When is the DNA replicated during meiosis?
As in mitosis, DNA is replicated during the S phase of the cell, between the G1 and the G2 phases of the cell. The only difference between mitosis and meiosis is that the cell that is dividing for mitosis is a regular somatic cell, while the cell used for meiosis is a specialized germ cell located in the sexual organs whose sole purpose is to grow and divide into 4 gametes.
Are homologous pairs of chromosomes exact copies of each other?
Homologous pairs of chromosomes are not exact copies of one another because one of the homologs is given to the cell from its father, and the other is from its mother. When DNA is replicated, the entire chromosomes are not replicated, only the sister chromatids from each of the parent's genes. There are many genetic variations between the chromosome belonging to the father and the chromosome belonging to the mother, and this in turn promotes genetic variation.
What is crossing over?
Crossing over is a process by which homologous pairs of chromosome form crossing points, called synapse, and exchange sets of DNA between each other. This means that the father's chromosome could trade its portion of DNA that codes for hair color, for example, with the mother's chromosomes gene for hair color. The resulting chromosomes are called recombinant chromosomes. This increases genetic variation because then it means that gametes can have random combinations of their parents DNA, not just be forced to have one or the other parents's genetic information.
What physical constraints control crossover frequency?
The physical constraint for crossing over are where the gene is on the chromosome. Because crossing over could lead to huge problems in a gametes DNA if a gene is spliced in half incorrectly and would therefore become defective, crossing over can only happen when two complete genes are switched. Therefore, only certain sections of the chromosome can swith, and if they do switch, the entire gene is moved.
What is meant by independent assortment?
Independent assortment is the process by which homologous chromosome pairs line up along the metaphase plate during meiosis I. When the chromosomes line up, they are not forced to have all of the maternal DNA on one side and all the paternal DNA on the other. Instead, because the homologs assort independently, there is a random mixture of paternal and maternal DNA on each side of the metaphase plate. Thus, in turn, when the homologs separate, a random mixture of DNA is put into each resulting gamete.
What happens if a homologous pair of chromosomes fails to separate, and how might this contribute to genetic disorders such as Down syndrome and cri du chat syndrome?
When a homologous pair of chromosomes fails to separate, the gametes resulting from that meiosis will either have one less or one extra chromosome. Without the correct number of chromosomes, if this gamete becomes a zygote, the child will face genetic disorders such as downs syndrome, which is having an extra chromosome 20, 21, or 22; or cri du chat syndrome, which is the deletion of the 5th chromosome. The child's DNA is messed up, and therefore their mental capabilities and many of their body's structures are abnormal.
How are mitosis and meiosis fundamentally different?
Mitosis and meiosis are fundamentally different in both their purpose and their products. For mitosis, the purpose is just to produce two identical, diploid cells to either increase the size of an organism as in a teenager going through puberty, or replace dead cells such as on a skinned knee after a bicycle accident. Meiosis, on the other hand, produces 4 haploid cells used for sexual reproduction. Each cell produced is different from the rest of the haploid cells produced because of processes that promote genetic diversity like crossing over and independent assortment.
Link to meiosis video:
https://www.youtube.com/watch?v=OZLBftmBsow
Tuesday, December 8, 2015
Sunday, December 6, 2015
Investigation 7: Part 2
Introduction
Cells, the building blocks of the body, are constantly dying and replacing themselves. Through a process called mitosis, one cell divides itself into two daughter cells that can then grow and divide as well. Though mitosis is the most lively stage of a cell's life cycle, it is also the smallest portion of that cell's life, with large phases of growth and DNA replication occurring alongside mitosis. Pictured below is a diagram representing a typical cell cycle.
As seen above, a cell's life is mostly growth and DNA replication that is punctuated by brief period of mitosis. So, this large time period, called Interphase, is just as important as mitosis, even though mitosis is where the action is at. Returning to the lab, Vikram, Vinay, Shreyan and I were tasked with the seemingly simple mission of counting cells in a growing root tip. In the root tip, because it is growing, there would be many cells performing mitosis and also quite a few in Interphase, as they are all over the organism's body. We had to find out how many cells were undergoing mitosis, and also we counted how many cells were in their Interphase stage.
Unfortunately for us, there isn't an exact indicator on when a cell is undergoing mitosis or not, so we had to use our eyes and make an educated guess on whether or not a certain cell was dividing or not. We also tried to compare the cells we saw under the microscope to diagrams such as the one below, which shows the different stages of mitosis so we could compare the two and determine if the cell in question was dividing or not.
As well as simply counting cells of a regular root tip for our control group, we also had to count cells of a root tip that had been specially treated as an experimental group. We were not told what the chemical was that the cells had been treated with, but we expected an increase in the number of cells undergoing mitosis because we believed that the chemical would mess with the cell cycle, but there was no explanation as to why we believed the rate of mitosis would increase.
Procedure
To begin, the group picked up two microscope slides from Mr. Wong, one regular, unlabeled slide, and a slide that had been treated with the unknown chemical and was labeled "T". Then, using the microscope, we took a picture of a certain part of each slide, and transferred the pictures to our iPads where we expanded the pictures and began labeling whether or not a cell was in mitosis. If the cell was undergoing mitosis, it was labeled "M", if it wasn't, it was labeled "I" for Interphase. After counting all of the cells that we could, we tallied up the number of cells in mitosis and not in mitosis, and then we shared our numbers with the class. The date is below.
Data/Pictures
Here is the table of all of the class's data from groups 1 to 6.
Here is a picture of the cells that we counted for our control group:
Here is the picture of the cells that we counted for our experimental group:
Conclusion
For our conclusion, rather than writing in the regular paragraph form, we were asked to answer the questions associated with our lab in our lab packet, and also some Postlab Review Questions posted on Canvas.
Investigation 7 Part 2
Collect the class data for each group, and calculate the mean and standard deviation for each group. Make a table. These are the observed values for the class.
Table 1
Use the data from Table 1 to calculate the totals using formulas found in Table 2
Table 2
Table 2 Completed
Use the totals from Table 2 to calculate the expected values (e) using the formulas from Table 3.
Table 3
'
Cells, the building blocks of the body, are constantly dying and replacing themselves. Through a process called mitosis, one cell divides itself into two daughter cells that can then grow and divide as well. Though mitosis is the most lively stage of a cell's life cycle, it is also the smallest portion of that cell's life, with large phases of growth and DNA replication occurring alongside mitosis. Pictured below is a diagram representing a typical cell cycle.
 |
| The Cell Cycle |
As seen above, a cell's life is mostly growth and DNA replication that is punctuated by brief period of mitosis. So, this large time period, called Interphase, is just as important as mitosis, even though mitosis is where the action is at. Returning to the lab, Vikram, Vinay, Shreyan and I were tasked with the seemingly simple mission of counting cells in a growing root tip. In the root tip, because it is growing, there would be many cells performing mitosis and also quite a few in Interphase, as they are all over the organism's body. We had to find out how many cells were undergoing mitosis, and also we counted how many cells were in their Interphase stage.
Unfortunately for us, there isn't an exact indicator on when a cell is undergoing mitosis or not, so we had to use our eyes and make an educated guess on whether or not a certain cell was dividing or not. We also tried to compare the cells we saw under the microscope to diagrams such as the one below, which shows the different stages of mitosis so we could compare the two and determine if the cell in question was dividing or not.
As well as simply counting cells of a regular root tip for our control group, we also had to count cells of a root tip that had been specially treated as an experimental group. We were not told what the chemical was that the cells had been treated with, but we expected an increase in the number of cells undergoing mitosis because we believed that the chemical would mess with the cell cycle, but there was no explanation as to why we believed the rate of mitosis would increase.
Procedure
To begin, the group picked up two microscope slides from Mr. Wong, one regular, unlabeled slide, and a slide that had been treated with the unknown chemical and was labeled "T". Then, using the microscope, we took a picture of a certain part of each slide, and transferred the pictures to our iPads where we expanded the pictures and began labeling whether or not a cell was in mitosis. If the cell was undergoing mitosis, it was labeled "M", if it wasn't, it was labeled "I" for Interphase. After counting all of the cells that we could, we tallied up the number of cells in mitosis and not in mitosis, and then we shared our numbers with the class. The date is below.
Data/Pictures
Here is the table of all of the class's data from groups 1 to 6.
|
|
Control
|
Experimental
|
||
|
Group
|
Interphase
|
Mitosis
|
Interphase
|
Mitosis
|
|
1
|
131
|
11
|
147
|
22
|
|
2
|
140
|
9
|
142
|
3
|
|
3
|
155
|
16
|
272
|
18
|
|
4
|
368
|
17
|
162
|
23
|
|
5
|
141
|
5
|
330
|
15
|
|
6
|
234
|
5
|
269
|
21
|
Here is a picture of the cells that we counted for our control group:
Here is the picture of the cells that we counted for our experimental group:
Conclusion
For our conclusion, rather than writing in the regular paragraph form, we were asked to answer the questions associated with our lab in our lab packet, and also some Postlab Review Questions posted on Canvas.
Investigation 7 Part 2
Collect the class data for each group, and calculate the mean and standard deviation for each group. Make a table. These are the observed values for the class.
Table 1
|
|
Control
|
Experimental
|
||
|
|
Interphase
|
Mitosis
|
Interphase
|
Mitosis
|
|
Mean
|
195
|
11
|
220
|
17
|
|
Standard Deviation
|
93
|
5.2
|
80
|
7.5
|
Use the data from Table 1 to calculate the totals using formulas found in Table 2
Table 2
|
|
Interphase
|
Mitosis
|
Total
|
|
Control
|
A
|
B
|
A + B
|
|
Treated
|
C
|
D
|
C + D
|
|
Total
|
A + C
|
B + D
|
A + B + C + D = N
|
Table 2 Completed
|
|
Interphase
|
Mitosis
|
Total
|
|
Control
|
195
|
11
|
206
|
|
Treated
|
220
|
17
|
237
|
|
Total
|
415
|
28
|
443
|
Use the totals from Table 2 to calculate the expected values (e) using the formulas from Table 3.
Table 3
|
|
Interphase
|
Mitosis
|
|
Control
|
[(A + B)(A + C)]/N
|
[(A + B)(B + D)]/N
|
|
Treated
|
[(C + D)(A + C)]/N
|
[(C + D)(B + D)]/N
|
Table 3 Completed
Enter the observed Values from Table 2 and the expected values from Table 3 for each group into Table 4. Calculate the chi-square value for the data by adding together the numbers in the right column.
Table 4
Chi-Square value = 0.473
Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Postlab Review
What was the importance of collecting class data?
The importance of collecting class data was to create a large enough sample size to calculate a valid chi-square value. If the sample size used for this value was extremely small, as it would have been had the values used in calculations had only come from one lab group, then the chi-square value would be less significant. This is because the single lab group could have been a fluke and then the rest of the class could have gotten completely different sets of data.
Was there a significant difference between the groups?
There was not a significant difference between the group. Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Did the fungal pathogen lectin increase the number of root tip cells in mitosis?
If this slides in this experiment were treated with lectin, I think we might have seen a change in the number of root tip cells in mitosis, but Mr. Wong revealed to us that the "treated" slides were not in fact treated with anything. Rather, Mr. Wong just labeled some control slides "T" and let our minds do the rest. During the experiment, as groups began reading off data, Mr. Wong actually told some groups, including mine, to recount their cells because their data was wildly off in that it showed a large increases in the number of cells in mitosis in the treated group as compared to the control. This is actually a phenomenon studied in psychology called observer bias, or the tendency of researchers to see data in a light that is favorable to their hypotheses. This is a common human error that the entire class made for this experiment.
What other experiments should you perform to verify your findings?
Other experiments that could be performed would be to look at more root tip cells or cells of a different plant and then counting cells and seeing if the chi-square value is different for those. Or we could actually use slides treated with something to determine the effects an environment can play on cells.
Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with a lower number of cells in mitosis?
An increased number of cells in mitosis does in fact mean that the cells are dividing at a faster rate than in roots with lower numbers of dividing cells because all cells are on their own specific timers of when to divide and when to not divide. The cells are not synced up at all, because that would be catastrophic if all of our cells divided at one time. Therefore, when the rate of all cells dividing speeds up, more cells are brought to mitosis around the same time, so during a snapshot look into the lives of the cells, as in the treated root tip slides, more cells would be undergoing mitosis than in a regular root tip.
What other way could you determine how fast the rate of mitosis is occurring in root tips?
Another way to measure the rate of mitosis in root tips is to measure the growth of the root tip as a whole. As cells divide and grow, more space is taken up and the tip of the root is pushed farther and farther along. So, as the tip expands, there are more cells performing mitosis.
|
|
Interphase
|
Mitosis
|
|
Control
|
192.98
|
13.02
|
|
Treated
|
222.02
|
14.98
|
Enter the observed Values from Table 2 and the expected values from Table 3 for each group into Table 4. Calculate the chi-square value for the data by adding together the numbers in the right column.
Table 4
|
Group
|
Observed (o)
|
Expected (e)
|
(o - e)
|
(o - e)2
|
(o – e)2/e
|
|
Control I
|
195
|
192.98
|
2.02
|
4.0804
|
0.021
|
|
Control M
|
11
|
13.02
|
-2.02
|
4.0804
|
0.167
|
|
Treated I
|
220
|
222.02
|
-2.02
|
4.0804
|
0.018
|
|
Treated M
|
17
|
14.98
|
2.02
|
4.0804
|
0.267
|
Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
Postlab Review
What was the importance of collecting class data?
The importance of collecting class data was to create a large enough sample size to calculate a valid chi-square value. If the sample size used for this value was extremely small, as it would have been had the values used in calculations had only come from one lab group, then the chi-square value would be less significant. This is because the single lab group could have been a fluke and then the rest of the class could have gotten completely different sets of data.
Was there a significant difference between the groups?
There was not a significant difference between the group. Because there was only one degree of freedom, the critical value for a p value of 0.05 is 3.84. The Chi-Square value is less than the critical value, the null hypothesis is not rejected and therefore the experiment cannot be definitively concluded to have not happened due to random chance.
If this slides in this experiment were treated with lectin, I think we might have seen a change in the number of root tip cells in mitosis, but Mr. Wong revealed to us that the "treated" slides were not in fact treated with anything. Rather, Mr. Wong just labeled some control slides "T" and let our minds do the rest. During the experiment, as groups began reading off data, Mr. Wong actually told some groups, including mine, to recount their cells because their data was wildly off in that it showed a large increases in the number of cells in mitosis in the treated group as compared to the control. This is actually a phenomenon studied in psychology called observer bias, or the tendency of researchers to see data in a light that is favorable to their hypotheses. This is a common human error that the entire class made for this experiment.
What other experiments should you perform to verify your findings?
Other experiments that could be performed would be to look at more root tip cells or cells of a different plant and then counting cells and seeing if the chi-square value is different for those. Or we could actually use slides treated with something to determine the effects an environment can play on cells.
Does an increased number of cells in mitosis mean that these cells are dividing faster than the cells in the roots with a lower number of cells in mitosis?
An increased number of cells in mitosis does in fact mean that the cells are dividing at a faster rate than in roots with lower numbers of dividing cells because all cells are on their own specific timers of when to divide and when to not divide. The cells are not synced up at all, because that would be catastrophic if all of our cells divided at one time. Therefore, when the rate of all cells dividing speeds up, more cells are brought to mitosis around the same time, so during a snapshot look into the lives of the cells, as in the treated root tip slides, more cells would be undergoing mitosis than in a regular root tip.
What other way could you determine how fast the rate of mitosis is occurring in root tips?
Another way to measure the rate of mitosis in root tips is to measure the growth of the root tip as a whole. As cells divide and grow, more space is taken up and the tip of the root is pushed farther and farther along. So, as the tip expands, there are more cells performing mitosis.
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