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  • Genetics
    • DNA
      • contains all the genetic information.
      • Has a double helix structure-2 strands coiled around each other. Strands of polynucleotides joined in a long chain.
      • A nucleotide is made from a phosphate group, a pentose sugar (with 5 carbon atoms) and a nitrogenous base. The base can vary.
        • Four bases: Adenine, Thymine, Cytosine and Guanine.
          • Specific base pairing occurs (Adenine can only join with Thymine) and (Cytosine and Guanine).
            • Adenine and Thymine join with 2 hydrogen bonds and Cytosine and Guanine join with 3 hydrogen bonds.
      • The double helix structure prevents damage from occurring.
        • It is also coiled which allows more DNA to be stored.
          • It has a paired structure which makes it easier to copy itself.
      • Eukaryotic and Prokaryotic Cells
        • Prokaryotic cells have DNA that is supercoiled.
        • Eukaryotic cells is linear DNA that exists in chromosomes. It wraps around proteins called histones that support the DNA.
    • Genes
      • Genes are sections of DNA.
      • Proteins are made from amino acids. The order of nucleotide bases that determines the order of amino acids.
      • Each amino acid is coded for by three bases ( a triplet code)
      • DNA is degenerative as more than one triplet codes for the same amino acid. There are 64 triplets and only 20 amino acids.
      • Pairs of matching chromosomes are called homologous pairs. They have the same shape and genes, but could have different alleles.
    • Meiosis and Genetic Variation
      • Gametes are sex cells; sperm and eggs.
      • Normal body cells have the diploid number but gametes have the haploid number.
      • Meiosis is a type of cell division that creates gametes with the haploid.
        • A cell with the diploid number replicates to form a cell with chromatids
          • Then the chromosomes arrange into homologous pairs
            • The first division occurs where the homologous pairs are separated.
              • The second cell division occurs where the sister chromosomes are separated.
      • Crossing over occurs when the homologous pairs come together. Parts of chromosomes touch and break off and join a different chromosome creating a new chromosome with different alleles.
        • Because the alleles have changed it causes variation as it is different to all other chromosomes.
      • The chromosomes can break in any order so the homologous pairs can go with any other chromosome. This process is random so variation is created.
    • Genetic Diversity
      • Genetic diversity is the amount of alleles in a population. It is important as it allows the species to survive diseases.
      • Mutations and gene flow both increase genetic diversity
      • Genetic diversity is reduced when a natural disaster or a disease wipes out a lot of the population. The number of alleles is reduced.
      • It is also reduced when part of the population moves to a new area. The population in the new area all have alleles from the founders.
      • Selective breeding also reduces the alleles in a population as undesired characteristics are breed out of the population.
      • Argument for and against selective breeding
        • For:
          • Increased resistance to disease.
          • Creates high yeilds
          • Increased tolerance to bad conditions.
        • Against:
          • Reduces genetic diversity
          • Causes health problems


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