Fertilization

it is great pleasur for me

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2. ALI RAZA BS ZOLOGY 6 th SEMESTER ROLL #4029

3. Fertilization Fusion of gametes

4. intoduction Fertilization accomplishes two things: Sex (combining genes from two genomes) Reproduction (produce offspring)

5. Fertilization OverviewNOTE Sperm formation and structure Egg structure and function Interactions between sperm and eggs Chemoattraction Acrosome reaction Binding and fusion Prevention of polyspermy Egg activation Pronuclear fusion Mammalian fertilization

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7. Sperm Formation

8. 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.

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

10. Egg Maturation at Sperm Entry

11. Egg Structure Egg jelly -glycoprotein meshwork-attract or activate sperm vitelline envelope extracellular(inverts)-fibrous mat-sperm-egg recognition-contains glycoproteins egg egg cell membrane binds sperm -fuses with sperm cell membrane

12. 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

13. . Pronuclear fusion: sperm and egg nuclei ( pronuclei ) meet, fuse; development initiated

14. Acrosome Reaction Acrosome reaction :fusion of acrosome and cell membranes-releases acrosome contents. Acrosome contains enzymes that digest jelly layer Exposed sperm membrane contains proteins that bind to egg receptors Sperm acrosomal process membrane fuses with egg

15. 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 polyspermy 2)Fast block of polyspermy

16. 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.

17. Fast block polyspermy Sperm-binding receptors Jelly coat Acrosome Actin Sperm head Basal body (centriole) Sperm plasma membrane Sperm nucleus Contact Acrosomal reaction Acrosomal process Contact and fusion of sperm and egg membranes Entry of sperm nucleus Cortical reaction Fertilization envelope Egg plasma membrane Vitelline layer Hydrolytic enzymes Cortical granule Fused plasma membranes Perivitelline space Cortical granule membrane EGG CYTOPLASM

18. 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 .

19. Cortical Granule ExocytosisHyaline

20. 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

21. Activation of the Egg The sharp rise in Ca 2+ in the egg’s cytosol increases the rates of cellular respiration and protein synthesis by the egg cell Chemical signals from cortical cause H+ to be transported out --> increase in pH Nuclei fuse Egg/sperm differences Egg contains proteins, mRNA not found in sperm Ca2+ injection, temperature shock can cause artificial activation With these rapid changes in metabolism, the egg is said to be activated

22. 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.

23. 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 .