The document summarizes key aspects of the cell cycle and its regulation: 1) The cell cycle involves duplication of chromosomes through the spindle and cytokinesis to split the cell into two. 2) Cytokinesis differs between animal and plant cells in how the cell divides. 3) Checkpoints at the G1, G2, and M phases determine whether the cell continues the cycle or stops, regulated by kinase and cyclin proteins. 4) External cues like growth factors, density, attachment, and disease can influence whether cells enter or continue the cell cycle.

1. 10/22/08
Lecture 19 – The Cell Cycle 2
− Spindle
− Composed of microtubules.
− Microtubules are part of the cytoskeleton.
− The single-stranded chromosome with a specific protein (associated with the kinetochore) is dragged along
the microtubules.
− The microtubule is disassembled into individual tubulin units.
− Cytokinesis
1. Animal cell
− The cell pinches into two cells.
− Involving microfilaments (actin and myosin)
2. Plant cell
− The cell wall (made of cellulose) is rigid, making pitching impossible.
− The membrane vesicles from the golgi apparatus form a cell plate at the center of the dividing cell.
− This results in two membranes.
− The cell divides into two.
− The cells secret new cell wall material.
− Control of the cell cycle
− In the cell cycle, there are check points.
− At these points, there is either a 'stop' or 'go ahead' signal.
− Three main check points are known.
1. G1 phase (has restriction point in the animal cell) – the most common check point.
2. G2 phase
3. M phase
− Two kinds of protein determine stop or go.
1. Kinase
− Attaches/Removes a phosphate from another protein.
− Changes the activity of the protein.
− Specific Kinases give the go/stop signal.
− Normally inactive.
− Has to be activated by attaching the protein, cyclin.
2. Cyclin (CDKs – Cyclin Dependant Kinase)
− MPF (M-phase promoting factor) – a CDK which gives the go signal at the G2 check point.
− Most cells in the human body that don't divide exit the cell cycle.
− Muscle and nerve cells are always in G0.
− Liver cells are normally in G0 but can exit G0 and reenter the cell cycle to divide.
− Bone marrow stem cells are continuously in the cell cycle.
− External cues to division
1. Growth factors
− PDGF (Platelet Derived Growth Factor)
− Some cell types have a receptor for PDGF.
− Leads to division.
2. Density (cell)
− If there isn't enough space, the cell will stop dividing.
3. Attachment
− Often there is a need for a substrate.

2. 4. Disease
− Ttumor formation.
− Benign: don't proliferate or spread.
− Malignant tumor: cells spread (metastasis)
5. Meiosis
− Sexual reproduction.