1. Cell Growth & Division
Chapter 5

2. The Cell Cycle
Eukaryotic cells undergo a predictable cycle of growth and division
This Cell Cycle has 4 main stages
G1 – growth
S – synthesis of DNA
G2 – more growth
M – mitosis

3. The Cell Cycle
G1 – the cell carries out its normal functions - GROWTH
The cell will also increase in size and organelles increase in number
Cells must pass a critical checkpoint in order to proceed to the next stage of the cell cycle

4. The Cell Cycle
S – synthesis of DNA
The cell will make a copy of its DNA
The DNA is arranged loosely inside the nucleus during the S stage
At the end there are 2 exact copies of the DNA

5. The Cell Cycle
G2 – additional growth occurs
Normal functions continue
Preparing for the M stage (mitosis) of the cycle
Must pass a critical checkpoint, just like in G1, in order to proceed to Mitosis

6. The Cell Cycle
M – mitosis – cell division
The nucleus divides in mitosis
The nuclear membrane dissolves
DNA condenses and separates
Two new nuclei form
Cytokinesis – the process that divides the cytoplasm into 2 new cells

7. The Cell Cycle
Cells all go through the cell cycle, but at different rates
The S, G2 and M stages in a human take about 12 hours.
The G1 stage varies widely with the type of cell
Skin cells live about 2 weeks

8. The Cell Cycle
Red blood cells live about 4 months
Liver cells live about days
Intestine (internal lining) cells live about 4-5 days
Intestine (muscle & other tissue) cells live about 16 years!

9. The Cell Cycle
Some cells do not divide or divide very rarely (neurons, for example)
These cells enter a stage called G0.
Cells in G0 continue to carry out normal functions, but do not proceed to the S, G2 and M stages

10. The Cell Cycle
Cell size is limited – they cannot be too small (the organelles won’t fit!) or too big (the cell would be inefficient)
The upper limit on the size of a cell is because of the cell “surface area to volume ratio”

11. The Cell Cycle
As a cell grows, its volume and surface area both increase
However, the volume increases faster than the surface area
The surface (cell membrane) of the cell is very important in getting materials in & out, so…

12. The Cell Cycle
If a cell has a large volume & not enough surface area, the cell could not get enough needed materials to the entire cell quickly enough
On page 137 in the textbook, there is a similar diagram

13. The Cell Cycle
Some cells must be large – for instance a neuron that needs to reach from your brain to your leg
This cell is not shaped like a sphere or cube, but instead is long & thin, making the surface area increase but with very little volume increase

14. The Cell Cycle
Cells have to regulate the amount of growth that occurs & the size of the cell before it divides
Cells that are more than twice as large as the original will produce daughter cells that are too big
Cells less than double the size will produce daughter cells that are too small

15. The Cell Cycle
Let’s take an in-detailed look at the last parts of the cell cycle – Mitosis & Cytokinesis
The cell must make sure that each new daughter cell has the exact same DNA as the other, if not, things could be disastrous!

16. Each human body cell has 46 chromosomes
Mitosis
Each human body cell has 46 chromosomes
A chromosome is a condensed strand of DNA that contains many genes along with other regulatory info
Chromosomes are tightly coiled DNA, wrapped around proteins

17. Histones are proteins that DNA wraps around at regular intervals
Mitosis
Page 139 in your text shows the organization of DNA from the basic strand to the tightly coiled chromosome (Fig 5.5)
Histones are proteins that DNA wraps around at regular intervals

18. One side of the “X” is called a chromatid.
Mitosis
Chromatin is the loose form of DNA (not as coiled as chromosomes, think spaghetti)
Chromosomes look like an “X” in which the left & right sides are identical
One side of the “X” is called a chromatid.

19. Mitosis Sister chromatids are held together at the centromere
The end of the DNA molecules form telomeres which are repeating nucleotides & do NOT code for genes
A portion of each chromosome is lost each time a cell divides, & it must be from the telomeres!

20. Interphase – G1, S & G2 stages
The Phases of Mitosis
Interphase – G1, S & G2 stages
Prophase – 1st official phase of mitosis, chromatin condenses, nuclear envelope breaks down, centrioles move to opposite sides of the cell, spindle fibers (microtubules) grow out of the centrioles

21. The Phases of Mitosis
Prophase Pictures

22. Metaphase – the chromosomes line up in the middle of the cell
The Phases of Mitosis
Metaphase – the chromosomes line up in the middle of the cell
Spindle fibers attach to each chromosome on either side

23. The Phases of Mitosis
Anaphase – The chromatids begin to separate to opposite sides of the cell
Cytokinesis will usually begin in late anaphase
Anaphase “bananaphase”

24. Telophase – the last phase of mitosis
The Phases of Mitosis
Telophase – the last phase of mitosis
2 new nuclear membranes start to form
Chromosomes begin to uncoil
Spindle fibers fall apart
The new cells start to pinch off

25. The Phases of Mitosis
Photos of Telophase

26. Regulation of the Cell Cycle
Both internal & external factors regulate the cell cycle
External factors come from outside the cell – messages from OTHER nearby cells or even from distant cells in the organism’s body
Internal factors come from inside the cell & include many different molecules found in the cytoplasm

27. Regulation of the Cell Cycle
External factors can be physical or chemical signals.
A physical signal could be cell-to-cell contact
If a cell is normal, once it touches another cell, it will stop growing/dividing

28. Regulation of the Cell Cycle
Growth factors are chemical signals that stimulate cell growth
Some hormones may also stimulate cell growth
External factors stimulate internal factors
Kinases and cyclins are 2 internal factors

29. Regulation of the Cell Cycle
Kinases are enzymes that are activated by cyclins to help regulate the cell cycle and move from stage to stage

30. Regulation of the Cell Cycle
Apoptosis: programmed cell death
Some cells are supposed to die at certain times
Genes are activated that produce self-destructive enzymes

31. Regulation of the Cell Cycle
Apoptosis occurs in developing babies between the fingers before birth
In the womb, babies have webbed fingers
At birth, babies do not have webbed fingers because those cells die

32. Regulation of the Cell Cycle
Sometimes, cells grow uncontrollably
This is commonly referred to as cancer
Cancer cells do not stop growing & dividing when they touch other cells, resulting in tumors

33. Regulation of the Cell Cycle
If a tumor is benign, the cells stay clustered together
If a tumor is malignant, the cells break off (metastasize) and spread to other parts of the body
Cancer cells take nutrients away from healthy cells

34. Regulation of the Cell Cycle
Cancer cells also do not perform regular cell functions
Cancer tumors can also put pressure on internal organs and cause them to not work properly
Carcinogens are cancer causing agents that can be environmental or even viruses

35. Regulation of the Cell Cycle
Cancer treatment often involves radiation therapy or chemotherapy
Radiation therapy uses radiation to shrink and kill cancer cells
Chemotherapy uses chemicals to kill the cells

36. Regulation of the Cell Cycle
Cancer treatments can also cause unwanted side effects because they kill healthy cells as well as the cancer cells
Radiation is localized (one spot) & chemotherapy is systemic (all over the body)