1. Mitosis and Cell Division

2. Mitosis Cells divide to make more cells.
-this allows organisms to grow, develop, maintain and repair themselves
Mitosis is Asexual (1 parent)
While all of the organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.

3. The Challenge of Mitosis
Normal human cells have 23 pairs of chromosomes
- each pair consists of one copy
of the chromosome from each
parent – 46 total
- after replication, the total number
of chromosomes = 92
Each cell produced by cell division
needs exactly the correct set
of 46 chromosomes

4. Still More Chromosomes
Each of the chromatids is a copy of the original chromosome
In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell.

5. Machinery of Mitosis
Cells build a structure called the spindle, which is made of microtubules to facilitate the movement of chromosomes during mitosis
The spindle fibers are attached to each chromosome at structures called the kinetochore found at the centromere

6. Machinery of Mitosis: The Spindle
In animal cells, the spindle fibers are
anchored at either end to a centrosome
- each centrosome is composed of
a pair of centrioles
Plants lack centrosomes and their
spindle formation is poorly
understood
spindle is thought to originate from the centrosomes. In plant cells, it is thought to originate from the region of the nucleus
In all cells, the opposite ends of the
cell are referred to as the poles

7. Mitosis Occurs in Phases
mitosis occurs in a logical sequence of steps
- the occurrences in one phase prepare the cell for the next stage in the process
-The phases of mitosis:
Prophase
Metaphase
Anaphase
Telophase

8. Interphase Preparatory phase Chromosomes replicate here
Where cell spends most of its time
G1-cell growth to mature size
S-cell DNA copied (synthesized)
G2- preparation for cell division

9. Prophase – the first step in cell division
1. DNA Condenses.
-Chromatin coils into chromosomes
2.The nucleus & nucleolus disappear
3. Centrioles migrate to poles
4. The spindle starts to form, chromosomes attach to spindle fibers
5. Longest Phase
The double membrane that surround the nucleus dissolves into a collection of small vesicles, freeing the chromosomes to use the whole cell for division
In plant cells, the microtubules seems to originate from the nucleus

10. Metaphase
The chromosomes line up along the middle of the cell – known as the equator
Everything is now aligned for the rest of the division process to occur. Shortest Phase.

11. Anaphase the chromosomes separate at the centromere.
- the sister chromatids move toward opposite poles
Then the spindle fibers shorten, and the chromosomes are pulled to opposite poles, towards the centrosomes.

12. Telophase The chromosomes uncoil back to chromatin
The spindle disintegrates
The nuclear envelope re-forms around the two sets of chromosomes, forming nucleus and nucleolus
The cytoplasm is divided into 2 separate cells, the process of cytokinesis begins.

13. Cytokinesis
The organelles 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.

14. Plant Cell Cytokinesis
No pinching because of cell wall.
Cell plate forms from the middle to split the cytoplasm.
Cell plate is formed by the Golgi Bodies.

15. Animal Cell Cytokinesis
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.
Cleavage furrow-cell membrane pinches inward.

16. Summary of Mitosis Prophase: Chromosomes condense
Nuclear envelope disappears
Centrioles 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 uncoil
Spindle disappears
Nuclear envelope reappears
Cytokinesis: the cytoplasm is divided into 2 cells

17. Vocabulary Histones nucleosomes chromosomes chromatin centromere
kinetochore
spindle
centrosomes
poles
Prophase
metaphase
anaphase
telophase