Every living organism is made up of, at least, a cell. Those that are made up of single cells are said to be unicellular while those with more than one cell are said to be multicellular. Within each cell, there is an organelle that directly or indirectly in charge of controlling the activities of the cell – the nucleus. Inside this nucleus are biomolecules referred to as the genetic materials which are responsible for the transmission of traits from one generation to another in living organisms. The study of these molecules, how they create variations, and how they dictate the inheritance in organisms is what is known as genetics.
The cell cycle
According to the cell theory, all cells arose from pre-existing cells. Hence, each cell has a cycle it observes throughout its life span. Some cells within the body of multicellular organisms divide at regular intervals while others do not divide except in special circumstances. A typical cell cycle consists of two major phases, namely:
- The interphase
- The m or the mitotic phase
The Interphase
The interphase represents the interval between the end of one active division and the beginning of another. Some people refer to the interphase as the resting phase. This is largely untrue due to the fact that many activities and processes take place during the interphase. Normal growth processes take place while the cell prepares itself for the active division. It is divided into sub-phases, including
- The G0 phase: this is essentially a resting or dormant phase. Cells in this phase do not divide unless when they are called upon by the body system. Most people do not include this phase in the cell cycle.
- G1 (gap 1) phase: the cell undergoes growth and development in this phase, especially at the biochemical level. Building blocks of essential biomolecules are accumulated and the cell itself experiences an increase in volume.
- S phase: This is also known as the synthesis phase. It is a stage during which the DNA of the cell is replicated or doubled. The centrosome is also duplicated during this phase. The amount of DNA in the cell is doubled without the ploidy level changing. That is, if 2 g of DNA is present in the cell before entering the S phase, 4 g would be present after exiting the phase while the chromosome number remains the same.
- G2 (gap 2) phase: cell growth continues in this phase and protein is also synthesized. Final preparation for an active division is made in this phase.
The m phase
The m phase is referred to as the mitotic phase and consists of several steps at the end of which a cell gives rise to two daughter cells. There are two main stages in the m phase: mitosis and cytokinesis.
There are four stages in mitosis which include
- Prophase: a stage in which the nuclear membrane surrounding the cell disappears and the genetic materials within the nucleus condenses and becomes visible as chromosomes. Other organelles of the cell become fragmented and dispersed toward the periphery of the cell. Centrosomes move to the opposite poles of the cell and give off spindles.
- Metaphase: the chromosomes align at the center or the equator of the cell while the spindle fibers extend and attach to the kinetochore regions of the chromosomes. The entire structure forms what is known as the metaphase plate.
- Anaphase: sister chromatids that make up each chromosome become pulled apart by the shortening of the spindle fibers from the opposing poles. They separate at the centromere and are pulled toward the centrosome at the poles.
- Telophase: sister chromatids complete their migration to the poles and begin to condense back. The nuclear envelop forms around the condensing chromosomes and nucleosomes appear within the nuclear area.
Cytokinesis
This is simply defined as the physical separation of the cytoplasm. Remember that the entire structure of the cell is still a unit even after telophase. During cytokinesis, the cytoplasm divides to give two daughter cells after the cell components have been apportioned to each of the daughter cells. This process is different in plant and animal cells.
Conclusively, cells generally spend about 78 percent of their cycle at the interphase with the remaining 22 percent spent in active division. The rate of division varies from cells to cells and there are checkpoints (at G1, G2, and metaphase) that ensure that everything is okay for the cycle to proceed, otherwise, the cell cycle stops and the cell stops dividing.
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