The Cell Cycle
Cytokinesis – cell
divides into two
DNA content = 20
G2 - Second growth phase - short
Short gap before mitosis (cell
division)
Cytoskeleton of cell breaks down
and the protein microtubule
components begin to reassemble
into spindle fibres – required for cell
division
DNA content = 40
G1 - First growth phase – longest
phase
Protein synthesis – cell “grows”
Most organelles produced
Volume of cytoplasm increases
Cell differentiation (switching on
or off of genes)
Length depends on internal and
external factors
If cell is not going to divide again
it remains in this phase
DNA content = 20 (arbitary)
S - Replication phase DNA
replication – this must occur if
mitosis is to take place
The cell enters this phase only if cell
division is to follow
DNA content = 40
G1 + S + G2 = INTERPHASE
No apparent observable activity
Before a cell divides, its chromosomes are copied exactly in INTERPHASE. This
process is called replication; ATP is synthesised – provides energy for cell division;
organelles are replicated and proteins are made
PROPHASE
The DNA of each chromosome is copied to form two chromatids (“sister”
chromosomes); chromosomes condense – becoming shorter and fatter – visible
under LM; nuclear envelope breaks down; chromosomes lie freely in cytoplasm;
centrioles move to opposite ends of the cell, forming protein (tubulin) fibres across
it called a spindle – fibres extend to the equator of the cell
METAPHASE
Chromosomes line up at the equator; the spindle fibres from each pole become
attached to the centromere of the chromosomes
ANAPHASE
The spindle fibres contract; the centromeres are split and the pairs of sister
chromatids are separated and dragged to opposite poles assuming a “V” shape – the
centromeres lead; a complete set of chromosomes is therefore found at each pole;
energy (ATP) is required
TELOPHASE
Chromatids reach their respective poles and uncoil – become thin and long again –...