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Genetics and evolution > Gametogenesis and fertilisation! > Flashcards

Gametogenesis and fertilisation! Flashcards

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1
Q

What are primordial germ cells (PGCs) and their role?

A

PGCs are the precursors of gametes. They are called this when the stem cells migrate (but when in gonad e.g testes, they’re called spermatogonia)

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2
Q

How does meiosis contribute to gametogenesis?

A

Meiosis reduces chromosome number by half and introduces genetic variability. MI separates homologous chromosomes (crossing over of chiasmata occurs); MII separates chromatids.

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3
Q

What is the main difference in the timing of meiosis in spermatogenesis vs. oogenesis?

A

In spermatogenesis, meiosis occurs before differentiation; in oogenesis, it occurs after differentiation.

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4
Q

What is the outcome of spermatogenesis from one primary spermatocyte?

A

Four haploid spermatozoa.

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5
Q

What is spermiogenesis?

A

The process by which spermatids differentiate into mature spermatozoa, involving acrosomal cap forming from the Golgi apparatus, flagellum development, cytoplasm removal, mitochondria coalesce (come together to form a mass) near the base of the flagella, histones are replaced by arginine-rich protamines and nuclear condensation and cytoplasmic bridges are lost.

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6
Q

What structures are in the sperm tail that aid motility?

A

The axoneme with a 9+2 microtubule structure powered by dynein and ATP hydrolysis.
Dynein uses energy from hydrolysis to allow MTs to slide past each other - causes flagellum to bend

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7
Q

What is the acrosome and what does it contain? What does it develop from?

A

A cap-like structure derived from the Golgi that contains lytic enzymes like acrosin (protein digestion), β-N-acetylglucosaminidase (carbohydrate digestion), and phospholipase C (lipid digestion)to digest egg coverings.

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8
Q

How many gametes are produced from one primary oocyte?

A

One ovum and two polar bodies.

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9
Q

What does mitotic division of PGCs produce?

A

Oogonia

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10
Q

When is meiosis I completed in oogenesis?

A

At ovulation.

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11
Q

When is meiosis II completed in oogenesis?

A

After fertilisation.

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12
Q

What features make the egg a specialized cell?

A

Contains nutrient yolk proteins, protein synthesis machinery (ribosomes, tRNAs, mRNAs encoding proteins for early development, morphogenetic factors to direct early development, protective chemicals: UV-filters and DNA repair enzymes, and an extracellular coat (zona pellucida).

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13
Q

What glycoproteins make up the zona pellucida?

A

ZP1, ZP2, and ZP3.

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14
Q

How do we know ZP3 is essential in formation of zona pellucida?

A

ZP3 ‘knockout’ mice produce oocytes lacking the zona
pellucida, and are therefore infertile
* Human ZP3 gene ‘knock-in’ rescues zona formation and fertility
* Mouse sperm can bind the human ZP3 ‘knock-in’ because it recognises the murine polysaccharide

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15
Q

What is the function of ZP3?

A

It acts as the sperm receptor, determining species specificity via its O-linked polysaccharide.

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16
Q

What is the cortical reaction?

A

Exocytosis of cortical granules triggered by Ca²⁺, leading to modification of the zona pellucida to block polyspermy.

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17
Q

What do cortical granules contain?

A

proteases and glycosides

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18
Q

What does GaIT stand for?

A

Sperm galactosyltransferase (GalT)

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19
Q

What happens once the sperm has penetrated the cumulus cells and hyaluronic acid?

A

GaIT on sperm recognises the N-aceylglucosamine residues on ZP3
GalT-ZP3 crosslinking causes GalT proteins to cluster, triggering G protein activation. The change in membrane potential opens voltage-gated calcium
channels, increasing intracellular Ca2+

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20
Q

What is the acrosomal reaction?

A

calcium-mediated exocytosis of the acrosomal vesicle

Acrosomal enzymes, including b-N- acetylglucosaminidase (digests oligosaccharide side chains) and acrosin (serine protease), lyse the zona
pellucida.

It triggers the cortical reaction

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21
Q

What molecules mediate sperm-oocyte membrane fusion?

A

Sperm protein Izumo binds to oocyte protein Juno, recruiting CD9 for membrane fusion.

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22
Q

What triggers calcium release in the oocyte during fertilisation?

A

Sperm entry triggers calcium release from the egg’s ER, initiating egg activation and development.

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23
Q

What is the speed of calcium release?

A

a wave of calcium crosses the egg at 5-10um s-1, followed by calcium oscillations

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24
Q

Why is a sharp increase of free ca2+ important?

A

Important for egg activation and for initiation of development

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25
How do cortical granules get to the plasma membrane?
Actin polymerises into microfilaments which transport cortical granules to the plasma membrane cortical granule content is released by exocytosis
26
What does Ca2+ activate?
A kinase that leads to the proteolysis of a cyclin and the metaphase 2 arrested oocyte completes meiosis
27
What happens after the sperm enters the oocyte?
The oocyte completes meiosis II, forming the definitive oocyte and second polar body.
28
What is the role of the sperm centriole after fertilisation?
A centrosome forms around the sperm centriole becoming the MTOC for the sperm aster for the first mitotic spindle.
29
What are the 10 steps of fertilisation in order?
1) Penetration of cumulus cells, 2) Binding to zona pellucida (ZP3), 3) Acrosomal exocytosis, 4) Passage through zona, 5) Membrane fusion (Izumo-Juno), 6) Ca²⁺ spiking, 7) Cortical granule release, 8) ZP modification, 9) Pronuclear DNA replication , 10) Mitotic spindle formation.
30
What happens in the pronuclei replication?
Pronuclei undergo DNA replication as they migrate along mitotic tubules towards each other Pronuclear envelope breaks down Centrosome replicates and organises a mitotic spindle Chromosomes align on common metaphase plate
31
What do the sperm and egg contribute to the zygote?
Sperm: haploid genome + centriole. Egg: haploid genome + mitochondria + mRNAs + proteins for early development.
32
What are the three key outcomes of fertilisation?
Restoration of diploidy, sex determination (XX or XY), and initiation of cleavage.
33
Where do PGCs originate?
They originate in the extra-embryonic tissue, migrate via the gut into the genital ridge, multiply by mitosis, and later enter meiosis to become gametes.
34
Where does spermatogenesis occur?
Seminiferous tubules
35
How does the sperm flagella bend?
Dynein uses the energy of ATP hydrolysis to slide the MTs past one another
36
Briefly state the 5 spermatogenesis steps
PGCs → Spermatogonia: Divide by mitosis. Spermatogonia → Primary spermatocytes (diploid). undergo meiosis 1 Primary spermatocytes → 2 Secondary spermatocytes (undergo Meiosis II). → 4 Spermatids (haploid). → Spermatozoa (mature sperm through spermiogenesis).
37
What is the acrosomal reaction? Describe how it happens
GalT-ZP3 crosslinking causes GalT proteins to cluster, triggering G protein activation. The change in membrane potential opens voltage-gated calcium channels, increasing intracellular Ca2+. Calcium-mediated exocytosis of the acrosomal vesicle is initiated: the acrosomal reaction
38
What allows the sperm and oocyte plasma membranes to fuse?
Sperm Izumo binds oocyte Juno, recruiting oocyte CD9, causing the plasma membranes to fuse, and the sperm enters the oocyte
39
What does the calcium wave in the oocyte trigger?
1. The cortical reaction 2. Completion of meiosis II
40
What is the cortical reaction?
The egg releases enzymes from cortical granules. These enzymes harden the zona pellucida and remove carbohydrate from ZP3, so no other sperm can enter. This blocks polyspermy
41
Where are sperm germ cells made and where do they move to?
Made outside gonad (testes) and move into gonad, give rise to gametes
42
what do cells need to have to become a germ cell?
Germ cell determinants
43
What is the genital ridge? What does it form?
GiveS rise to somatic cells of gonads, can form testes or ovaries which germ cells have to migrate into (via gut and into genital ridge)
44
What do PGCs divide by as they migrate?
Mitosis
45
What do PGCs divide by to produce gametes?
Meiosis
46
Can meiosis be paused?
Yes and cell can stay like that for years before meiotic division carries on
47
What are spermatogonia called when they divide and the daughter cells enter meiosis?
Spermatocytes
48
What is the spermatic ridge? What are the cells then called?
Linking between four spermatocytes after meiosis --> they're then called spermatids
49
What do spermatids share?
Cytoplasm- to share gene products of the y chromosome (as some chromatids only have an x chromosome and need y gene products for speriogenesis)
50
Where do PGCs migrate to?
From origin site to genital ridge via gut
51
How do PGCs multiply as they migrate?
By mitosis
52
How do PGCs multiply to produce gametes?
By meiosis
53
Give the steps of PGCs becoming spermatozoa
PGCs -> mitosis into spermatogonia -> mitosis into spermatocytes -> incomplete cytokinesis into syncytia -> differentiation into 4 spermatozoa
54
When do secondary oocytes arrest?
Metaphase II
55
Outline the key points that turn PGCs into an ovum and 2 polar bodies
PGC -> mitosis makes oogonia -> mitosis makes primary oocytes -> STOP in prophase I -> ovulation means meiosis I completes -> secondary oocyte and 1st polar body created, STOP -> fertilisation completes meiosis II -> creates 1 ovum and 2 polar bodies
56
How do we know ZP3 contains the O linked polysaccharide that sperm attaches to?
ZP3 ‘knockout’ mice produce oocytes lacking the zona pellucida, and are therefore infertile Human ZP3 gene ‘knock-in’ rescues zona formation and fertility
57
Where do the cortical granules evolve from?
Golgi apparatus
58
What is the oocyte surrounded by?
cumulus cells from the follicle, in a matrix of hyaluronic acid
59
What activity on the sperm head allows it to penetrate the cumulus cells and hyaluronic acid matrix?
Hyaluronidase activity
60
Give the 10 basic steps of fertilisation
Sperm penetration of the cumulus cell layer Sperm binding to the zona pellucida (ZP3) Exocytosis of the acrosomal vesicle (acrosome reaction) Passage of the sperm through the zona pellucida Fusion of sperm and oocyte membranes (Izumo–Juno interaction) Intracellular calcium spiking Exocytosis of cortical granules Enzymatic modification of the zona pellucida (block to polyspermy) Pronuclear DNA replication Chromosome alignment on a single mitotic spindle
61
What do the polar bodies do?
Eliminate one half of the diploid chromosome set produced by meoitic division in the egg, leaving a haploid cell - cell has to divide asymmetrically to do this
62
What happens after the spermatocytes enter meiosis?
Incomplete cytokinesis produces a syncytia of haploid spermatids held together by cytoplasmic bridges
63
What is the point of the syncytia and cytoplasmic bridges?
Shared cytoplasm allows synchronisation of maturation and sharing of Y chromosome gene products and proteins.
64
What is the zona pellucida for?
Glycoprotein coat for species-specific sperm recognition and fusion
65
how many eggs are females born with?
born with 1-2 million which declines - at 20 weeks gestation, have 6-7 million oocytes
66
Why is sperm a streamlined structure?
To optimise speed, efficiency and genetic delivery
67
What are the limitations to sperm structure?
Limits sperm cytoplasmic contents and organelles - relies on egg for post-fertilisation support
68
What are the three pieces to sperm structure?
Head Midpiece - mitochondria wrapped AROUND axoneme Tail - axoneme and flagellum
69
What are gametes essential role?
Sexual reproduction
70
What is the acrosomal cap and enzymes for?
Acrosomal reaction
71
How is the nucleus specialised?
Haploid and compact chromatin to minimise volume and protect (during transport of sperm) In females, allows meiotic arrest to prevent premature chromosomal divison and maintain genomic integrity
72
Why do sperm need lots of mitochondria?
For travel and flagellum (strong, directional propulsion) as sperm can travel through the reproductive tract for up to a few days
73
What is the zona pellucida?
A glycoprotein coat
74
What is the layer of cumulus cells?
Corona radiata is innermost layer of cumulus cells - metabolic support and protection during ovulation
75
What is anisogamy for?
Divergence of this along with evolutionary pressures maximises fertilisation success and ensures viable offspring - sperm designed for competition - egg designed for selectivity and support
76
What is asthenozoospermia?
Defective tails - leads to infertility
77
What do defects in the zona pellucida cause?
Polyspermy risk or implantation failure
78
What is gametogenesis?
The process by which diploid germ cells divide into haploid gametes
79
What do Leydig cells produce?
Testosterone
80
Which hormone impacts Leydig cells?
LH
81
What do Sertoli cells produce?
Androgen-binding peptide
82
Which hormone impacts Sertoli cells?
FSH
83
How many mature ovum are released over a life time?
400
84
What is capacitation?
Changes to sperm membrane in the female tract - increases motility - more responsible to chemotactic signals from egg - prepare sperm for acrosome reaction - fertilisation can only occur after capacitation

Genetics and evolution (16 decks)

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