All language subtitles for 27281x_Cell_Cycle-en

Our bodies consist of trillions of cells.

These cells need to divide when our bodies grow,

replenish the cells we've lost from injuries,

or need to maintain cell populations, like that

of the skin.

The mitotic cell cycle is the process

by which all eukaryotic cells replicate and divide

to produce daughter cells for growth, repair,

and development.

Because the cell cycle is vital to our survival,

our cells tightly regulate the process

to prevent uncontrolled cell division or incorrect DNA

replication, which can lead to diseases, like cancer.

This video gives an overview of the steps

within the cell cycle, and introduces

some of the key proteins that help our bodies regulate

cell division.

We will use language that is defined in our cell cycle terms

video.

So we recommend that you watch that video,

if you haven't already done so.

How, exactly, does the cell cycle

enable DNA replication and cell division to produce more cells?

We divide the cell cycle into four main phases--

G1, S, G2, and M. We group these phases

into two more general phases--

interphase, during which the cell

prepares for cell division, and the mitotic phase,

when cell division occurs.

In experimental assays, populations of cells

can be either synchronous or asynchronous.

A synchronous population of cells

contains cells that are all in the same phase of the cell

cycle--

for example, a cell culture with many cells

that are all beginning mitosis simultaneously.

Cells in an asynchronous population

are spread throughout the cell cycle stages.

For example, some cells will be in S phase,

while others are in M phase and actively dividing.

This video focuses on one phase of the human cell cycle

at a time, paying specific attention

to how much genetic material is in the cell at each point.

Note that when cells are not dividing,

they often enter a quiescent stage, called the G0 phase.

These cells are performing their unique functions,

but are not undergoing cell growth or division.

Because these cells are not dividing,

they will have 46 homologous chromosomes, or 2N chromosomes,

no sister chromatids, and 2C DNA content,

because the cell is diploid and no DNA

replication has occurred.

The G1, or Gap 1, phase involves cell growth and preparation

for DNA replication, which will take place in S phase.

This preparation includes checking

for cells with DNA damage, making

sure there are enough nutrients in the cell for cell division,

and verifying that the cell is big enough to divide.

During the G1 phase, the cell also

prepares for initiation of DNA replication.

Specifically, proteins identify origins of replication and load

multi-protein complexes onto this DNA,

for example, the DNA helicase.

How many chromosomes are currently

in the cell after G1 phase?

Try to fill out this table.

Right now, the diploid eukaryotic cell

has 2N chromosomes, 46 in humans.

The cell modified the DNA to prepare for DNA replication.

But no replication has occurred and, therefore,

the number of chromosomes and genetic material

is still the same as in G0.

That is, sister chromatids have not formed

and the cell has 2C DNA content because the cell is diploid

and no DNA replication has occurred.

At the end of G1, there's a point of no return,

often called the restriction point,

where the cell commits to completing the rest of the cell

cycle and transitions into the S phase.

DNA replication occurs in the S, or synthesis, phase.

The cell now activates the DNA helicases that the cell loaded

onto DNA in G1 phase.

This process separates the double-stranded DNA

into single-stranded DNA to provide the templates

for DNA replication.

The sites of helicase action are junctions

between double-stranded and single-stranded DNA,

which form replication forks.

Many other proteins assemble at these sites, like DNA

polymerase, primase, and others, to replicate the DNA strands.

The cell does not load helicases during S phase,

to ensure that the DNA is replicated once and only once.

DNA replication results in duplicated chromosomes,

sister chromatids that are held together

by proteins that promote sister chromatid cohesion.

Remember that sister chromatids are identical copies

of a single chromosome.

Now how many chromosomes are currently in the cell,

after S phase?

And how many copies of the DNA?

Try to fill out this table.

Right now, the cell is still 2N, with 46

unique homologous chromosomes.

The DNA replication duplicates each

of these individual chromosomes to form 46 pairs

of identical sister chromatids.

And there's 4C DNA content now, because DNA replication has

occurred in this diploid cell.

G2 phase is a gap phase that allows the cell

to prepare for N phase, during which the cell will divide

in two.

This preparation mainly includes verifying

that S phase completed correctly and that the chromosomes were

completely and accurately replicated,

with no need for DNA repair or further DNA replication.

Now how many chromosomes are currently in the cell after G2,

and how many copies of the DNA?

Try to fill out this table.

Right now, the cell is still in the exact same state

as it was at the end of S phase.

There are 2N, or 46 unique homologous chromosomes,

that have been duplicated to form

46 pairs of sister chromatids.

Because DNA replication occurred in S phase,

there's still 4C DNA contents in the cell.

Chromosome segregation occurs during M phase, or mitosis.

We divide mitosis into subphases--

prophase, metaphase, anaphase, and telophase, directly

followed by cytokinesis, or cell division.

Prophase is when the pairs of sister chromatids

condense to form the familiar chromosome shapes we

see in textbook illustrations.

The nuclear envelope in the cell also breaks down.

To simplify these animations, we only

show 3 of the 46 pairs of sister chromatids in a human cell.

But keep in mind that the remaining pairs are still

there.

Metaphase is when the mitotic spindle forms.

This structure is a molecular machine

that separates the sister chromatids,

ensuring that each daughter cell gets

one copy of each chromosome.

During metaphase, the filaments--

microtubules from the poles of the mitotic spindle,

centrosomes--

attach to the pairs of sister chromatids

on both sides of the region called the centromere.

The tension from mitotic filaments,

pulling the pairs of sister chromatids

toward the opposite poles of the mitotic spindle,

aligns the chromosomes in the center of the cell.

This is called the metaphase plate.

Anaphase is when the sister chromatids separate and are

pulled to opposite sides of the cell by the mitotic spindle.

Now each side of the cell, which will

become one of the two daughter cells,

has a complete set of 46 chromosomes.

During telophase, the nuclear envelope

begins to re-form around each group

of segregated chromosomes.

These chromosomes also begin to decondense once more.

Cytokinesis is not technically a phase of mitosis,

but follows immediately afterward.

During cytokinesis, the plasma membrane of the mother cell

fuses in a way that physically separates

the cytoplasm of the mother cell to form

two daughter cells, each with their own nucleus

and chromosomes.

Now how many chromosomes are currently

in each daughter cell?

And how many copies of the DNA?

Try to fill out this table.

Each daughter cell is 2N, with 46

unique homologous chromosomes.

The pairs of sister chromatids have separated and are now

just considered to be individual chromosomes in each

of the daughter cells.

Each new cell now only has two 2C DNA content,

like all diploid cells.

Now the cell cycle is ready to start all over again,

with each of these daughter cells,

or the cells can stop growing and arrest in the G0 phase.

The cell cycle involves many steps, all of which

are equally important to ensure that the replication

of genetic material and cell division goes correctly.

Now that you're familiar with the phases of the cell cycle,

let's talk about what regulates these phases.

How does the cell know what phase it's in,

and how does it know that it's safe to proceed

to the next phase?

The cell cycle is regulated by a class of proteins called

cyclins which, in turn, activate cyclin-dependent kinases,

or CDKs.

The name cyclin reflects their property

of fluctuating in abundance in specific ways

during the cell cycle, as you can see in this diagram.

The abundance of cyclin and cyclin CDK activity

divide the cell cycle into the distinct phases

we described before--

G1, S, G2, and M.

In addition, there are checkpoints during the cell

cycle that check the work of the cell

and ensure that one phase is complete

before the next is started.

For example, there's a checkpoint

that prevents cells from entering mitosis until DNA

replication is complete.

This prevents catastrophic errors,

such as segregating incompletely replicated chromosomes, which

would lead to chromosome breaks and DNA damage,

which could in turn lead to diseases like cancer.

The cell cycle is essential for our growth and survival.

The cell cycle can appear complicated

because it integrates many smaller, equally complicated

cellular processes, such as DNA replication and mitosis.

Now that you've finished this video,

can you explain the four different stages

of the cell cycle, and what occurs in each step?

And do you have an idea of what role cyclin and CDKs play

in regulating the cell cycle, and why

they are important to our overall health?

Thanks for watching.