BIOL2161 Lecture Notes - Lecture 3: Dna Supercoil, Topoisomerase, Pyrimidine

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13 Jun 2018
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BIOL6: Genes: Replication and Expression
Block 1: DNA Structure, Replication and Repair
DNA Structure
General
DNA is a macromolecule comprised of many nucleotides (monomers) joined together forming
strand (polymer)
General structure of a nucleotide: has a sugar, a base and a phosphate
o
o Have 4 nitrogenous bases: Adenine (A), guanine (G), cytosine (C) and thymine (T)
A fifth nucleotide also exists, and replaced T in an RNA strand, uracil (U)
Bases are either purines or pyrimidines
Pyrimidine bases: C, T and U
Purine bases: G and A
o Sugar is a pentose sugar, either deoxyribose (in DNA) or ribose (in RNA)
DNA must be stable and protected to prevent damage to genetic information, yet accessible
for information to be used
Eukaryotes have more DNA than prokaryotes
DNA is a double helix of complementary antiparallel chains
o Linked by hydrogen bonds between bases
3H bonds in a G-C pair, 2H bonds in a A-T pair
DNA is packed with proteins
o Double helices are wound up further in supercoils
o Supercoiling is regulated by enzymes (topoisomerases and gyrases)
Topoisomerases can induce or undo supercoiling by breaking the nucleotide
strands, rotating the ends and re-joining the ends
o It is due to DNA being over/underwound
o Allows further packing of DNA as it occupies less space than when relaxed
o In bacteria, DNA is packaged as twisted loops
Organisation of DNA
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