AS Biology Fertilization Revision

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Information about AS Biology Fertilization Revision

Published on January 28, 2014

Author: mirjanagn



Revision cards to help revise fertilization in mammals and angiosperms!

Fertilisation Mammals & Flowering Plants Unit 2 Topic 3

• • • • Fertilisationthe fusion of gametes. Mammalian gametessperm & ovum Flowering plant gametespollen & egg The importance of reproduction in sexual reproduction: – it restores full competent (diploid number) of chromosomes (46 in humans); – it allows genetic variation.

Fertilisation in Mammals Stage Acrosome reaction Description of events • • Acrosome releases digestive enzymes when sperm head meets zona pellucida (jelly layer) of oocyte (egg cell before having gone through complete meiosis). Enzymes digest a channel in zona pellucida for sperm to burrow through to the cell surface membrane of oocyte. Membranes fuse • Cell surface membrane of sperm and oocyte fuse enabling haploid nucleus from sperm to enter cytoplasm of oocyte. Egg cell response (cortical reaction) • Special vesicles (cortical granules) move towards and fuse with cell surface membrane. They release their contents (by exocytosis)cause changes in surface layers of oocyte that stop other sperm from entering oocyte. • Meiosis restarted Fertilisation • The presence of the sperm nucleus in the cytoplasm of the oocyte causes the second division of meiosis to occur providing a haploid egg nucleus to fuse with haploid sperm nucleusforms an ovum (egg cell that has completed meiosis). • Chromosomes from the haploid sperm nucleus and from the haploid egg nucleus combine to restore the full complement of chromosomes, the diploid number.

Fertilisation in Flowering Plants  Flowering plantsangiosperms  Male gametepollen grain  Female gameteembedded deep in ovary tissueovule • • • • • • • Pollen grain lands on surface of stigma of the flower during pollination. Molecules on surface of pollen grain and stigma interact. If they are of the same speciespollen grain begins to grow (germinate). Pollen tube begins to grow out from the tube cell of the pollen grain down the stylesecretes enzymes to digest style tissue and uses products to continue tube growth. During the growth and extension of the tube, the generative nucleus, behind the tube nucleus, divides by mitosis to produce 2 male haploid gametes. Pollen tube enters ovule through micropyle and penetrates the embryo sac wall. The tip of the tube bursts open, tube nucleus dies and what follows is called double fertilisation: – 1 male gamete fuses with female gamete nucleusdiploid zygote (2n) – 1 male gamete fuses with the two polar nucleitriploid primary endosperm nucleus (3n) – Immediately after fertilisation, the ovule is known as the seed.

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