BIOL 411 Lecture Notes - Lecture 7: Phosphodiester Bond, Dna Replication, Proofreading
Bio411.01
2/15/16
• Structure of DNA:
o Phosphate sugar backbone is on the outside
▪ Every phosphate group has a negative charge, so negative charges are on
the outside- overall, DNA is negatively charged
▪ Proteins are positively charged, leads to formation of chromatin
o Nitrogenous bases are stacked in the middle
▪ The bases are hydrophobic- stacked and in the middle leads to least
amount of contact
o Minor grove: smaller/skinnier grove
o Major grove: thicker grove (major grove and minor grove alternate)
• *Know: what holds together the 2 strands? Aka why is duplex more stable than single
strands
o 1. Complementary base pairs
o 2. Stacked bases that keep hydrophobic rings hidden from water
• DNA replication: duplex separates, each strand serves as a template for a
complementary new strand
o Mode of replication- explained by complementary base pair
o Nature of replication- explained by sequence
o Mutation- explained by changes in sequence
• Two requirements of DNA synthesis:
o 1. Adding nucleotides in 5’ → 3’ direction only
o what you make (the new duplex) has to end up antiparallel
• *know how to draw a replication fork filling in the direction of replication
• As replication progresses, the fork is progressing upwards (toward original duplex)
o At replication fork, you have synthesis going in opposite directions
▪ Synthesis going towards fork: leading strand
▪ Synthesis going away from fork: lagging strand
• Discontinuous synthesis on the lagging strand because there is a
lagging period before synthesis on this strand
• DNA polymerase is responsible for
o Catalysis of formation of phosphodiester bond
o Fidelity of phosphodiester bond
▪ Proofreading
▪ There are still mistakes →variations in genome →life evolved
• DNA helicase- unwinds duplex
• Chromatin- the stuff of DNA in eukaryotic chromosomes
o Make up chromosomes
o Mixture of DNA and histomes (about 150 basepairs of DNA + histone octamer=
nucleosome)
o Nucleosomes are the units of chromatin
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