3A: Observing Mitosis in Plant and Animal Cells Using Prepared Slides of the Onion Root Tip and Whitefish Blastula
Purpose
For this experiment we were required to identify what stages of mitosis each of the onion root tip and whitefish blastula’s cells were in. We were expected to understand and identify each of the stages of mitosis of both organisms cells. And after the cells were identified they needed to be counted to estimate the length of time the cells spent in the each stage of the cell cycle.
Introduction
Mitosis is an essential process to all organisms whether they are plants, animals, bacteria, etc. because it allows organisms to grow with the use of identical somatic cells. This experiment observes the steps throughout the process of mitosis in samples of onion root tip and whitefish blastula. Onion root tip contains three different regions that can be easily identified in the provided slide. There is a region of maturation, elongation, and cell division. As cells are located more towards the terminal of the onion root tip, they progress to later stages of mitosis. The whitefish blastula samples lacked these distinct regions, but the different stages could still be readily identified.
Methods
The procedure for studying the progression of plant cells through mitosis using these samples was fairly simple. After placing each sample under a microscope, a 10X objective was used to find the areas of the dividing cells. A 40X sample was then used to identify cells in each of the five stages of mitosis (interphase, prophase, metaphase, anaphase, telophase) as described by biologists.
The tip of the needle under the microscope points to the cell undergoing each phase in the following pictures. Pictures were taken using a phone’s camera held up against the eye lens of the microscope. The phones had to be moved around and their lenses had to be zoomed in in order to show the cells clearly.
(Onion Root Tip)
Interphase
Prophase
Metaphase
Anaphase
Telophase
(Whitefish Blastula)
Interphase
Prophase
Metaphase
Anaphase
Interphase
In the second part of this experiment, onion root tip cells were examined to calculate the estimated time (in minutes) cells spend in each stage of mitosis. These amounts of time can be inferred from counting the number of cells in each stage because there will be more cells in a certain phase at one time if that phase takes longer to complete.
Two lab group members were needed to do the counting. One person would examine the cells under the microscope, identifying the phase of each cell out loud. The second person would then tally each cell under its respective phase of mitosis. The data collected from this follows.
Data
Graphs & Charts
Discussion
The counted number of onion root tips cells in each phase can be subject to some error, however. The approximate number of cells depends on the ability of the person counting to correctly identify the stage and the ability of the person keeping track of the counting to keep up with them. As seen from the chart, data from three different fields of view were used in counting. This was done because one field of view would give an inaccurate idea as to how long cells throughout the sample would undergo each phase. For example, a field of view closer to the very tip of the sample would contain more cells in the latter stages, anaphase and telophase. Multiple fields of view spaced across the sample would give more accurate numbers.
Judging by the calculated numbers of minutes for each phase, the amount of time in the phase generally shortens as mitosis progresses. It takes more than half of the time phrase for interphase to occur while it takes only 2% of the time for anaphase to occur. From this, it can be inferred that it takes longer for the nondividing cell to prepare for splitting than for a cell to actually split.
Conclusion
As seen visually from the number of onion root tip cells in each phase, the greatest number of cells were counted under interphase, and this signified that this phase took the longest amount of time to complete. Assuming that the entire cycle would take a 24-hour day, based on the number of minutes in a day (1,440), it would take 58.1% of the time or 836.64 minutes of the day for a cell to undergo interphase. In order from greatest to least amount of time, the stages are interphase, prophase, metaphase, telophase, and anaphase.
References
- The procedure and chart included here were obtained from the "Lab Three: Mitosis and Meiosis" document from CollegeBoard.
3B: Simulation of Meiosis
Purpose
To better understand meiosis and crossing over within meiosis lengths of beads were provided to simulate chromosomes. As instructed the beads were placed into a structure replicating the chromosomes in the different stages of meiosis I and meiosis II. In part two, crossing over was further examined when we counted the Asci in order to count the centromere distance (map units) between genes.
Introduction
Unlike mitosis, meiosis goes through two nuclear divisions ending with four different daughter cells increasing genetic variation. The differences in cells is a result from crossing over. Meaning chromatids of homologous pairs exchange physical parts of genes. Crossing over occurs during prophase I. Meiosis I reduces chromosome numbers from diploid to haploid and separates homologous pairs. While meiosis II separates the sister chromatids and ends with the four new cells.
Methods
Special meiosis simulation kits were used in imitating the different stages of meiosis. Two strands of one color represented one chromosome, and two pairs of the two colors represented homologous chromosomes. The creation of each stage had specific instructions.
Interphase includes DNA replication. The formation of identical chromatids was simulated by bringing the magnetic centromeres of strands of the same color together so that they connected.
Crossing over, which is unique to meiosis I, takes place in prophase I. This was simulated by popping off beads from one strand of each chromosome and swapping those with its homologous chromosome.
In metaphase I, the chromosomes line up in the center of the cell.
Also unique to meiosis I is the way in which whole chromosomes are pulled to opposite poles during anaphase I in the cell rather than single chromatids.
Formation of the first two daughter cells from cytokinesis and later re-development of the nuclear envelope takes place in telophase I. Note that there is no interphase during the two stages of meiosis because DNA is being further divided rather than replicated.
Prophase II includes movement of the two daughter cells to different areas within the organism so their further process of meiosis take place farther apart.
In metaphase II, the chromosomes are aligned in the center of each daughter cell.
In anaphase II, the chromatids of the chromosome within each daughter cell move to separate poles.
Meiosis ends after telophase II when the nuclear envelope forms, cytokinesis occurs, and the two daughter cells each split to form four daughter cells.
Data
Graphs & Charts
Graphs and charts were not necessary to this portion of the lab.
Discussion
Taken from this experiment were three major differences of mitosis and meiosis. In prophase I of meiosis each pair of chromosomes come together in a process called synapsis. A second difference happens during anaphase I, homologous chromosomes separate and eat the spindle fiber towards the opposite sides of the cell. DNA replication does not occur during interphase II, renamed interkinesis. This is the final difference of mitosis and meiosis. There are also a few differences in meiosis I and II; meiosis I starts with one parent cell that has 46 chromosomes and meiosis II starts with two parent cells that each have 23 chromosomes.
The oogenesis is the female gamine division although it produces four cells ,only one cell has the correct amount of organelles while the other three do not have organelles and therefore are at a disadvantage. The spermatogenesis is the male gamete division and the four haploid daughter cells are fully equipped for fertilization. Resulting in meiosis being important for sexual reproduction, it produces gametes with 23 chromosomes allowing different daughter cells with 23 gamete chromosomes to create an organism.
Conclusion
We learned about the differences between mitosis and meiosis, that crossing over only occurs during meiosis and all of the phases of each of them.
References
- The procedure and chart included here were obtained from the "Lab Three: Mitosis and Meiosis" document from CollegeBoard.
