DNA Replication
Objectives
• Understand the process of DNA replication
• Be able to describe the role of :
– Helicase
– Single Strand Binding Proteins *
– Primase *
– DNA Polymerase
– DNA Ligase
• Understand why DNA is synthesized from the 5’
end to the 3’end
• Recognize the function of telomeres
DNA Replication
• DNA replication is semiconservative in that a each
new molecule incorporates and old strand that serves
as a template
• Requires many enzymes for assistance
• Few mistakes (~1/billion nucleotides)
DNA Replication
• Origins of
Replication: regions
on the DNA where
synthesis begins
• synthesis occurs in
both directions of the
“bubble” along the
replication fork (site
of DNA elongation)
DNA Replication
• Elongation of DNA
• Catalyzed by DNA
Polymerase and driven
by the hydrolysis of
phosphate groups from
nucleosides (a nucleoside
is a nucleotide with three
phosphate groups)
DNA Strands are Antiparallel
• New DNA grows from 5’3’ as DNA Polymerase
only adds nucleotides to the 3’ end of the DNA
strand (leading strand)
• Okazaki fragments, short pieces of synthesized
DNA, are formed and joined together by DNA ligase
to form the lagging strand of DNA
DNA must be Primed
• DNA Polymerase is unable
to replicate DNA directly
and requires that the original
DNA be primed
• Primase makes the initial
nucleotide (RNA primer) to
which DNA polymerase
attaches
• RNA primer is replaced
with DNA nucleotides later
Protein Summary
• Additional to
Primase, DNA
Polymerase and
ligase proteins are
2 others
– Helicase:
responsible for
unwinding the
DNA
– Single-strand
binding proteins:
keep original
complimentary
strands separated
Other things to consider
• Placement of mismatched
nucleotides during
synthesis is not rare and is
repaired by DNA
Polymerase through a
mechanism called
mismatch repair
• Excision repair takes
place in DNA to repair
damaged DNA (not
related to replication) that
could eventually lead to
problems
Other things to consider *
• RNA polymerase cannot synthesize
the extreme ends of a DNA
molecule
• Gradual shortening with each
replication could lead to deletion of
important information
• Telomerase adds many copies of
TTAGGG nucleotide sequence
(Telomere) to ends of DNA
• Telomerase is usually only found in
germ cells and sex cells
• Presence in cancerous cells may
lead to proliferation of tumors
* discussed on p190