1. Cell Division
Lab 10

2. Cell Division
According to cell theory – cells only come from other living cells.
Information – encoded within DNA – must be replicated so each new cell has a copy.

3. Eukaryote Cell Cycle
DNA is contained in multiple chromosomes (a single long DNA molecule packaged with proteins into a compact shape).

4. Cell Division in Eukaryotes
Mitosis – cell division in somatic (nonreproductive) cells.
Meiosis – cell division in reproductive cells.
Results in eggs or sperm.

5. The Cell Cycle Interphase – G1, S, and G2 together. M phase – mitosis
G1 phase – first growth phase
S phase – synthesis phase, replication of DNA.
G2 phase – second growth phase, replication of mitochondria, chromosomes condense.
M phase – mitosis
C phase - cytokinesis

6. Chromosomes
Homologous chromosomes or homologues – two nearly identical versions of a chromosome.
Diploid cells have two of each type of chromosome ( 23 pairs or 46 in humans).

7. Chromosome Structure
During S phase, each of the 2 homologues replicates, resulting in identical copies called sister chromatids.
Chromatids remain connected at a linkage site called the centromere.

8. Cell Division
Interphase – Cells grow, DNA replicates and winds up tightly (condensation).

9. Cell Division Mitosis – nuclear cell division in somatic cells.
Prophase
Metaphase
Anaphase
Telophase
Continuous process where stages flow one into the next.

10. Cell Division
Prophase – Chromosomes condense enough to be seen with a light microscope.

11. Cell Division
Metaphase – Alignment of the chromosomes along center of cell.

12. Cell Division Anaphase – Separation of the chromatids.
Sister chromatids split apart – one moving to each pole.

13. Cell Division
Telophase – re-formation of the nuclei.

14. Cell Division Cytokinesis – division of the cytoplasm.
Two complete, diploid cells.

15. Cytokinesis In plants, a cell plate forms dividing the cell in two.
Cellulose is laid down to form new cell wall.
In animal cells, the cell pinches in two.
This produces a cleavage furrow which deepens until the cell splits.

16. Reproduction
When organisms reproduce sexually, a cell from each parent fuses together.
In order to avoid a doubling of chromosome number, there must be a way to reduce the number of chromosomes in the gametes (eggs & sperm).

17. Meiosis
Meiosis is the special type of cell division that produces eggs and sperm.
In meiosis, a diploid cell with two sets of homologous chromosomes will divide so that the daughter cells are haploid and have one set of chromosomes.

18. Fertilization
One haploid egg and one haploid sperm can then fuse in the process called fertilization forming a diploid zygote.

19. Meiosis
Chromosomes have replicated during interphase just as in mitosis.
Meiosis actually consists of two separate divisions.
Meiosis I – serves to separate the two versions of the chromosome (homologues).
Meiosis II – serves to separate the two replicas of each version (sister chromatids).

20. Meiosis
Because there is only one replication of DNA but two cell divisions, each of the four daughter cells is haploid – has only one set of chromosomes.

21. Meiosis I Prophase I – Chromosomes become visible.
The 2 versions of each chromosome pair up and exchange segments. This is called crossing over.

22. Meiosis I Metaphase I –Chromosomes line up in the middle.
Which chromosome faces which pole is random. This is called independent assortment.

23. Meiosis I
Anaphase I
Homologues are pulled apart and move toward opposite poles.
Sister chromatids NOT separated yet.
Each pole has half as many chromosomes (one set rather than two) as the original cell.
Telophase I – the chromosomes gather at the two poles and wait for the onset of meiosis II.

25. Meiosis II
After a brief interphase in which NO DNA synthesis occurs, meiosis II begins.
Meiosis II is just like mitosis except that the sister chromatids are no longer identical due to crossing over.

26. Meiosis II Prophase II – nuclear envelopes break down.
Metaphase II – chromosomes line up in the middle of the cell.

27. Meiosis II Anaphase II – sister chromatids move to opposite poles.
Telophase II – The nuclear envelope reforms around the four sets of daughter chromosomes.

28. Meiosis II The resulting 4 daughter cells are haploid.
No 2 cells are alike due to crossing over.
In animals, these cells develop directly into gametes (eggs & sperm).

30. Unique Features of Meiosis
Synapsis – occurs when homologous chromosomes line up along their entire length.
Crossing over - occurs when segments of the chromosomes are then exchanged.

31. Unique Features of Meiosis
Another unique feature is that the homologous chromosomes separate in meiosis I rather than sister chromatids.
Sister chromatids separate in meiosis II.
Meiosis is a reduction division.