Supplemental Lecture (97/02/16 update) by Stephen T. Abedon (abedon.1@osu.edu)

1. Chapter title: Mitosis
1. A list of vocabulary words is found toward the end of this document
2. Were eucaryotic cell division to occur as it does in procaryotes, i.e., DNA replication, followed by separation of replicated DNA (due to attachment to growing cell envelope), then one would expect eucaryotic DNA to be attached to the cell nuclear membrane, and division of the nucleus to effect the pulling apart of DNAs until they were segregated into two separate nuclei. However, that's not the way eucaryote cell division works. For one thing, nuclei, unlike mitochondria and chloroplasts, don't divide (or, at least, this is true for most eucaryotes including plants, animals, and most protozoa). Instead, the membrane surrounding eucaryotic DNA is transient, usually existing to separate the DNA from the cytoplasm, but removed when it is time for DNA to be segregated into newly created daughter cells.
3. Another consideration is that eucaryote DNA is different from procaryote DNA. First, eucaryote DNA is linear rather than closed, circular (i.e., a very long "string" or "stick" rather than a "hoop" or "loop"). Second, eucaryotes tend to have more than one pair of DNA molecules (many chromosomes), unlike procaryote's for whom the bulk of their DNA is located in a single pair of molecules (i.e., one chromosome). Finally, eucaryotes tend to have a lot more DNA than do procaryotes; in some cases on the order of 1000-fold more.
4. Perhaps befitting these differences, as well as the generally increased complexity and size of eucaryotic cells relative to procaryotes, eucaryotic cell division is a much more complicated process than the relatively simple binary fission exhibited by many procaryotes. The complex overlapping series of events that constitutes nuclear duplication during eucaryotic cell division is called mitosis. We consider the general mechanism of mitosis in this lecture. In a subsequent lecture we will consider a related process called meiosis.
2. Cell division [cellular replication]
1. The various steps involved in converting a single cell into two daughter cells, called division because in many cases the cell literally divides down the middle with half of its contents going to one daughter cell and the other half going to the other.
2. A crucial aspect of eucaryote cell division is the duplication of the nucleus which occurs by a process called mitosis.
3. Daughter (parent) cell
1. A daughter cell is an offspring of a cell resulting from cell division.
2. Following the occurrence of cell division a parent cell no longer exists and in its place are two daughter cells.
3. Note, however, that in a different process of cellular replication called budding (which occurs, for example, in some yeasts), cell division occurs but the parent cell remains intact and a daughter cell smaller than the remaining parent cell is the product.
4. Segregation
1. The placement in daughter cells, in the course of division, of half of the contents of the daughter cell . . .
2. . . . including, but not limited to, a complete set of chromosomes.
5. Cell cycle
1. The vegetative cell cycle in eucaryotes generally follows a trajectory whereby periods of nuclear duplication (mitosis), followed by actual (cytoplasmic) cell division (C phase, a.k.a., cytokinesis), are interspersed with periods during which cells are not dividing (interphase).
2. Variable interphase:
1. Cell division proper involves mitosis and cytokinesis. Interphase separates cell divisions.
2. Depending upon circumstances and the type of cell, the periods between cell divisions can range from very short to very long to effectively infinite (i.e., no additional cell division).
3. The period during which cell division, or any of the steps leading up to cell division are not occurring are called G1 phase.
4. The period during which nuclear DNA replication occurs is called the synthesis or S phase.
5. The gap between S phase and mitosis.
6. G1 phase is followed by a period of DNA synthesis, G2 phase (all three cumulatively referred to as interphase), then mitosis (nuclear duplication including prophase, metaphase, anaphase, and, finally, telophase), and, finally, cytokinesis (actual cytoplasmic division).
7. See illustration below.
6. Illustration, cell cycle



7. Interphase
1. Period of no nuclear nor cytoplasmic segregation:
1. The periods in the cell cycle during which the nuclear membrane is intact, subsequent to the segregation of nuclei to daughter cells.
2. Consequently, interphase is the time between the creation of a daughter cell (a product of cell division) and some point into the division of that daughter cell.
2. Interphase consists of G1 phase, S phase, and G2 phase, in that temporal order.
8. G1 (gap) phase
1. The period in the cell cycle during which neither cell division nor preparation for cell division occurs.
2. In other words, it is during the G1 phase that a cell does whatever it is that a cell does other than dividing.
9. S (synthesis) phase
1. The end of G1 phase occurs with the replication (synthesis) of the cell DNA. S phase ends at the point of the completion of this replication task.
2. That is, at the end of S phase the cell contains two complete sets of DNA.
10. Sister chromatids [centromere]
1. Post S phase paired chromatin:
1. The two DNA copies that result from S phase are not visible through a light microscope because they have not yet condensed to form chromosomes (i.e., they remain chromatin).
2. They are also joined together, via specific proteins, at a short sequence of DNA (which is found on each double helix) and called a centromere.
3. Together these chromatin are called sister chromatids.
2. Chromatin/chromosome accounting:
1. Note that the number of pairs of sister chromatids found in a cell at this point is equal to the number of chromosomes (i.e., double helical pairs of DNA molecules) found in the cell prior to S phase.
2. For example, if the parent cell had 46 chromosomes (as do most human cells), then following S phase the cell would have 46 pairs of sister chromatids (for a total of 92 pairs of DNA molecules, 92 double helices, 92 sister chromatids, and 184 individual DNA molecules--make sure you know where all of these numbers come from).
11. G2 (gap) phase
1. The period occurring between the DNA replication and the various steps of mitosis.
2. Condensation to chromosomes:
1. During G2 phase chromatin begins to condense into the relatively compact structures called chromosomes (which, as a result of condensation, become visible through a light microscope).
2. These chromosomes remain attached through their centromeres.
3. Preparation for segregation:
1. Preparation is made for chromosomal segregation, though actual segregation is not yet initiated (e.g., the nuclear membrane remains intact).
2. For example, the centrioles or their equivalents form.
3. See illustration below.
12. Illustration, G2 phase



13. M phase [mitosis]
1. Duplication of nucleus occurs during M phase.
2. In particular, the steps of the cell cycle during which chromosomes are segregated toward what will be two daughter cells (i.e., following cytoplasmic division as occurs during C phase).
3. M phase consists of prophase, metaphase, anaphase, and telophase. These separate phases of mitosis occur in order and individually describe specific aspects of the segregation process.
4. Note, however, that though mitosis is considered to occur in multiple phases, this is not meant to imply that the process is one that starts and stops. Rather, each stage of mitosis begins as the previous ends thus resulting in smooth transitions and what, overall, flows as an essentially a continuous process.
14. Prophase
1. Chromosome formation through condensation of chromatin continues during prophase.
2. Also during prophase the nuclear membrane disappears.
3. However, the key function of prophase is the development of scaffolding (spindle apparatus) necessary to move sister chromatids about the (now nucleus-less) cell.
4. At the end of prophase (by definition) sister chromatids are lined up in the middle of the cell . . .
5. . . . with each individual chromosome attached to through spindle fibers . . .
6. . . . to centrioles now located at opposite poles of the cell (the spindle fibers bind at the centromere).
7. See illustration below.
15. Illustration, prophase



16. Spindle apparatus [spindle fiber]
1. Microtubule chromosome movers:
1. To effect chromosome segregation during mitosis an adjunct cytoskeleton (microtubules) develops which is referred to as the spindle apparatus.
2. One aspect on the spindle apparatus are spindle fibers which are directly in contact with chromosomes and thus directly effect their movement.
3. See line between rectangles (centrioles) and chromosomes in illustration above.
17. Centriole
1. Animal cell spindle fiber making machines:
1. A spindle fiber making machine found in the cytoplasm of animal cells (though absent from many other kinds of eucaryotic cells).
2. See rectangles in illustration above and below.
18. Metaphase [metaphase plate]

1. Metaphase plate:

1. Metaphase begins with the formation of metaphase plate.
2. This is a plane found in the center of a dividing cell, half-way between the centrioles and perpendicular to a line connecting the two centrioles.
3. Within (and, in fact, defining) the metaphase plate are found all of the sister chromatids.

2. Sister chromatids then separate and begin to be pulled to the oppositely located centrioles, thus defining the end of metaphase.
3. See illustration below.

19. Illustration, metaphase



20. Anaphase

1. Anaphase consists of the further movement of sister chromatids toward their respective poles.
2. In addition, the poles themselves move farther apart, thus elongating the cell.

21. Telophase

1. Reformation of the pre-mitosis cell structure . . .
2. . . . including the reformation of the nucleus and the decondensation of the chromosomes back into chromatin.
3. See illustration below.

22. Illustration, telophase/cytokinesis



23. C phase [cytokinesis]

1. Cytoplasmic division:

1. The actual division of the cell cytoplasm following duplication and reconstitution of the cell nucleus.
2. Note that in order for two viable daughter cells to result from this division, both cells must contain, in addition to a nucleus, a compliment of mitochondria, endomembrane system, etc.

2. See illustration above.

24. Vocabulary
1. Anaphase
2. Cell cycle
3. Cell cycle, illustration
4. Centriole
5. Centromere
6. Cytokinesis
7. Cytokinesis, illustration
8. C phase
9. Daughter cell
10. G1 phase
11. G2 phase
12. G2 phase, illustration
13. Interphase
14. M phase
15. Metaphase
16. Metaphase, illustration
17. Mitosis
18. Prophase
19. Prophase, illustration
20. S phase
21. Segregation
22. Sister chromatids
23. Spindle fiber
24. Telophase
25. Telophase, illustration
26. Telophase/cytokinesis, illustration
25. Practice questions

1. In a diploid organism containing 32 chromosomes per haploid, during metaphase how many sister chromatid pairs line up within a plane defining the center of a dividing cell? [PEEK]

1. 16
2. 32
3. 64
4. 128
5. all of the above
6. none of the above

2. True or false, mitotic division cannot directly follow mitotic division (circle correct answer). [PEEK]
3. Which phase of the cell cycle is often the longest of all those which occur? [PEEK]

1. prophase
2. cytokinesis
3. G2
4. M phase
5. none of the above
6. all of the above

4. In animal cells, which of the following is a characteristic of the end of prophase? [PEEK]

1. nuclear membrane is intact
2. sister chromatic pairs separated
3. crossing over completed
4. centrioles not yet duplicated
5. all of the above
6. none of the above

5. Cytokinesis is [PEEK]

1. separation of duplicated nuclei to opposite poles
2. cytoplasmic division
3. separation of divided cells
4. movement of material within eucaryotic cells
5. all of the above
6. none of the above

26. Practice question answers

1. iii, 64
2. false
3. v, none of the above, interphase or G1 phase are appropriate answers
4. vi, none of the above
5. ii, cytoplasmic division

27. References

1. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 202-217.