DNA - REPLICATION
This page takes a very simplified look at how DNA replicates
(copies) itself. It gives only a brief over-view of the process, with
no attempt to describe the mechanism. As such, it is only
suitable for 16 - 18 year old chemistry students. If you are doing
biology or biochemistry, you are going to need more detail than
this page covers.
Note: If you have come straight to this page from a search
engine, you should be aware that this is the second page in
a sequence of pages about DNA. Unless you already
understand the structure of DNA, follow this link to read that
page first.
If you need detailed information about the replication of
DNA, a Google search for DNA replication will lead you to
some well thought out and very clear animations of the
process. The best one to start with can be found by following
this link. This second link perhaps gives a better view of the
over-all process, but is more difficult to understand if you
don't look at the other one first. If you are an A level (or
equivalent) chemistry student, only look at these if you are
interested, not because you are likely to need it for exam
purposes.
Both of these links involve Flash animations. Unfortunately
some devices such as the iPad don't support Flash, and you
won't be able to watch these animations on them. There is
no way around that apart from using a standard computer.
I am a bit wary of suggesting links to other sites, because
these sites can change. If you find that either of these links
don't work (apart from problems with Flash), please e-mail
me via the address on the about this site page.
Semi-conservative replication
A very simple look at the process
We'll explain exactly what "semi-conservative" means when we
have got some diagrams to look at. First imagine what happens
if the two individual strands in the DNA double helix start to
unzip.
The diagram shows this happening in the middle of the DNA
double helix - you mustn't assume that the top of the diagram is
the end of the chain. It isn't. Further up the double helix, the two
strands will still be joined together.
In fact, this is happening lots of times along the very long DNA
molecule. Lengths of chain become separated to form what are
known as "bubbles". If you feel the need to see this in more
detail, read the rest of this page, and then have a quick look at
the links above.
Some of the hydrogen bonds get broken and the two strands
become partly separated.
The red dotted lines on the diagram just point out the original
base pairs. These are not bonds in any form. These base pairs
are now much too far apart for any sort of bonding between
them.
Now suppose that you have a source of nucleotides - phosphate
joined to deoxyribose joined to a base, including all the four
sorts of bases needed for DNA.
The next diagram shows what would happen if a nucleotide
containing guanine (G) and one containing cytosine (C) were
attracted to the top two bases on the left-hand strand of the
unzipped DNA - and then joined together.
How did they end up joined together? This is all under the
control of a number of enzymes, one of which (DNA
polymerase) is responsible for joining up nucleotides along the
chain in this way.
Now suppose the same sort of thing happened at the top of the
right-hand strand. You would end up with . . .
Now compare the double strands that you are forming on the
left- and right-hand sides. They are exactly the same . . . and if
you were to continue this process, they would continue to be the
same.
And if you compare the patterns of bases in the new DNA being
formed with what was in the original DNA before it started to
unzip, everything is the same. This is inevitable because of the
way the bases pair together.
What does semi-conservative replication mean?
Let's simplify the last diagram, and assume that the whole
copying process is complete. The next diagram focusses on the
short bit of the total DNA molecule that we have been looking at.
A typical human DNA molecule is around 150 million base pairs
long - you will have to imagine the rest of it!
You have also got to remember that in reality the whole thing
would have coiled into its double helix. Trying to draw that just
makes everything look messy and complicated!
The original DNA is shown all in blue. The red strands in the
daughter DNA are the ones which have been built on the
original blue strands during the replication process.
You can see that each of the daughter molecules is made of half
of the original DNA plus a new strand. That's all "semiconservative replication" means. Half of the original DNA is
conserved (kept) in each of the daughter molecules.
The red and blue, of course, have no physical significance apart
from as a way of making the diagrams clearer. All three of these
DNA molecules will be identical in every way.