(4) Fertilization: Beginning a New Organism Part 2

Question 1

consequences of polyspermy

Answer 1

1. entry of more than 1 sperm into egg
2. triploid fertilized egg
3. abnormal segregation of chromosomes during mitotic cell divison (3 sets of chromosomes divided amongst 4 daughter cells)
4. will not develop past first few divisions

Question 2

consequence of a triploid egg

Answer 2

1. 3 chromosome sets (2 paternal) and
2. 2 sperm centrioles (a. 2 mitotic spindles form at first cell division; b. 4 poles toward which chromosomes can migrate)

Question 3

prevention of polyspermy in sea urchins

Answer 3

a. polyspermy must be avoided for correct development to proceed
b. 2 mechanisms ensure prevention of polyspermy in sea urchins (fast block and slow block)

Question 4

fast block to polyspermy

Answer 4

1. occurs within 1-3 sec of sperm entry
2. sperm entry causes depolarization fo egg PM from -70mV to +20mV (Na_ channels open in egg PM upon fertilization Na+ enters; makes egg more positive)
3. depolarization inhibits additional sperm entry (unclear why)
4. transient; only lasts ~1 min

Question 5

slow block to polyspermy

Answer 5

1. slower mechanical block (occurs ~20 sec after fertilization)
2. also called the cortical reaction
3. fusion of cortical granules with egg PM and release of contents into space between PM and vitellin membrane (the perivitelline space)

Question 6

contents of cortical granules

Answer 6

1. cortical granule serine proteases (CGSPs)
2. glycosaminoglycans (GAGs)
3. peroxidase and transglutaminase
4. hyalin

Question 7

cortical granule serine proteases (CGSPs)

Answer 7

1. degrade protein-protein connections between VM and PM
2. degrade bindin receptors

Question 8

glycosaminoglycans (GAGs)

Answer 8

increase osmolarity of perivitelline space (make hyperosmatic)
water enters space, pushing VM farther away from PM
the raised VM referred to as the fertilization envelope

Question 9

peroxidase and transglutaminase

Answer 9

cross-links proteins in fertilization envelope, making it impenetrable to additional sperm

Question 10

hyalin

Answer 10

coats the egg
will provide sites of attachment for blastomeres during cleavage stage

Question 11

udx1

Answer 11

egg PM enzyme that forms hydrogen perozide which is required substrate of TG and OVOP

Question 12

OVOP

Answer 12

ovoperoxidase; enzyme that crosslinks proteins of VM to harden it

Question 13

cortical reaction is Ca2+ dependent

Answer 13

intracellular ca2+ release starts at point of sperm entry; wave of ca2_ release occurs across entire egg within 30 sec; Ca2+ is rapidly re-sequestered

Question 14

where does the ca2+ come from?

Answer 14

1. from the egg’s own endoplasmic reticulum
2. what does it do? causes cortical granules to fuse to egg PM and release their contents

Question 15

what triggers ca2+ release in sea urchins?

Answer 15

1. sperm receptor= believed to be a G-protein coupled receptor (GPCR)
2. TK= tyrosine kinase
3. PLC= phospholipase C
4. PIP2= phosphatidylinositol 4,5 bisphosphate
5. IP3= inositol 1, 4, 5 trisphosphate

Question 16

IP3 and Ca2+ release

Answer 16

IP3 triggered release of Ca2+ from ER results in wave of Ca2+ release across entire egg; cortical granules released from egg, beginning at point of sperm entry

Question 17

evidence for role of Gq

Answer 17

sperm is loaded with Ca2+ sensitive dye and then with Gq inhibitory and then it is allowed to be fertilized
bight red dluroscnece shows high concentration of ca2+; no red fluroscence in presence of inhibitor; hence, gq is required for Ca2+ release into cytoplasm

Question 18

roles of Ca2+

Answer 18

release of cortical granules (slow block to polyspermy)
egg activation (reentry of cell into cell cycle- resumption of mitosis; reactivaion of egg protein synthesis (translation) from stored materal mRNAs

Question 19

role of DAG (diacylglycerol)

Answer 19

both DAG and Ca2+ activate the Na+/H+ exchange pump
PLC activated DAG activated PKC activated Na+/H+ exchange=> increase in intracellular pH-> stimulation of protein synthesis, DNA replication, and cytoplasmic movements of morphogenetic material

Question 20

inside a newly fertilized sea urchin egg:

Answer 20

a. sperm mitochondrioa and flagellum break down
b. burst of protein synthesis (translation) from maternal mRNAs
(histones, tubulins, actins, morphogenic factors, membrane protein for mitosis)
c. sperm centriole produces microtubules that draw sperm and egg nuclei toward each other and nuclei fuse
d. chromosomes duplicate
e. sperm centriole duplicates
f. mitotic spindle forms
g. nuclear envelope breaks down
h. duplicated material and paternal chromosome align on mitotic spindle
i. first mitosis and cleavage occurs