Cell cycle. Cell division. Mitosis. Endomitosis. Amitosis

The cell cycle is the series of events that takes place in the cell as it grows and divides. Its duration is 15 to 20 h and it has 2 phases: mitosis, the M phase, lasting 1 h, and interphase, everything else.

Interphase is G₁, presynthetic, about 10 h and the longest period; S, synthetic, 8 h, when DNA is replicated; and G₂, postsynthetic, 2 to 4 h, preparing for mitosis. G₀ is a specialised resting stage in which cells exit the cell cycle, permanently in neurons and heart muscle cells.

Three checkpoints halt progression until conditions are favourable: G₁, G₂ and M.

Mitosis has four stages: prophase, metaphase, anaphase, telophase, followed by cytokinesis. Endomitosis and amitosis are two special forms.

The cell cycle is a series of events that takes place in the cell as it grows and divides.

• Its duration is 15 to 20 h.
• It has 2 phases: mitosis, the M phase, lasting 1 h, and interphase.

G₀ phase is a specialised resting stage, an inactive stage in which cells exit the cell cycle when they are not actively preparing to divide.

It is usually a temporary stage, but some cells remain permanently: neurons and heart muscle cells.

That permanence has a consequence worth carrying forward. A tissue whose cells sit in G₀ for good cannot replace what it loses, which is why damage to the heart and to nerve tissue does not heal the way damage to skin does.

Checkpoints halt progression of the cell cycle to the next stage until conditions are favourable. There are three.

The G₁ checkpoint evaluates damage to DNA and other external factors. If conditions are unfavourable, the cell is not allowed to continue to S phase.

The G₂ checkpoint ensures all chromosomes have been replicated, and that the replicated DNA is not damaged, before entering mitosis.

The M checkpoint controls whether all sister chromatids are correctly attached to spindle microtubules before entering the irreversible anaphase stage.

The word irreversible is what makes the M checkpoint matter. Once anaphase begins there is no undoing it, so everything has to be verified beforehand.

Cyclin is any of a group of proteins that regulates the cell cycle by forming a complex of kinases.

Cell division is the process in which a parent cell divides into two or more daughter cells.

Mitosis, the M phase, is the cell cycle of somatic cells. It has four stages.

• Rupture of the nuclear envelope.
• Replicated chromatin condenses into chromosomes.
• 2 centrioles duplicate, get separated and move to opposite poles, to organise the mitotic spindle.

• Chromosomes condense further, becoming much thicker.
• They are arranged in the equatorial plane, and the cell is now more spherical.
• Nucleolus and nuclear envelope disappear.
• The spindle is completed and binds to kinetochores.

Sister chromatids are pulled toward opposite spindle poles by microtubules.

Reconstruction.

• Chromosomes revert to an uncondensed state.
• Microtubules depolymerize.
• Nuclear envelope and nucleolar formation.

A belt-like contractile ring of actin filaments with myosins develops and creates a cleavage furrow.

It progresses until the cytoplasm and its organelles are divided into two daughter cells, each with one nucleus.

Endomitosis is a special form of mitosis, seen in megakaryocytes.

• Replication of chromatids without cell division.
• Chromosomes separate without spindle apparatus.
• Result: a big cell with a lot of chromatids.

Amitosis is a special form of mitosis, seen in osteoclasts.

It is cell division by division of cytoplasm without spindle formation, without the appearance of chromosomes, and without breakdown of the nuclear envelope.

The two special forms are opposite failures of the ordinary process: endomitosis copies the chromosomes without dividing the cell, and amitosis divides the cell without properly handling the chromosomes.