1. The Cell Cycle
How Cells Divide
and Grow

2. Humble Beginnings
 Every living thing on
earth starts as a
single cell

3. All Cells Have a “Life Cycle”
• Includes periods of growth, rest & replication
• Can be short (minutes) or long (years)
• Different cell types have very different cycles

4. Cells Divide for Different Reasons
• To replace
old/worn out
cells (red blood
cells)
• To repair injured
tissues or
combat illness
• To reproduce
(create another
entire organism)

5. Three Types of Cell Division
 Binary Fission
• Used by prokaryotes (unicellular: bacteria)
• Fast & simple: every 20 – 30 min.
 Mitosis
• Used by eukaryotic, somatic cells (body)
• Much more complex
• Highly regulated
 Meiosis
• Even more complicated (cells divide twice)
• Used by eukaryotic germ cells (reproductive)

6. Binary Fission in Prokaryotes
 Bacteria only have 1 chromosome
• A single strand of DNA
• Circular in shape

7. Binary Fission in Prokaryotes
 DNA duplicates
• Two circular DNA strands move apart
• Cell doubles in size

8. Binary Fission in Prokaryotes
 DNA moves to opposite ends of cell
 Cell membrane grows inward
 Cell wall grows from membrane materials &
pinches cell in two

9. Binary Fission in Prokaryotes
 Two new cells are formed
• Identical genetically (same DNA)
• Cytoplasm contents vary

10. Types of Cell Division
 Binary Fission
• Used by prokaryotes (unicellular: bacteria)
• Fast & simple: every 20 – 30 min.
 Mitosis
• Used by eukaryotic, somatic cells (body)
• Much more complex
• Highly regulated
 Meiosis
• Even more complicated
• Used by eukaryotic germ cells (reproductive)

11. Cell Cycle in Eukaryotes
 Interphase
• Longest stage
• 90% of cell’s life
• G - no division
0
• 3 checkpoints:
• G – gap 1
1
• S - synthesis
• G – gap 2
2
 Mitosis
• 4 stages
• Cytokinesis completes
the cycle

12. Interphase – G1 (1st Gap or Growth phase)
 Cell Growth
• Organelles replicate
• Cell volume doubles
 Cell does its job
• Normal metabolic
processes
 Longest phase
 Some cells stay here
permanently
 G1 checkpoint
• Is cell big enough?
• Environment good?

13. Interphase – S (Synthesis phase)
 Chromosome
Replication
• Loose bundles of
chromatin
• All DNA in nucleus
copied resulting in…
• Two identical strands
of each chromosome
• Sister chromatids
joined by centromere

14. Interphase – G2 (2nd Gap or Growth phase)
 More growth
• Cell volume
increases
 G2 Checkpoint
• DNA mistakes?
• Cell volume OK?
• Environment
good?
 Shortest phase

15. Mitosis
4 Stages:
• Prophase
• Metaphase
• Anaphase
• Telophase

16. Mitosis - Prophase
 DNA coils & condenses (like
thread on a spool) to form
distinct chromosomes
 Nuclear membrane disappears
 Spindle fibers form out of
centrosomes (2 centrioles ea.)

17. Metaphase
 Spindle fibers attach to
kinetochores on the
centromeres
 Chromosomes line up
at the equatorial plane
(middle) of the cell
 Mitotic spindle
checkpoint
• Are all chromosomes
lined up?
• Are they all attached
to spindle fibers?

18. Anaphase
 Spindle fibers shorten, pulling
sister chromatids apart
 Chromatids are drawn to
opposite ends (poles) of the cell
 Centromere connections are cut

19. Mitotic Spindle
 Microtubules of the spindle
attach to kinetochores on
?
chromosomes
 Sister chromatids are
pulled apart by
?
spindle fibers
 Other unattached spindle
fibers push against one
another to elongate
(stretch out) the cell

20. Telophase
 Chromosomes reach
the poles
 New nuclear
membranes form
around chromatids
 Chromosomes
unwind to become
thread-like chromatin
again
 Nucleolus forms

21. Cytokinesis
 The cytoplasm is
evenly divided
 New cell membrane
forms in animal cells
 New cell wall is built
across for plant cells

22. Cytokinesis in Animal Cells
 Microfilaments assemble around cell middle
 Actin& myosin
microfilaments
form a ring
around cell
 ATP (energy)
used to
squeeze cell
in half

23. Cytokinesis in Plant Cells
 Rigid cell wall can’t just be squeezed in half
 Golgi apparatus sends
vesicles filled with cell
wall building materials
to center
 Cell plate forms at
equatorial plane
 Cell wall fills in gaps,
cleaves cell in two

24. Interphase – G0 (pre-growth or resting phase)
 Mitosiscreates two
identical daughter
cells that go on to
enter Interphase
• Some enter G 1 and
divide again
• Some enter G0 - a
rest phase where
they do their job

25. Mitosis summarized
Eukaryotic
(plant &
animal)
Somatic (non-
reproductive)
Regulated by
checkpoints
PMAT:
Prophase
Metaphase
Anaphase
Telophase

26. Week 11 Lab
Review
Microscope Lab: onion root tip mitosis

34. Cell Cycles vary by Type of Cell
 Nerve, liver & muscle cells
• Most stay in G0 (never divide)
• Some divide only after years, if
there are injuries
• Brain & spinal chord injuries are
often permanent because of this
 Skin, hair, eye, cheek
& intestinal lining cells
• Exposed to more wear
& tear than other cells
• Replaced every 1 - 35 
Red blood cells (hemoglobin)
days
• Most rapidly dividing • Formed from bone marrow stem cells
cells in humans • Never divide once formed
• Die & replaced after ~ 4 months

35. Cancer - Cell Cycle out of Control
 Cancer cells
• Tumors result from rapid, out of
control cells dividing
• Bypass normal G1, G2 and/or mitotic
spindle checkpoints
• Sloppy cell division = big trouble
 Lack of quality controls
• More mutations
• Disorganized/deformed cells
• Signal proteins lost/changed
• Contact inhibition lost

36. Your Turn…
 What is binary fission? What type of cells
use this process?
 Draw and label the cell cycle…include G1,
S, G2, M and cytokinesis…what happens at
each stage?
 What are the stages of Mitosis?
 What disease is associated with unregulated
mitosis?