Cell Cycle: The cell cycle was
proposed by Mazia in the 1960s
based on data from DNA synthesis of
cultured cells.  The cycle
represents a pie chart of a clock
for a dividing cell. The S
standsfor DNA synthesis or
replication.  This is the time in
the life of a dividing cell when it
produces more DNA.  DNA synthesis
is very rapid in eukaryotes because
of the multiple origins of
replication.  The rate is about
3 kb/min.  The average length in
human DNA is about 3 billion bases. 
Over a period of several hours the
whole genome is replicated.  The
time it takes depends upon many
factors.  In adult fruit flies it
takes 10 hrs while in embryonic
flies only 5 min. The M for
mitosis. This is when the
chromosomes separate and daughter
cells are formed.  Note that there
is some time between these 2
phases.  Mazia realized this and
referred to them as gaps.  There
was G1, or gap 1, between mitosis
and the next synthesis of DNA and
G2, or gap 2, between synthesis and
mitosis.  It was later determined
that during G1 each of the daughter
cells were growing in volume, up to
the original size of the mother
cell, before the start of the next
round of synthesis.  Many books,
therefore, talk about the G in G1
standing for GROWTH.   During early
cell division in embryos there is
almost no G1.  Cells keep getting
smaller with each round of
division, but the rate of cell
cycle is shorter so more divisions
occur.  More recent evidence
suggests that G1 is very complex.
There are several substages in
adult cells.   During the time that
histone are made and, cells prepare
for S, G2 is relatively constant;
it is this time that it takes to
prepare the cell for mitosis.   All
parts of the cycle except M are in
INTERPHASE.  The rest of the part
of mitosis ( prophase, metaphase,
anaphase and telophase) occur
during M.  If the entire cycle is
24 hr the G1 phase is about 9 hrs
(in adults) , the S about 9 , G2
about 5 hrs and M about 1 hr.  A
special phase was added to the cell
cycle to explain cells that do not
divide all the time.  This phase is
called G0.  Cells like neural cells
and muscle cells seldom divide, 
they are in G0, a state of
differentiation.  When these cells
become cancerous they enter the
cell cycle.  We do not know
completely how this occurs. 
However the genes responsible for
this rentry are often natural
GROWTH FACTORS which for some
reason get out of control and
become ONCOGENES ( GENES THAT CAUSE
CANCER ).   Many oncogenes have
been identified whose natural gene
( protoncogene ) has a role in
development.  Over the next few
years scientists may discover how
these genes work and methods to
keep them from causing tumors.  The
cell cycle is important because it
explains why cells do different
things at different times in their
life cycle.  It allows us to
compare cells at the same stage and
better understand development,
cancer and aging.  An interesting
protein has been identified as a
controlling factor in going from
Interphase to M.  It is called
CYCLIN.  The level of active cyclin
increase to start M.  The addition
of phosphate groups to cyclin may
be what activates this protein. 
When cyclin is deactivated the cell
returns to interphase.  