Fertilization

Question 1

What are the three regions of a sperm cell?

Answer 1

The head portion, The mid portion, The tail portion.

Question 2

If you take a cross-section of the tail of a sperm cell, You will see an axoneme. What is an Axonome?

Answer 2

Axoneme - A cytoskeleton component formed by nine doublets of microtubules surrounding a centre doublet

Question 3

What is the function of the axoneme?

Answer 3

With the help of the motor protein dynein, It gives the Sperm tail its coordinated movement.

Question 4

dynein walks on the doublets of the Axoneme in order to create sperm movement. This requires ATP. Where does dynein get this ATP?

Answer 4

There’s a lot of mitochondria that is condensed in the mid portion of the sperm cell.

Question 5

In the head region of a sperm cell, at the very distal end near the tip, we have a vesicle called what?

Answer 5

An acrosome. Do not confuse this with the axoneme.

Question 6

The biggest cells are ____

Answer 6

eggs

Question 7

The egg contains maternally-inherited stores. What are some things that are within an egg that are maternally-derived?

Answer 7

Nutritional proteins, mRNAs, ribosomes and tRNAs, and morphogenetic factors

Question 8

What are morphogenetic factors within an egg?

Answer 8

Factors that have been maternally-inherited that are involved in cell differentiation in some way

Question 9

True or false. A zygote is created when the sperm pronucleus fuses with the egg pronucleus, to create a diploid nucleus

Answer 9

true

Question 10

What are the different layers on the exterior of an egg? From outermost to innermost…

Answer 10

From outermost to innermost, we have…

1. The egg jelly
2. Vitelline envelope
3. The plasma membrane of the egg

Question 11

What happens to the vitelline envelope when the egg is fertilized?

Answer 11

After fertilization, the vitelline envelope separates from the plasma membrane of the egg, and hardens into the fertilization envelope.

Question 12

The outermost layer of cytoplasm just behind the plasma membrane of the egg is called what?

Answer 12

the cortex

Question 13

What stuff does the cortex of an egg have?

Answer 13

1. The cortical granules
2. Actin that helps form the microvilli and the fertilization cone
3. Polysaccharides

Question 14

True or false. The surface of the egg is not smooth, it is covered in microvilli

Answer 14

True

Question 15

What does chemotaxis mean? to

Answer 15

Chemotaxis refers to the movement towards or away from a chemical.

Question 16

How do eggs attract sperm? Where are these things located? How do they work?

Answer 16

By using Sperm Attracting Peptides (SAPs) and chemotaxis. SAPs are located in the egg jelly, and they create a concentration gradient when the highest concentration at the egg, and it gets more diluted with distance. Sperm will move from low to high concentration of SAPs, thus getting them closer to the egg.

Question 17

What makes sperm attraction species-specific?

Answer 17

Different species’ eggs have different SAPs (Sperm Attracting Peptides) in their egg jelly.

Question 18

Why doesn’t the SAP of an egg in species A work for attracting sperm in species B?

Answer 18

Species B sperm doesn’t have the receptor for the SAP of species A.

Question 19

What happens after SAP binds to its receptor on the cell membrane of the sperm?

Answer 19

It triggers a biochemical cascade that eventually leads to an activation of Ca++ channels and an increase in the influx of Ca++ into the sperm cell. The Ca++ appears to be involved in increasing sperm motility by enhancing ATP production in mitochondria in the midpiece of the sperm cell.

Question 20

What is the acrosomal reaction?

Answer 20

This is the reaction by which the acrosomal vesicle within the sperm head releases proteolytic enzymes to digest a path through the egg jelly to the cell membrane of the egg.

Question 21

What triggers the acrosomal reaction?

Answer 21

The reaction is triggered by the binding of sulphated polysaccharides (that exist in the egg jelly) to the receptors for them on the sperm cell. This again increases Ca++ influx through the opening of Ca++ channels, and this appears to be needed to trigger the acrosomal reaction (and the release of proteolytic enzymes from the acrosomal vesicle in the sperm head).

Question 22

Is the triggering of the acrosomal reaction species-specific?

Answer 22

Yes. The sulphated polysaccharides are species-specific, and need to be correct in order to bind with the receptor on the sperm

Question 23

Where does the Ca++ come from for all these influxes in the sperm cell?

Answer 23

debated. Some say sea water, others say it’s in the egg jelly.

Question 24

After the release of enzymes from the acrosomal vesicle, the acrosomal process is formed. Describe how it is formed, and what it is used for.

Answer 24

The acrosomal process forms when actin just behind the acrosomal vesicle begins to polymerize and assemble to form microfilaments that create an extension from the head = an acrosomal process. The acrosomal process is covered in Bindin protein, which will bind to its receptor at the vitelline membrane.

Question 25

Is the binding of bindin to the receptor on the vitelline envelope species-specific?

Answer 25

Yes. The receptor for bindin is species-specific

Question 26

What happens when you add bindin to a petri dish of eggs that have the receptor for it?

Answer 26

The eggs will clump together.

Question 27

What happens when you add bindin to a petri dish of eggs that DO NOT have the receptor for it?

Answer 27

Nothing will happen.

Question 28

How does the cell membrane of the sperm fuse with the cell membrane of the egg? Describe the fusion of the cell membranes of the egg and sperm stage.

Answer 28

After the acrosomal process' bindin proteins make contact with the receptor, bindin initiates its secondary function, which is to recruit lysosomes (it's a fusogenic protein). Lysosomes break down phospholipid bilayer and help with fusion of the cell membranes of the egg and sperm. At the same time, actin within the cortex cytoplasm assembles and forms a fertilization cone at the site of sperm fusion. the actin of the fertilization cone of the egg and the actin in the acrosomal process of the sperm will join together, widening the cytoplasmic bridge between sperm and egg. This allows the injection of the sperm pronucleus and its centrioles.

Question 29

True or false. Bindin has two functions, one to bind the sperm to the egg, and two to recruit lysosomes to the area and facilitate fusion of the cell membranes of the egg and sperm

Answer 29

true

Question 30

How does the sperm pronucleus meet the egg pronucleus?

Answer 30

The centriole that comes with the sperm pronucleus would assemble microtubules of the egg into an aster (a MT track) that is used by the sperm pronucleus as a track to the egg pronucleus, where they will fuse to form the diploid nucleus.

Question 31

what is polyspermy?

Answer 31

when an egg is fertilized by more than one sperm cell.

Question 32

Why is polyspermy bad?

Answer 32

The nucleus of the zygote will be of unfavourable ploidy, and with cell divisions, we'll have an uneven number of chromosomes in the daughter cells, resulting in embryo death.

Question 33

Name but don't describe the two mechanisms in place to stop polyspermy

Answer 33

1. the fast block | 2. the slow block

Question 34

Describe the fast block

Answer 34

mystery chemical from sperm is released into egg after fusion, that opens up Na+ channels. Intracellular [Na+] increases, causes the Vm of the egg to become positive. The membrane of sperm heads are also positively-charged, so new sperm cells will be repelled by the egg electrostatically because like charges repel.

Question 35

Describe the initiation of the slow block in mammals and sea urchins

Answer 35

In mammals, a (unknown) chemical is released by the fused sperm and results in activation of tyrosine kinase (TK). In sea urchins, bindin binding to its receptor activates TK. TK activates PLC (Phospholipase C), which breaks apart PIP2 into both DAG and IP3 (inositol trisphosphate). IP3 goes on to activate Ca++ channels on the ER. Ca++ influx increases in egg cytoplasm as Ca++ leaves ER lumen stores.

Question 36

In the sea urchin, the endoplasmic reticula that contain all that Ca++ are located near what structures of the egg?

Answer 36

The cortical granules

Question 37

What are two things the increase of Ca++ from the lumen of the ER into the egg cytoplasm do?

Answer 37

1. Initiation of embryo development through triggering DNA and protein synthesis (unrelated to slow block) 2. Exocytosis of the contents of cortical granules (initiates slow block)

Question 38

What are the chemicals released from the cortical granules?

Answer 38

1. CGSPs (cortical granule serine proteases) 2. Mucopolysaccharides 3. Udx1 4. Hyalin

Question 39

What is the function of CGSPs (cortical granule serine proteases)?

Answer 39

Cleaves structural proteins between vitelline envelope and cell membrane, separating these two membranes. CGSP also cleaves bindin receptors to prevent more sperm binding.

Question 40

What is the function of mucopolysaccharides?

Answer 40

Secreted between the separating cell membrane and vitelline envelope. It’s presence draws in water to that space (forms osmotic gradient), to make that space swell.

Question 41

What is the function of the enzyme Udx1?

Answer 41

Catalyses a reaction that forms H2O2, which reacts with ovoperoxidase (OVOP). OVOP reacts with transglutaminases (TG) to harden vitelline envelope into the fertilization envelope.

Question 42

What is the function of the protein hyalin?

Answer 42

Hyalin makes contact with water in the space between the cell membrane and vitelline envelope and forms the gel-like hyaline layer