The cell cycle

A human being started as one single cell. Today they have tens of trillions, and replace millions every second. All of them come from successive divisions.

Three reasons to divide

GROWTH       : going from a fertilised egg to a complete organism
RENEWAL      : replacing worn-out cells
               (skin: 28 days, intestine: 4 days, red blood cells: 120 days)
REPAIR       : healing a wound

In an adult, growth is over but renewal never stops: about two million cells are produced every second.

The cell cycle

A cell spends only a small part of its life dividing. The rest of the time it works and prepares.

                    ___________
                  /             \
                 |   INTERPHASE  |          about 90 % of the time
                 |               |
       G1 -------|-> S ------> G2|
       (growth)  | (replication) |
                  \_____________/
                        |
                        v
                     MITOSIS           about 10 % of the time
                    (division)

G1: the cell grows and makes its proteins
S : the DNA is REPLICATED — each chromosome is copied identically
G2: final checks before division
M : the division itself

Phase S is decisive: without it, each division would halve the amount of DNA. It is because DNA is copied beforehand that both daughter cells can each receive a full set.

The chromosome, before and after replication

BEFORE replication (G1)         AFTER replication (G2)

      |                              |     |
      |                              |     |
      |          ------>             +--●--+     <- centromere
      |                              |     |
      |                              |     |
  1 chromosome                  1 chromosome
  with ONE chromatid            with TWO IDENTICAL chromatids

This is a point of vocabulary that trips everyone up: after replication, the number of chromosomes has not changed. It is the amount of DNA that has doubled, each chromosome now carrying two attached copies.

Checkpoints

The cycle has checkpoints that verify things before allowing it to continue:

end of G1: is the cell big enough? is the DNA intact?
end of G2: is replication complete and correct?
in mitosis: are the chromosomes properly attached before being separated?

If damage is detected, the cycle halts for repair — or the cell triggers its own suicide, apoptosis.

Cancer is precisely the failure of this system: a cell that divides without control, ignores stop signals and refuses apoptosis. The gene p53, nicknamed "guardian of the genome", is mutated in about half of human cancers — it codes for exactly the protein charged with halting the cycle when DNA is damaged.

Summary

• Cells divide for growth, renewal and repair.
• The cell cycle alternates interphase (G1, S, G2) and mitosis.
• In phase S, DNA is replicated: each chromosome goes to two identical chromatids.
• The number of chromosomes does not change during replication; the amount of DNA doubles.
• Checkpoints verify DNA integrity; their failure is central to cancer.