1. 01 Introduction to Cell Respiration STUDENT HANDOUTS
DNA Part IV:
Cellular Reproduction- Mitosis and Cytokinesis

2. Eukaryotic Cell Reproduction
Cell division includes mitosis and cytokinesis.
Mitosis it the division of the nucleus.
Cytokinesis is the division of the cytoplasm.
Daughter cells are genetically identical.
Cell replication is often called mitosis; however, it is possible to have mitosis without cytokinesis, resulting in a single multinucleated cell.
Cytokinesis (division of the cytoplasm).
Two main purposes for cell reproduction:
Unicellular organisms reproduce to continue the species.
Multicellular organisms to make more cells from a fertilized egg, for growth, or for repair.

3. Unicellular vs. Multicellular Mitosis
Unicellular organisms undergo cell division to reproduce themselves.
Multicellular organisms undergo cell division for growth or repair, or to make a new organism from a fertilized egg,.
Emphasize
Mitosis is asexual reproduction which produces daughter cells that are genetically identical to one another.
Meiosis is a special part of the sexual life cycle that produces haploid cells from diploid cells that are genetically different from one another. In animals these cells are gametes (sperm and egg cells).
Mitosis has distinct phases. Students are not responsible for memorizing these names. It is most likely that this is prior knowledge
Prophase, Prometaphase, Metaphase, Anaphase, Telophase
All followed by Interphase (composed of G1, S, and G2)
Interphase was once referred to as the resting phase, but no longer, as during this time the cell grows, differentiates and DNA is replicated. Note that some books are now including an additional phase “C” for cytokinesis.
Graphic

4. Purpose of Chromosomes
There are six feet of DNA in a human somatic cell.
DNA is wrapped around histone proteins and coiled forming chromosomes.
Packaging DNA in chromosomes prevents DNA breakage and helps ensure that each cell gets one copy of each chromosome.
The DNA in humans must be coiled into 46 chromosomes. The number of chromosomes varies for different species.
humans- 46
chimpanzees- 48
elephants- 56
cabbage- 18
Hedgehogs- 90
During the course of evolution, chromosomes can break, making two smaller chromosomes OR fuse, taking two chromosomes and making one larger chromosome. Example, human chromosome # 2 is a fusion event from two smaller chromosomes found in chimpanzees.
The DNA is wound around organizing proteins (histones) forming nucleosomes. The strands of nucleosomes coil and then coiled to second level forming a supercoil or chromosomes. Chromosomes prevent breakage during mitosis. Eukaryotes have histones but prokaryotes do not.
Graphic:

5. Coiling of DNA to Form Chromosomes
During interphase, the DNA forms chromatin. There are areas of DNA that are tightly wound around histones, and there are areas of DNA that are just loosely wound around histones. This depends on the DNA is being transcribed or replicated.
Chromatin
Complex of approximately 40% DNA and 60% protein
Also RNA is associated with chromatin as that is where transcription takes place.
Heterochromatin is DNA that is not expressed into proteins.
Euchromatin that is expressed into proteins.
About 200 nucleotides are wrapped around a core of 8 histone proteins.
Histone proteins have a positive charge and is attracted to the negative DNA.
The DNA wrapped around histones is called a nucleosome. Looks like beads on a string.
Now imagine taking a string of pearls and coiling them around a wooden dowel. That forms a solenoid.
Look at the next slide and then imagine the solenoid like pipe cleaner. This can form loops around a protein scaffold which will eventually form the chromosome structure.

6. DNA Coiling to Form Chromosomes
The “c” words in this unit are just plain brutal! Chromatin, chromatid, chromosome, centromere, etc.
It’s a good idea to keep a running list of them with a quick definition on the board throughout this lesson. Have your students keep the list as well.

7. Chromatids Versus Chromosome
Double stranded chromosomes are held together by centromere.
One half of a double stranded chromosome is called a chromatid.
The lengths of the chromatids may also be held together by proteins called cohesins.
When chromosomes are formed in prophase, they are double stranded because the DNA has already been replicated in S of the cell cycle and the two identical copies are still held together to form a double-stranded chromosome. Each strand of the double-stranded chromosome is called a chromatid (the two sister chromatids are identical). In most cells the chromatids are held together with a protein called cohesins. There is a place were the chromatids seem to be “pinched” together. This structure is the centromere. In vertebrates, the cohesin is removed before mitosis begins and is only located at the centromere holding the sister chromatids together, in other cells the cohesins run the length of the chromatids and remain there until anaphase, occurs.
Compare the two chromosomes in this slide versus the next. One the cohesin is removed and the other it is present.
A centromere is visible point of constriction on a chromosome. The DNA found in the centromere region consist of DNA repeats. Within the centromere region, there is a disc-shaped structure made of proteins call the kinetochore. Microtubules attach to the kinetochore during mitosis. Each chromatid has its own kinetochore. Quite often the term centromere and kinetochore are used interchangeably but technically they are two different structures. To count the number of chromosomes in a cell, count the number of centromeres.

8. Chromatids Versus Chromosome
This is a nice graphic that succinctly illustrates the process of mitosis. This chromosome is held together by cohesins and in the previous slide, the chromatids were only held together by centromere.

9. Centromere vs. Kinetochore
This graphic illustrates the difference between a kinetochore, the centromere and cohesin protein. This chromosome is held together by cohesins and the kinetochore attaches to kinetochore spindle fibers.
Graphic

10. Prophase
Chromatin fibers begin to condense into chromosomes and are visible under the microscope.
Cohesins hold chromatid arms together (vertebrates only at the centromere).
Nucleoli disappears.
Mitotic spindle forms asters radiating out from the centrosome.
After replicating, the centrosomes are moving to opposite poles.
Chromatin fibers begin to condense into chromosomes and wind around histone proteins.
Nucleolus disappears.
Chromosomes become visible. Chromatid arms held together by cohesins (vertebrates only at the centromere).
Mitotic spindle forms, extending from the centrosomes outside the nucleus forming asters. Centrioles (found in animal and lower plant cells) are in the center of the centrosome. Point out that centrosomes are microtubule organizing centers. These centers organize the microtubules to either attach to the kinetochore or overlap one another. These overlapping microtubules will interact with one another to push the poles of the asters further apart.
At first microtubules are being formed outside the nucleus.
Centrioles are composed of nine triplets of microtubules and at the start of mitosis there are two of these centrioles found at right angles to one another.
Centrioles are found in animal cells, lower plant cells and certain Protista. Cells can replicate without centrioles but there is an increase in the number errors that occur when cells try to replicate without their centrioles
Centrioles seem to also be associated with formation of cillia and flagella in certain cells.
The centrosomes are moving to opposite poles.

11. Prometaphase Prometaphase
Nuclear envelopes fragments and nucleolus is no longer visible.
Centrosomes are at opposite ends of the nuclear area.
The microtubules extend through the nuclear area
Two opposing kinetochores form on the centromere on each chromatid.
Kinetochore microtubules attach to the kinetochores. Moving the chromosomes back and forth until they reach the middle of the cell.
Prometaphase
Nuclear envelopes fragments and nucleolus is no longer visible however in most fungi, and many protists, the nuclear membrane remains intact and the microtubules penetrate the nuclear membrane. The nuclear membrane does not break down. If the nuclear membrane breaks down, it is termed “open mitosis” but if the nuclear membrane remains intact, it is termed “closed mitosis”.
Point out to the students how the chromatid arms are together in the illustration because of the cohensins.
Centrosomes are at opposite ends of the nuclear area.
The microtubules extend through the nuclear area
Two opposing kinetochores form on the centromere on each chromatid.
Kinetochore microtubules attach to the kinetochores. Moving the chromosomes back and forth until they reach the middle of the cell.
Nonkinetochore microtubules will overlap one another at the metaphase plate.

12. Nonkinetochore Microtubules
Nonkinetochore microtubules overlap from opposite poles.

13. Metaphase Metaphase Longest phase of mitosis.
Double stranded chromosomes line up on the metaphase plate.
Metaphase
Longest phase of mitosis.
The “tug and pull” of the kinetochore microtubules brings the twin chromatids to the metaphase plate.
Ask the students how many chromosomes are in the diagram on the right. Answer is six double stranded chromosomes.
Ask the students how many chromatids are present in the diagram on the right. Answer is 12 chromatids.

14. Anaphase
Anaphase-
Cohesin proteins are cleaved and the sister chromatids separate.
The chromosomes are pulled to opposite poles.
The kinetochore microtubules are disassembled at the chromosome end.
Spindle poles move apart by interacting with nonkinetochore microtubules.
Anaphase-
Cohesin proteins are cleaved and the sister chromatids are held together by separate cohesins.
Signal for this is transmitted through anaphase-promoting-complex APC.
APC “deactivates” securin protein. Securin inhibits a protein protease called separase.
Now separase is activated and degrades the cohesin protein complex holding the chromatids together and now they can separate.
APC also marks proteins for destruction by tagging them with ubiquitin.
Ask the students how many chromosomes are in the diagram on the right. Answer is 12 single stranded chromosomes.
Ask the students how many chromatids are present in the diagram the right. Answer is 0 chromatids.
The chromatids become chromosomes in their own right.
The chromosomes begin to move to opposite poles. The chromosomes are “walking” up the kinetochore microtubules.
The kinetochore microtubules are disassembled at the chromosome end.
The nonkinetochore microtubules move further apart as they interact with one another. More subunits are attached to the overlapping end. This causes the cell to enlarge.

15. Which End of the Microtubule is Shortened?
Experiment
During anaphase, mark the microtubules to form a stripe.
Observe which side of the microtubules shorten.
They shortened on the side of the chromatids, so therefore the kinetochores are disassembling the kinetochores microtubules and not the centrosome.
This experiment shows that the microtubules are disassembled at the chromosome end and not the centrosome end. The spindle fibers were marked in the middle. If the microtubules shorten between the mark and centrosome, then the microtubules are being reeled in but if the microtubules shorten between the mark and the chromosomes, then microtubules are being disassembled at that end. The latter is the case. The kinetochore disassembles the microtubules at the chromosome end, as the chromosome “walks” up the microtubule. So, the chromosomes are not “pulled” toward opposite poles, but walk themselves toward the poles.

16. Telophase Two daughter nuclei form in the cell.
Nuclear envelope forms from the fragments of the disassembled nuclei and the endomembrane system.
Chromosomes unwind forming chromatin.
Beginning of cytokinesis
Two daughter nuclei form in the cell.
Nuclear envelope forms from the fragments of the disassembled nuclei and the endomembrane system in closed mitosis; in open mitosis, something similar to binary fission occurs to separate the two nuclei.
Chromosomes unwind forming chromatin.
Beginning of cytokinesis (if the cell is going to divide).

17. Cytokinesis in Animals
Mitosis without cytokinesis results in multinucleated cells. This happens in certain algae, plants, fungi, and even a few animals.
Animals cells do cytokinesis by the pinching in of the cell membrane.
Rings of actin form under the cell membrane associated with myosin contracts like a “pull-string” purse forming a cleavage furrow.
Mitosis without cytokinesis results in multinucleated cells. This happens in some plants, fungi, algae and even a few animals. Human examples are skeletal muscle cells, which are long and multinucleated because they undergo many episodes of mitosis without the cell dividing.
Animals cells do cytokinesis by the pinching in of the cell membrane.
In animal cells, rings of actin form under the cell membrane associated with myosin (much like skeletal muscles) contracts like a “pull-string” purse. This forms a cleavage furrow.

18. Cytokinesis in Algae
In algal cells, cytokinesis occurs by an inward growth of new cell wall and membrane.

19. Cytokinesis in Higher Plants
In higher plants, cytokinesis begins in the middle and proceeds toward the periphery as membranous vesicles fuse to form the cell plate..

20. Comparing Cytokinesis in Plants and Animals

21. Mitosis in a Plant Root Tip
Ask students, “Why?” Hopefully, they respond with answers relating to the fact that those are the fastest growing portion of a plant.
Mitosis in a plant occurs in the root tips, shoot tips and other specialized areas.

22. Prokaryotes Reproduce by Binary Fission
This is a much more primitive, thus simpler mechanism of cell division!
Chromosome attaches to the plasma membrane at the origin of replication
DNA Replication occurs in both directions.
The cytoplasm divides and pinches in forming two new cells.

23. Evolution of Mitosis
Yeast cells and diatoms do not breakdown the nuclear membrane. Instead it pinches inward like the binary fission of prokaryotes. The microtubules are contained within in the nucleus.
Dinoflagellates also do not breakdown the nuclear membrane but the spindle fibers penetrate the nuclear membrane and attach to the plasma membrane.
Mitosis is much more complicated than binary fission.
Major differences
Linear chromosomes as opposed to one circular chromosome
Separating and coordinating the separation and coordination of so many chromatids.
Existence of centrosomes or microtubule organizing centers.
Disintegration of the nuclear membrane and it reformation.
Replication of centrioles
Some connections
The observed movement of bacterial chromosomes which is reminiscent of the poleward movements of the centromere regions of linear chromosomes however the bacterial chromosomes are anchored to the plasma membrane by certain proteins instead of microtubules
There are some proteins used in binary fission that seem to be related to some proteins used in mitosis.
The examples of dinoflagellates and diatoms seem to suggest an earlier more primitive type of mitosis.
In diatoms and yeast cells the spindle fibers remain inside the nucleus and give spatial orientation to the separation of the chromatids.
The nucleus then divides, similar to binary fission
In dinoflagellates the microtubules extend through the nuclear membrane, but the nuclear membrane does not break down.
There is still much to learn about this relationship of binary fission and mitosis.