Though DNA polymerase can elongate a polynucleotide strand by adding new
nucleotides, it cannot start a strand from scratch because it can only
bond new nucleotides to a free sugar (3') end of a nucleotide chain. DNA
polymerase requires the assistance of a
primer, a previously existing
short strand of DNA
(or RNA) that is complementary to the first part of the DNA segment being
This small strand of nucleotides anneals (binds) by complementary base
pairing to the beginning of the area being copied. With the primer
in place, DNA polymerase is then able to continue adding the rest of the
pairs of the segment until a new double strand of DNA is completed.
Primers are formed from free nucleotides in the cell by enzymes called
Replication occurs differently on antiparallel strands of DNA.
nucleotides can be added only to the sugar or 3' end of the growing
complementary chain presents no problem for the side of the DNA chain
opening at its phosphate or 5' end. The primer that binds to the
first few exposed bases will end with a sugar (3') where the phosphate of
a new nucleotide can be attached. From there on, DNA polymerase can
continuously synthesize the growing complementary strand. This
strand of DNA is called the leading strand. A nice little animation
of DNA synthesis on the leading strand can be seen at the Nobel Prize
e-museum site at
different challenge faces DNA polymerase when the complementary side of
the DNA molecule begins unzipping from its sugar (3') toward its phosphate
(5') end. A primer of complementary molecules attaching to the
opening end of this chain would have a phosphate not a sugar at its
exposed end so that new nucleotides could not be joined. To get around
this problem, this strand is synthesized in small pieces backward from the
overall direction of replication. This strand is called the
The short segments of
newly assembled DNA from which the lagging strand is built are called
As replication proceeds and nucleotides are added to the 3' end of the
Okazaki fragments, they come to meet each other. The primer
fragments are then booted out by enzymes and replaced by appropriate DNA
nucleotides. The whole thing is then stitched together by another
enzyme called DNA ligase. The Nobel e-museum also has an animation
of this process at
Replication occurs simultaneously at multiple places along a DNA
Because human DNA is so
very long (with up to 80 million base pairs in a chromosome) it unzips at
multiple places along its length so that the replication process is going
on simultaneously at hundreds of places along the length of the chain.
Eventually these areas run together to form a complete chain. In
humans, DNA is copied at about
pairs per second.
The process would take
a month (rather than the hour it actually does) without these multiple
places on the chromosome where replication can begin.
DNA replication is
DNA polymerase makes very few errors, and most
of those that are made are quickly corrected by DNA polymerase and other
enzymes that "proofread" the nucleotides added into the new DNA strand.
If a newly added nucleotide is not complementary to the one on the
template strand, these enzymes remove the nucleotide and replace it with
the correct one. With this system, a cell's DNA is
copied with less than one
mistake in a billion nucleotides.
This is equal to a person copying 100 large
(1000 page) dictionaries word for word, and symbol for symbol, with only
one error for the whole process!
Again for the fortunate
with high speed Internet connections, I have included a
Quick Time video animation of
the process of DNA replication. I'd like to credit it to its
original creator but have no idea who that would be. It has been
around since the 1970's or 80's as I remember it in a video I used in my
classroom that long ago.
Another video animation
of DNA replication can be found at
Unfortunately, this one
is 9.63 MB:(
An smaller file size (~
4MB) animation with text narration can be seen at: