Phillips - Genetics Flashcards
Gametogenesis
- Devo of eggs (ova) or sperm from primordial germ cells -> both require meiosis, but difference in TIMING (and other details)
- Germ cells originate in endoderm of the yolk sac at the 4th week of embryonal life
- 6th week: migrate to genital ridge, and associate with somatic cells to form the primitive gonad
- Gonad differentiates into ovary or testis in accordance w/sex-determining genetic guidance
SRY
- Sex-determining region on the Y chromosome
-
46 XY: SRY presence indicates that indifferent gonad will become testis
1. Leydig cells -> testosterone
2. Sertoli cells -> AMH (SALT memory tool) -
46 XX: NO SRY
1. Indifferent gonads default to ovary - Not the only sex-determining gene, but the most important one
46 XY F discovered to have a mutation in SRY. What does this mean?
- Mutation in SRY causes gonad to remain an undifferentiated that will not produce any hormones
What are the 2 purposes of meiosis?
- Reduce chromosomal number from diploid (2n or 46 in humans) to haploid (n or 23 chromosomes)
- Shuffling of genetic material to increase genetic variation
Spermatogenesis
- Seminiferous tubules of testes -> prior to puberty, primordial germ cells undergo mitosis and line the tubules
- At onset of sexual maturity (testosterone-driven):
1. Spermatogonia (2N) -> 1o spermatocytes (2N); there can be TONS of mitosis here
2. Meiosis I: division into 2o spermatocytes (N); reduction step
3. Meiosis II: division into spermatids (N); homologues separate
4. Spermatid (N) to sperm (N) w/o further div - ONGOING throughout life
Mature sperm
-
Capacitation: develop head and tail
1. Shed all unnecessary baggage to become efficient swimmers, i.e., lose mitochondria - Spermatogenesis takes 64 days
1. Hundreds of mitotic divisions, and 200 million sperm per ejaculate
45-y/o new dad and his 37-y/o wife gave birth to baby with achondroplastic dwarfism. Can we postulate a cause?
- Mitotic errors are common in sperm and the mutations accumulate in older dads and put offspring at risk
- New auto dominant mutation that likely came from the sperm
What kinds of genetic risks might an AMA pregnancy pose for the child?
Risk of chromosome abnormalities, like trisomy 21
Oogenesis
- Largely completed by birth: 30 mitoses
1. 2.5 million oocytes at birth, but most degenerate (atresia) -> 400 mature to ova for repro age range - Oogonia in F embryo begin Meiosis I in month 3 of embryologic prenatal development
1. Meiosis I arrested in 1o oocyte at diakenesis of prophase I by birth -> completed at ovulation
2. Meiosis II is completed at fertilization - Polar bodies inactive, and NOT fertilizable
What happens in interphase before meiosis?
- DNA replication: each chromosome goes from univalent to a bivalent unit
- Identical sister chromatids joined by a centromere
- Diploid # of chromosomes (2N; 46)
What are the stages of prophase I?
- Leptotene: chromosomes have already replicated, but lie on top of each other
- Zygotene: homologous pairs move together and pair or synapse
- Pachytene: first time you can see the bivalent chromosome (cheerleader pose) -> 2 homologous chromosomes now look like a tetrad
- Diplotene: crossing over occurs (KNOW THIS)
- Diakinesis: oogenesis frozen here until ovulation
When does crossing over occur in prophase I?
Diplotene
In what stage of prophase I is oogenesis arrested?
Diakinesis: until ovulation
What is recombination? When does it occur? How often?
- Crossing over that occurs at chiasmata (areas of contact) during Diplotene (Prophase I)
- Occurs (on average) on one arm of each chromosome
What is the purpose of crossing over?
- INC genetic variation
- Multiplies the already huge # of different gamete types produced by independent assortment
What happens after prophase in meiosis I?
- Metaphase: nuclear membrane disappears, spindle appears, and pairs align on metaphase plate
- Anaphase: bivalents disjoin, and are pulled apart by spindles attached to chromosomal centromere
- Telophase and cytokinesis: chromosomes gather into nuclei and cell division occurs
How is meiosis II different from meiosis I?
- Same steps, but much more rapidly
- Chromatids of same chromosome are pulled apart (going from N to N)
1. NOT identical because crossing over occurred in Diplotene of Prophase or Meiosis I - Occurs at fertilization in oogenesis
Woman has a child with an extra chromosome that is maternal. How can we tell which meiotic stage (1 or 2) that this nondisjunction resulted from?
- If all chromosome 21s are different DNA fingerprints, then its meiosis I
- 95% of maternal nondisjunction is meiosis I
What is the key difference between mitosis and meiosis?
- The way chromosomes uniquely pair and align in meiosis
How are the products of spermatogenesis and oogenesis different (image)?
What are the key hormones that determine the fate of the bipotential gonad?
- AMH: anti-mullerian hormone (MIS = mullerian inhibiting substance)
- Insl3 (insulin-like substance 3): may play a role in testicular descent (produced by the gonad)
