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--
interface, 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 condense 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.
