Fertilisation Flashcards
LO
- Understand the concept of two divisions during meiosis
- Explain how PLC zeta and Ca2+ cause egg activation.
- Explain how polyspermy is prevented.
- Understand what is meant by capacitation
How do cells become haploid and what is meant by this?
Tell me about the major differences between males and females
- Meiosis - the most revolutionary invention of the eukaryote- the halving of genetic material to allow sexual reproduction. Meiosis is an initial duplication of the whole genome followed by 2 cell divisions with no intervening DNA replication
- Commitment to differentiation - Primary spermatocytes/ oogonia enter a pre-meiotic S-phase (all chromosomes replicated, = 4n) MALES after puberty, FEMALES in utero(arrest)
- Primary spermatocytes/ oocyte (4n) undergoes a first meiotic division to form…
- Secondary spermatocytes/ oocyte (2n) undergoes a second meiotic division to form, FEMALES ovulate (arrest)
- Spermatid or ovum (=n, haploid- in female this completes at fertilization)
- Major differences in male and female - female needs to produce an egg with sufficient nutrients and organelles for later development – hence 4 sperm: 1 egg
Tell me the steps to meiosis
1. Prophase I
- prior to prophase, chromosomes replicate to form sister chromatids
- NE disintegrates and the chromosomes condense
- spindle fibres appear
- homologous chromosomes exchange parts of themselves such that one chromosome contains both maternal and paternal DNA (crossing-over)
2. Prometaphase I
- spindle fibres attach to the centromeres
- chromosomes continue to condense
3. Metaphase I
- chromosomes align along the equator of the cell
- independent assortment occurs
4. Anaphase I
- homologous chromosomes pulles in opposite directions towards poles as the spindle fibres retract to divide the DNA between the two cells which will be formed
5. Telophase/ cytokinesis I
- Telophase: NE reforms, spindle fibres disappear
- cytokinesis: two, haploid cells
6. Prophase II
- same as prophase I
7. Prometaphase II
- same as prometaphase I
8. Metaphase II
- chromosome line up in single file along the equator of the cell
9. Anaphase II
- sister chromatids pulled to opposite poles of the equator
10. Telophase/ cytokinesis II
- Telophase: same as telophase I
- cytokinesis: 2 non-identical haploid daughter cells
net= 4 non-identical haploid daighter cells
Tell me the steps to mitosis
Tell me the stages of the cell cycle
- Meiosis – allows recombinatory rearrangement at prophase 1
- S phase – DNA replicated
- Males – symmetry, Females – asymmetrical– (fish/ chicken egg has to supply all nutrients till free living, mammal till implantation)
Tell me about the process of spermatogenesis
- A1-4 as they divide here
- Recombination occurs as part of the first meiotic division – first meiotic division after S phase duplication of genome
- Increased testosterone production (intertiail cells) at puberty- via FSH leads to retinoic acid formation by sertoli cells
- Spermatids mature physically – nucleus condenses, mitochondria coalesce in mid piece and tail forms – Spermiogenesis- but not functional
Can freshly ejaculated mammalian sperm fertilise eggs?
Freshly ejaculated mammalian sperm cannot fertilize eggs. Need time in female reproductive tract.
Whats sperm Capacitation?
- Freshly ejaculated mammalian sperm cannot fertilize eggs. Need time in female reproductive tract.
- Capacitation name given to this process. Can be triggered in vitro by raising sperm calcium levels.
- Hyperactivation capacitated sperm swim faster. May help sperm swim to egg.
- Capacitation – also destabilises the acrosomal membrane breaks down the Zona Pellucida
Label this sperm
What material is IVF carried out on?
IVF – on glass / plastic surfaces in culture raised Ca levels also trigger
What does an Acrosome contain?
Acrosome- contains hydrolases, glycodases and proteases
Whats an acrosome?
The acrosome is an organelle that develops over the anterior half of the head in the spermatozoa (sperm cells) of many animals including humans. It is a cap-like structure derived from the Golgi apparatus. In Eutherian mammals the acrosome contains degradative enzymes (including hyaluronidase and acrosin).
Tell me about PGC in female meiosis?
~1000 PGC –> 7million oogonia in gestation but most die, those that survive enter meiosis
tell me about female meiosis gestation
~3-7 months gestation- remain paused at 4n (diplotene) stage as primary oocytes –for up to 50 years
- at onset of puberty waves of primary oocytes resume meiosis due to the action of LH
Female meiosis
Whats oogonia?
an immature female reproductive cell that gives rise to primary oocytes by mitosis.
About 1,000,000 PGC form what at birth?
oogonia
At birth all PGCs have developed into what?
At birth all PGCs have developed to oogonia (or died) and are at the 4N stage – compare with the male
Tell me about oogonia meiotic arrest
- Arrest at 3 month stage for 12-50 yrs!
- Activate due to variable numbers of granulasa cells – reduce cAMP (have gap junctions to oogonia) if falls enough LH receptors form and oogonia leave arrest – usually 4-5/cycle – competition for FSH most mature follice produces estrogen in induces down regulation of FSH
- At ovulation –Arrests again in the second meiotic division at ovulation until fertilized- sperm breaks that arrest
Are female meiotic divisions symmetric or asymmetric?
Female meiotic divisions are asymmetric
- Two meiotic divisions (unequal sizes) to become haploid. PB1 (first polar body); PB2 (second polar body).
Meiosis is evolutionarily conserved, but the timing of fertilisation is not
Tell me about this…
- I and II are positions of meiotic arrest –All species show S phase arrest in the first meiosis but there is great variation after this and Fertilisation* can occur are different points during meiosis
- Clam Starfish – has no second arrest
- Note Clam – 5 copies of genome!! Before mitosis restarts
- Mouse -2 (?3)
- In all cases except star fish – breaking of arrest caused by fertilisation – how???
Tell me about sea urchin eggs
- Large eggs 150-200um diameter
- Quantity
- Simple to use (keep in seawater)
- Very simple system – inj KCl – releases millions of eggs into sea water – very much used experimental system
- Sold as fish roe!!!- sushi
When do sea urchin eggs arrest?
Eggs arrest having completed meiosis. Eggs are shed and fertilized at interphase (G1) of the first mitotic cell cycle (mammals arrest but in second meiotic cycle)
What are sea urchin eggs arrested by?
Arrest by cytoplasmic acidification. Unfertilized egg cytoplasm is acidic. Fertilization increases egg pH 0.3 units (1-4 min). Sperm activates a Na+/H+ exchanger in the plasma membrane (mammalian arrest does not cause by this mechanism)
