The only way to create a new cell is to duplicate a pre-existing one.
The original cell is called the parent cell, and the two new cells, which are genetically
identical, are called daughter cells.
The series of steps leading to and involving the duplication of a cell is called the cell cycle.
The cell cycle involves cell growth, chromosome replication, chromosome segregation, and cell division
These processes take place over the course of the four sequential phases of the cell
cycle: G1, S, G2, and M. The first three phases are together called interphase.
Most cells need more time to grow and double their organelles and biomolecules than
they need to duplicate and divide up their chromosomes, the cell cycle has the extra
gap phases, called G1 and G2.
These gap phases, in addition to compensating for the time needed for growth, also give
the cell time to assess whether the environment is favourable for division and whether all
the preparations for cell division have been appropriately completed.
Eukaryotic cells have a complex system of regulatory proteins, called the cell-cycle
control system, to make sure that cell division occurs correctly and only when appropriate.
Taking place between G1 and G2, S phase is when DNA replication and chromosomal duplication
takes place.
It is critical that this replication is accurate, and that every nucleotide is copied exactly
once.
Here it’s important to discuss two closely-related, and commonly confused terms – chromosomes
and chromatids.
A chromosome can be one of two things: a continuous strand of DNA or two sister chromatids linked
together with a centromere.
These sister chromatids include the original chromosome and its identical replica.
So to recap: you have a chromosome, then after DNA replication you have a chromosome made
up of sister chromatids.
When cell division takes place, the sister chromatids will be separated, and you now
have two daughter chromosomes.
Also, keep in mind that in diploid cells, there would be two copies of
each chromosome present.
Finally, we come to M phase, which includes mitosis and cytokinesis.
Mitosis is the process by which sister chromatids are split up equally between the two new daughter
nuclei, so that each new cell can have an identical and complete version of the genome.
Cytokinesis is the division of the cytoplasm.
Before we describe what happens during mitosis, it is important to define the mitotic spindle
and centrosomes.
The mitotic spindle is what eukaryotes use to separate chromosomes into two new daughter
cells.
It is a bipolar array of microtubules that pulls sister chromatids apart.
These microtubules have minus ends focused at the “spindle poles” and plus ends that
radiate outward.
Microtubules are composed of tubulin subunits, which can get added or removed from the plus
end of the microtubule, making it grow or shrink.
There are three types of microtubules in the mitotic spindle.
Interpolar microtubules are those whose plus ends interact with the plus ends of interpolar
microtubules originating from the opposite spindle pole.
Astral microtubules radiate outward from the poles to contact the cell cortex.
Their job is to position the spindle in the cell.
Finally, there are the kinetochore microtubules, whose positive ends are attached to sister
chromatid pairs at kinetochores.
Kinetochores are large protein structures located at the centromere of each sister chromatid.
The kinetochores are built on the heterochromatin that forms at the centromeric region of the
chromosome.
The kinetochores allow the plus ends of kinetochore microtubules to be embedded in special attachment
sites in the kinetochore.
An animal cell kinetochore can have 10-40 of these attachment sites.
The attachment sites contain a protein collar that secures the microtubule but still allows
the addition or removal of tubulin subunits to its end.
Kinetochores allow kinetochore microtubules to attach correctly to sister chromatids.
How?
Well, if there is an imbalance in microtubule attachment, such as if two microtubules from
opposite spindle poles attach to the same sister chromatid, the attachment is unstable.
However, the correct attachment gets locked in place.
In animal cells, at each spindle pole, there is a single centromere, which consists of
two centrioles oriented perpendicular to one another, surrounded by the pericentriolar
matrix.
The pericentriolar matrix nucleates a radial array of microtubules, whose negative ends
are associated with the centromere.
Like the genome, the centromere must be duplicated exactly one time prior to mitosis.
An incorrect number of centromeres would result in errors in spindle assembly and chromosome
segregation.
This duplication occurs at the start of S phase.
Finally, we come to mitosis, the division of identical genomic information between two
new nuclei!
Mitosis has 5 stages: prophase, prometaphase, metaphase, anaphase, and telophase.
During prophase, chromosomes condense within the nucleus, with each chromosome consisting
of two linked sister chromatids.
Outside the nucleus, the mitotic spindle assembles between two centrosomes.
Next we have prometaphase, which begins when the nuclear envelope breaks down.
This means that chromosomes can now attach to spindle microtubules.
This occurs via kinetochores and results in the movement of the chromosomes.
Metaphase takes place when the chromosomes are aligned along the equator of the spindle,
with microtubules attaching sister chromatids to opposite poles of the spindle.
Anaphase involves the synchronized separation of sister chromatids to form two daughter
chromosomes, which are pulled towards the spindle poles.
During this process, not only do the kinetochore microtubules shorten, but the spindle poles
also move apart.
Telophase is the last stage of mitosis.
At this point, the two sets of daughter chromosomes arrive at the spindle poles and decondense.
The formation of the two nuclei, which occurs when a nuclear envelope assembles around the
two genomes, means that mitosis is over.
Mitosis is followed by cytokinesis, which is the division of the cytoplasm.
This occurs thanks to the formation of a contractile ring of actin and myosin filaments, which
form a rapidly deepening furrow and pinch the cell into two new cells.
The contractile ring actually begins to assemble during anaphase, just beneath the plasma membrane.
As the ring contracts, there is fusion of intracellular vesicles to add more plasma
membrane to compensate for the extra surface area.
So cytokinesis has 4 steps: initiation, contraction, membrane insertion, and completion.