1. THE CELL CYCLE AND MITOSIS

2. PART 1: THE CELL CYCLE

3. 3 stages of the cell cycle:
Interphase – cell grows and carries out its normal functions, replication of chromosomes
Mitosis – genetic material in the nucleus is divided equally into two new nuclei
Cytokinesis – rest of the cell divides
-each ‘daughter’ cell is identical to the parent cell

4. Cell Cycle Animation: http://www.cellsalive.com/cell_cycle.htm
The Cell Cycle
Interphase:
in between stages of dividing
G1—beginning cell growth
S—DNA duplicates (makes a copy)
DNA is in the form of chromatin
G2—cell undergoes its function
Purpose: to replenish dead or dying cells, to allow an organism to grow and develop
- results in somatic cells such as internal organs, skin, bones, blood, etc.
Cell Cycle Animation:

5. Checkpoints
regulatory points at which the cell examines internal and external cues and decides whether or not to move forward with division.
The three most important ones are:
The G1​ checkpoint: at the G1/S transition
The G2 checkpoint​​: at the G2/M transition
The spindle checkpoint: at the transition from metaphase to anaphase.

6. G1 Checkpoint
What is being checked?
Size: Is the cell large enough to divide?
Nutrients: Are all needed nutrients present in the environment?
Molecular signals: Is the cell receiving positive cues (such as growth factors) from neighbors?
Mechanical signals: Is the cell attached to a support? Is it crowded by neighbors?
DNA integrity: Is any of the DNA damaged?
If cell doesn’t pass G1 checkpoint it may enter G0 resting phase
Once cell passes G1 it is committed to dividing

7. G0 Phase
Resting phase
E.g., nerve cells in the brain stop dividing when the brain is developed and cannot re- enter the cell cycle.
Liver cells can be in G0 but re-enter the cell cycle if the liver is damaged or needs repaired

8. G2 Checkpoint What is being checked?
DNA integrity: Is any of the DNA damaged?
DNA replication: Was the DNA completely copied during S phase?
If errors or damage are detected, the cell will pause for repairs.
If the problem is fixed, the cell will re-enter the cell cycle and divide
If the damage is irreparable, however, the cell may instead undergo apoptosis, or programmed cell death
This ensures that damaged DNA is not passed onto daughter cells and is important in preventing cancer

9. Cell Death - 2 Types Apoptosis: Programmed and targeted. Necrosis:
E.g., White blood cells divide to fight viral infections. When they are no longer required, they undergo apoptosis.
Necrosis:
Cells may die due to external factors.
E.g., Toxins, infections, trauma.

10. Spindle Checkpoint What is checked?
Alignment: chromosomes are lined up at the metaphase plate.
Attachment: Each chromosome is attached to a spindle fibre
Cells look for "straggler" chromosomes that are in the wrong place
cell will pause mitosis, allowing time for spindle to capture stray chromosome.
If a cell fails to pass the spindle checkpoint, it may undergo apoptosis.

11. Part 2: Mitosis

12. Vocabulary
Diploid – contains 2 complete sets of chromosomes – 46 in humans (2n)
Body cells (somatic cells) are diploid
Haploid – contains a single set of unpaired chromosomes – 23 in humans (n)
Sex cells (gametes – sperm and egg) are haploid

13. Vocabulary a chromatid is one strand of a chromosome
2 chromatids that are the same are called sister chromatids

14. Mitosis Mitosis animations: Beginning of second class
(1:22 min)
(1:30 min)
Beginning of second class 14

15. Interphase occurs just before Mitosis begins:
DNA is replicated along with organelles and other cellular components and the cell prepares for division. 15

16. Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Interphase
Animal cell
Plant cell
Photographs from:

17. Prophase (preparation phase)
DNA recoils and the chromosomes condense and become shorter and thicker
the nuclear membrane dissolves and disappears
the spindle fibres (which attach to chromosomes during cell division) begin to form

18. Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Prophase
Animal cell
Plant cell
NOTE: In animal cells, centrioles are involved in the formation and organization of spindle fibres.
Photographs from:

19. Metaphase (organizational phase)
chromosomes line up in the middle of the cell with the help of spindle fibers attached to the centromere of each replicated chromosome (sister chromatids)

20. Metaphase Animal cell Plant cell
Photographs from:

21. Anaphase (separation phase)
chromosomes split in the middle (centromeres divide) and the sister chromatids are pulled by the spindle fibres to opposite poles of the cell

22. Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Anaphase
Plant cell
Animal cell
Photographs from:

23. Telophase
chromosomes reach opposite poles of the cell and begin to unwind
spindle fibres dissolve and nuclear membrane forms around the chromosomes

24. Photographs from: http://www.bioweb.uncc.edu/biol1110/Stages.htm
Telophase
Animal cell
Plant cell
Photographs from:

25. After Mitosis: Cytokinesis
cytokinesis in a plant:
cell plate forms
The actual splitting of the cytoplasm and all the organelles into two separate cells called daughter cells.
Occurs differently in both plant and animal cells.
cytokinesis in an animal cell: cleavage furrow forms