BIOL2161 Lecture Notes - Lecture 3: Dna Supercoil, Topoisomerase, Pyrimidine
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
find more resources at oneclass.com
find more resources at oneclass.com