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ALI RAZA

BS ZOLOGY 6th

SEMESTER

ROLL #4029

FERTILIZATIONFusion of gametes

INTODUCTION Fertilization accomplishes two things:

Sex (combining genes from two genomes)

Reproduction (produce offspring)

FERTILIZATION OVERVIEWNOTE 1) Sperm formation and structure2) Egg structure and function3) Interactions between sperm and eggs4) Chemoattraction5) Acrosome reaction Binding and fusion 6) Prevention of polyspermy 7) Egg activation8) Pronuclear fusion9) Mammalian fertilization

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SPERM FORMATION

THE EGG All materials necessary to begin development

are stored in the egg. Proteins yolk (made in other organs (liver, fat bodies),

transported to egg. Ribosomes and tRNA -burst of protein synthesis after fertilization. mRNA -encode proteins for use in early development -some localized regionally.

Protective chemicals UV filters -DNA repair enzymes- antibodies -alkaloids

EGG MATURATION AT SPERM ENTRY

EGG STRUCTURE

Egg jelly -glycoprotein

meshwork-attract or activate sperm

vitelline envelope extracellular(inverts)

-fibrous mat-sperm-egg recognition-contains glycoproteinsegg

egg cell membrane

binds sperm -fuses with sperm

cell membrane

INTERACTIONS BETWEEN EGG AND SPERM

1. Chemoattraction of sperm to egg -soluble molecules released by egg 2. Exocytosis of the acrosome -stimulated by binding of egg molecules 3. Binding of sperm to the extracellular

envelope -usually a multi-step process -binding molecules and receptors located on

each gamete 4. Passage of sperm through the extracellular

envelope 5. Fusion of the egg and sperm cell membranes

.

Pronuclear fusion: sperm and egg nuclei (pronuclei) meet, fuse;

development initiated

ACROSOME REACTION 1) Acrosome reaction :fusion

of acrosome and cell membranes-releases acrosome contents.

2) Acrosome contains enzymes

3) that digest jelly layer4) Exposed sperm membrane5) contains proteins that

bind6) to egg receptors7) Sperm acrosomal process 8) membrane fuses with egg

PREVENTION OF POLYSPERMY Why? More than one sperm entering an egg results in polyploidy; Usually eventual death. There are two types of block of

polyspermy.1)Slow block of polyspermy2)Fast block of polyspermy

FAST BLOCK OF POLYSPERMY

. The fast block is electrical and is mediated by sodium ions the egg membrane resting potential rises, and sperm can no longer fuse with the egg.

FAST BLOCK POLYSPERMY

Sperm-bindingreceptors

Jelly coat

Acrosome

Actin

Spermhead

Basal body(centriole)

Sperm plasmamembrane

SpermnucleusContact

Acrosomalreaction

Acrosomalprocess

Contact and fusionof sperm and eggmembranes Entry of sperm

nucleus

Cortical reaction

Fertilizationenvelope

Egg plasmamembrane

Vitelline layer

Hydrolytic enzymes

Corticalgranule

Fused plasmamembranes

Perivitellinespace

Cortical granulemembrane

EGG CYTOPLASM

SLOW BLOCK OF POLYSPERMY

The slow block is physical and is mediated by calcium ions. A wave of calcium ions propagates from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane. The released contents of the granules cause the vitelline membrane to rise and to harden into the fertilization envelope.In mammals, blocks to polyspermy include the modification of the zona proteins by the contents of the cortical granules. Sperm can no longer bind to the zona.

CORTICAL GRANULE EXOCYTOSISHYALINE

ACTIVATION OF EGG METABOLISM

Fertilization results in: 1. merging of two haploid nuclei 2. initiating the processes that start

development These events happen in the cytoplasm -occur without nuclear involvement Sperm fusion activates egg metabolism stimulates a preprogrammed set of metabolic

events into action

ACTIVATION OF THE EGG

The sharp rise in Ca2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cellChemical signals from cortical cause H+ to be

transported out --> increase in pH Nuclei fuse Egg/sperm differences

Egg contains proteins, mRNA not found in spermCa2+ injection, temperature shock can cause

artificial activation With these rapid changes in metabolism, the

egg is said to be activated

FUSION OF GENETIC MATERIAL

15) The male pronucleus and the female pronucleus migrate toward each other, replicating DNA as they move.

16 ) In sea urchins, the two pronuclei merge and a diploid zygote nucleus is formed. In mammals, the pronuclei disintegrate as they approach each other, and their chromosomes gather around a common metaphase plate.

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17) Some genes are transmitted differently depending on whether they are from the egg or the sperm. Methylation differences determine if these genes are to be expressed in the early embryo.18) Micro tubular changes cause cytoplasmic movements. These rearrangements of cytoplasm can be critical in specifying which portions of the egg are going to develop into which organs.