lec 5- fertilization in animals

Question 1

what is fertilization?

Answer 1

the process by which two haploid gametes fuse together to create a new individual with genes that come from both parents

Question 2

how does sperm, like from a sea urchin, that is secreted into water find the right egg?

Answer 2

the egg secretes soluble molecules that serve as chemoattractants for sperm of the same species

Question 3

what only allows compatible sperm to fuse with an egg?

Answer 3

species-specific gamete-recognition proteins

Question 4

what triggers the release of digestive enzymes from the acrosomal part of the sperm and what do the digestive enzymes do?

Answer 4

when the sperm reaches the jelly layer, release of digestive enzymes allows the sperm to penetrate the jelly layer to the egg

Question 5

what happens after the sperm enters the jelly layer?

Answer 5

it reaches the vitelline envelope and passes through it, allowing for the fusion of the sperm and egg plasma membrane

Question 6

what on the sea urchin sperm interacts with the EBR1 receptor in the vitelline envelope of the egg to allow passage?

Answer 6

bindin on the sperm membrane binds to the EBR1 receptor allowing passage

Question 7

is bindin species-specific?

Answer 7

-yes, it only binds to eggs of the same species of sea urchin
-e.g. bindin that was on sperm only binded to dejellied eggs of the same species but not those of others

Question 8

what happens if sperm doesn’t have bindin?

Answer 8

an egg can not be fertilized, making it essential

Question 9

how do you know sperm with bindin fertilized an egg?

Answer 9

you will see a fertilization membrane around the egg

Question 10

what is the jelly layer around eggs called in humans and other mammals?

Answer 10

zona pellucida

Question 11

what are the 5 steps to fertilization in mammals?

Answer 11

1. binding of sperm to zona pellucida
2. acrosomal reaction occurs to release digestive enzymes
3. released digestive enzymes allow penetration through zona pellucida
4. the protein fertilin is species-specific and binds to the a6b1 integrin on the egg membrane, allowing fusion
5. sperm nucleus enters egg

Question 12

what does the sperm entering the egg cause?

Answer 12

it initiates a signal transduction pathway which causes calcium to be released to activate the egg

Question 13

what happens to calcium in the egg?

Answer 13

it enters at sperm-egg fusion site then travels across the egg, allowing the activation of development

Question 14

can eggs be artificially activated? how so?

Answer 14

yes, by increasing calcium concentrations inside the egg

Question 15

what blocks activation in eggs?

Answer 15

calcium chelators, which bind calcium and prevent it from interacting with other molecules in the cell

Question 16

what is polyspermy?

Answer 16

when more than one sperm binds to an egg

Question 17

what are the two steps to blocking polyspermy?

Answer 17

1. fast block, an immediate form of signalling that tells that a sperm has fused with an egg right when it occurs
2. slow block, provides a permanent barrier to prevent sperm from fertilizing the egg

Question 18

what is the resting membrane potential value for eggs?

Answer 18

-70mV

Question 19

what occurs during fast block in sea urchins?

Answer 19

-when the sperm fuses to the egg, it causes a Na+ influx resulting in a change in resting membrane potential from -70mV to +20mV
-this is a rapid depolarization that changes the membrane potential so sperm cant fuse anymore, the value gradually returns to normal

Question 20

what occurs during slow block in sea urchins?

Answer 20

-when the egg slowly repolarizes, an impenetrable membrane reforms called the fertilization membrane which prevents sperm from entering
-fast block only last for a minute, so a second block is needed
-it occurs a minute after fast block

Question 21

what is the slow block to polyspermy also known as?

Answer 21

cortical granule reaction

Question 22

what triggers the fertilization membrane to form?

Answer 22

-when the sperm fuses to the egg, calcium is released which causes cortical granules to release their content
-this causes the vitelline envelope to lift off of the plasma membrane
-the contents become cross linked together forming the fertilization membrane

Question 23

once the fertilization membrane forms, what do the excess granules do?

Answer 23

form a jelly like hyaline layer between the fertilization layer and the plasma mebrane

Question 24

has fast block been observed in mammals?

Answer 24

not really, up for debate if it even occurs