• Unicellular organisms do it to make new individuals
• Multicellular organisms do it to grow, repair, and maintain-
– there are about 10,000,000 mitotic cell divisions/
sec in the human body!

• PROKARYOTES use binary fission :
– DNA attaches directly to membrane
• DNA replicates and attaches to different points on membrane
• Plasma membrane grows between these two points
• A cross wall begins to form between the DNA

• Use Mitosis and Meiosis
– Nuclear contents (DNA) are copied
& divided
– Cell splits into two by cytokinesis
– The process is complex in eukaryotes:
• they are big and contain many organelles
• they contain 700x the DNA of Proks
• they contain many chromosomes
(not just one)

– cell division
– growth
– DNA replication
– preparation for the next division

• Why is the cell cycle called a “cycle”?
– It represents recurring events
• Why do you think that it is important for a cell to grow in size during its cell cycle?
– If a cell did not grow in size, each cell division would produce progressively smaller cells
• What might happen to a cell if all events leading up to cell division took place as they should, but the cell did not divide?
– The cell would grow increasingly larger--- to a point at which the cell could no longer exchange materials with the environment efficiently enough to live.
– Animation

• M Phase-
– mitotic phase
– We’ll talk about this at length next week.
• Interphase-
– majority of life cycle, preparation for cell reproduction

• G
1
- Gap
1
Phase-
– cell grows and carries out normal metabolism
– organelles duplicate
– some cells stay here ex. muscle & nerve cells
• S (synthesis) Phase
– all DNA in chromosomes replicates
• G
2
– Gap
2
Phase
– Cell grows and preps for mitosis
– Animation

• Label the cell cycle diagram on page 3 of your packet and complete worksheet on page 4.

• Cyclins-
– proteins that regulate the progression through the cell cycle by activating kinases (enzymes that transfer a phosphate group from ATP to other enzymes) so that reactions can happen
• G
1 cyclins-
– Accumulate in late G
1 peak during S phase and reach a
• Mitotic cyclins-
– Accumulate after DNA synthesis until they peak at metaphase
Cyclins and kinases work together to regulate events of the cell cycle
Cell Cycle Checkpoints
Consists of proteins that detect mistakes and damage. They halt the cell cycle until repairs are made
(cell cycle arrest) animation

• CANCER:
– Occurs when checkpoint controls are damaged and cell repeatedly divides forming a tumor
– Video
• Proto-oncogenes-
– signal the nucleus to promote growth and cell division, but can be mutated to become oncogenes.
• Oncogenes –
– are cancer genes. They signal cells to leave G
0 and divide whether or not they are signaled.
• Tumor Supressors-
– are genes that encode the checkpoint proteins.
These can be inactivated by mutations.
NIH video