0:00:06.414,0:00:08.748
The DNA in just one of your cells

0:00:08.748,0:00:12.997
gets damaged tens of thousands [br]of times per day.

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

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and you've got a quintillion [br]DNA errors everyday.

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And because DNA provides the blueprint

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for the proteins [br]your cells need to function,

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damage causes serious problems,[br]such as cancer.

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The errors come in different forms.

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Sometimes nucleotides, [br]DNA's building blocks, get damaged,

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other times nucleotides [br]get matched up incorrectly,

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causing mutations,

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

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or even cause sections [br]of DNA to get mixed up.

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Fortunately, your cells have ways[br]of fixing most of these problems

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most of the time.

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These repair pathways [br]all rely on specialized enzymes.

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Different ones respond [br]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[br]is supposed to bring in the right partner

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to pair with every base [br]on each template strand.

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Adenine with thymine,[br]and guanine with cytosine.

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But about once every [br]hundred thousand additions,

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it makes a mistake.

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The enzyme catches [br]most of these right away,

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and cuts off a few nucleotides[br]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 [br]comes behind it to check.

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If they find a mismatch,

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they cut out the incorrect nucleotide[br]and replace it.

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This is called mismatch repair.

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Together, these two systems reduce[br]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[br]after replication, too.

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Lots of different molecules[br]can cause chemical changes to nucleotides.

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Some of these come [br]from environmental exposure,

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like certain compounds in tobacco smoke.

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But others are molecules that are found[br]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[br]to reverse the damage.

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But the cell also has more general[br]repair pathways.

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If just one base is damaged,

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it can usually be fixed by a process[br]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[br]the site and replace the nucleotides.

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

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Sometimes, it causes two adjacent[br]nucleotides to stick together,

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distorting the DNA's double helix shape.

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Damage like this requires [br]a more complex process

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called nucleotide excision repair.

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

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and replaces them with fresh ones.

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Very high frequency radiation,[br]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 [br]or both strands of the DNA backbone.

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Double strand breaks[br]are the most dangerous.

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Even one can cause cell death.

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The two most common pathways[br]for repairing double strand breaks

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are called homologous recombination[br]and non-homologous end joining.

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

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Enzymes interlace the damaged[br]and undamgaed strands,

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get them to exchange sequences[br]of nucleotides,

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and finally fill in the missing gaps

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to end up with two complete[br]double-stranded segments.

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Non-homologous end joining,[br]on the other hand,

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doesn't rely on a template.

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Instead, a series of proteins[br]trims off a few nucleotides

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and then fuses the broken ends [br]back together.

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This process isn't as accurate.

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

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

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

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Beneficial mutations [br]can allow a species to evolve.

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

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Defects in DNA repair are associated[br]with premature aging

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and many kinds of cancer.

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So if you're looking for [br]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.