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  • Created by: ernily
  • Created on: 22-04-15 19:16


  • DNA is made up of lots of nucleotides joined together, called a polynucleotide.
  • Each nucleotide is made from a pentose sugar, a phosphate group, and a nitrogenous base.
  • The sugar is called a deoxyribose sugar.
  • DNA nucleotides join together to form polynucleotide strands.
  • They join between the phosphate group of one nucleotide and the sugar of another, forming a sugar-phosphate backbone.
  • Two polynucleotide strands join by formation of hydrogen bonds between the bases, this is specific base pairing.
  • DNA molecules are long and coiled tightly, so lots of information is stored in a small space.
  • DNA molecules have a paired structure, which makes it easier for it to copy itself.
  • The double helix structure means that DNA is very stable in the cell.
  • Eukaryotic cells contain linear DNA molecules which exist as chromosomes.
    • DNA is really long, so must be wound up to fit in the nucleus.
    • DNA is wound around proteins called histones, which help to support the DNA.
    • DNA and histones are coiled up tightly to make a compact chromosome.
  • Prokaryotes also carry DNA as chromosomes, but the DNA molecules are shorter and circular.
  • The DNA isn't wound around the proteins; it condenses to fit by supercoiling.
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  • Genes are sections of DNA that code for proteins.
  • Proteins are made from amino acids.
  • The order of the nucleotide bases in a gene determines the order of amino acids in a protein.
  • In eukaryotic DNA, introns don't code for amino acids; exons do.
  • Introns are removed during protein synthesis.
  • Eukaryotic DNA also contains regions of multiple repeats which don't code for anything either.
  • Enzymes control our metabolic pathways and so contribute to our development.
  • All enzymes are proteins, and genes decide which types of protein are made.
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  • Genes can exist in different forms called alleles.
  • Alleles code for different versions of the same characteristic.
  • Alleles coding for the same characteristic will be found at the same position on each chromosome.
  • Mutations are changes in the base sequence of DNA.
  • Mutations can produce new alleles of genes.
  • If the sequences of bases in a gene changes, a non-functional or different protein could be produced.
  • All enzymes are proteins. A mutation could prevent the enzyme from folding up properly.
    • This may produce an active site that's the wrong shape, and so a non-functional enzyme.
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Meiosis & Genetic Variation

  • Gametes join together at fertilisation to form a zygote.
  • Gametes have a haploid (n) number of chromosomes, so normal body cells have the diploid (2n) number of chromosomes.
  • Gametes are formed by meiosis:
  • DNA unravels and replicates so that there are two copies of each chromosome, called chromatids.
  • DNA condenses to form double-armed chromosomes.
  • Meiosis I: Chromosomes arrange into homologous pairs.
  • The homologous pairs are then separated, halving the chromosome number.
  • Meiosis II: The pairs of sister chromatids in each chromosome are separated.
  • Four genetically different haploid cells are produced.
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Meiosis & Genetic Variation

  • Meiosis produces genetically different cells:
  • Crossing Over Of Chromatids:
    • This occurs in Meiosis I.
    • It means that the 4 daughter cells that are formed from meiosis contain chromatids with different alleles.
  • Independent Segregation Of Chromosomes:
    • The four daughter cells have completely different combinations of chromosomes.
    • All of your cells have a combination of chromosomes from your mother (maternal) and father (paternal).
    • When gametes are produced, different combinations of those maternal and paternal chromosomes go into each cell.
    • This is called independent segregation.
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Genetic Diversity

  • Genetic Bottlenecks Reduce Diversity:
    • A genetic bottleneck is an event that causes a big reduction in a population.
    • e.g. when a large number of organisms die before reproducing.
    • This reduces the number of different alleles in the gene pool, and so reduces genetic diversity.
    • The survivors reproduce, and a larger population is created.
  • The Founder Effect Is A Type Of Genetic Bottleneck:
    • The founder effect describes what happens when a few organisms from a population start a new colony.
    • Only a small number of organisms have contributed their alleles to the gene pool.
    • There's more inbreeding, which can lead to a higher incidence of genetic disease.
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Genetic Diversity

  • Selective Breeding Involves Choosing Which Organisms Reproduce:
    • This leads to reduction in genetic diversity, as only organisms with desired characteristics will be bred.
    • It reduces the number of alleles in the gene pool.
    • It produces high yielding animals and plants.
    • It produces organisms with high resistance to disease.
    • This means that fewer drugs and pesticides will be used.
    • It can cause health problems.
    • It reduces genetic diversity, which increases genetic diseases and also increases the susceptibility to new diseases due to the lack of alleles.
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