1. Mitosis

2. Genes and Proteins
Proteins do the work of the cell: growth, maintenance, response to the environment, reproduction, etc.
Proteins are chains of amino acids. The sequence of amino acids in each protein is coded in the DNA as a specific sequence of A, C, G and T bases: a gene.
Each gene codes for a different protein.
Key points:
All cells within an organism have the same genes.
What makes cells different from each other is that different genes are turned on and turned off in different cells.
The DNA must be copied and then divided exactly so that each cell gets an identical copy.

3. Mitosis
Mitosis is the normal cell division that occurs throughout life in all parts of a multicellular body. The process by which cells divide to make more cells.
All the other organelles (lysosomes, golgi apparatus, endoplasmic reticulum, mitochondria,etc.) can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.
Remember:
Humans have 46 chromosomes, 23 from each parent.
Every cell has the same 46 chromosomes
Each species has a characteristic number of chromosomes eg. corn has 20, house flies have 10, chimpanzees have 48, some ferns have over 200.

4. Chromosomes
A chromosome is made up of DNA wrapped around proteins, it’s DNA in it’s condensed form.
The DNA is supported and neatly packaged by proteins bound to it. At different times, these proteins cause the DNA to be spread out like spaghetti in a bowl, or tightly condensed into the X-shaped chromosomes we can see in the microscope.
Each chromosome has a central constricted region called a centromere that serves as an attachment point for the machinery of mitosis.

6. More information on Chromosomes
Before replication, chromosomes have one chromatid.
After replication, chromosomes have 2 sister chromatids, held together at the centromere.
In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell.

7. Cell Cycle
Some cells divide constantly: cells in the embryo, skin cells, gut lining cells, etc. Other cells divide rarely or never: only to replace themselves, brain.
If a cell is not undergoing mitosis it is performing it’s destined function in the body, each chromosome has one chromatid (G1).
Actively dividing cells go through a cycle of events. The first part of the cycle is called “interphase”.
During interphase the cell increases in size, and the DNA replicates (S) but the chromosomes are not visible under a microscope.

8. Machinery of Mitosis
The chromosomes are pulled apart by the spindle fibres, which is made of microtubules. The spindle fibers are attached to each centromere (which is part of the chromosome), and anchored on the other end to a centrosome (which is the organizing center for the spindle).
There are 2 centrosomes, one at each pole of the cell.

9. Stages of mitosis:
interphase
prophase
metaphase
anaphase
telophase

10. Summary of Mitosis Prophase: Metaphase Anaphase Telophase
Chromosomes condense
Nuclear envelope disappears
centrosomes move to opposite sides of the cell
Spindle forms and attaches to centromeres on the chromosomes
Metaphase
Chromosomes lined up on equator of spindle
centrosomes at opposite ends of cell
Anaphase
Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes
Chromosomes pulled to opposite poles by the spindle
Telophase
Chromosomes de-condense
Nuclear envelope reappears
Cytokinesis: the cytoplasm is divided into 2 cells

11. Prophase In prophase, the cell begins the process of division.
1. The chromosomes condense. The proteins attached to the DNA cause the chromosomes to go from long thin structures to short fat one, which makes them easier to pull apart.
2. The nuclear envelope disappears. The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division
3. The centrosomes move to opposite poles. During interphase, the pair of centrosomes were together just outside the nucleus. In prophase they separate and move to opposite ends of the cell.
4. The spindle starts to form, growing out of the centrosomes towards the chromosomes.

12. Metaphase
Metaphase is a short resting period where the chromosomes are lined up on the equator of the cell, with the centrosomes at opposite ends and the spindle fibers attached to the centromeres. Everything is aligned for the rest of the division process to occur.

13. Anaphase
In anaphase, the centromeres divide. At this point, each individual chromosome goes from:
1 chromosome with 2 chromatids
to:
2 chromosomes with one chromatid each.
Then the spindle fibers contract, and the chromosomes are pulled to opposite poles, towards the centrosomes.

14. Telophase In telophase the cell actually divides.
The chromosomes are at the poles of the spindle.
The spindle disintegrates
The nuclear envelope re- forms around the two sets of chromosomes.
The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

15. Cytokinesis
The organelles (other than the chromosomes) get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in.
Plant and animal cells divide the cytoplasm in different ways.
In plant cells, a new cell wall made of cellulose forms between the 2 new nuclei, about where the chromosomes lined up in metaphase. Cell membranes form along the surfaces of this wall. When the new wall joins with the existing side wall, the 2 cells have become separate.
In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half.