Meiosis Lecture Sep 25 Flashcards
At the most basic level, how to mitosis and meiosis differ?
Meiosis involves one round of S and then two division: meiosis 1 and meiosis 2. THis results in 4 haploid cells.
Mitosis is an S and one division, resulting in 2 diploid cells.
How do mitosis and meiosis differ in terms of prophase 1?
For meiosis Prophae 1, homologous chromosomes will pair up and undergo crossing over/homologous recombination to promote genetic variability
In mitosis, homologous chromosomes do NOT pair up and will align single file during metaphase
What two characteristics of meiosis result in increased genetic variability?
- random assortment of the homologous chromosomes along the metaphase plate during metaphase 1. It is totally random which chromosome will face which spindle pole.
- Homologous recombination - crossing over of homologous chromosomes during prophase 1.
In meiosis, cohesins connect both the homologous chromosomes and the sister chromatids.
How then does the cell manage to have only the homologous chromosomes separate during anaphase 1 and not the sister chromatids?
A protein called Sgo complexes with the cohesins between the sister chromatids but not the homologous chromosomes.
The Sgo blocks APC activity against the cohesion between the daughter chromatids, but it isn’t present between the homologs, so APC ubiquinates the cohesins and breaks those down
Where does recombination occur along the homologous chromosomes?
at recombination molecules
there are usually about 2-3 per chormosome
what is another name for a homologous chromosome pair?
a bivalent
WHat causes nondisjunction?
When does it occur?
Nondisjunction results from a failure of chromosomes to separate normally in anaphase.
This can occur in meiosis 1 or 2! (can also occur during mitosis)
What does nondisjunction result in?
aneuploidy
What will happen to the number of chroomsomes in the 4 daughter cells after a nondisjunction in Meiosis 1?
After nondiscjuntion in Meiosis 2?
In meiosis 1: You’d get 2 daughter cells with n+1 chromosomes and 2 daughter cells with n-1 chromosomes
In meiosis 2: you’d get 2 daughter cells with the appropriate number of chromosomes, 1 daughter cell with n+1 chromosomes and 1 daughter cell with n-1 chromosomes
In what chromosomes do nondisjunctions usually occur?
the sex chromosomes
What is the attachment structure connecting the 2 homologous chromosomes together in prophase 1 called?
synaptonemal complex
Describe the steps and timing of spermatogenesis.
- Primordial germs cells develop and migrate to the developing gonads in the 4th week of embryonic development
- THe PGCs develop into spermatogonia
- The spermatogonia will reproduce via mitosis during fetal development.
- Eventually they will arrest after Mitosis and will remain dormant until puberty
- At puberty, the spermatogonia are stimulated to resume mitosis and increase in number again (this will continue in the seminiferous tubules of males for the rest of their life)
- Subsequently, subsets of spermatogonia (the primary spermatocytes) will enter Meiosis 1 (this continues through the entire life of the male). This forms secondary spermatocytes
- The secondary spermatocytes will enter meiosis 2 and become haploid spermatids
- Morphological transformation turns spermatids into spermatozoa (mature sperm)
Describes the steps and timin of oogenesis.
- primordial germ cells migrate to developing gonads at week 4 of development
- The PGCs develop into oogonia, which replicate through mitosis to form 7 million oogonia
- Between the 3rd and 5th months of fetal development, the oogonia will enter meiosis 1, arresting in prophase 1, becoming primary oocytes.
- At birth, a huge number of primary oocytes die off, leaving only about 400,000 - these are all the female will ever have
- THe primary oocyte will remain arrested in prophase 1 until puberty, protected within a follicle
- At puberty,, just before ovulation, a few primary oocytes will complete meiosis 1, becoming secondary oocytes.
- The larger secondary oocyte will enter meiosis 2, but will arrest in metaphse 2 during ovulation.
- If the ooctye is fertilized by sperm, it will complete meiosis 2
What enzyme induces the gonial cells to enter meiosis 1?
Why does this not occur in males until puberty when it happens during development in females?
Retinoic acid is what induces the gonial cells to enter meiosis 1.
This doesn’t happen until puberty in males because they have a cytochrome that inhibits retinoic acid until its broken down in puberty
Females don’t have thi cytochrome, so the retinoic acid is never inhibited and the oogonia enter meiosis 1 during fetal development
What protein is involved in the primary ooctye arrest in prophase 1 until puberty?
high cAMP levels are produced and maintained by the primary occyte and the surrounding follicle cells.
This keeps M Cdk activity low, arresting further progress.
However, at the beginning of puberty, the cAMP levels with drop in a select few follicles in response to the FSH/LH surge in each menstrual cycle.
This relieves the inhibition of M Cdk activity and meiosis re-commences
