What are cell cycle and cell division

We are aware that a group of cells forms an organism. You must wonder how this single cell helps in the formation of an organism. Moreover, how does this organism divide and grow into an adult and reproduce? Read on to understand the concepts of cell cycle and cell division.

The Cell 

The cell is the fundamental building block that forms the functional and structural unit of a living organism. Similar cells combine to form an organ and tissue of an organism. Several organelles are present in a cell, such as the cytoskeleton, cytoplasm, Golgi apparatus, endoplasmic reticulum, mitochondria, lysosomes, nucleus, ribosomes, plasma membrane, etc.

Cell Cycle

The cell goes through several events resulting in the duplication of cells and DNA. This cycle is known as the cell cycle. It also includes the division of organelles and cytoplasm. The end product of this cycle forms two daughter cells. The cell cycle includes the growth stages and replication of DNA and division that results in two genetically identical cells. There are two major phases of the cell cycle:

• Interphase
• Mitotic phase

Human cells take 24 hours to complete one cycle of division and growth. Overall, this cycle is responsible for the division and production of new cells. The main reason for the growth of an entire population is the cell cycle. For example, many cell divisions occur simultaneously when a baby grows into an adult. There are millions of cells in a fully grown adult. These can also include cells that are harmful, like cancer cells. 

Interphase

The interphase is the resting phase of the cell cycle. During this phase, the cell prepares itself for further phase division. In the interphase, the cell grows, and DNA replication occurs. 95% of the time in the overall cycle is spent in the interphase. The interphase is divided into three phases: G1, S, and G2.

The G1 or Gap 1 phase occurs between mitosis and before or starting of replication of genetic material. During G1, the cell is metabolically active and continues growth without DNA replication.

DNA replication occurs in the synthetic phase or S phase. It replicates in multiples of 2. If the initial quantity of cells of DNA is 2N, then, by the end, it becomes 4N. The cells with centrioles also double after division, however, the chromosome number does not vary. These remain the same from the G1 phase until the end of the S phase. 

In the G2 or Gap 2 phase, proteins, macromolecules, RNA necessary for the multiplication of spindle formation, and cell organelles grow, and the cell prepares and grows for the mitotic phase.

Mitotic Phase

Cell division that is important for repair and growth is called mitosis. The M or mitotic phase is a multistep process. It is also known as equational division, as the number of chromosomes of daughter cells is equal to the parent cell. In this phase, the chromosomes go through a series of steps, including alignment, separation, and moving towards opposite poles at the final formation of two daughter cells. 

The first step is mitosis in the mitotic phase, which includes nuclear division. The second step is cytokinesis. Cytokinesis includes the separation of cytoplasmic components into two daughter cells. There are four stages in the mitotic phase:

• Prophase
• Metaphase
• Anaphase
• Telophase

At times, the prophase is divided into early prophase and prometaphase. During mitosis, the DNA of a cell condenses into chromosomes, which are set apart with the help of the mitotic spindle. Mitosis helps replace cells that wear out with time with new ones. 

Cytokinesis

The cytoplasm of the cell splits into two, forming new cells. Cytokinesis, the physical process of cell division, starts as mitosis ends. Cytokinesis is different in plant cells and animal cells. This difference is due to the rigidity of the cell wall and high internal pressure.

Cytokinesis in Animal Cells

In animals, cell division happens once a band of cytoskeletal fibres known as the contracted ring contracts inward and pinches the cell in two, in a method known as the contracted cytokinesis process.

Cytokinesis in Plant Cells

Plant cells tend to be much stiffer than animal cells. They are covered by a rigid cell wall and have internal pressure that is high. Due to this, cells divide into two by building a new replaced structure in the centre of the cell. This structure, referred to as the cell plate, is formed from the cell wall and plasma membrane parts delivered in vesicles and results in the partition of cells into two.

Conclusion

Human cells take 24 hours to complete one cycle of division and growth. There are two major phases of the cell cycle, the interphase and the mitotic phase. During mitosis, the condensation of chromosomes and the assembly of the spindle occurs. Cytokinesis, the physical process of cell division, starts as mitosis ends, dividing the parental cell’s cytoplasm into two daughter cells.