|The Cell Cycle & Cell Division
Mitosis is a type of cell division by binary fission (splitting in two) which occurs in certain eukaryotic cells.
Functions of mitosis:
- Mitosis generates new body cells (somatocytes) for renewal and repair.
- During development and growth new cells in the developing embryo are produced by mitosis.
The cell cycle is the life-cycle of the cell. Cells in multicellular animals, like humans, cycle between two phases:
interphase (I phase), the interval between mitosis, and mitosis (M phase) itself (including nuclear division and cell
division). Most cells are in interphase most of the time. In interphase cells perform their normal function or grow
and get ready for mitosis. Mitosis is the process of cell division in which a mother cell splits into two daughter
cells which are gentically identical copies of itself. Thus mitosis clones cells - it is a copy division. Interphase is
split into several sub-phases: in G1 the cell grows by forming new organelles. In S-phase (synthesis phase) it
duplicates (synthesises) its DNA in preparation for mitosis. In G2 the cell makes final preparations for mitiosis,
which might involve shape-changes as the cell rounds-up into a ball prior to division.
The cell cycle has very variable duration, but is ~24 h in most mammalian cells that are actively cycling. It may be
as short as 8 minutes in some fly embryos, when cells are needed to multiply very quickly. It may be as long as a
year in some liver cells, which spend most of their energies performing other functions apart from cell growth and
Interphase (I phase): cells in interphase are metabolically active, they grow and synthesise enzymes,
especially those enzymes required for DNA replication (like DNA polymerase) as they prepare for mitosis. In most
mammalian cells, interphase occupies ~18-20 h of the 24 h cycle. Interphase is subdivided into the: G1, S and
G1 and G2 are gaps or growth phases. G1 is often the longest phase (~ 10 h / 24 h) as the cells use this as an
opportunity to resume growth following mitosis. It is here that the cell prepares for DNA replication. G2 (~ 3-4 h /
24 h): a short gap between DNA synthesis and the onset of mitosis. Cells may exit the cell cycle, usually at G1
and enter a phase called G0. G0 cells may be quiescent (resting) cells, they may be busy performing other tasks
(like metabolising glucose in the liver) or their DNA might be too damaged to enable them to replicate. Terminally
differentiated cells have left the cell cycle permanently. Some differentiated cells are not in terminal stages, and
can be induced to re-enter the cell cycle. For example, damage to the liver and kidneys can induce liver and
kidney cells to re-enter the cycle in order to replace those cells that have been destroyed (regeneration). Most
cells in the nervous system, especially the central nervous system (CNS) are terminally differentiated.
S phase (~ 5-6 h / 24 h): this is the DNA synthesis phase. The cell’s DNA replicates (duplicates) during this
Is the process by which cells develop into specific cell types by taking on shapes and expressing the specific
enzymes required for their different roles within the body. E.g. cells in the embryo may develop into skin, gut,
blood, muscle or nerve cells, etc. Often differentiation occurs in stages, e.g. stem cells in the bone marrow may
undergo mitosis and some of the daughter cells become new stem cells, whilst others become differentiated into
myeloid and lymphoid stem cells, which then differentiate into the various types of blood cell: