Overview of Cell Biology/Mitosis
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.
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:
- Gap 1 Phase (G1) - A period of growth for the cell (Mitochondria, chloroplasts, ribosomes, etc... increase in numbers).
- Synthesis Phase (S) - Replication and synthesis of DNA.
- Gap 2 Phase (G2) - Prepares for cell division.
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.
One last difference between plant and animal mitosis:
- Plants form their mitotic spindle from the "unknown".
- Animals form their mitotic spindle using "microtubules".
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.