The Cell Cycle

1
The Cell Cycle

• The Process of Cell Division

2
Purpose of Mitosis

• Growth
• Embryonic development
• Tissue repair
• Asexual reproduction

3
The Role of the Chromosome
Chromosomes are a scaffolding that hold,
carry and protect DNA
4
Chromosomes

• Exist as Chromatin for most of the cell cycle
• Unwound, string-like (like a stretched out
  Slinky)
• DNA replication and RNA synthesis occur when like
  this
• DNA in each cell is approximately 2 meters long
• Prior to cell division, Chromatin begins to coil
  and thicken (super coiling)
• Now called Chromosomes (sister chromatids)
• Initially joined all along their length by
  proteins called cohesins, later centromere
• Human somatic cells have 46 chromosomes
• Gametes have 23

5
The Cell Cycle

• 2 general periods exist Interphase and mitotic
  (M) phase
• 22-24 hour cycle
• 21 hours of Interphase (Growth)
• 1 hour of Mitosis (Division)

6
The Cell Cycle
7
Interphase

• 21 hours long
• Three Phases
• G1, S, G2
• Cells grow in size
• Carries on metabolism ATP synthesis, excretion of
  waste, new organelle synthesis, new proteins
• Chromosomes are duplicated

8
G1 (5-6 hrs long)(a.k.a.-Gap 1)

• Rapid growth and metabolic activity
• Multitudes of proteins being synthesized
• Think Magic E word

9
S (10-12 hours long)(a.k.a.-Synthesis)

• More growth
• DNA synthesis/replication

10
G2 (4-6 hrs long)(a.k.a.-Gap 2)

• Growth
• Centrosomes replicate
• In animal cells, each centrosome has 2 centrioles
• Organelle manufacturing
• Final preparations for cell division

11
G0 Molecular Control of Cell Cycle

• Different cells different cell cycles
• Cytoplasmic signals seem to be most valid theory
  of cell division (think ECM extra cellular
  matrix)
• Cell cycle control system
• Checkpoints dictate stop and go
• Can be internal and/or external signals
• G1 checkpoint seems to be most important
• Go G1, S, G2 and Mitosis
• Stop G0 (Most cells in the human body)

12
Cyclins and Cyclin-Dependent Kinases

• Protein kinases (phosphotransferase)
• Enzymes that activate or inactivate other
  proteins
• Go signal at G1 and G2 checkpoints
• Cyclins (think major milestones)
• Proteins that attach to kinases
• Concentration fluctuates during cell cycle
• Cyclin-dependent kinases (Cdks)
• Example of this relationship shown in Metaphase
  to Anaphase conversion

13
continued

• Density-dependent inhibition
• Release of growth factors or growth inhibiting
  factors
• Surface proteins seem to be the receptors and
  messengers of this phenomena
• Anchorage dependence
• Substratum is essential for cell division
• Cancer cells do not respond to these mechanisms
• HeLa cells (Henrietta Lacks, 1951)
• 20-50 cell divisions is the norm before autophagy
• Transformation forms cancer cells
• Benign Does not migrate
• Malignant affects multiple tissues/organs
• metastasis

14
Mitosis Cell Division

• Approximately 1 hour long
• Has 5 phases
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase

15
Prophase

• Chromatin thickens into chromosomes
  (supercoiling)
• Sister Chromatids visible
• Held together by a centromere
• Mitotic spindle begins to form (other microtubles
  dissociate to help form spindle)

16
Prophase (continued)

• Nucleoli disappear
• Centrioles migrate to opposite ends of the cell
• Spindle begins to form (cage-like structure,
  aster)

17
Prometaphase

• Nuclear envelope fragements
• Microtubles (spindle makes its way towards
  nuclear region)
• Kinetochore present on each sister chromatid
• Microtubels attach to kinetochore and begin to
  loosen sister chromatids

18
Metaphase

• Longest phase of mitosis (20 minutes)
• Centrosomes at opposite poles
• Spindle arranges the chromosomes at the
  equatorial plane of cell
• Chromosomes are fully attached to micortubles at
  kinetochore

19
Anaphase

• Shortest stage
• Sister chromatids separate via cohesin separation
• Microtubles begin dissociating, moving
  chromosomes to poles
• Cell lengthens as structural microtubles lengthen

20
Telophase

• Chromatids reach opposite ends of the cell
• Nucleus and nucleolus form (nucleus via ER)
• Chromosomes lengthen into chromatin again
• Spindle breaks down
• Plasma membrane begins to separate (cytokinesis)
• Animal cells pinch (cleavage furrow)
• Plant cells form new cell wall

21
Telophase in plant and animal cells
22
Identical Offspring

• Asexual reproduction results in offspring
  identical to their parents
• Types of asexual reproduction
• Binary Fission
• Budding
• Spore Formation
• Regeneration
• Vegetative Reproduction

23
Binary Fission

• Parent cell increases in size and splits into two
  equal but smaller parts
• No parent cell is left
• Occurs in single-celled organisms amoeba,
  protozoa and certain algae
• Prokaryotes (bacteria and archea) divide via
  binary fission without mitosis
• One gene containing chromosome
• Origin of replication
• Cell elongates and separates (tubulin? Actin?)

24
Bacteria Protozoa Binary Fission
25
Evolutionary Mitosis

• Ancestral mechanisms remain intact
• Dinoflagellates Replicated chromosomes attach to
  nuclear envelope and separate with nuclear
  division
• Diatoms and Yeasts Spindle within nucleus
  separates chromosomes