosomes.
The binding of nonsister chromatids by a synaptonemal complex draws the homologs into close contact (synapsis)
recombination nodules
appear in the central element of the synaptonemal complex at intervals during pachytene
play a pivotal role in crossing over of genetic material between nonsister chromatids of homologous chromosomes.
The number of nodules correlates closely with the average number of crossover events along each homologous chromosome arm.
two important observations
1) their appearance and location within the synaptonemal complex is coincident with the timing and location of crossing over
2) they seem to be present in organisms that undergo crossing over and absent in those that dont
chiasmata
after synaptonemal complex starts dissolving, homologs pull apart slightly in diplotene phase.
chiasmata are the resulting visible contact points between nonsister chromatids from the pulling apart.
they are located along chromosomes where crossing over has occurred.
mark the locations of DNA strand exchange between nonsister chromatids of homologous chromosomes.
Dissolved at onset of Metaphase I
spores
haploid gametes formed by meiosis in yeast
temporarily contained in an ascus
ascus
sac-like structure containing spores
