01 The Structure and Replication of DNA

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01 The Structure and Replication of DNA

  1. 1. GENETIC CONTROLThe structure and replication of DNA ALBIO9700/2006JK
  2. 2. Structure of DNA and RNA• DNA – deoxyribonucleic acid• RNA – ribonucleic acid• Polynucleotides/nucleic acids• Nucleotides – Made up of 3 smaller components: nitrogen- containing base; pentose sugar; phosphate group) – 5 bases: adenine, thymine/uracil, guanine, cytosine – Pentose (5-carbon): ribose (RNA) and deoxyribose (DNA) ALBIO9700/2006JK
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  4. 4. • Polynucleotides – Many nucleotides linked in a long chain (during interphase of cell cycle) – Formed of alternating sugars and phosphates linked together, with the bases projecting sideways – DNA molecule: 2 polynucleotide strands lying side by side running in opposite directions (held together by hydrogen bonds between bases) – Bases can be purines (adenine and guanine) or pyrimidines (cytosine and thymine) – Complementary base pairing (A with T, C with G) – Double helix: the 3D shape that DNA molecules form – RNA molecules remain as single strands of polynucleotide and form a different 3D structure ALBIO9700/2006JK
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  7. 7. DNA replication• Ability to be copied perfectly many times over• 1953: James Watson and Francis Crick (Rosalind Franklin)• Semi-conservative replication : half of the original molecule is kept (conserved) in each of the molecules ALBIO9700/2006JK
  8. 8. 1. The DNA double helix unwinds and ‘unzips’ as the hydrogen bonds between the bases break2. In the nucleus, there are nucleotides to which 2 extra phosphates have been added. The extra phosphates activate the nucleotides, enabling them to take part in the following reactions3. The bases of the activated nucleotides pair up with their complementary base on each of the old DNA strands. An enzyme, DNA polymerase, links the sugar and innermost phosphate groups of next-door nucleotides together. The two extra phosphates are broken off and released into the nucleus. • DNA polymerase will only link an incoming nucleotide to the growing new chain if it is complementary to the base on the old strand. Thus very few mistakes are made, perhaps around one in every 108 base pairs. ALBIO9700/2006JK
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