1. Cell cycle and cell division

2. Cell division?

3. What is the cell in the forefront of this picture doing?
What is a cycle?
What is the cell in the forefront of this picture doing?
Why do cells do this?

4. The cell cycle
This graph roughly shows how much time a cell spends in each section of it’s cycle.
Most of it is in the three blue sections, which together are called interphase.

5. Interphase, part 1 (G1)
In the dark blue section of the cycle, a cell is growing a replicating its organelles.
This helps ensure that each of the new daughter cells will have enough of each type of organelle to survive.
Some cells, such as nerve cells do not go past this stage of the cycle. This is why nerve damage can be so harmful – you won’t grow new ones!

6. Interphase, part 2 ( part S)
In this second part of interphase,
The cell keeps growing but now
it also replicates the DNA. The
DNA coils up and forms
chromosomes. As identical
chromosomes join they are
called sister chromotids and are
held in the middle by a centromere.

7. G2 – the last part of interphase
In the last stage of interphase, the cell begins separating the organelles between what will become the two daughter cells.
The cell is also busy storing energy to use during the mitotic phases of the cell cycle.

8. The mitotic phase
Note that the mitotic phase is subdivided into mitosis and cytokinesis.
While mitosis is also subdivided into 4 stages, the last two phases overlap with cytokinesis.
Mitosis is the division of the nucleus into 2 new nuclei.
Cytokinesis is the stage where the cytoplasm divides.

9. Mitosis : Prophase
The nuclear membrane breaks apart in prophase, and the spindle fibers begin to form.

10. Mitosis - metaphase

11. Daisy Mae – totally unrelated to mitosis, but she’s a good dog.

12. Mitosis - anaphase
The spindle fibers pull the sister chromotids apart, and the cell starts to stretch out

13. Mitosis - telophase
In this final section of mitosis, nuclear membranes form around the two new nuclei. The spindle fibers disappear.
Why would some people consider telophase the reverse of prophase?

14. Cytokinesis
cells can’t “pinch” in the middle to split. Instead a cell plate forms which will eventually become part of the cell wall.

15. The end results of cell division:
Reproduction: for one-celled organisms, cell division is how they reproduce. Other organisms that have only one parent may also reproduce this way.

16. The end results of cell division:
Reproduction:
Growth: We all started out as a single cell. Cellular reproduction has allowed each of us to grow, as well as allowing cells to differentiate into different types of cells.

17. The end results of cell division:
Reproduction:
Growth:
Replacement: Most organisms lose cells frequently. Our outer layer of skin is constantly being rubbed off, and new cells are growing to replace that layer.

18. The end results of cell division:
Reproduction:
Growth:
Replacement:
Repair: cuts, scrapes, and broken bones can heal when the body makes new cells to replace the ones that were damaged. Nerve cells usually can’t be replaced if they are actually killed so it’s important to protect your head and spine!

19. Homework!
Read pages 85 – 93.
On page 92, answer the practice question in the “math skills” box. If you don’t show your work, you won’t get credit.