|Lab 3 Mitosis & Meiosis|
All cells come from other cells. New cells are formed during cell division which involves both the replication of the cell’s nucleus and division of the cytoplasm. The two kinds of cellular division are mitosis and meiosis. Mitosis usually makes body cells, somatic cells. Mitosis is used in adult cells for asexual reproduction, regeneration, and the maintenance and repair of body parts. The process called meiosis makes gametes, sperm and eggs, and spores in plants. Gamete or spore cells have half the chromosomes that the parent cell has.
Mitosis is the first of the cell divisions studied in this lab. It is easily observed in cells that are growing at a fast paces such as whitefish blastula or onion root tips, which are used in this lab. The onion root tips have the highest percentage of cells going through mitosis. The whitefish blastula is formed directly after the egg is fertilized. This is a period of a fast paced growth and numerous cellular divisions where mitosis can be observed. Just before mitosis the cell is in interphase, a part of the cell cycle where the cell has a distinct nucleus and nucleoli. Next is prophase, where the chromatin thickens into distinct chromosomes and the nuclear envelope breaks open releasing them into the cytoplasm. The first signs of the spindle apparatus begin to appear. Next the cell begins metaphase, where the spindle attaches to the centromere of each chromosome pair and moves them to the middle of the cell. This level position is called the metaphase plate. Then anaphase begins when the chromatids are separated and pulled to the opposite poles. The final stage is telophase where the nuclear envelope is reformed and the chromosomes gradually uncoil. Cytokinesis then may occur forming a cleavage furrow and then the two daughter cells will separate.
Meiosis is more complex and involves two nuclear divisions. The two divisions are called Meiosis I and Meiosis II. These two divisions result in the production of four haploid gametes. This process allows increased genetic variation due to crossing over where genes can be exchanged. The process, like mitosis, depends on interphase to replicate the DNA. Meiosis begins with prophase I. In this stage, homologous chromosomes move together to form a tetrad. This is where crossing over occurs resulting in the recombination of genes. Metaphase I moves the tetrads to the metaphase plate in the middle of the cell, and anaphase I reduces the tetrads to their original two stranded form and moves them to opposite poles. Telophase I then prepares the cell for its second division. Meiosis II is just like mitosis except that the daughter cells are haploid instead of diploid. DNA replication does not occur in interphase II, and prophase II, metaphase II, anaphase II, and telophase II occur as usual. The only change is the number of chromosomes.
Mitosis is easily observed in the whitefish blastula and the onion root tip. Meiosis and crossing over occurs in the production of gametes, in animals, and spores, in plants.
Lab 3A. 1
The materials used in this lab are as follows: light microscopes, prepared slides of whitefish blastula and onion root tips, pencil, and paper.
Lab 3A. 2
The materials used in this lab are as follows: light microscopes, prepared slides of onion root tips, paper, and pencil.
Lab 3B. 1
The materials used in this section of the lab are as follows: a chromosome simulation kit, pencil and paper.
Lab 3B. 2
The materials used in this section of the lab are as follows: light microscopes, prepared slides of Sordaria fimicola, pencil, and paper.
Lab 3A. 1
Observe prepared slides of whitefish blastula and onion root tips under the 10X and 40X objectives. Sketch and identify each section of cell division.
Lab 3A. 2
Observe every cell and determine what stage the cell is in. Count at least 200 cells total, separating them into groups of the same phase. Consider it takes 24 hours for the onion root-tip cells to complete the cell cycle.
Lab 3B. 1
Use the lab book to show how to make the chromosomes. The simulation kit has plenty of beads to use. There are red and yellow beads to be used to show the different chromatids. There is also a piece that resembles half of a centromere which has a magnet to connect to another one.
Lab 3B. 2
Use a light microscope to observe the prepared slide and record all data.
Lab 3A. 1
The sketches below show the phases of mitosis for the onion root-tip.
The sketches below show the phases of mitosis for the whitefish blastula.
Why is it more accurate to call mitosis “nuclear replication” rather than “cellular division”?
In mitosis, two new nuclei are formed and the cytokinesis is just part mitosis.
Explain why the whitefish blastula and onion root tip are selected for a study of mitosis.
The blastula is generally a ball of cells that are rapidly going under mitosis. The onion root-tip is an area where mitosis also occurs very rapidly.
Lab 3A. 2
The table below shows the data collected for this lab.
Number of Cells
|Field 1||Field 2||Field 3||Total|
If your observations had not been restricted to the area of the root tip that is actively dividing, how would your results have been different?
There would have been less cell division since most of the plant’s growth occurs at the root-tip.
Based on the data in the table above, what can your infer about the relative length of time an onion root-tip cell spends in each stage of cell division?
The most time of an onion root-tip cell’s life is in interphase. Prophase is the next most common phase that the cells spend in.
Lab 3B. 1
List three major differences between the events of mitosis and meiosis.
Mitosis has only one nuclear division while meiosis has two. Mitosis also makes two diploid cells and meiosis makes four haploid cells. Crossing over is also only found in meiosis.
Compare mitosis and meiosis with respect to each of the following:
|Chromosome number of parent cells||2n||2n|
|Number of DNA replications||1||1|
|Number of divisions||1||2|
|Number of daughter cells produced||2||4|
|Chromosome number of daughter cells||2n||n|
|Purpose||Repair and growth||Gamete and spore production|
How are Meiosis I and Meiosis II different?
Meiosis I ends in two chromosomes with two chromatids and Meiosis II ends in four chromosomes with only one chromatid.
How do oogenesis and spermatogenesis differ?
Oogeneis forms the eggs and spermatogenesis forms the sperm.
Why is meiosis important for sexual reproduction?
Meiosis makes the chromosome number come out in half so that fertilization can come back and restore the diploid number.
Lab 3B. 2
|Number of 4:4||Number of Asci Showing Crossover||Total Asci||% Asci Showing Crossover Divided by 2||Gene to Centromere Distance (Map Units)|
|39||19||58||9.5||9.5 map units|
Draw a pair of chromosomes in MI and MII, and show how you would get a 2:4:2 arrangement of ascospores by crossing over.
No errors were reported in this lab but there could have been. The cells in Lab 3A. 1 could have been miscounted or not counted.
From the data collected in this experiment, it can be concluded that the mitotic stages of the whitefish blastula and the onion root-tip can be observed with a light microscope. The time spent in each phase of mitosis can be recorded and it is concluded that the most time spent in a stage is in interphase. It can also be concluded that the least time spent in a stage is in telophase. It is also understood that someone can simulate meiosis using a chromosome simulation kit. On the last part of the lab, Lab 3B. 2, one could conclude that more asci do not cross over than do the number of asci that do cross over.