DNA - Chapter 8 AQA AS Biology

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  • Created by: H.N
  • Created on: 09-11-12 19:38
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  • DNA
    • structure
      • Phosphate - deoxyribose sugar- base
      • Single ring bases: C (cytosine) and T (thymine)
      • Double ring bases: A (adenine) and G (guanine)
      • A + T = 2 H bonds
      • G+ C = 3 H bonds
      • long and coiled - lot of info in small space in cell nucleus
      • contains genes - sections of DNA on chromosomes
    • Functions
      • stable -pass down generations
      • H bonds allows it to separate during DNA replication and protein synthesis
      • large molecule - lots of info
      • base pairs in helical cylinder - protected from outside
      • paired structure - easier to copy itself (self-replication)
    • The triplet code
      • 3 bases code for each amino acid (a.a)
      • 1 sequence can code for many different a.a
      • to find the minimum no. of bases needed to code for a molecule that is 127 a.a long = 127 x 3
      • to find out how many a.a 33 base pairs code for = 33 / 3
    • Chromosomes
      • eukaryotic cells - large DNA molecules, linear, chromosomes
      • structure
        • 2 threads = chromatids, single point = centomere
        • visible when cell dividing
        • DNA-protein complex, coiled + looped, packed inside
        • contains a single molecule of DNA
      • homologous pair - 2 sets of chromosomes, total no. = diploid no.
      • genes exist in different forms = allele
      • different base sequence of allele = difference a.a sequence = production of different protein
        • enzyme-substrate complex no longer formed
    • meiosis
      • 4 daughter cells, half no. of chromosomes as parent cell
      • 1 chromosome from each pair enters each gamete
      • 2 nuclear divisions
        • 1) homologous chromosomes pair up and chromatids wrap around each other
          • 2) equivalent portions exchanged by crossing over
            • 3) homologous pair separated, 1 chromosome from each pair into 1 of the 2 daughter cells
        • 2) chromatids move apart - 4 cells formed
      • genetic variation
        • independent segregation  of homologous chromosomes
          • chromosomes randomly line up, so combination of chromosomes going into daughter cells is random
        • new genetic combinations
          • alleles may differ - random distribution
        • crossing over
          • recombination - produces 4 different cells, not just 2
  • polynucleotide
    • structure
      • Phosphate - deoxyribose sugar- base
      • Single ring bases: C (cytosine) and T (thymine)
      • Double ring bases: A (adenine) and G (guanine)
      • A + T = 2 H bonds
      • G+ C = 3 H bonds
      • long and coiled - lot of info in small space in cell nucleus
      • contains genes - sections of DNA on chromosomes
  • join to form sugar-phosphate backbone
    • polynucleotide
      • condensation reaction
      • Double helix

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