So here I've got a picture of an average person.
They're a little more than 5 feet tall.
So if we were to convert that into meters,
the average person is a little less than
about two meters tall.
Now, a person is the largest unit of life we consider.
When we talk about the smallest unit of life,
we're talking about a cell
that relatively looks like that.
And while a human being is about two meters tall,
a cell has a diameter of about 100 micrometers,
which, to put into perspective relative to a human being
is about one one-millionth of the size.
And yet there's so much that goes on here.
Just as human beings grow and maybe will have babies,
cells do the exact same thing.
They grow and have babies, as well,
or they undergo cell division.
So, in the next couple of videos,
we're going to talk about how a cell grows and divides.
So let's zoom in on the cell right here
and spend the next couple of videos talking about
how a cell grows and divides.
The life span of a cell can be described by what's called
the cell cycle.
The cell cycle can be thought of as seasons in a year.
Just as we have seasons such as the spring or the summer
where things grow, versus fall and winter where they don't,
the cell has times when it grows and divides
and other times when it doesn't divide.
There are two main overarching seasons,
or types of seasons,
that we can talk about here.
There's this period here that's more like
the fall or the winter, where you don't have
as much cell division, but you have
more growth of the cell.
This period is where the cell spends most of its time,
and it's called interphase. Interphase: where we
primarily have cell growth occur,
but not cell division.
Interphase is where cells spend most of the time,
so most cells live here.
But there is one key exception.
What do you think that one exception might be?
I think I heard you correctly if you said, "Cancer."
You are absolutely right.
Cancer cells have some defect in them that causes
them to want to divide more so than grow,
and we'll talk more in detail
about how that occurs in a minute.
The other main phase of the cell cycle here
is where you have active cell division,
and it's called mitosis.
Mitosis, or sometimes it's abbreviated with just an "M."
Mitosis is the time where you have active cell division.
Now, there are a few other phases that occur within interphase.
The first part of interphase is a growth phase.
It's usually abbreviated, "G1."
As you can see here, G1 is the
longest phase of the cell cycle, so
most of a cell's life is spent here,
and it's in this phase that we produce
extra organelles, such as ribosomes and proteins.
So we make proteins that will be useful when
we get to the point of cell division.
>From here, the cell has a choice.
If it wants to continue growing
and move towards the direction of cell division,
it will move forward this way
to the next phase that's called the, "S phase."
The S phase just stands for synthesis,
more specifically DNA synthesis,
because here we're going to have DNA replication.
That's where we take 23 pairs of chromosomes,
so 23 pairs. We call them pairs because half of them
are from your mom, half of them are from your dad,
and we duplicate them, we replicate them,
and we end up with 46 pairs.
and most cells go in this direction
as I mentioned over here.
Some cells however, instead of going forward,
from G1 they'll go in another direction here
to a phase that's called, "G not," or G 0,"
where you have no more division.
No more cell division.
Because there are certain cells in the body that
don't like to divide or don't tend to divide,
and you can think of a quick example like neurons in the brain.
Once your brain is formed
it doesn't necessarily need to divide any more.
You just have cells grow.
So that means it's sort of
the end point for these types of cells.
They won't usually come back and enter
the cell cycle in this way.
But let's continue as if it had.
The next step, or the next phase of the cell cycle,
is called, "G 2," which is another growth phase
where we are more directly preparing for mitosis.
So, we prepare for mitosis in a couple of ways.
Perhaps a good example is we make microtubules,
which will be used to pull our chromatids apart
when it comes time for anaphase,
and we'll talk about what that means in a separate video.
And finally, to be complete, we have our last phase
right here, which is just mitosis,
which is our final season of the cell cycle
where our cell will divide.
And once it's divided and turned into two cells,
each of the cells will next enter the G1 phase,
where they will grow and produce
extra organelles and proteins that will eventually
allow them to divide again. So, as you can see,
just like seasons in the year,
the cell cycle goes around and around as we
divide cells further and further,
unless it's a cell that goes off to this G not or G0 phase