11. DNA polymerase adds DNA
nucleotides to the RNA
primer. Proofreading activity
checks and replaces
incorrect bases just added.
Leading (continuous)
strand synthesis continues
in a 5′ to 3′ direction.
Helicase separates strands.
Binding proteins prevent
single strands from rejoining.
Primase makes a short stretch
of RNA on the DNA template.
Overall direction of replication
Discontinuous
synthesis of the lagging
strand produces
Okazaki fragments.
After RNA primer is replaced with DNA,
ligase seals the sugar–phosphate backbone.
Steps in DNA replication
RNA primers
New DNA
strands
Okazaki
fragment
Leading strand
Lagging strand
3’
3’
3’
3’
3’
3’
3’
3’
3’
3’
3’
3’ 3’
3’
3’
3’
3’
3’
5’
5’
5’
5’
5’
5’
5’
5’
5’
5’
5’
5’
5’5’
5’
5’
5’5’
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