1. Cell Division Topics covered: Why is DNA replication important?
Why is cell division important?
How do mitosis and meiosis compare?
How do asexual and sexual reproduction compare?

2. DNA Replication and Cell Division
DNA must replicate so that during cell division, the new cells formed each receive a complete set of genetic information

3. Cells must divide for:
Reproduction (e.g. unicellular organisms)

4. Growth (e.g. 1 fertilized egg --> human of ~100 trillion cells)

5. Healing and tissue repair (e.g to replace dead or damaged cells)

6. Mitosis & the Cell Cycle
Mitosis occurs when a parent cell divides to produce two identical daughter cells
Mitosis refers to the process of dividing the nuclear material
Cytokinesis refers to the process of separating the cytoplasm and its contents into equal parts
The cell cycle consists of mitosis, cytokinesis and interphase
The Cell Cycle

7. Interphase G1 phase: cell grows S phase: DNA is replicated
G2 phase: cell prepares for mitosis
DNA is visible in the nucleus as strands called chromatin

8. Phase 1 of Mitosis: Prophase
Centrioles move to opposite poles of the cell
Chromatin condenses and shortens into chromosomes
Spindle fibres form between the centrioles
Nuclear membranes starts to disappear

9. Phase 2 of Mitosis: Metaphase
Spindle fibres attached to centromeres pull chromosomes into place
Chromosomes line up across the equator of the cell
Centrioles duplicate

10. Phase 3 of Mitosis: Anaphase
Chromatids separate at the centromere
Single-stranded chomosomes are pulled to opposite poles by spindle fibres contracting

11. Phase 4 of Mitosis: Telophase
Two nuclear envelopes form
Single-stranded chromosomes uncoil to become chromatin
Cytokinesis occurs after telophase:
Organelles are distributed between the two daughter cells and the cell membrane pinches inward

12. Mitosis in Onion Root Cells

13. Meiosis vs Mitosis
The purpose of mitosis is to maintain genetic continuity (the number of chromosomes in each daughter cell stays the same)
The purpose of meiosis is to produce gametes (sex cells) which unite during sexual reproduction (the number of chromosomes in each sex cell is half of parent cell)

14. Stages of Meiosis We will look at the details later
The key is that the four daughter cells produced are NOT identical

15. Asexual & Sexual Reproduction
Asexual reproduction is any reproduction that does NOT involve meiosis.
Sexual reproduction is any reproduction that does involve meiosis.

16. Asexual Reproduction
1. A single parent gives rise to offspring that are genetically identical to the original parent (clones)
2. Often produces many offspring rapidly
3. There are no specialized structures required by the parent.

17. For example:
Binary fission of Amoeba;
Budding of yeast;

18. fragmentation of sea stars;
spore formation by ferns
vegetative propagation by strawberries;

19. Sexual Reproduction
Genetic information from parent cells are combined to produce a new organism
(offspring are genetically different from parent)
Requires more energy and time than asexual reproduction

20. Sexually reproducing organisms are better able to adapt to changing environments because of differences between individuals
Individuals that are better adapted will survive and perpetuate the species

21. Parthenogenesis Sexual reproduction involving only one individual
Egg is made by meiosis and then it duplicates its chromosomes without being fertilized
common in lizards & salamanders in deserts