Speed and precision of DNA replication | Biomolecules | MCAT | Khan Academy

- [Voiceover] In the earlier
video on DNA replication,

we go into some detail

about leading strands and lagging strands

and all of the different actors,

all of these different enzymatic actors.

But I left out what is probably

the most mind-boggling
aspect of all of this,

and that's the speed and the precision

with which this is actually happening.

As we talked about in that video,

it feels pretty complex.

You have this topoisomerase
that's unwinding things,

the helicase is unzipping it.

Then you have the
polymerase that can only go

from the five prime to
three prime direction,

and needs a little primer to get started,

but then it starts adding the,

it starts adding the nucleotides.

On the lagging strand,

you have to have the R,

you get the RNA primer,

but then it's going from,

once again, from five
prime to three prime,

so you have these Okazaki fragments.

And all of this craziness
that's happening,

and remember, these
things don't have brains.

These aren't computers.

They don't know exactly where to go.

It's all because of the chemistry.

They're all bumping into each other

and reacting in just the right way

to make this incredible thing happen.

Now what I'm about to tell you

is really going to boggle your mind.

Because this is happening incredibly fast.

DNA polymerase has been clocked,

at least in E. coli,

has clocked at approaching
1,000 base pairs per second.

I think the number that I saw

was 700-something base pairs per second.

So polymerase, let me write this down.

This is worth writing down,

because it's mind-boggling.

Gives you a sense of just how amazing

what the machinery in your cells are.

So it's been as high as,

and it can change.

It can speed up and slow down,

and that's actually been observed.

But polymerase as fast as,

as fast as

700-plus base pairs per,

per second.

So if this, on this diagram,

it, man, it's just zipping,

it's just zipping along,

at least from our perceptual
frame of reference.

A second seems like a very
short amount of time to us,

but on a molecular scale,

these things are just bouncing around

and just getting this stuff done.

Now the second thing that
you might be wondering,

okay, this is happening fast,

but surely it has a lot of errors.

Well, the first thing you might say,

well, if it had a lot of errors,

that would really not be good for biology,

because you always have,
you have DNA replicating

all throughout our lives.

And at some point you
just have so many errors

that the cells wouldn't function any more.

And so lucky for us

that this is actually a
fairly precise process.

Even in the first pass of the polymerase,

you have one mistake,

you have one mistake,
let me write this down,

'cause it's amazing.

One mistake for every,

for every approximately 10 to the seventh.

So this is 10 million,

10 million in nucleotides.

Nucleotides.

And that might seem pretty accurate,

but you gotta remember,

we have billions of
nucleotides in our DNA.

So this would still
introduce a lot of errors.

But then there's
proofreading that goes back

and makes sure that those
errors don't stick around.

And so once all the
proofreading takes place,

it actually becomes one mistake,

one mistake for every

approximately 10 to the ninth nucleotides.

So approximately, you can,

it would do this at an
incredibly fast pace,

as fast as 700-plus approaching
1,000 base pairs per second.

And you have one error
every billion nucleotides,

especially after you go through
these proofreading steps.

And so it's incredibly fast,

and it's incredibly precise.

So hopefully that gives
you a better appreciation

for just the magic that's literally,

I would look at your hand,

or just think about,

this is happening in all of the cells

or most of the cells of
your body as we speak.