OCR GCSE Biology B3 Processes

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Nuclear Transfer (B3)

  • 1.      An egg cell is extracted from Animal A, the nucleus is removed from this egg cell.
  • 2.       A nucleus is removed from a diploid, somatic skin cell is taken from Animal B
  • 3.       The nucleus from the skin cell is inserted into the enucleated egg cell from Animal A and an electric shock is applied to make the newly combined cell divide into an embryo.
  • 4.       The embryo is then placed into a surrogate Animal C where it then gestates before being born as a clone of the animal from which the nucleus was obtained.
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Mitosis (B3)

  • 1.       The cell is resting.
  • 2.       The chromosomes in the nucleus duplicate and then shorten and fatten.
  • 3.       The nuclear membrane disintegrates.
  • 4.       The centrioles duplicate and move to opposite ends of the cell.
  • 5.       The chromosomes line up along the cell equator.
  • 6.       Spindle fibres extend from the centrioles and attach to the centromeres of the chromosomes.
  • 7.       The chromosomes are pulled apart at the centromeres by the spindle fibres to opposite ends of the cell.
  • 8.       The cell divides into two and the nuclear membrane reforms around each cluster of chromosomes.
  • 9.       Two new genetically identical diploid daughter cells form.
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DNA replication (B3)

  • 1.       The DNA double helix unwinds.
  • 2.       The enzyme DNA helicase ‘unzips’ the two strands of DNA.
  • 3.       New bases found in the nucleus attach to their complementary bases on the single DNA strands.
  • 4.       The DNA re-zips and re-winds back into two double helix structures.
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Enzyme Reactions (B3)

  • 1.       Each enzyme has a unique sequence of amino acids, this results in each enzyme having a different shape and active site
  • 2.       Only one type of substrate is complementary to the active site of one enzyme. This makes enzymes specific to reactions.
  • 3.       If the substrate is complementary to the active site, it will fit into the enzyme to form the enzyme-substrate complex.
  • 4.       The enzyme acts on the substrate and turns it into a product.
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Selective Breeding (B3)

  • 1.       The desired characteristics of both species are identified.
  • 2.       The species are cross-bred by mating a male member with one characteristic with a female member with another characteristic.
  • 3.       The offspring that best express the desired characteristics are selected.
  • 4.       The selection and the breeding process is repeated for a number of generations.
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