biology

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The process of meiosis in cells

Introduction:

In this experiment we observed the process of meiosis by looking at different slides. Meiosis is a process in which a diploid (2n) parent cell is divided into four haploid (n) daughter cells. The daughter cells have half the number of chromosomes as the parent cell. Meiosis mainly occurs in sex cells (gametes) of humans through the process of spermatogenesis (males) or oogenesis (females). It is essential for sexual reproduction, and thus is seen in all eukaryotes that reproduce sexually. Before the cell undergoes meiosis, it first replicates its DNA. Meiosis includes 2 cycles of division- meiosis I and meiosis II. After meiosis I is completed, DNA is not replicated, which leads to the final daughter cells being haploid (n). The first step of meiosis I is prophase I. During prophase I, DNA can be exchanged between homologous chromosomes by tetrads “crossing over”, a process referred to as recombination. The new combination of DNA provides for genetic variation for the daughter cells. In addition, in prophase I, the nuclear envelope disintegrates, and the two centrioles move to opposite ends of the cell. In metaphase I, homologous chromosomes are aligned at the metaphase plate (site where the cell will divide) in pairs. The side at which homologous pairs will lineup is random and further improves the chances for genetic variation. The centrioles attach kinetochore microtubules to the chromosomes, so that they can be pulled apart to the different ends as the cell divides. In anaphase I, the microtubules shorten, pulling the pairs of homologous chromosomes apart from one another. In telophase I, the chromosomes arrive at their respective ends and the cell divides to form two haploid cells. The nuclear membrane is reformed, and the microtubules disappear. The chromosomes uncoil back into chromatin. Note, that even though the first meiotic division led to two haploid cells, each chromosome still contains a pair of sister chromatids. Thus, meiosis II begins without DNA replicating beforehand. The steps in meiosis II are very similar to the ones in meiosis I. In prophase II, the nuclear envelope disintegrates, the centrioles move to the opposite end of the pole, the chromosomes condense and prepare for the second division. In metaphase II, the chromosomes again line up randomly at the plate, but this time independently, not in pairs. The spindle network is formed is also formed. In anaphase II, the sister chromatids are pulled apart and move toward the opposite ends of the pole. Lastly, in telophase II, the cells are cleaved and the nuclear envelope reappears. The chromosomes uncoil and the end result is 4 haploid daughter cells. In spermatogenesis, the 4 daughter cells are the spermatids. However, in oogenesis, even though 4 haploid daughter cells are created, 3 are polar bodies, while the last is an ootid (egg), which might be fertilized by a spermatid. During fertilization (when the spermatid and ootid join), the number of chromosomes reverts back to 2n (diploid). The random alignment and crossing over are very important to the process of meiosis because they provide for greater genotypic diversity. However, if the chromosomes are not able to separate, several errors can arise. Klinefelter and Turner syndromes are due to nondisjunction, during which there is an extra X chromosome present in males, or missing an X chromosome in females, respectively (Russell, 346-349). We also observed the life cycle of the insect drosophila. We will be experimenting on them in the coming weeks. This insect serves as a great experimental organism in the field of genetics due to its short, unique life cycle, and since Mendel’s laws of inheritance (law of segregation, law of independent assortment) are clearly visible when they mate. The law of segregation states that when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy. The law of independent assortment states that alleles of different genes assort independently from each other during gamete formation. The purpose of this experiment was to familiarize ourselves with the process of meiosis and the insect drosophila, as we will be working with them in future experiments. We used slides from human testis, rat testis, and chorthippus testis, to compare the process of meiosis in different eukaryotes. I predict that I will be able to see the stages of prophase, metaphase, anaphase and telophase in the slides.

Hypothesis:

I believe that the process of meiosis will be the same in all three eukaryotes, and I will be able to view the cells differentiating.

I should be able to see the different structures of the insects and be able to distinguish male and female drosophilas based on their appearance.

I believe that I will be able to witness the different stages of meiosis in the slides.

Methods:

Obtain the slides and the compound light microscope from the instructor. Place the first slide on the stage of the microscope (the microscope should be on the lowest power- 40x) and use the coarse adjustment knob to focus the slide. Turn to the next highest power (100x), and this time, use only the fine adjustment knob to bring the slide into focus. Turn the microscope to the 400x power, and again focus the slide. Sketch what you see on a separate sheet of paper and label the different structures. Before moving on to the oil immersion power, put a little drop of oil in the middle of the slide. Focus the image under oil immersion and sketch the results once again. After you’re done sketching the slide, lower the stage and put the microscope back to the lowest power (CAUTION: be careful not to get oil on the 400x power when turning the objectives as this will ruin the lens). Repeat these steps for the rest of the slides (NOTE: for the drosophila male and female slide, the lowest power, 40x, is good enough to get a good overview). The slides we viewed were: chorthippus testis, generalized animal cell, human chromosome (metaphase state), turtle liver mitochondria, drosophila chromosome, drosophila (male and female), rat testis, and human testis. At the end of the experiment, clean all the slides that have oil on them, wipe the oil immersion lens, and return the materials to the instructor.

Results:

Drawings- See Attached Papers

Questions:

1. What major chromosomal event occurs between leptonema and zygonema?

Between leptonema and zygonema, the major chromosomal event that occurs is the pairing of the homologous chromosomes.

2. Do any of the chromosomes at zygonema appear to consist of two parallel parts?  How do you account for this appearance?

Yes, chromosomes at zygonema appear to consist of two parallel parts, which is probably due to the paired homologues.

3. Consult your textbook for a definition of the term chromomere.  Can you detect chromomeres in any of the meiotic cells you are examining? At what substages of prophase I are chromomeres evident?

Chromomeres are dark regions of chromatin condensation. Yes, you can detect chromomeres in meitotic cells; they are usually seen in zygonema of prophase I.

4. Do you observe a large, darkly staining structure in the nucleus during leptonema and zygonema? This body represents an already highly condensed (heterochromatic) X chromosome. Can you follow the fate of this chromosome through the rest of the substages of prophase I and metaphase I?

Yes, it should be possible to follow the fate of this chromosome through the rest of the substages of prophase I and metaphase I. This X chromosome will not align with the rest of the chromosomes at the metaphase plate and will be near one end of the splitting cell or the other.

5.  Briefly list major differences between zygonema and pachynema.

At zygonema, the chromosomes are much less condensed than those at pachynema. Crossing over occurs at pachynema. The number of chromosomes can be determined at pachynema, but not at zygonema.

6. Locate cells in diplonema.  Can you observe a) the two homologous chromosomes in a pair? b) individual chromatids in a chromosome? c) chiasmata?

a) Yes, the homologous chromosomes in pairs are visible. b) Yes, the chromatids are also visible, since the chromosomes at this stage are much coiled. c) Yes, the chiasmata is visible, it is the point where the pair of homologous chromosomes exchange genetic material.

7. Because of the degree of condensation of the chromosomes, diakinesis is an ideal stage at which to determine the chromosome number.  Count the chromosomes in a grasshopper cell at diakinesis. Record the number here.  Does this represent the diploid number? Justify your answer.

Note that sex in grasshoppers is determined by an XO mechanism in which the female is XX, but the male has a single X chromosome.  Therefore, the X chromosome that you observe in diakinesis is not a tetrad. What is the significance of this information for determining chromosome number in grasshopper males versus females?

Since grasshopper males are missing an X chromosome, to find their diploid number of chromosomes, one would have to count the haploid number (n), double it (2n), but then subtract 1, since it is missing an X chromosome. In females, the subtraction will not be necessary; they will always have double their haploid number of chromosomes (example- if haploid number equals 14 chromosomes, male diploid number will equal (2n-1 = 28-1) 27 chromosomes, while the females will have 28 chromosomes in a diploid cell).

8. Observe several cells in metaphase I. Do you notice a chromosome in an unusual position with respect to the other chromosomes in the cell? What chromosome might this be?

Yes, this chromosome could be the X or Y chromosome.

9. Can you find cells in other stages of meiosis or sperm differentiation?  If so, briefly describe their appearance and state what stages you think they might be.

Yes, it is possible to find other stages of meiosis. In metaphase, the chromosomes are lined up at the metaphase plate. In anaphase, the chromosomes are being pulled apart, and in telophase the cells should be separating via cytokinesis.

Conclusion:

The process of meiosis is very complicated, but is necessary for sexual reproduction. There are five substages of prophase I in meiosis. Prophase I is the most important stage in meiosis, since this is the stage where crossing over occurs between homologous pairs of chromosomes, which is essential for genetic variation. The first substage is leptonema where chromosomes begin to condense into long strands and begin to look for their homologous pair. In the second substage, zygonema, the chromosomes have found their pairs. The third substage, pachynema, is where crossing over occurs. In addition, the chromosomes are condensed enough so that one can count the number of chromosomes. In the fourth substage, diplonema, portions of the chromosome begin to separate, and the chiasmata (the site where crossing over takes place) is made visible. The last stage, diakenisis, is where the nucleoli disappears, the nuclear membrane disintegrates, and the four tetrads of a pair of homologous chromosomes are clearly visible (the chromosomes are fully condensed) (Meiosis Prophase I).

When looking at the drosophilas, males were easily distinguishable from females. Males were smaller in size compared to the females. The end of the male was more rounded, while the female was pointier. Females had more of a striped pattern on their ends, while males have black as the dominant color. Lastly, males have a sex comb at the joint of each front leg (males also have a penis) (Hammersmith & Mertens, 5).

In the generalized animal cell, I was able to identify the nucleus and the nuclear envelope. In the human chromosome slide of metaphase, the chromosomes were lined up, which means they were about to be separated. In the human, rat and chorthippus testis, I had a difficult time identifying the different cell types, or cells in different phases of meiosis. Meiosis is an essential process, and if an error occurs, the consequences could be lethal.


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