Video Transcript
When errors in DNA replication
occur, the newly formed strand can be recognized as not being complementary to the
original strand. Which enzyme is responsible for
correcting these errors?
You may recall that each DNA
nucleotide contains a phosphate group, a pentose sugar, and one of four possible
nitrogenous bases, either guanine, adenine, cytosine, or thymine. The bases form hydrogen bonds with
each other. And it is these bonds which hold
the two strands of the double helix together in a DNA molecule, but they don’t do
this randomly. As you can see in the diagram,
guanine forms bonds with cytosine and adenine forms bonds with thymine, and this is
always the case. This phenomenon is known as
complementary base pairing. Complementary can be spelled two
different ways with two very different meanings. Notice how it’s spelled with an E
here rather than an I because we’re referring to pairs of bases which go
together.
Now, let’s consider how errors can
lead to a newly formed DNA strand not being complementary to the original
strand. During DNA replication, the two
strands of the double helix are unwound by an enzyme called DNA helicase. Another enzyme called DNA
polymerase then moves along each strand, adding in nucleotides with complementary
bases to build two new strands. But DNA polymerase is not perfect,
and about once every 100,000 nucleotides, it makes an error. This means a nucleotide is added to
the new strand, which isn’t complementary to the corresponding nucleotide on the
original strand.
Imagine if you were asked to copy
out an entire novel word for word, you would probably make some mistakes too. Pause the video for a moment and
see if you can spot the error on one of the new DNA strands. Remember, in our diagram, orange
pairs with blue and green pairs with pink. So, look out for the colored bases
which have been paired incorrectly. Hopefully, you spotted that thymine
has been incorrectly paired with cytosine here. It should’ve been guanine. So, let’s correct the mistake.
The clever thing about DNA
polymerase is that it can proofread and correct its own mistakes as it goes
along. Every time it adds a nucleotide to
the new strand, it checks to make sure its base is complementary to the nucleotide
on the original strand. If it isn’t, DNA polymerase removes
the incorrect nucleotide and replaces it with the correct nucleotide before
synthesizing the rest of the new strand. Although errors like these often
have no consequence, they can sometimes lead to life-threatening diseases such as
cancer, which is why this proofreading ability is so important.
Therefore, the enzyme that is
responsible for correcting errors during DNA replication is DNA polymerase.
