Fertilization

Question 1

How are the gametes attracted during EXTERNAL fertilization

eg water by sea urchins

Answer 1

• female gamete releases specific substances from extracellular jelly for sperm to recognize- positive chemotaxis
• acrosomal reaction occurs
• Digestion of jelly layer(ALL SPERM GOES IN)
• Receptor mediated binding of sperm to vitelline envelope of egg
• fusion of acrosomal process and egg membrane

Question 2

How to prevent polyspermy

Answer 2

fast block is when the egg plasma membrane changes electric potential

slow block is when cortical granule release

Question 3

What is main difference between external fertilization and internal fertilization (mammals)

Answer 3

In external, acrosomal reaction triggered by extracellular coat of egg

In internal, acrosomal reaction triggered by zona pellucida, which is after the extracellular cumulus layer

Question 4

What is acrosomal reaction (im guessing in mammlas?)

Answer 4

occurs after binding to zona pellucida
induced by zona proteins
releases enzymes :

hyaluronidase= penetration of the corona radiata barrier

trypsin-like substance =digestion of the zona pellucida

acrosin (zonalysin) =helps the spermatozoon cross the zona pellucida

Question 5

What is sperm capacitation

Answer 5

When sperm acquires capacity to perform in fertilisation

• Final stages of sperm maturation that occurs in female repo tract
• ATP generating system gives sperm motility (due to modificstion of dyenin)
• Changes in plasma membrane- replacing cholesterol making membrane more fluid

Question 6

What are 2 mechanisms to attract spermatozoa

Answer 6

Thermotaxis:capacitated spermatozoa can
sense thermal gradient along fallopian tube (in the
final part the temperature is about 2oC higher)

Chemotaxis: secreted molecules from cumulus
cells and oocyte.

Question 7

What increases the hyperactivity of the sperm as it moves along fallopian tube

Answer 7

Ca 2+

Question 8

How long is the process of fertilization in humans

Answer 8

24 hours- from when gametes fuse to first zygote division

Question 9

Stages of fertilization

Answer 9

• First sperm moves across cumulus layer to zona pellucida. ZP contact triggers acrosomal reaction (hyaluronidase= corona radiata, acrosin= ZP)
• gametes fuse and polyspermy prevented
• Male and fem pronuclei formed.They possess nuclear envelope and contain haploid sets of chromosomes. They move towards each other.
• OOcyte finishes meiosis II and releases polar body
• The first mitotic spindle is arranged, then the zygote is divided into two blastomeres.

Question 10

What happens when more than 1 sperm fuse

Answer 10

wrong number of chromosomes

spindle becomes multipolar rather than bipolar

Question 11

What glycoproteins are in Zona pellucida

Answer 11

Zp glycoprotein matrix made during oocyte growth

contains ZP (ZP1, ZP2, ZP3 …).

Question 12

What are the 2 major roles of ZP

Answer 12

Binds sperm

Activates acrosomal reaction after sperm bound

Question 13

How does spermatozoon sense if
it is at the right place for activation of
acrosomal reaction.

Answer 13

Because there is A receptor on the sperm membrane
has to bind ZP in order to initiate the
acrosomal reaction.

ZP3 binding proteins in sperm plasma membrane (so acrosomal process not formed like in external)

Question 14

Which sperm protein is essential for sperm egg fusion

Answer 14

IZUMO 1: immunoglobin at sperm head

localized at both the inner and
outer acrosomal membranes

Question 15

What is the receptor for IZUMO 1 on the egg membrane and what is its role in preventing polyspermy

Answer 15

JUNO

JUNO disappears within 40 minutes of egg sperm fusion- prevents other sperm binding

Question 16

Which region of the sperm head does membrane fusion occur

Answer 16

equatorial region- bc membrane on head tip already destroyed

Question 17

What are some proteins involved in membrane fusion on sperm and egg

Answer 17

sperm: IZUMO 1, fertilin, ADAM family proteins

Egg: JUNO, integrins, cd9

Question 18

How does IZUMO and JUNO cause membrane fusion

Answer 18

JUNO first binds to monomeric IZUMO1. The interaction is specific but weak.

An increased amount of JUNO gradually gathers to the attached site of the spermatozoon.

By the action of other proteins, IZUMO1 is folded and stays in firmly connected dimers.

The dimers are shorter than monomeric IZUMO1. That reduces the distance between egg and sperm
membranes. IZUMO1 no longer binds to JUNO and it leaves the complex.

Question 19

What is the role of cd9

Answer 19

Involved in creation of microvilli and
microexesomes (small membrane vesicles
that facilitate membrane contacts).

Question 20

What is the cortical reaction

Answer 20

cortical granules released upon fusion of of gamete membrane- BLOCKS POLYSPERMY

Content of granules:

• removes carbohydrates from ZP3 ( not able to bind to other sperm)
• Partly cleaves ZP2, hardening the zona pellucida

Question 21

What happens to the oocyte after gamete fusion

Answer 21

• finshes second meiotic division and releases polar body
• formation of pronucleus
• replication of chromosomes
• Pronucleus migrates to centre

Question 22

What happens to spermatozoon after gamete fusion

Answer 22

• protamines replace histones -compaction
• formation of male pronucleus
• Formation of microtubule aster
• replication of chromosomes
• Pronucleus migrates to centre

Question 23

Why doesn’t the oocyte posses centrioles

Answer 23

To prevent parthenogenesis

Question 24

How is the first centrosome formed in the egg?

Answer 24

Centriole imported from sperm to egg

Centriole duplicated during pronuclear phase = centrosome

Question 25

What is the second centrosome needed for

Answer 25

needed to
arrange the fist mitotic spindle in the
zygote,

Question 26

How do pronuclei move to each other

Answer 26

Microtubule aster bring the pronuclei to center of zygote

During this time, both pronuclei replicate DNA

Process mediated by cytoplasmic dyenin

Question 27

Do the pronuclei ever fuse their nuclear envelopes?

Answer 27

NO

Only come into close contact

They wait until centrosome duplicated ( to arrange mieotic spindle) only then will their nuclear envelopes breakdown