1. Cell Division
Chapter 10

2. Why do cells divide?
Each cell divides for a different reason. Some of the reasons include:
Growth
Replace dead or dying cells
Repair damaged tissue
Reproduction
In order to remain an efficient size

3. Growth Are you the same size today as the day you were born?
What happens as an organism grows? 2 options
The cells grow larger.
The number of cells grows larger.

4. Growth The cells of a baby are basically the same size as an adult.
Babies are smaller because they have fewer cells.

5. Growth
So, in order for an organism to grow, the number of cells that make up that organism must increase.
The number of cells increases with each cell division.

6. Replace dead/dying cells
An average human red blood cell lives 120 days.
Without red blood cells our bodies would not be able to transport oxygen through our blood stream.
As old cells die, new ones must take their place in order for an organism to remain healthy.

7. Repair damaged tissue
Raise your hand if you’ve ever broken a bone in your body. (keep your hand raised)
Raise your hand if you’ve ever had a cut somewhere on your body—paper cuts count!
Cell division allows an organism to repair wounds or injuries.

8. Reproduction
In order for some prokaryotes and simple eukaryotes to produce offspring, cell division must occur.
It is a form of asexual reproduction, where only one parent is needed to produce offspring.

9. In order to remain small
A cell grows larger by building more organelles and producing items necessary for it to function.
These functions are carried out by proteins produced by the cell.

10. In order to remain small
As a cell gets larger, many things happen:
More nutrients are required by the cell, which must also be processed and gotten rid of as extra waste.
The cell needs to make more proteins to maintain its function.

11. In order to remain small
BUT with a larger cell….
Substances needed by the cell must pass through the surface of the cell and travel farther to get where they need to be.
The amount of DNA doesn’t change. DNA determines the amount of proteins produced.

12. In order to remain small
Therefore, in order to remain efficient, a cell must:
Keep a higher surface area to volume ratio.
Higher surface area to volume ratios are found in smaller cells
Be able to support the protein requirements with a set amount of DNA.

13. In order to remain small
Both of those problems are solved through cell division.
Daughter cells have a higher surface area to volume ratio than its parent cell.
Each new cell gets an entire copy of the parent cell’s DNA

15. How do cells divide?
The Cell Theory states that all new cells come from pre-existing cells.
So in order to make a new cell, an existing cell must divide.
But we can’t just cut a cell in two pieces. The new cells have to contain all of the equipment needed to stay alive.

16. How do cells divide? What type of equipment does a cell need?
Organelles such as mitochondria, vacuoles, lysosomes, golgi bodies, cytoplasm, endoplasmic reticulum and of course….a nucleus!
Each new cell must contain enough of each organelle to carry out its function.

17. Of course…. the nucleus
A cell’s activity is directed by its DNA, which is organized into genes.
These genes and DNA are found within the nucleus of a cell.
The simplest organisms have thousands of genes.
All this DNA must fit into a very small volume (nucleus).

18. Of course…. the nucleus Most of the time, the DNA exists as chromatin.
DNA coiled around proteins, which densely fill the nucleus.
But all of this information must be passed onto the new daughter cell in order for it to carry out its function.

19. Of course…. the nucleus
In its current form, passing on the DNA is a difficult task!
If the DNA inside one cell is stretched out it would be almost 2 meters long (78 inches).
This would get all tangled up during cell division if kept in the form of chromatin.

20. How can that be prevented?
In preparation for cell division, chromatin coils up and condenses into compact structures called chromosomes.
DNA coils around proteins and condenses into chromosomes.

21. How do cells divide?
Cell division requires a bit of preparation in eukaryotic cells.
The life of a eukaryotic cell cycles through phases of growth, DNA replication, preparation for cell division, and division of the nucleus and cytoplasm.

22. Interphase
This is where the cell is functioning, growing, and preparing to divide.
Consists of 3 stages: G1
S and G2

23. G1 Interphase Called the first gap phase.
Nucleus
Nucleolus
Called the first gap phase.
Cell is growing and building more organelles.
Chromatin
Only one copy of the DNA exists.
Phase where a cell spends most of its life.

24. Chromosome with 2 copies of DNA
S Interphase
Called the synthesis phase
Cell’s DNA is copied during this phase.
Chromatin begins to package into chromosomes.
Chromosome with 2 copies of DNA

25. Chromosomes
Duplicated
Sister Chromatid

26. G2 Interphase Called the second gap phase
Cell continues to grow and prepares to divide.
Spindle fibers and other special structures that aid in cell division are formed.
Duplicated DNA continues to condense into chromosomes.

27. What now?
The cell has built a complete set of organelles for the new cell.
There is twice as much DNA and it’s packaged in such a way that it can move around easily.
Now its time to divide up the nucleus and all its contents (DNA and chromosomes).

28. Mitosis Prophase Metaphase Anaphase Telophase
Stage of the cell cycle where the nucleus is divided.
Consists of 4 phases:
Prophase
Metaphase
Anaphase
Telophase

29. Prophase
Chromosomes are fully condensed and begin to move around the cell.
Spindle fibers start to lengthen.
Distinct nucleus disappears.

30. Metaphase Chromosomes line up in the middle of the cell.
Spindle fibers attach to the centromeres of the chromosomes.

31. Anaphase Spindle fibers begin to shorten.
This action causes the sister chromatids to separate and move toward opposite ends of the cell.

32. Telophase Chromosomes have reached the ends of the cell.
Distinct nucleus begins to re-form.
Spindle fibers disappear.
Cytoplasm begins to divide.

33. Cytokinesis Final stage of the cell cycle.
Cytoplasm is divided, with each daughter cell receiving half of the original cell’s organelles.
Daughter cells are physically separated from each other, resulting in two new cells.

34. Cytokinesis
In animals, cytoplasm is divided by the formation of a cleavage furrow which pinches the cell in half.
In plants, cytoplasm is divided by a cell plate, or new cell wall.

35. After Cytokinesis…
The daughter cells are both the same size—half the size of the original cell—and IDENTICAL genetically to the parent cell.
The chromosomes unwind and return to chromatin.
The daughter cells enter the G1 stage of Interphase.