HRR: meiosis Flashcards
What are gametes?
Germ cells generated in the gonads
What is oogenesis?
Generation and maturation of female gametes aka ova
What is spermatogenesis?
Generation and maturation of male gametes aka spermatozoa
What are tetrads?
4-strand intermediates formed by the alignment of homologous chromosomes during prophase I
What is chiasma?
Crossover points in tetrads made by homologous recombination between DNA strands during prophase I
What is ploidy?
The number of sets of chromosomes
What is haploid?
1 set of 23 chromosomes
What is diploid?
2 sets of 23 chromosomes
Generally, what happens in prophase I?
Chromosomes condense and homologous chromosomes pair up to form synaptonemal complexes. Chiasmata also form.
What are the stages of prophase I?
Leptotene, zygotene, pachytene, diplotene, diakinesis
What happens in the leptotene stage of prophase I?
Condensation of replicated chromosomes
What happens in the zygotene stage of prophase I?
Synaptonemal complexes form between homologous chromosomes
What happens in the pachytene stage of prophase I?
Synapsis completes and crossing over via homologous recombination
What happens in the diplotene stage of prophase I?
Synaptonemal complexes disappear, but chiasmata remain and are now the main connection between homologous chromosomes
What happens during diakinesis?
Condensation of chromosomes for meiosis I
What is synapsis?
Sister chromatids pair to form tetrads during prophase I and a synaptonemal complex tethers them together to facilitate crossing over
Describe the process of crossing over.
- Homologous chromosomes form tetrads
- Spo11 functions as an endonuclease to form DNA double strand breaks
- Rad51 and Dmc1 catalyze DNA strand invasion and exchange by homologous recombination
- Crossover points aka chiasmata generate 4-strand intermediates called holiday junctions
- Holiday junctions are resolved via cleavage and ligation. Incorrect base pairings are repaired via mismatch repair
What are cohesins?
Proteins that hold sister chromatids together
Generally, what happens in metaphase I?
Homologous chromosomes align on the metaphase plate and the mitotic spindle forms
What is the spindle assembly checkpoint?
Maintains metaphase I until homologous chromosomes are correctly aligned, and kinetochores attach to the spindle
Generally, what happens during anaphase I?
Homologous chromosomes are pulled to opposite poles, and the chromosome number is reduced by half
Entry into anaphase is induced by…
Activation of anaphase-promoting complex
Describe the anaphase promoting complex.
It functions as an E3 ubiquitin ligase that ubiquitinates cyclin B and other proteins to tag them for degradation
Where does cohesion remain during anaphase?
At the centromere region
Generally, what happens during telophase I?
Chromosomes are at the poles of the daughter cells, mitotic spindle disappears, cytokinesis occurs
Generally, what happens in prophase II?
The mitotic spindle reforms. The nuclear envelope does not reform and chromatin remains condensed
Generally, what happens in metaphase II?
Chromosomes align on the metaphase plate, sister chromatids are attached at the centromere, the kinetochore of sister chromatids attaches to microtubules from opposite spindle poles
Generally, what happens in anaphase II?
Sister chromatids are pulled apart and cohesin is released
What happens in telophase II?
Cytokinesis produces haploid daughter cells
What is the difference between oogonia and primary oocyte?
A primary oocyte has committed to meiosis I
A primary oocyte will develop into…
A secondary oocyte and 1st polar body
Primary oocytes are ___ploid and _n.
Diploid; 4n
When does a primary oocyte undergo its first division to become a secondary oocyte?
Right before ovulation
Secondary oocytes are ___ploid and _n.
Haploid; 2n
Secondary oocytes are arrested in…
Metaphase II
An ovum is ___ploid and _n.
Haploid; 1n
When does meiosis II occur?
After a secondary oocyte has been fertilized, it will then become an ovum and another polar body
What keeps a primary oocyte in prophase I?
High levels of cAMP from a constitutively-active GCPR in the oolemma. Follicular cells also produce cGMP that inhibits a phosphodiesterase 3A to keep cAMP from breaking down.
What is cdc25b and how does PKA impact it?
A protein involved in the progression through meiosis; PKA phosphorylates it, causing inactivation and keeping the cell in prophase I
Describe what happens in terms of meiosis right before ovulation.
The LH surge drops cAMP levels, allowing the activity of cyclin B/CDK1 to increase and allow the cell to move through prophase I
Describe arrest in metaphase II after ovulation.
Progesterone increases a kinase called Mos, part of cytostatic factor. This protein activates MAP kinase that activates p90rsk. This inhibits APC and keeps the secondary oocyte in metaphase II
What does APC do?
Needed for ubiquitination and degradation of cyclin B
How does the secondary oocyte resume meiosis II following arrest?
Sperm binding to the ovum causes a raise in intracellular calcium, activating APC. APC ubiquitinates and degrades cyclin B, allowing the oocyte to progress to anaphase II
What develops in the fertilized zygote?
2 haploid pronuclei develop and come together over time and eventually fuse by formation of microtubules
Describe spermatocytogenesis.
The mitotic division of spermatogonia into spermatogonia A and eventually B
Describe spermatogonia A.
There are stem cells maintained by mitotic divisions and progenitor cells that become committed to becoming spermatogonia B
What is special about the division of spermatogonia A progenitor cells?
They stay connected via cytoplasmic bridges during replication
Describe spermatogonia B.
Cells developed from spermatogonia A progenitor cells that after one round of mitosis enter meiosis by developing into primary spermatocytes
Meiosis I in males produces…
Secondary spermatocyte from primary spermatocyte
Meiosis II in males produces…
Spermatids from secondary spermatocytes
Spermatids develop into spermatozoa via ___
Differentiation
