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The DNA in just one of your cells

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gets damaged tens of thousands 
of times per day.

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Multiply that by your body's
hundred trillion or so cells,

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and you've got a quintillion 
DNA errors everyday.

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And because DNA provides the blueprint

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for the proteins 
your cells need to function,

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damage causes serious problems,
such as cancer.

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The errors come in different forms.

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Sometimes nucleotides, 
DNA's building blocks, get damaged,

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other times nucleotides 
get matched up incorrectly,

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causing mutations,

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and nicks in one or both strands
can interfere with DNA replication,

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or even cause sections 
of DNA to get mixed up.

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Fortunately, your cells have ways
of fixing most of these problems

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most of the time.

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These repair pathways 
all rely on specialized enzymes.

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Different ones respond 
to different types of damage.

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One common error is base mismatches.

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Each nucleotide contains a base,

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and during DNA replication,

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the enzyme DNA polymerase
is supposed to bring in the right partner

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to pair with every base 
on each template strand.

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Adenine with thymine,
and guanine with cytosine.

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But about once every 
hundred thousand additions,

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it makes a mistake.

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The enzyme catches 
most of these right away,

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and cuts off a few nucleotides
and replaces them with the correct ones.

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And just in case it missed a few,

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a second set of proteins 
comes behind it to check it.

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If they find a mismatch,

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they cut out the incorrect nucleotide
and replace it.

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This is called mismatch repair.

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Together, these two systems reduce
the number of base mismatch errors

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to about one in one billion.

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But DNA can get damaged
after replication, too.

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Lots of different molecules
can cause chemical changes to nucleotides.

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Some of these come 
from environmental exposure,

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like certain compounds in tobacco smoke.

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But others are molecules that are found
in cells naturally,

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like hydrogen peroxide.

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Certain chemical changes are so common

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that they have specific enzymes assigned
to reverse the damage.

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But the cell also has more general
repair pathways.

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If just one base is damaged,

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it can usually be fixed by a process
called base excision repair.

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One enzyme snips out the damaged base,

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and other enzymes come in to trim around
the site and replace the nucleotides.

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UV light can cause damage 
that's a little harder to fix.

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Sometimes, it causes two adjacent
nucleotides to stick together,

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distorting the DNA's double helix shape.

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Damage like this requires 
a more complex process

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called nucleotide excision repair.

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A team of proteins removes a long strand
of 24 or so nucleotides,

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and replaces them with fresh ones.

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Very high frequency radiation,
like gamma rays and x-rays,

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cause a different kind of damage.

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They can actually sever one 
or both strands of the DNA backbone.

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Double strand breaks
are the most dangerous.

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Even one can cause cell death.

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The two most common pathways
for repairing double strand breaks

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are called homologous recombination
and non-homologous end joining.

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Homologous recombination uses an undamaged
section of similar DNA as a template.

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Enzymes interlace the damaged
and undamgaed strands,

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get them to exchange sequences
in nucleotides,

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and finally fill in the missing gaps

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to end up with two complete
double-stranded segments.

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Non-homologous end joining,
on the other hand,

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doesn't rely on a template.

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Instead, a series of proteins
trims off a few nucleotides

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and then fuses the broken ends 
back together.

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This process isn't as accurate.

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It can cause genes to get mixed up,
or moved around.

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But it's useful when 
sister DNA isn't available.

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Of course, changes to DNA 
aren't always bad.

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Beneficial mutations 
can allow a species to evolve.

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But most of the time, 
we want DNA to stay the same.

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Defects in DNA repair are associated
with premature aging

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and many kinds of cancer.

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So if you're looking for 
a fountain of youth,

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it's already operating in your cells,

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billions and billions of times a day.