Cell Division: Mitosis & Meiosis, Different Phases of Cell Cycle

The cell theory proposed by schleiden, schwann, & R.Virchow explains that, all cells arise from their pre-existing cells. Thus, cells are considered as basic functional and structural unit of life and a single cell gives rise to a complete organism.

• Cell division and cell cycle is considered as a basis for reproduction and growth in an organism. 
• A series of repeated and synchronised events that takes place during the time of cell divison is called cell cycle. 
• In living cell, mitosis and meiosis are the modes of cell division. 
• The Cell cycle divison takes place both in prokaryotes and eukaryotes.

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Cell Division Definition 

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The series of events that occur in a living cell to form daughter cells is called cell division. The process of cell division class 9 begins with the division of the genome followed by the development of cellular components that ultimately give rise to new cells.

• The new cells that develop after cell division are called daughter cells in biology. 
• Depending on the type of cell division, the number of daughter cells produced varies.
• There are two types of cell division through which a cell divides: meiosis and mitosis. 
• Identical daughter cells are the end result of mitotic cell division. 
• In case of meiosis, genetic diversity and variation is maintained.

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Cell Cycle Definition

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A bacterial cell cycle is an orderly series of events that was first introduced by Prevost and Dumas in 1824. These events occur repeatedly when a cell divides.

• In prokaryotes, the cell cycle is simple wherein a cell grows, its DNA replicates and eventually, the cell divides, on the contrary, eukaryotes have a complex mechanism.
• The cell cycle helps in the growth of cells and the process of DNA replication, subsequently producing two daughter cells.
• This process helps in the formation of a matured organism from a single-celled fertilized ovum.
• Cell cycle and cell division is important for regeneration and renewal of blood cells, internal organs, and the skin.

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Types of Cell Division

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Mitosis and meiosis are two types of class 9 cell division that play important roles in different stages of the cell cycle. The definition of mitotic and meiotic cell division is discussed in detail below:

Mitotic Phase 

Mitotic divisions are seen in plants and they usually exist in both haploid and diploid cells. M phase is also termed as equational division as the number of parent and progeny cells are seen to be identical to one another. The mitotic phase is further classified into 4 main sub-categories:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

Figure: Mitotic Cell Division 

Prophase

It marks the beginning of Mitosis, and helps in the separation of duplicated genetic material present in parent cell’s nucleus into two new and identical daughter cells.

• The complex material present in the cell called chromatin condenses during this phase.
• Formation of mitotic splindle and microtubules occur.
• The cell organelles like golgi bodies, endoplasic reticulum, nucleolus, etc, disappears.

Metaphase

In this phase, the nuclear envelope disintegrates completely and the spindle matures, whilst the chromosomes line up at the metaphase plate. 

• Two sister chromatids are present in the metaphase chromosome.
• These chromatids are attached to the splindle fibres from opposing ends at kinetochores.

Anaphase

This phase marks the splitting of the sister chromatids paired chromosomes to further divide, and continue moving to opposite poles of an individual cell.

Telophase:

Telophase is responsible for the chromosomes being segregated into two newly emerging identical daughter cells. 

Cytokinesis:

Telophase is usually followed by the process of Cytokinesis which basically refers to the segregation of the cell's cytoplasm.

• This usually initiates prior to the end of the mitosis process.
• Cytokinesis occurs differently in plant and animal cells.

Meiosis

Meiosis is basically responsible for the production of haploid gametes that further help in creating diploid zygotes.

• The zygote is typically formed by fusion that occurs due to sexual reproduction.
• Meiosis interphase is known to share a resemblance with mitosis interphase.
• In a nutshell, it is best known to reduce the number of chromosomes, thus forming haploid daughter cells.

Figure: Meiosis Cell Division

The stages of meiosis can be further divided into the following sub-categories. 

Meiosis I

• Prophase I: It is the first stage of the meiosis cell cycle and is a complex phase as compared with prophase in mitosis. Leptotene, zygotene, pachytene, diplotene, and diakinesis are the further steps in prophase I
• Metaphase I: Chromosomes that are bivalent assemble themselves with the equatorial plate. The microtubules from the opposite poles of the spindle, binds themselves to the kinetochore of homologous chromosomes.
• Anaphase I: Separation of homologous chromosomes takes place, and the sister chromatids remain unseparated.
• Telophase I: Nuclear membrane and nucleolus reappear and cytokinesis follows after this phase. 

Meiosis II

• Prophase II: This stage is less complex than prophase I, here the nuclear membrane disappears and chromosomes compact.
• Metaphase II: We can see an attachment of microtubules to the kinetochores of sister chromatids and alignment of chromosomes at the equator.
• Anaphase II: The centromere of each chromosome splits and they travel to the opposite poles.
• Telophase II: Termination of telophase results in the closure of meiosis. Here, the nuclear envelop reappears and covers the two groups of chromosomes. Finally, cytokinesis follows forming four daughter cells.

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Phases of Cell Cycle

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Cell cycle or cell division is an array of processes that occurs in a cell resulting in its maturity and subsequent division. As stated earlier, eukaryotic organisms like humans, have a complex mechanism of cell division which occurs in two stages:

1. Interphase 
2. Mitotic Phase (M Phase)

Interphase

Interphase is the first stage of cell cycle where the cell is prepared for division. During this stage, growth of cell and replication of DNA takes place. It is further divided into the following three stages:

G1 Phase

The G1 Phase or the Gap 1 Phase occurs between mitosis and DNA replication. Rapid growth of cell occurs during this phase

• During this phase, formation of organelles, synthesis of amino acids, production of vital molecular building blocks, like RNA takes place.
• If the cells do not divide for example the cardiac cells in humans, they enter into a resting phase called the G₀ phase.

S Phase

The synthesis phase or the S phase is where the synthesis or replication of DNA occurs.

• In this stage, DNA replication results in the formation of daughter cells.
• Therefore, at the end of this stage, the quantity of DNA in each cell has doubled. 

G2 Phase

This is known as the Gap 2 Phase and is the last stage of interphase where the cell is prepared to enter mitosis.

• During this phase, organelles, RNA, proteins, and other components are formed necessary for cell division and mitosis. 

Figure: Cell Cycle

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Things to Remember

• An array of events that helps a cell to replicate its genome, consequently dividing into two new daughter cells is known as cell cycle. 
• It is mainly composed of three stages: G1 phase, S Phase, and G2 phase.
• The multiplication of a parent cell into two or more new daughter cells is called cell division
• Formation of microtubules in G2 phase helps in splindle formation.
• The cells that do not divide in G1 phase enters the quiescent stage.