Mitosis is one of the phases of the cell cycle, which is described in below:
The cell cycle is a concept that describes the life of a cell, from its “birth” to its “death”. Although this introduction will be of a basic nature, cell cycling problems are of extreme interest to scientists. Cells that will not progress normally through their life cycle at the heart of many human diseases, such as neurogenerative disease, tuberous sclerosis, arthritis, glomerular disease, cardiovascular diseases, and cancer (just to name a few). For example, a cancerous cell is simply a cell has speeded up the reproductive process and it will not die easily.
The “brain” of a cell is its DNA. DNA stands for DeoxyriboNucleic Acid, a chemical name that describes the nature of DNA molecules. DNA has four different nucleic acids, which are referred to by their first letters: A (Adenosine), T (Tyrosine), G (Guanine), and C (Cytosine). The chains of nucleic acids are stored in the nucleus as coiled helices called chromosomes. Humans have 46 chromosomes which appear X-shaped under the microscope, with the exception that the 46th chromosome may instead be the Y-shaped chromosome which establishes the gender of its career as male. The four acids in different combinations provide the code so that the cell may synthesize all the materials (proteins) which make up the body.
- Grow [Interphase]
- Copy [Interphase]
- Split into 2 daughter cells [Mitosis]
The larger a cell becomes, the more stress there is on a cell and on its DNA. As a cell gets larger, the ratio of surface area to volume will decrease. This means that it will take longer for nutrients and waste to move from the cell membrane to the interior of the cell and vice versa. The cell's ratio of surface area to volume changes as the cell grows larger: Volume increases faster than surface area. This can be thought of like a city: The larger city vs. smaller city - which has faster moving traffic?
If a cell reaches a point where it can no longer handle the amount of growth that has occurred (to itself), it will go through cell division.
Cell division is when a cell splits to create two new identical daughter cells. Before a cell will divide, it will replicate all of its DNA so that the two new daughter cells will have its own set of DNA. The new cells are genetically identical to the parent cell (as this is asexual reproduction). These two cells will each have the same number of chromosomes as the parent cell.
This section will describe each stage of Cell Division:
Interphase (I-Phase), otherwise known as Pre-Mitosis, is where 90% of a cell's life grows and replicates itself in preparation for cell division. Organelles double in number. Interphase begins in the daughter cells. It includes the following sub-phases [if the cell fails at G1 checkpoint, it gets sent to the G0 stage for repairment. If fails at G2 checkpoint, it goes back to the G0 stage. If DNA is damaged beyond repair than the cell will die [apoptosis]]:
- First gap phase
- Grows larger, copies organelles, and creates the molecular blocks ready for the next stages.
- G1 Checkpoint: Checks for cell density, cell adhesion, and environmental conditions. (must have enough nutrients and be large enough to divide).
- Makes another copy of the DNA in the nucleus.
- Duplicates centrosomes (microtubule - organizing structures).
- Prepares for mitosis
- G2 Checkpoint: Checks for organelles, size, and mistakes in DNA [Requirement: Accumulate cyclin molecules + recycled Cdk molecules --> MPF. All needed for mitosis].
- More on G2 Checkpoint
- Cyclin - A protein that cyclically fluctuates in concentration in the cell. Activates kinases, which activates Cdks, kinases that get activated by cyclin.
- MPF [Maturation Promoting Factor] - Cyclin-CDK complex. Moves the cell past the G2 checkpoint in mitosis by adding P groups to proteins. Corresponds to peaks of cyclin concentration.
Mitotic Phase (M-Phase) is 10% of a cell's life where the DNA and cytoplasm are divided into 2 new cells. Mitosis is the stage in which the nucleus/DNA is divided equally into 2 cells. This occurs in 4 phases-
- Prophase - Chromatin (uncoiled DNA) condenses (coils) to chromosomes, nuclear membrane disappears, nucleolus disappears, centrioles move to opposite ends of cell and mitotic spindle starts to form.
- Metaphase - Chromosomes attach to the mitotic spindle and line up at the middle of the cell, or metaphase plate.
- Anaphase (prefix ana-: back) - Centromeres split and sister chromatids are pulled apart (or moving apart) and move towards the poles of the cell.
- Telophase - Chromosomes uncoil into chromatin, nuclear membrane reappears, nucleolus reappears, centrioles disappear and cleavage furrow (animal cells)/cell plate (plant cells) starts to form.
- Mitosis checkpoint: All chromosomes be attached to the mitotic spindle with the help of a kinetochore (structure of proteins; develops on centromere; attaches sister chromatids to mitotic spindle).
- Animals: The contractile ring (band of cytoskeletal fibers) contracts inwards. The indentation produced as a result of this is the cleavage furrow.
- Plants: Divide into 2 by building a cell plate down the middle.
Cytokenisis is the stage in which the cell's cytoplasm is divided. This occurs differently in plants and animals. In animals, the pinching of the membrane and cytoplasm is called the cleavage furrow. In plants, it is called a cell plate.