As Biology Unit 2 - Fertilisation

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Specialised Gametes

  • Sperm:
    • Acrosome = contains digestive enzymes which enable the sperm to penetrate the zona pellucida (jelly layer) of the ovum.
    • Nucleus = contains a haploid number of chromosomes so that the full compliment is resotred upon fertilisation.
    • Mitochondria = found in the mid region and provide useable energy which drive the flagellum.
    • Flagellum = Whips back and forth to move the sperm towards the ovum.
  • Ovum:
    • Nucleus = contains a haploid number of chromosomes so that the full compliment is resotred upon fertilisation.
    • Cytoplasm = is full of energy rich material.
    • Cortical Granueles = contain a substance that prevent more than one sperm from fertilising the egg.
    • Zona Pellucida (Jelly Layer) = To stop more than one sperm from entering the ovum.
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Fertilisation in Mammals

  • Acrosome Reaction
    • Acrosome releases digestive enzymes when sperm head meets zona pellucida (jelly layer) of the egg.
    • Enzymes digest a channel in zona pellucida for sperm to burrow thruogh to the cell surface membrane of the egg cell.
  • Membrane Fuses
    • Cell surface membrane of sperm and eg fuse enablign haploid nucleus of sperm to to enter the cytoplasm of egg cell.
  • Cortical Reaction
    • Cortical granules move towards and fuse with cell surface membrane.
    • Tehy releae their contents (through exocytosis) causing a change in surface layer of egg to prevent other sperm from entering.
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Fertilisation in Mammals 2

  • Meiosis Restarted
    • The sperm nucleus in the cytoplasm of the egg cell cause the secon ddivision of meiosis to occur.
  • Fertilisation
    • The chromosomes from the haploid sperm nucleus and the haploid egg cell combine to reach the full compliment of chromosomes, diplod.
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Fertilisation in Flowering Plants

  • Pollen lands on the stigma and begins to grow towards the embryo.
  • The pollen tube grows down the style.
  • It secretes enzymes and digests style tissue using the products to continue growth.
  • There are 2 haploid nuclei near the tube tip;
    • 1 controls the growth - the pollen tube nucleus
    • 1 is the genervative nucleus.
  • The pollen tube reaches the embryo sac.
  • It fuses with it and the pollen tube nuclesu breaks down.
  • The Genervative nucleus divides to produce 2 male gametes.
    • The first male gamete fuses with the female gamete nucleus and produces a diploid zygote (2n).
    • The second male gamete fuses with the 2 polar nuclei producing a triploid primary endosperm nucleus (3n).
      • This develops into the seed's storage tissue, endosperm.
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Genetic Variation

  • Crossing Over:
    • During the first division in meoisis, pairs of chromosomes, known as homologous chromosomes, line up and swap part of their genetic material.
    • After, all four components (chromatids) are now different from eachother as two have swapped their alleles.
  • Independant Assortment:
    • During the first division of meiosis, each homologous pair of chromosomes can line up.
    • Since eahc of the 23 paris of homologous chromosomes in humans can line up either way around, it is highly unlikley that daughter cells will end up with different chromosome combinations when teh homologous chromosomes are seperated. 
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