The cell cycle is a series of ordered events involving the growth of the cell and cell division, producing two new daughter cells. All cells reproduce by dividing into two more new daughter cells from each parent cell. 

Cell division

Cell division is a very important process in all living organisms, and this repeating cell division process is called the cell cycle. During the cell division, the growth of the cell and DNA replication occur. This ordered event of dividing the cells and forming two new cells is called a cell cycle.

The cell cycle is classified into two phases which are:

1. Interphase

2. Mitosis phase or M phase

Interphase: The interphase is the time during which the cells are preparing for cell division by undergoing cell growth and replication of the DNA in a proper sequence. The interphase is divided further into three other phases:

• G1 Phase (Gap 1)

• S Phase (synthesis)

• G2 phase (gap 2)

G1 phase 

During the G1 phase, the cells are metabolically active and continuously grow and increase in size but do not replicate the DNA. The G1 stage takes place between mitosis and DNA replication.

S Phase (Synthesis)

This is a phase or period where replication or synthesis of DNA occurs. During the Synthesis period, the amount of DNA per cell will double in this phase.

G2 phase

During this phase, there is a gap between mitosis and DNA synthesis, the cell will continue to grow, and proteins are synthesized to make sure that everything is ready to enter the mitosis stage.

Some cells in the organisms do not seem to exhibit cell division because of injury or cell death. The cells that do not divide exit the G2 phase to enter an inactive stage called the Quiescent stage also called the G0 phase of the cell cycle.

M Phase 

It is the phase where the cell growth stops, and the cell energy will be distributed into two new daughter cells. After cell division, each new daughter cell will start the interphase of the new cycle making another cell cycle. 

The mitosis will be divided into four stages of nuclear division, also called karyokinesis. The karyokinesis is further divided into four stages, which are:

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Let us know more about each stage of karyokinesis:

Prophase

The first stage of karyokinesis or mitosis is called prophase. It is a process that will separate the duplicated genetic material present in the parent cell into two identical daughter cells. The new DNA molecules formed are intertwined. The prophase is that whenever chromosomal material commences condensing. During the chromatin condensation process, the chromosomal material becomes untangled. The centrosome reproduced during the S phase of interphase begins to move towards the cell’s opposite poles. Prometaphase, the second phase of mitosis, follows prophase.

Metaphase

This is the stage at which the morphology of chromosomes can be studied the most easily. The metaphase chromosome comprises two sister chromatids held together by the centromere at this stage. Kinetochores are small disc-shaped structures on the surface of centromeres. The spindle fibers are attached to the chromosomes moved into position at the cell’s center by these structures. As a result, all of the chromosomes come to lie at the equator during metaphase, with one chromatid of each chromosome connected to spindle fibers from one pole and its sister chromatid connected to spindle fibers from the opposite pole by its kinetochore.

Anaphase

Each chromosome on the metaphase plate is split into two daughter chromosomes at the beginning of anaphase. Furthermore, chromatids are identified as daughter chromosomes and are also the daughter chromosomes of a cell. The future daughter nuclei start migrating towards the mother nucleus. The two opposites, each chromosome moves away from the others. Each chromosome’s centromere is positioned on the equatorial plate. It remains primarily directed at the pole and, as a result, at the front edge, with the chromosome arms trailing behind.

Cytokinesis

Mitosis typically involves the segregation of duplicated chromosomes into daughter nuclei (karyokinesis) and the division of the cell into two daughter cells (cytokinesis), at which point cell division is complete. The appearance of a furrow in the plasma membrane in an animal cell accomplishes this. The furrow deepens over time and eventually joins in the midpoint, dividing the cell cytoplasm in half. Plant cells, on the other contrary, have a relatively inextensible cell wall, so they go through cytokinesis differently. The appearance of a simple precursor known as the cell plate is the first step in forming the new cell wall, the middle lamella, between the walls of two adjacent cells.

Meiosis

The fusion of two gametes, each with a complete haploid set of chromosomes, is considered necessary for sexual reproduction to produce offspring. Diploid cells with specialized functions produce gametes. The production of haploid daughter cells results from a technical cell division that reduces the chromosome number by half. Meiosis is the specific type of division. Meiosis is a process that takes place during gametogenesis across both plants and animals.

 Compared to mitosis, the prophase of the first meiotic division is typically longer and more complex. 

Metaphase I 

On the equatorial plate, the bivalent chromosomes align. Microtubules from complete opposites of the spindle attach to homologous chromosomes’ kinetochore.

Anaphase I 

Sister chromatids remain attached at their centromeres as homologous chromosomes separate.

Telophase I 

The nuclear membrane and nucleolus reappear, followed by cytokinesis and the formation of a dyad of cells. Even though chromosomes disperse in many cases, they do not reach the interphase nucleus’s extremely extended state. Interkinesis is a stage that occurs between the two meiotic divisions and lasts only a few minutes.

Meiosis II

Prophase II

Meiosis II begins soon after cytokinesis, usually before the chromosomes have fully extended. Meiosis II, in contrast to meiosis I, resembles a normal mitosis. By the end of prophase II, the nuclear membrane has disappeared completely. As a result, the chromosomes are compacted once more.

Metaphase II

The chromosomes align at the equator at this juncture, and microtubules from opposite spindle poles attach to the sister chromatids’ kinetochores.

Anaphase II

It tends to start with the centromeres of each chromosome splitting simultaneously, allowing them to move toward opposite poles of the cell by shortening microtubules attached to kinetochores.

Telophase II

Meiosis finally ends with telophase II, when the two groups of chromosomes are once again wrapped in a nuclear envelope; cytokinesis then takes place. As a result, a tetrad of cells, or four haploid daughter cells, is formed.

Conclusion

Meiosis is the mechanism by which sexually reproducing organisms maintain their specific chromosome number across generations, even though the process itself paradoxically results in a halving of chromosome number. It also increases the genetic variability of an organism’s population from generation to generation. In the evolution process, variations are extremely important.

Mitosis usually produces diploid daughter cells with the same genetic complement as the mother cell. Mitosis is the process by which multicellular organisms grow. The ratio between the nucleus and the cytoplasm is disrupted due to cell growth.