Do individual cells have a life history?
The Cell Cycle
INTERPHASE
• 90% of a cell’s life
cycle
• Comprised of three
subphases
– G1
–S
– G2
G1
• Primary Activity is
Protein Synthesis and
Cell Growth.
• At end of G1, cells
reach a
RESTRICTION
POINT
• Cells Either Pass the
restriction point,
entering the S
subphase or enter G0
WHY DO CELLS EVER HAVE TO DIVIDE?
Surface Area To Volume
• As the cell increases in size, the volume (cytoplasm) of
the cell grows faster than the surface area
• With more cell volume to feed, cell is required to do more
diffusion, but can’t speed up RATE of diffusion
• Cell also cannot excrete wastes via diffusion effectively
G0
• If cells do not pass
the restriction point at
the end of G1, they
enter G0
• Cells in G0 will never
divide again
• Most somatic (body)
cells are in G0
STEM CELLS
STEM CELLS
S Phase
• Cells that DO pass
the restriction point
enter the S phase
• In S phase, DNA is
replicated
• Control over S phase
regulated by the
enzyme kinase
G2
• At the end of S
phase, cells re-enter
a period of protein
synthesis and
organelle
development
M Phase = Mitosis
• NOTE: NOT A PART
OF INTERPHASE
• If cells pass the
restriction point, they
must undergo S, G2
and finally, the
division of the
genetic material
known as MITOSIS
Mitosis (M Phase)
• To ensure a high surface
area to volume ratio (and
to reduce the need for
more DNA), the cell
undergoes mitotic division
• Mitosis is comprised of
four subphases;
prophase, metaphase
anaphase and
telophase
Early Prophase
• chromatin begins to coil
and condense to form
chromosomes
• each chromosome
appears to have two
strands (each containing
a single DNA molecule)
• each strand is called a
chromatid
• each chromatid is
attached to its sister
chromatid at the
centromere
Late Prophase
• the nuclear envelope
and nucleolus
disappear
• in cytoplasm, the
spindle apparatus
forms
• eventually the spindle
guides the separation
of sister chromatids
into the two daughter
cells
METAPHASE
• spindle grows and forms
attachments to the
chromosomes at the
centromeres
• chromosomes move to an
equatorial plate which is
formed along the midline
between the poles
• chromosomes are at their
most condensed state
ANAPHASE
• centromeres divide to
create two chromosomes
instead of a pair of attached
chromatids
• spindle fibers shorten and
the sister chromosomes are
drawn to the opposite poles
of the cell
• poles of the spindle
apparatus are pushed apart
as the cell elongates
• anaphase results in the
exact division of genetic
information to each
daughter cell
TELOPHASE
• nuclear envelopes
reassemble and surround
each set of daughter
chromosomes
• nucleoli reappear inside
the newly formed nuclei
• chromosomes
recondense in the
daughter cells to become
chromatin
CYTOKINESIS
• in animal cell, a
furrow appears
around the cell that
eventually pinches
the cell into two new
cells
• in plants, a cell plate
forms between the
two daughter nuclei
as the cell wall
divides the cell
The length of the cell cycle varies
on the species
Length of cell cycle also depends on
the cell function WITHIN an organism
Variances In Mitosis
• At any given time,
different parts of an
organism may be in
different parts of the
cell cycle
• Different organs,
tissues and structures
in the body also
perform mitosis at
different rates
Lab: Comparitive Cell Cycles In
Allium cepa
Comparative Cell Cycles in Allium cepa
• Purpose: To compare the relative
proportion of time required to complete the
cell cycle in two different regions of an
organism’s anatomy
• Two zones of root growth in Allium cepa
– Zone of elongation
– Zone of maturation
How do the lengths of the various parts of the
cell cycle (including mitotic subphases)
compare? Why are they different?
Protocol
• For each lab pair, one lab partner will assess the
zone of elongation, one will assess the zone of
maturation
• While the other partner tallies the results, the
other partner should view the required region of
the slide and classify each of the cells in the field
of view based on its place in the cell cycle
• Continue until 100 cells have been assessed
• Move the slide to the zone of maturation and
switch roles with your partner. Repeat
Analysis
• Based on your collected results, construct two pie charts
for the percentage of time spent in each of the cell cycle
phases (interphase, prophase, metaphase, anaphase
and telophase).
• In a paragraph, contrast the reasons behind the
differences. Use the following terms: G1, G2, S, G0, zone
of elongation, zone of maturation, mitosis, anaphase,
telophase, metaphase, prophase, cytokinesis,
• Explain the differences regarding the function of each
subphase and the actions in each of the subphases