Lecture notes: Biochemistry And Molecular Biology (BIOC0001) - Cell and Molecular Biology

DNA Replication
DNA Replication
 Base-pairing
o DNA double helix – 1 strand = contains a sequence of nucleotides complementary to the sequence
on the partner strand
 Semiconservative replication
o Each strand = serve as a template – for synthesis of new strand
 Allows a cell to replicate its genes before dividing
 Produces 2 double helixes from the original DNA molecule
o Each parental strand serves as a template for a new strand
o Each one of the daughter DNA double helixes = one new strand + one old
strand = semiconservative
o Replication machine
 Cluster of proteins that performs DNA replication
 Replication machine
o DNA primase
 Primers – made by enzyme primase
 Short lengths of RNA
 Provides a starting point for DNA polymerase to work
 Removed before DNA replication is completed – gaps are filled by DNA polymerase
o DNA polymerase
 Types
 DNA polymerase III
o Adds complementary nucleotides to the template stand
o Works from 5’ to 3’
 DNA polymerase I
o Removes RNA primers from the strand
o Replaces gaps with complementary nucleotides
 Self-correcting
 DNA polymerase minimises the chance of incorrect base pairing
o DNA polymerase carefully monitors which base is used
o Once the correct base is separated > DNA polymerase adds another
nucleotide
 Proofreading
 Before adding another nucleotide – DNA polymerase checks correcting pairing
o If wrong nucleotide was added > nuclease cleaves phosphodiester backbone
= removing nucleotide > DNA polymerase adds another
o DNA helicases
 Separate double stranded DNA into single strand = allows each strand to act as a template
strand
 Breaks hydrogen bonds between bases
 Creates replication bubble = 2 replication forks
o DNA topoisomerase
 Participate in the overwinding or unwinding of DNA – relieves tension due to excess twisting
in front of the replication fork
 As DNA helicase separates the DNA – DNA ahead of the replication fork becomes
overwound (wound tightly)
o Single-strand DNA-binding proteins
 Binds to DNA tightly – to protect single stranded DNA from degrading or rebinding
o DNA ligase
 Joins okazaki fragments on lagging strand together