Cell Division: Mitosis and the Cell Cycle Explained
The Cell Cycle: A Detailed Explanation
Role Play: This is the period of time from when a cell divides until it gives rise to other cells, encompassing all processes within that timeframe.
Interface: The Longest Stage
The interface is the longest stage, featuring the cell nucleus and consisting of three phases:
G1 Phase
During this stage, mRNA synthesis, and therefore protein synthesis, takes place. The cell contains a Diplosoma. Towards the end of this phase, there’s a point of no return, called the R point or point of restriction. If the cell reaches this tipping point, it inevitably continues. Some cells, before reaching the R point, express certain genes and become specialized, remaining for some time without progressing to the R point. This is known as entering the G0 phase, from which they can return to the G1 phase and reach the R point. Some cells, like neurons, remain detained in the G0 phase and never proceed to divide. In a 24-hour cell cycle, this phase lasts approximately 11 hours.
S Phase
DNA replication occurs, along with the synthesis of mRNA and proteins, particularly histones. Next to each centriole, another called a procentriole begins to form. This phase lasts about 8 hours in a 24-hour cycle.
G2 Phase
The synthesis of mRNA and protein continues, including H1 and histone proteins that form the mitotic spindle microtubules. The cell has two still immature Diplosoma, taking about four hours in a 24-hour cycle.
Cellular Division or M Phase
A phase where a stem cell gives rise to two daughter cells with the same chromosome number. It distinguishes the division of the nucleus (mitosis) and cytoplasmic division (cytokinesis). In a 24-hour cycle, this phase lasts from one to two hours.
Mitosis: Nuclear Division Explained
Mitosis: A process of nuclear division that occurs when cells need to generate the same number of chromosomes as the parent cell. In unicellular organisms and some simple multicellular organisms, this kind of division is asexual reproduction. However, in multicellular organisms, it’s involved in individual growth and tissue regeneration. Successive mitotic divisions occur from the zygote. Therefore, all of the individual’s cells have the same genetic information. Cells appear different because of cell differentiation processes, where each cell type expresses different genes.
Four Phases of Mitosis
Prophase
Starts when the chromatin condenses and chromosomes with two chromatids become visible; the nucleolus disappears. The two immature Diplosoma are surrounded by pericentriolar material, which acts as a microtubule organizing center. These are surrounded by aster fibers, forming the aster or centrosphere. The Diplosoma with pericentriolar material is the centriolar complex. Polar fibers or continuous fibers form between the two asters, extending them so the two asters are located on opposite sides in the two poles of the cells, forming the mitotic spindle. We now have two pairs of mature centrioles. In late prophase, the core swells due to water entry until the nuclear envelope fragments. The kinetochore forms around the centromere in chromosomes, and microtubules form from the kinetochore. Microtubules separate the chromosomes to mix with the mitotic spindle and orient the chromatids.
Metaphase
The kinetochore microtubules elongate, positioning the chromosomes at the central plane of the cell to form the equatorial plate.
Anaphase
This is when the separation of sister chromatids occurs, so each chromosome has one chromatid. Microtubules pull the chromatids to each pole. At the end of this phase, kinetochores disappear.
Telophase
Begins when the two sets of chromosomes are at the two poles of the spindle apparatus. Nuclear envelopes form from the saccules of the endoplasmic reticulum and remnants of the mother cell’s nuclear envelope. The chromosomes desespiralize, and nucleoli reappear. The polar microtubules are near the interzone, the area between the two chromosome bodies. Dense substance cylinders also form in the interzone. Cytokinesis takes place in telophase.