CELL CYCLE
Cell Cycle
How the Cell Cycle Works
Cell cycle
• The period between successive mitoses is
known as the interphase of the cell cycle.
• In rapidly dividing cells this lasts for between
16 and 24 hours.
• Interphase start with the G1 (G = gap) phase
during which the chromosomes become thin
and extended.
• This phase of the cycle is very variable in
length and is responsible for the variation in
generation time between different cell
populations.
The period between successive mitoses is known as the interphase of the cell cycle (Fig. 3.18). In rapidly dividing cells this lasts for between 16
and 24 h. Interphase commences with the G1 (G = gap) phase during which the chromosomes become thin and extended. This phase of the cycle
is very variable in length and is responsible for the variation in generation time between different cell populations. Cells which have stopped
dividing, such as neurons, usually arrest in this phase and are said to have entered a non-cyclic stage known as G0.
Cell cycle
• Cells which have stopped dividing, such as neurons,
usually arrest in this phase and are said to have
entered a non-cyclic stage known as Go.
• The G1 phase is followed by the S phase (S =
synthesis), when DNA replication occurs and the
chromatin of each chromosome is replicated.
• This results in the formation of two chromatids which
give each chromosome its characteristic X-shaped.
• Interphase is completed by a relatively short G2
phase during which the chromosomes begin to
condense in preparation for the next mitotic division.
The G1 phase is followed by the S phase (S = synthesis), when DNA replication occurs and the chromatin of each chromosome is replicated. This
results in the formation of two chromatids which give each chromosome its characteristic X-shaped configuration. The process of DNA replication
commences at multiple points on a chromosome (p. 15).
Usually homologous pairs of chromosomes replicate in synchrony. However, one of the X chromosomes is always late replicating. This is the
inactive X chromosome (p. 99) which forms the sex chromatin, or the so-called Barr body, which can be visualized in interphase in female somatic
cells. This used to be the basis of a rather unsatisfactory means of sex determination based on analysis of cells obtained by scraping the buccal
mucosa - a 'buccal smear'.
Interphase is completed by a relatively short G2 phase during which the chromosomes begin to condense in preparation for the next mitotic
division.
Cell cycle
Cell Cycle
Figure 3.18 Stages of the cell cycle. G1 and G2 are the first and second 'resting' stages of interphase. S is the stage of DNA replication. M = mitosis.
Cell cycle
Cell cycle
Cell cycle
State
quiescent/
senescent
Interphase
Cell division
Phase
Description
Gap 0
A resting phase where the cell has left the cycle and has stopped
dividing.
Gap 1
Cells increase in size in Gap 1. The G1 checkpoint control mechanism
ensures that everything is ready for DNA synthesis.
Synthesis
(S)
DNA replication occurs during this phase.
Gap 2
During the gap between DNA synthesis and mitosis, the cell will continue
to grow. The G2 checkpoint control mechanism ensures that everything
is ready to enter the M (mitosis) phase and divide.
Mitosis
(M)
Cell growth stops at this stage and cellular energy is focused on the
orderly division into two daughter cells. A checkpoint in the middle of
mitosis (Metaphase Checkpoint) ensures that the cell is ready to
complete cell division.
Cell Cycle
Control of the Cell Cycle