Nucleic Acid, DNA and Replication

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  • Created by: farahh24
  • Created on: 12-09-18 18:48
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  • Nucleic Acid & DNA Replication
  • Contain carbon, hydrogen, oxygen, nitrogen and phosphorous.
  • Sugar
    • Base
    • Phosphate
    • Phosphate
      • Sugar
        • Base
  • Pentose mono-saccharide, either ribose or deoxyribose.
    • Made up of:
      • Phosphate head forming 'backbone'
      • Nitrogenous base: adenine, guanine, cytosine and thymine.
  • Phosphate group at carbon-5 forms covalent bond with hydroxyl at carbon-3 of next nucleotide.
  • Phospho-diester bonds.
  • Deoxyribose has one less oxygen atom than ribose.
  • Pyrimidines: single carbon ring structure. Thymine and cytosine.
    • Purines: double carbon ring structure. Adanine and guanine.
  • Two hydrogen bonds: Adenine and thymine.
    • Three hydrogen bonds: Cytosine and guanine.
  • Phosphate backbone provides rigidity and creates structure.
  • Two strands of polynucleotide coiled tightly into a helix to make it more compact.
    • Double Helix
    • Two strands of the helix are held together by hydrogen bonds.
  • Each polynucleotide strand has a phosphate group on one end (5') and a hydroxyl group on the other end (3')
    • The two  polynucleotide strands run anti-parallel - running in opposite directions.
  • Semi-conservative replication
    • One strand of DNA untwists and unzips with the help of DNA helicase.
      • Free nucleotides match up with the bases, and joined up by DNA polymerase.
        • Two identical molecules formed. Each has half of the original strand, and half of a new strand.
  • DNA helicase 'unzips'.
  • DNA polymerase 'zips up' again.
  • Meselson-Stahl Experiment.
    • First replication -  100% in middle
      • Second replication - 50% in middle, 50% at top
        • Later replication - 75% at top, 25% middle.
  • Continuous v. Discontinuous Replication
  • DNA polymerase can only travel from 3' to 5'.
    • Leading strand (3' to 5') can be replicated continuously.
      • Lagging strand (5' to 3') uses discontinuous replication.
        • In lagging strand, DNA polymerase must wait until a section of the strand has been zipped before working backwards.
          • Forms Okasaki fragments, which then have to be joined.
  • Genetic code: series of bases coding for a sequence of amino acids.
  • Start codon: ATG
  • Stop codon: TAG. TGA, TAA
  • Degenerate: More codons than amino acids. 20 amino acids, 64 codons. Some codons code for the same amino acid.
  • Non-overlapping, universal, degenerate
  • Replication errors, e.g. missing stop codon, skipping codon, can cause mutations.
    • Mutations may be silent, beneficial, or harmful.
  • ATP: Adenine triphosphate
    • Nitrogenous base, pentose sugar, three phosphate groups
  • Small amount of energy required to break bond of last phosphate.
    • Releases 30.6kJ per mole.
  • PROS: small enough that energy is not wasted as heat, water soluble, easily regenerated by phosphorylation.
    • CONS: weak bonds = unstable. Not long-term energy store.


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