THE CELL CYCLE

• The cell cycle is the regular sequence of events that takes place between one cell division and the next. 
• Cell cycle consists of two major phases namely

1. Interphase 
2. Mitotic phase (M phase).

INTERPHASE

Interphase is the time when the metabolic activity of cell is very high, as it performs its various functions. It is further divided into three phases namely;

1. G1 (first gap) phase
2. S (synthesis) phase
3. G2 (second gap) phase

G1 PHASE

• After its production, a cell starts its cell cycle in G1 phase.
• During this phase, cell increases the number of its organelles (such as mitochondria, ribosomes etc) and grows in size. 
• This phase is also marked by the synthesis of various enzymes that are required in the next phase i.e. S phase for the duplication of chromosomes.

G0 PHASE

• In multicellular organisms, cells enter G0 phase from G1 and stop dividing.
• Some cells remain in G0 for indefinite period e.g. neurons.
• Some cells enter G0 semi-permanently I.e. some cells of liver and kidney.
• Many cells do not enter G0 and continue to divide throughout an organism’s life i.e. epithelial cells.

S PHASE

• In this phase, cell duplicates its chromosomes.
• As a result, each chromosome consists of two sister chromatids.

G2 phase

• In the G2 phase, cell prepares proteins that are essential for mitosis, mainly to produce spindle fibres.

• During G2 the cell continues to grow and new DNA is checked and any errors are usually repaired.
• Preparations are also made to begin the process of division. For example, there is a sharp increase in production of the protein tubulin which is needed to make microtubules for the mitotic spindle.
• After the G2 phase of interphase, cell enters the division phase I.e. M phase.

MITOTIC PHASE

• M phase is characterized by mitosis, in which cell divides into the two daughter cells.
• Nuclear division occurs in mitotic phase.
• mitotic phase is a relatively short period of cell cycle. 
• It alternates with much longer interphase, where cell prepares itself for cell division

The length of the cell cycle is very variable, depending on environmental conditions and cell type. 

• On average, root tip cells of onions divide once every 20 hours; epithelial cells in the human intestine every 10 hours.
• In animal cells, cell division involves constriction of the cytoplasm between the two new nuclei, a process called cytokinesis.
• In plant cells, it involves the formation of a new cell wall between the two new nuclei.

APOPTOSIS AND NECROSIS
Apoptosis and necrosis are two phenomena of cell death. 

APOPTOSIS

• Apoptosis is one of the main types of programmed cell death.
• During apoptosis, cells shrink and becomes rounded due to the breakdown of cytoskeleton by enzymes.
• Its chromatin undergoes condensation and nuclear envelope breaks.
• In this way, nucleus spreads in the form of several discrete chromatin bodies.
• Cell membrane makes irregular buds known as blebs.
• Blebs break off from the cell and are now known as apoptotic bodies, which are then phagocytosed by other cells.
•  Apoptosis occurs when cell is damaged or undergoes stress conditions.
• Apoptosis removes the damaged cell, preventing it from getting further nutrients or to prevent the spread of infections.
• Apoptosis also gives advantages during development for example during the formation of fingers; the cells between them undergo apoptosis and the digits separate.

NECROSIS

• Necrosis is the accidental death of cells and living tissues. Necrosis is less sequential than apoptosis. 
• There are many causes of necrosis including injury, infection, cancer etc. 
• Necrosis may occur when a cell is given hypoxic (with less oxygen) environments.
• During necrosis, there is a release of special enzymes from lysosomes. Lysosomal enzymes break cellular components and may also be released outside cell to break surrounding cells.
• Cells that die by necrosis may also release harmful chemicals that damage other cells.