• Every cell in our body produced by a pre-existing cell.
• process of cell formation from pre-existing cells is known as the cell cycle.
• Involves in many repetitions of cellular growth and reproduction, nuclear division.
• Almost all the living cells undergo a cell cycle (some exceptions like RBC).
• The cell cycle is separated into two phases:
• Interphase and mitosis

Different stages of the mitotic cycle and time duration:

Part of cell cycle	Phase	Description of phase	Duration hr. (humans)
Interphase	G1	Pre-DNA-synthesis	12
	S	DNA-synthesis phase	10
	G2	Post-DNA synthesis phase	3
Mitosis	M	Mitotic phase	1

Interphase:

• Interphase of a cell cycle includes three-phase:
• G1 phase, S phase, and G2 phase.

G1 phase

• After the completion of a cell cycle, the daughter cell begins the G1 phase of the new cell cycle.
• G1 is the resting phase.
• It is named as first gap phase.
• No DNA synthesis takes place during this stage.
• It involves the synthesis of RNA.
• During this phase, chromatin is fully extended.
• G1 involves transcription of rRNA, tRNA, and mRNA.

S phase (Synthesis phase)

• Replication of DNA starts.
• Synthesis of histone protein starts.
• S phase occupies 30-35% of the cell cycle.

G2 phase

• This a second gap or growth phase.
• During this phase, the synthesis of RNA and protein.
• Restores energy.
• It takes 10-25% time of the cell cycle.

Mitosis

• In Latin mito: threads.
• During the mitosis process, the nuclear material becomes visible and appears like threadlike chromosomes.
• Occurs in Somatic cells for the growth of cell number during embryogenesis and blastogenesis in plants, and animals.

M phase is split into the following steps:

Prophase (pro: before, phasis: appearance)

• The nuclear membrane starts disappearing.
• Formation of spindle fibers.
• Each chromosome of prophase is consisting of two coiled filaments.(Chromatids).
• The region where two chromatids joined is known as Centromere.
• Formation of sister chromatids.
• Cells become spheroid, more refractile, and viscous.
• During prophase, microscopic bodies are called Centrioles.
• Causes by an increase in cytosolic Ca²⁺  

Prometaphase, Metaphase (Gr: meta: after, phasis: appearance)

• Breakdown of the nuclear envelope.
• Chromosome line up at the center of the cell.
• Spindle fiber interacts with chromosomes.
• When centrioles reach to poles of the cell, they’re surrounded by series of radiating microtubules recognized as Aster.
• The region where spindle fibers attached to the centromere acknowledged as kinetochore.

Anaphase (Gr: ana: up)

• Spindle filer pulls the chromosomes towards opposite poles.
• Splitting of the chromosomes into its sister chromatids (daughter chromosomes).
• Each daughter chromosomes having one kinetochore.

Telophase

• Chromosomes finally reach the opposite poles.
• Nuclear envelope reforms.
• The disappearance of spindle fibers.

Cytokinesis

• Process of the division of cytoplasm.
• Formation of two separate cells.
• Cytoplasm divides by a process recognized as Cleavage.
• puckering and furrowing of the plasma membrane is the first sign of cleavage in an animal cell.
• Cell pinch apart in animal cell and plant cell plate become the new cell wall
• In-plant cell cytokinesis occurs by other different processes due to a rigid wall.
• Cleavage doesn’t occur in plant cells.
• Formation of the Daughter cell.
• Daughter cells have a full set of chromosomes which is identical to the mother cell.

QUESTIONS:

• The number of centrosomes in animal cells?
• Purpose of mitosis?
• The process of nuclear division is known as?