1. Cell Division IB Topic 2.5 Chapter 12 Campbell
New cells arise by division of existing cells .

2. Overview
Multicellular organisms begin life as a single cell which grows and divides
It is important that division of a nucleus produces two new cells (daughter cells) that contain identical sets of chromosomes
Achieved by mitosis

3. The cell division cycle
The cycle of growth and division of a cell
See Fig in text
Three main stages
1. interphase
2. division of the nucleus by mitosis
3. cytokinesis (cell division)

4. Interphase Longest part of the cell cycle Not resting!!
Consists of three phases:
1. G1: new organelles formed, growth
2. S: cell copies all genetic material (duplicate)
3. G2: more growth

5. Mitosis
Division of a eukaryotic nucleus into two genetically identical nuclei.
4 phases (PMAT)
2 copies of each chromosome are needed
Each chromosome initially consists of a single DNA molecule; this needs to be replicated
Two identical DNA molecules called sister chromatids (see next slide)

6. Prophase Chromosomes become visible as long threads
They shorten and thicken by supercoiling
Figure 12.3 in textbook
Only at the end of prophase is it possible to see the two chromatids held together at the centromere
Nuclear membrane breaks down
Spindle microtubles are growing

7. Metaphase Centrioles move to opposite ends of the cell (poles)
Spindle microtubules form and attach to the centromeres of each pair of chromatids
Chromosomes are moved to the equator of the cell

8. Anaphase Centromeres divide Spindle fibers shorten
Chromatids are pulled by their centromeres to opposite poles
Once separated, the chromatids are referred to as chromosomes

9. Telophase
Nuclear envelope (or membrane) reforms around both groups of chromosomes
Chromosomes decondense by uncoiling, becoming chromatin again

10. Cytokinesis Division of the cytoplasm
Cell organelles are evenly distributed between cells
Pinching of cytoplasm (cleavage) in half
Two daughter cells enter interphase again

11. Images and animations links

12. The significance of mitosis
Daughter cells produced by mitosis have a set of chromosomes identical to each other and to the parent from which they are from
This occurs because:
Exact copy of each chromosome is made
Chromatids remain attached during metaphase
Centromeres divide
New nuclei
Division of cytoplasm

13. The significance of mitosis (cont.)
Growth and development of an embryo
Repair of damage or worn out cells
Growth, development, and repair
Asexual reproduction
Why is mitosis critical for items stated above?

14. Cancer …
Cells divide by mitosis repeatedly without control or regulation, forming a tumor
Benign vs. malignant
Metastasize
Caused by damage to DNA of chromosomes  different types of mistakes, which explain why cancer is not one single disease
Environmental, genetic, and viral influence

15. Cancer

16. Test yourself
In each of the following combinations of words or phrases, one word or phrase does not belong. Identify which one it is and explain your answer.
1. spindle microtubule formation, replication, growth
2. spindle microtubule formation, supercoiling of chromosomes, attachment of microtubules to centromeres
3. maintenance, repair, formation of gametes