Eddy Egan January 14, 2009Contreras Pd. ¾ EvenLab 3

Observing mitosis of prepared Whitefish Blastula cell slides and Calculating the time (in minutes) that each stage of Mitosis undergoes in Onion root tip prepared cell slides

Cell division is an integral part of the cell cycle in all plants, organisms and all

living things in general. Passing genetic information from parent cells to offspring allows

plants and organisms to grow and growth is vital to life. In both plants and animals the

process involves the distribution of genetic material to daughter cells. The process of cell

division involves two different types: mitosis and meiosis. Simply stated, Mitosis divides

the nucleus of a cell to produce these two daughter cells and consists of five stages. The

stages in an animal cell include Prophase- chromatin is condensing and the mitotic

spindle begins to form, but the nucleus is still intact, Metaphase- spindle is complete and

the chromosomes are all aligned at the metaphase plate, Anaphase- the chromatids of

each chromosome have separated and the daughter chromosomes are moving back to the

poles of the cell, Telophase- daughter nuclei are forming and Cytokinesis has begun. In

plants, this process is quite the same except for Cytokinesis. Instead, while in Telophase,

vesicles from the Golgi apparatus move along microtubules to the middle of the cell,

producing a cell plate. Cell wall materials are carried in the vesicles that make up the cell

plate and then are released, and actually form two cell walls. (Campbell pg. 223)

Meiosis is a process that is similar to mitosis and even has a few of the same

processes as mitosis does. Meiosis has the stages of Prophase, metaphase, anaphase,

Telophase and Cytokinesis but these processes happen twice. Meiosis I separates

homologous chromosomes and meiosis II separates sister chromatids. Meiosis is different

from Mitosis in a few procedures that happen during the phases. During Prophase I of

meiosis, homologous chromosomes loosely pair along their lengths, precisely aligned

gene by gene. Then, the nonsister chromatids break at corresponding place and then

rejoin to the other’s DNA. Each tetrad has more than one of these places of crossing over

called chiasmata. The same process as Mitosis then happens, movement of centrosomes,

formation of spindle microtubules, breakdown of the nuclear envelope, and dispersal of

nuclei. In metaphase I, Pairs of homologus chromosomes, in the form of tetrads, line up

along the metaphase plate with one chromosome of each pair facing each plate. This

allows one of Mendel’s laws to come into affect, the law of independent assortment:

tetrads align independently of each other during metaphase I ( meaning 8,000,000

possible alignments with the 23 human tetrads 2 to 23rd power) Because alleles can

separate independently, there are more chances for variation among the products of

meiosis. In mitosis, this stage lines up the chromosomes and separates the sister

chromatids from each other and no variation takes place. Now in anaphase the

chromosomes more toward the poles as in Mitosis but sister chromatids stay attached to

one another but homologous chromosome separate. The law of segregation that Mendel

had made can be observed during anaphase I since the homologous chromosomes can

independently assort, segregate, and randomly unite. Telophase and Cytokinesis are

relatively the same. The process now starts over with these two cells, thus making four

daughter cells. Due to the crossing over, there will be a larger amount of diversity

compared to Mitosis which makes replicas of itself. Prophase II allows the spindle to

form. Metaphase II and Anaphase are exactly like Metaphase and Anaphase in Mitosis.

Nuclei then form during Telophase and Cytokinesis and the cells split. Each of the four

daughters have a haploid set of unreplicated chromosomes as compared to the Diploid

replicated that Mitosis produces. (Campbell pg. 245) I have mentioned a few differences

between Mitosis and Meiosis above but the key ones include the number of divisions in

each process: Two in meiosis and one in Mitosis. The number of daughter cells of each

process: two in Mitosis and four in meiosis. The functions of each: Mitosis forms somatic

cells, any cells that form the body of an organism while meiosis forms a gamete which

fuses with another gamete during fertilization in organisms that reproduce sexually. The

last key difference is the number of chromosomes at the end of the processes: Mitosis

generates 46 chromosomes in each daughter cell (2n) while meiosis generates 26

chromosomes (n) in four daughter cells. (Campbell pg. 246)

In animals, the process of making gametes through meiosis is called

gametogenesis. It includes the same thing we are studying and the same processes as

meiosis but relates the function to the name a lot clearer than meiosis gives. In plants the

process of sporogenesis is the same way and is another name for meiosis but instead

produces spores. This can be found in plants and includes algae and fungi. The pictures

below demonstrate both of these processes clearly and in simplified detail.

Picture 1.1 - Gametogenesis represents the collective processes of mitosis, meiosis, and developmental events necessary for the production of male and female gametes in sexual reproduction. Spermatogenesis is the production of sperm cells in the male testis, or sex organ; oogenesis is the production of eggs in the female ovary.

Picture 1.2 – “At the beginning of spore formation, a septum forms, separating the nascent spore from the rest of the cell and all of the genetic material of the cell is copied into the newly-forming cell. the spore contents are dehydrated and the protective outer coatings are laid down. Once the spore is matured it is released from the cell. On germination, the spore contents rehydrate and a new bacterium emerges and multiplies.”

Procedure1. Acquire both onion root tip cells (plant cells) and whitefish blastula cells (animal

cells)2. Use the scanning lens of the microscope to locate a viable specimen to view of the

whitefish blastula3. Switch to the highest power lens of the microscope and view your specimen.4. Next, you will be sketching the cells that you are observing. This means that you

must be able to distinguish all of the different characteristics of both mitosis and meiosis. It would be helpful to have a textbook or another reference open in order to fully know which steps are taking place.

5. Observe the Onion root tip specimen next and view under high power after finding a viable sample.

6. Looking at the slide, count and record the number of cells in the field of view that are in each phase.

7. Determine the total number of cells counted.8. Determine the percent of cells that are in each phase. 9. To calculate the time (in minutes) for each phase, multiply the percent of cells in

that phase by the number of minutes for the whole cycle using the equation below.

Time in each stage calculation =Percentage of cells in stage (in decimal form Ex. 50% = .5) x 1,440 minutes _____ minutes of cell cycle spent in stage

Data

Number of Cells in each stage, percentage of cells in each stage, and calculated time in each stage of Mitosis in a prepared slide of Onion Root tips

Number of Cells Percent of Total Cells

Counted

Time in Each Stage (minutes)Field 1 Field 2 Total

Interphase 215 239 454 91.5% 1,317.6Prophase 12 9 21 4.2% 60.48

Metaphase 6 7 13 2.6% 37.44Anaphase 2 3 5 1.0% 14.4Telophase 1 2 3 .06% 8.64

- Total Cells Counted 496Table 3.1

Time in each stage calculation =Percentage of cells in stage (in decimal form Ex. 50% = .5) x 1,440 minutes _____ minutes of cell cycle spent in stage

Relative Time in Each Stage of the Cell Cycle of Onion Root Tip Cells

Interphase

Anaphase

TelophaseMetaphaseProphase

Interphase Prophase Metaphase Anaphase Telophase

Pie Cart 1.1

Analysis/Discussion

Our data clearly showed trend of data that we had expected before going into the

lab. Interphase is the longest part of the cell cycle and our data was supported by this fact.

The rest of the data showed a decline in cells in each stage and that is exactly what

usually happens in the cell cycle, Interphase being the longest and then in sequential

order the number diminishes.

Mitosis produces two daughters cells that are both diploid and genetically

identically to the original cell and each other. In order to achieve this, the process of

Mitosis must carefully regulate all of its phases for complete accuracy. The cell is in

Interphase for 90% of its cycle. During this very long phase the cell grows and copies its

chromosomes in preparation for cell division. Interphase is then divided into three

separate sections or subphases: the G1 phase (aka first gap: early scientists saw it as a

gap), the S phase (synthesis occurs here), and the Gf2 phase (aka second gap). During

these phases the cell grows and produces proteins and doubles its organelles in

preparation for division. The S phase allows for chromosomes to be replicated. In these

subphases are checkpoints. These checkpoints allow for the cell cycle to be regulated and

act as a “quality control” of the cell and verify whether the processes at each phase of the

cell cycle have been accurately completed before progression into the next phase. After

the long period of Interphase, the cell enters mitosis and the process of cell division takes

place. The cell undergoes five more stages and the daughter cells are produced, the

introduction fully covers this topic and what each stage is composed of. (Campbell pg.

221)

Plants and animals have quite a few differences in their processes of mitosis. In

prophase, both animals and plants condense their chromosomes but in animals the mitotic

spindle forms between centrosomes on one side of the nucleus. Plants instead form their

spindle around the nuclear envelope with microtubules and Actin microtubules form

under the plasma membrane at the place where then new cell wall will be formed when

the cell divides. The nuclear envelope in both plants and animals breaks down, but in the

plant the actin microtubules in the membrane break down just after prophase. Metaphase

and Anaphase are completely alike between plants and animals. In Telophase the nuclear

envelope reforms, the chromosomes begin to decondense and the spindle breaks down in

both. During this phase, plants begin the task of setting up a new cell wall. The proteins

of actin, myosin, and protein made microtubules form in the center of the cell where the

new cell wall will be formed. Animals have contractile ring made of actin and myosin

that forms about midway between the two nuclei. When Cytokinesis takes place and the

real division of the two cells takes place, animal cells contract the center ring of actin and

myosin and it in turn pinches the two daughter cells apart. In plants, the actin, myosin,

and microtubules extend to the cell wall on both sides of the original cell and a new cell

wall between the two daughter cells is completed. (Campbell pg. 226)

The centrosome is present in the cytoplasm of all eukaryotic cells and plays a

major part of cellular division and especially for mitosis. The centrosome is where the

microtubules in the cell assemble and connect to the chromosome. It is usually called the

microtubule organizing center for this capability and plays a key part in mitosis. Without

the centrosome, the chromosomes would not be capable to being separated and no

division would take place. (Campbell pg. 221)

The tip of the onion roots provided an excellent medium for studying cell

division. Since roots are constantly growing and moving further into the ground, the roots

will have to get performing cell division to achieve these longer lengths. We easily

observed this high activity under our microscopes and could see all of the phases of cell

division since so much activity was taking place. This is only one of the two places on a

plant that replication occurs. That means that if we were to take a sample from another

pant of the plant, no activity or division would take place and we wouldn’t be witnessing

much to do with the experiment. Our results would have come up as inconclusive

because they would all be in the G0 phase, or simply stated, the phase that only steadily

survives for awhile but no reproducing takes place.

Our data strongly supports that Interphase is a very long process and the cell

spends over 90% of its time in it. (Campbell pg. 221) The mitosis process is very brief

and accounts for a small part of the cells life cycle but is ultimately a very large part of it.

Looking at our textbook page of 221, it clearly states that Interphase occupies 90% of the

cell cycle, meaning our data was strongly supported and had a small percentage of error

in relation to the Interphase stage.

Although there are quite a few differences between the events of mitosis and

meiosis, three major ones that can be recognized include the following. In mitosis, the

nucleus is only divided once but happens in all cells, while in meiosis the nucleus is

divided twice but only happens in cells in the gonads (sex cells). Another difference is

that mitosis produces two identical daughter cells each with 46 chromosomes (2n), but

meiosis produces up to four different daughter cells each with 23 chromosomes (n). Also,

synapses’ and crossing over do not take place in mitosis, but do take place in meiosis and

are called chiasmata as stated before.

Mitosis MeiosisChromosome

Number of Parent Cells

46 46

Number of DNA

1 1

ReplicationsNumber of Divisions

1 2

Number of Daughter Cells

Produced

2 4

Chromosome Number of

Daughter Cells

46 23

Purpose/ Function

Mitosis produces the cells of the body. When the cells go through mitosis they

split and form exact copies of themselves with a complete set of DNA.

The process can be used to replace or repair damaged or destroyed tissues of your body. An example can be a cut on

your arm. Mitosis takes over and produces epithelial cells that heal they

would by replicating themselves.

Meiosis produces the gametes or the "sex cells" such as sperm and eggs. Meiosis consists of two divisions that produce 4 daughter cells, each with different genetic material. This genetic variation allows for variation in a population. At fertilization male and female gametes come together to form an offspring with half of its mothers genes and half of its fathers. Meiosis keeps chromosome number constant across the generations, produces genetic variation, makes sure that each gamete contains only one member of each homologous pair and eventually leads to the creation of offspring

When meiosis I is completed, to daughter cells are haploid but the chromosomes

are still doubled stranded as they have their sister chromatids connected. The homologous

pairs that had been attached to them have already been separated. The original cell had 23

pairs of chromosomes but at the end of meiosis I the cells now have 23 chromosomes,

not pairs. Meaning they still look like an X in configuration. In short, meiosis I reduces

the ploidy level from 2n to n.

In meiosis I, the DNA had been replicated but does not happen in meiosis II. At

the end of meiosis II, there are a total of four daughter cells, each of which is haploid. At

this point, the sister chromatids have separated from each other. This means that the

gametes each have 23 chromosomes, and each has one chromatid. Also, crossing over

occurs during prophase I of meiosis I but does not happen in meiosis II.

In spermatogenesis, the primary spermatocyte grows larger in order to

accommodate the splitting of the cytoplasm. The spermatocyte then goes through meiosis

and splits the beginning sperm cell into 2 and then 4 cells. Since the cytoplasm grew

extra large, each spermatocyte was able to survive and have an equal amount of

cytoplasm and in turn into four separate sperm cells.

In oogenesis, the beginning egg cell grows and matures to a larger state with a lot of

cytoplasm. The cell then goes through meiosis I and the product includes one cell that is

larger than the other one. The other cell (now called a Polar body 1) becomes small and

useless. Both cells, the larger cell and polar body one, goes through meiosis II. The first

polar body creates to smaller polar bodies that will eventually die. The larger cell divides

into two cells with one being larger than the other again. One cell will receive more

cytoplasm than the other and a total of three polar bodies are formed that will die. The

final cell differentiates and eventually turns into an ovum or an egg.

So in spermatogenesis, four viable small same sized sperm cells are produced and in

oogenesis, one big viable egg is produced along with three unviable cells. The sperm

cells are only created in male organisms that sexually reproduce while eggs are produced

only in female organisms that sexually reproduce.

Each cell has two copies of each chromosome, making it diploid. Meiosis changes

this ploidy level into haploid cells (n) each with 23 chromosomes. A haploid cell from

each parent, the sperm and the ova, fuse together (fertilization) to make one diploid cell

which then produces a whole new organism or offspring. The offspring can have the

genes of both parents due to meiosis and the fertilization of the cell. Without meiosis,

genetic variation would not be possible as only mitosis would occur and every organism

on the planet would consist of one type of cell. Meaning no organs could be assembled to

make complete organisms. Variation would be inexistent. Since mating would do

absolutely nothing to improve the fitness of a species, it would not occur even if it was

possible. Again, if only one type of cell survived then pandemics would become fierce

and could certainly wipe out our entire globe if it got bad enough. Our bodies would be

ineffective at fighting anything off since we would be comprised of one type of cell that

could not be mutated into a cell that was resistant to certain strains. Meiosis allows for

these mutations and adaptations to exist. Meiosis constantly increases genetic diversity

especially in today’s times when interracial couples are extremely prevalent and we have

planes, trains, and buses that can take us to all types of foreign lands. Meiosis allows for

evolution to continue therefore increasing survival rates against plagues and making

future generations better adapted and maintains the survival of entire species.

Other Experiments

Other experiments that could lead from this one could be many. One that would

really interest me and inspire my full attention would be to witness cells going through

the cell cycles before our eyes. Of course we can watch videos of this on the internet but

when something is brought out in front of you to your attention it just pulls you in and

engulfs your curiosity. Like our teacher had stated while we were in our labroatoy, we are

going to be spending countless ours with ours eyes looking through microscope lenses so

why not start on that now. Instead of finding which cells are doing what on a prepared

slide, we could witness the entire process before our eyes.

Another experiment that would be exciting to do is watching the fertilization on

an egg. Although this would require a lot of work and would probably be very expensive,

the whole process is just mind blowing. Two of the smallest cells in the body fuse

together and eventually create and living, breathing person. It’s both a thing of science

and a thing of art in my mind. Fusion of the gametes and then an explosion of cell

division that then divide into all of the tissues, organs and everything else you need to

create a full, live human being complete with thoughts, memories, and the ability to

gather unlimited knowledge.

References

Campbell, Neil A., and Jane B. Reece. Biology: AP* Student Edition. 7th ed. San

Francisco, CA: Pearson Education, Inc., 2005.

Mitosis and Meiosis. Lab Three Handout.