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Notes by Zainab Ahsan

Cell cycle
"The cell undergoes a sequence of changes, which involves a period of
growth, DNA replication followed by cell division
...
In human cells, the average cell cycle is about 24 hours
...

• Interphase
• Mitosis

Interphase
“The period of cell cycle between the end of one cell division and start of next
division”
It is the longest phase of cell cycle during which a cell prepares itself for cell
division by undergoing cell growth and DNA replication
...

Somatic cells, or normal diploid cells of the body, go through mitosis in order to
reproduce themselves through cell division, whereas diploid germ cells (i
...
,
primary spermatocytes and primary oocytes) go through meiosis in order to create
haploid gametes (i
...
, sperm and ova) for the purpose of sexual reproduction
...

1
...
S- phase
3
...
G1 – Phase / Gap phase
It is the period of extensive metabolic activity which takes place from few hours to
several days
...
During this phase, following
major events take place
...
S - phase / Synthesis phase
During S – phase
• DNA is synthesized / replicated
...
If the initial amount of DNA is denoted as 2C then it increases
to 4C
• Each chromosome replicates to form two sister chromatids held at
centromere
...

3
...
5 hours
...
For instance, many cell types
in an early embryo divide rapidly, and so do cells in a tumor
...
These cells may exit the G1

Notes by Zainab Ahsan
phase and enter a resting state called GO
...
For instance, it might conduct signals as a neuron
(like the one in the drawing below) or store carbohydrates as a liver cell
...


Mitosis

Notes by Zainab Ahsan
"Mitosis is a type of cell division in which one cell (the mother) divides to
produce two new cells (the daughters) that are genetically identical to itself
...
"

Mitosis is also known as equational division
...
This term was based
on the warped thread appearance of the chromatin of the cell nucleus in the first
stages of mitosis
...
Prophase
Prophase is the first step of mitosis
...

During interphase, the parent cell’s chromosomes are replicated, but they aren’t yet
visible
...

During prophase, that loose chromatin condenses and forms into visible, individual
chromosomes
...
Once the
chromatin has condensed into individual chromosomes, the genetically-identical
chromosomes come together to form an “X” shape, called sister chromatids
...
The centromeres will
serve as anchors that’ll be used to pull the sister chromatids apart during a later
phase of mitosis
...
As they move to opposite sides of the cell,

Notes by Zainab Ahsan
the centrosomes form mitotic spindle
...


Late Prophase: Prometaphase
Prometaphase is the phase of mitosis following prophase and preceding metaphase
...

Once the nuclear envelope breaks apart, the sister chromatids that were stuck
inside the nucleus break free
...

Microtubules can bind to chromosomes at the kinetochore, a patch of protein
found on the centromere of each sister chromatid
...


Notes by Zainab Ahsan

2
...

the spindle has captured all the chromosomes and lined them up at the middle of
the cell, ready to divide
...

Before proceeding to anaphase, the cell will check to make sure that all the
chromosomes are at the metaphase plate with their kinetochores correctly attached
to microtubules
...
If a chromosome is not properly aligned or attached, the
cell will halt division until the problem is fixed
...
Anaphase
In anaphase, the sister chromatids separate from each other and are pulled
towards opposite ends of the cell
...


Notes by Zainab Ahsan

4
...
Telophase is when the newly separated
daughter chromosomes get their own individual nuclear membranes and
identical sets of chromosomes
...

Two new nuclei form, one for each set of chromosomes
...

The chromosomes begin to decondense and return to their “stringy” form
...
Sometimes, the
occurrence of the events of cytokinesis overlaps with telophase and even anaphase,
but cytokinesis is still considered a separate process from mitosis
...
At
the end of mitosis, there are two new nuclei contained within the existing parent
cell, which has stretched out into an oblong shape
...

Once the contractile ring forms down the middle of the cell, it starts shrinking,
which pulls the cell’s outer plasma membrane inward
...

The end of cytokinesis signifies the end of the M-phase of the cell cycle, of which
mitosis is also a part
...


Notes by Zainab Ahsan

When cytokinesis finishes, we end up with two new cells, each with a complete set
of chromosomes identical to those of the mother cell
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Notes by Zainab Ahsan

Importance of Mitosis in Living Process
Genetic stability- Mitosis helps in the splitting of chromosomes during cell
division and generates two new daughter cells
...
Therefore, the daughter
cells formed as genetically uniform and identical to the parent as well as to
each other
...


Growth- Mitosis help in increasing the number of cells in a living organism
thereby playing a significant role in the growth of a living organism
...
Mitosis helps in the production of identical copies of cells and thus
helps in repairing the damaged tissue or replacing the worn-out cells
...
For example, the mitosis process is used in order to regrowth the legs of
newts and crustaceans
...

Asexual reproduction- Mitosis is used in the production of genetically similar
offspring
...




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