10/11/13 9:54 PMBIO156 – Lab 6
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Lab 6 Cell Division, Mitosis, and Meiosis
Introduction: Connecting Your Learning
All cells, including those in the human body, have a cell cycle. This cycle involves preparing for cell division and eventually dividing. Coupled with cell division is nuclear division. Nuclear division, either mitosis or meiosis, is the process by which the nucleus of a cell divides. Mitosis results in two identical daughter cells; each containing the same number of chromosomes as the parent cell. (In humans, this is 46.) In comparison, meiosis results in four daughter cells, each containing half the number of chromosomes as the parent cell (23 in humans). Meiosis is essential to sexual reproduction and the inheritance of genes. This lab examines cell division, nuclear division, and the concepts associated with the study of inheritance or genetics.
Resources and Assignments
Multimedia Resources Virtual Microscope
Required Assignments Lesson 7 Lab 6
Laboratory Materials None
Focusing Your Learning
Lab Objectives
By the end of this lesson, you should be able to:
1. Describe the molecular structure of DNA. 2. Identify and describe the stages of mitosis, meiosis, and cell division. 3. Distinguish between cell division and mitosis. 4. Identify the stages of mitosis in onion root tip cells, observed under a
microscope. 5. Explain the process of crossing over.
Background Information
Deoxyribonucleic acid (DNA) is an important component that determines who an individual is and what he or she looks like. But DNA is much more complex than simply defining the external features of an individual. DNA is responsible for controlling the complex processes involved in living organisms.
DNA is composed of a coiled double helical strand of nucleotides that are bonded together in a specific pattern. The backbone of the double helix is composed of linked deoxyribose sugars and phosphorus atoms, and cross-links form between two nitrogenous bases. The four nitrogenous bases consist of Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). An image of a DNA molecule is seen below. Note that the sugar-phosphate backbones are the blue ribbons, and the nitrogenous bases are the cross-links seen in shades of green and orange.
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Click on image to enlarge.
Three adjacent bases compose what is known as a codon, which codes for a particular amino acid. The sequence of bases thus determines the sequence of amino acids for different proteins. These proteins eventually are demonstrated as traits in the organism.
It is important to note that DNA does not reside by itself in the nucleus. Instead, it is associated with proteins. When a cell is not dividing, the DNA and associated proteins are uncondensed in the nucleus into a structure called chromatin. When the chromatin condenses and coils on itself, the structure is called a chromosome.
All human cells that are not sex cells contain two sets of 23 chromosomes (for a total of 46 chromosomes per cell in human cells), and are called diploid cells. When cells divide, if the daughter cells are to be functional, they must possess all genetic material found in the parent cell. Therefore, the DNA of the parent cell must be duplicated prior to cell division. For all body cells except sex cells (e.g., sperm or eggs in humans), the process by which cells reproduce is called mitosis. Mitosis plays an important role in cell growth, tissue repair, and asexual reproduction.
Compared with body cells, sex cells only contain one set of chromosomes (23 chromosomes in humans), and are called haploid cells. Haploid cells are formed through a process called meiosis. In the process of meiosis, which is explained in additional detail below, chromosome numbers from parent cells are halved, yielding one pair of chromosomes (for a total of 23 chromosomes per cell).
Click on image to enlarge.
Organismal cells undergo a cycle of events, beginning at the point when the cell first forms from a parent cell, through the time when it divides into two daughter cells. This cycle is called the cell cycle. Most of a cell’s life is spent in interphase, which is the longest phase of the cell cycle. This is the stage where the cell is metabolically active and performs its normal functions. Several stages of interphase are seen above, which include the following:
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G1 – The cell grows and is metabolically active. Organelles are duplicated in preparation for the S stage. In this stage, the DNA is present in the form of chromatin.
S – The DNA and chromosomes are replicated but are not distinguishable because they are still in the form of chromatin fibers.
G2 – The cell continues to grow and is prepared for cell division.
Click on image to enlarge.
Once a cell is ready to divide, the process of mitosis begins. Mitosis consists of four stages: prophase, metaphase, anaphase, and telophase. A visual representation of the events that occur during each stage is seen above.
During prophase, the chromatin fibers coil and condense forming chromosomes that are now visible with a compound light microscope. The chromosomes are held together at the centromere, a pinched region of the chromosome. While connected, each individual chromosome is referred to as a sister chromatid. During prophase, the mitotic spindle forms as outgrowths from the centrosomes, the nuclear envelope begins to disappear, and the centrosomes move to the poles.
In the next stage of mitosis, metaphase, the sister chromatids line up at the center (equator) of the cell. This area is called the metaphase plate. In addition, the mitotic spindle is completely formed. These fibers extend from the perpendicular to the plane of the centrioles and attach to the centromeres of the sister chromatids. Chromosomes move along these fibers during the subsequent stage of mitosis.
In anaphase, the centromeres split, and one copy of each chromosome (chromatid) is pulled to each centriole due to the contraction of the spindle fibers. Once the chromatids are separated, they are called chromosomes again.
In telophase, separated chromosomes have migrated to opposite ends of the cell, the nuclear envelopes form, chromosomes uncoil, and the mitotic spindle disappears. In this stage, the division of the nuclear material has been completed, along with division of the cytoplasm. Cytokinesis is the name for the process by which the cytoplasm is divided. This process occurs during telophase, along with the process of nuclear division. It is important to understand the difference between cytokinesis (division of cytoplasm) and mitosis (nuclear division). Nuclear division results in the separation of the information within the nucleus, specifically the replicated chromosomes containing the DNA. In comparison, cell division (cytokinesis) refers to the formation of two cells from one, or the splitting of the cell and cytoplasm. While the two are related, they are separate processes that occur simultaneously.
In animal cells, cleavage furrows start to appear during telophase. The original cell pinches off into two daughter cells, starting with an indentation at the cell equator called the cleavage furrow. The furrow deepens as microfilaments in the cytoplasm contract, pinching the parent cell into two cells. This process does not occur in plant cells. Rather, in plant cells, a cell plate forms from cell wall material that collects in the middle of the cell. The cell plate grows outward until its membrane fuses with the parental cell wall, resulting in the formation of two daughter cells. The comparison of these two processes can be seen in the images below. Animal cytokinesis is seen in the image on the left, and plant cytokinesis is seen in the image on the right.
Click on image to enlarge. Click on image to enlarge.
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Meiosis
As discussed previously, cells involved in the process of sexual reproduction also must divide; however they do so through a process called meiosis. The steps of cellular division of meiosis resemble the steps of mitosis but there are two distinguishing characteristics of meiosis. The first difference between mitosis and meiosis relates to the number of cell divisions and resultant chromosome number found in cells. Sex cells undergo two cell divisions instead of the single division that occurs in mitosis. This results in haploid daughter cells that contain half the number of chromosomes as the diploid, parent cell. The process starts with a single diploid (2n) cell and ends with four haploid (n) cells. Remember that the process of mitosis results in diploid daughter cells each containing the same number of chromosomes as the parent cell.
The second distinction between mitosis and meiosis is that genetic material is exchanged between chromosomes in cells undergoing meiosis. This process where chromosomes exchange material is called crossing over. The process of crossing over, as seen in the image below, leads to an increase in genetic diversity.
Click on image to enlarge.
In sexual reproduction, the gamete (sperm, egg, or pollen, for example) contains only one copy of each chromosome pair (homolog). If both chromosomes originally had the same characteristics (genes) then all gametes produced would have that characteristic. If the original genes were different, then two distinct gametes could be produced. The characteristics expressed depend upon the interaction of the genes. If a characteristic is expressed and it is found on only one of the chromosomes of the pair, it is said to be dominant over the other characteristic on the other chromosome of the homologous pair. If the characteristic is expressed only in the absence of the dominant characteristic, it is said to be recessive; therefore it must appear on both chromosomes of the pair.
By knowing the composition of the possible gametes, the frequency of a characteristic in the offspring can be calculated. To predict the probability of traits being passed on to offspring, a Punnett square is employed. Punnett squares help to identify the both the phenotypes (an organism’s physical appearance) and genotypes (an organism’s genetic makeup) of offspring, based on the genotypes of the parents. To construct a Punnett square, possible gametes from one parent are written horizontally across the top of the Punnett square, and gametes from the second parent are written vertically along the side of the Punnett square. The example Punnett square below details a monohybrid cross of the character for plant color of the F1 generation with another F1 generation, resulting in the F2 generation. Note that purple is the dominant color and is identified with a P, while white is the recessive color and is identified with a p. The genotype for both hybrids from the F1 generation is Pp.
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Click on image to enlarge.
The result of the Punnett square for flower color indicate that in the F2 generation, the ratio of purple to white plants (the phenotypic ratio) is 3:1, and the ratio of varying genotypes (genotypic ratio) is 1 (PP) :2 (Pp) :1 (pp). These ratios aligned with the ratios Mendel observed in his experiments. These inheritance patterns were consistently observed for each character that Mendel studied and form the basis for the law of segregation that applies to organisms that reproduce sexually.
In this laboratory, the concepts of mitosis and gamete frequency will be investigated. In the first part of the lab, a visual model will be developed, detailing each stage of the cell cycle for a typical cell undergoing mitosis. This virtual exercise will be a drag and drop activity.
In the second step of the laboratory, microscope slides of an onion root tip and blastula cells are analyzed to count the number of cells observed in each stage of the cell cycle. Then, the percentage of time that cells spend in each stage will be calculated.
Cell Mitosis Examples
In the third part of the lab, images of corn ears will be examined to calculate the frequency of two corn kernel characteristics: color and texture. In performing this part of the lab, the frequency of the characteristic observed will be expressed as a ratio and then compared to the expected ratio, as determined by a Punnett square.
Procedures
PART I: Building models of the cell cycle stages
1. Review the stages of the cell cycle to review the major events that occur during each stage. Also, the student should reference the image below which shows some of the visual differences between the stages of mitosis.
Click on image to enlarge.
Click on image to enlarge.
2. Using the model cells located above, the pool of chromosomes, spindle fibers, and centrosomes, build a visual model of each stage of the cell cycle that occurs in mitosis. Drag the components into the cells below for each stage until all components are in their correct positions. Finally, provide each stage with the correct name from the pool of cell cycle stages.
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PART II: Identifying and counting cells
For this section of the lab, you will view a micrograph of an onion root tip that shows cells in the different stages of mitosis. Before beginning the steps below, view the following micrograph of an onion root tip to become familiar with the different stages of mitosis, as seen in a micrograph of a cell. Once the micrograph is opened, use the mouse to scroll over a cell. Once the cursor is placed over a cell that should be identified, click on the cell and then identify the stage of mitosis for the cell. Immediate feedback will be provided. Repeat this procedure for all of the cells that are identified on the micrograph. Completing this practice exercise will allow the student to become proficient in correctly identifying cells with their respective stage of mitosis.
1. Using the Virtual Microscope, view an image of an onion root tip and count the number of cells that are in each stage of the cell cycle. As a reminder, the cell cycle consists of the following stages: interphase, prophase, metaphase, anaphase, and telophase/cytokinesis. It may be helpful to record the data in a table similar to the one below to assist in compiling results.
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2. Repeat Step 1 three more times, each time selecting a different slide from the virtual microscope. Count the cells observed in each stage of the cell cycle for each slide and record the information.
3. After the counts have been completed for all four slides, total the number of cells in each stage and find the average. To find the average, add up the number of cells observed in each stage and divide this number by four. Using the table above as a reference, to calculate the average number of cells in interphase, add the numbers found in locations 1, 7, 13, and 19 and then divide this number by four. Enter that result into location 25 above. Repeat this process for each column (stage in the cell cycle).
4. Finally, calculate the percent of time that cells are actually in each stage of the cell cycle. To determine the percentage of time cells are in each stage, divide the average number of cells in each stage by the total average number of cells in each field of view. Using the table above as a reference, first determine the total average number of cells by adding across the row entitled Average Number of Cells/Slide. Add up locations 25, 26, 27, 28 and 29. Enter the total obtained into location 30 above. To calculate the percentage of time cells were in telophase, divide the number in location 29 by the number in location 30, then multiply this number by 100 to obtain the percentage. The equation for calculating this percent is shown below.
PART III: Calculating the frequency of corn kernel characteristics
In this part of the lab, frequencies of two corn kernel characteristics will be determined: kernel color and kernel texture. In observing the ears of corn, note that the kernels are either purple or yellow and that they are either smooth or wrinkled. With respect to color, purple is the dominant color, and yellow is the recessive color. With respect to texture, smooth is the dominant texture, and wrinkled is the recessive texture. Use the following designations for each trait: purple (P); yellow (p); smooth (S); wrinkled (s). Keep this information available as it will be needed later on in the lab.
1. Observe the image of a corn ear below. Notice that the ear of corn contains purple and yellow kernels. Study the image and then count three rows of kernels, tallying the number of purple and the number of yellow kernels. Record this information on a piece of paper to refer to later in this laboratory. Review the results and indicate the frequency (ratio) of purple to yellow kernels.
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Click on image to enlarge.
2. Observe this second image of a corn ear below. Notice that the ear of corn contains purple and yellow kernels. Study the image and then count three rows of kernels, tallying the number of purple and the number of yellow kernels. Record this information on a piece of paper. Review the results and indicate the frequency (ratio) of purple to yellow kernels.
Click on image to enlarge.
3. Observe this third image of a corn ear below. Notice that the ear of corn contains purple and yellow kernels. Additionally, notice that some of the corn kernels are smooth and some of the corn kernels are wrinkled. Study the image and then count three rows of kernels, tallying the number of kernels. For this exercise, there are four different kernels to count: purple and smooth, purple and wrinkled, yellow and smooth, or yellow and wrinkled. Record this information on a piece of paper. Review the results and indicate the frequency (ratio) of corn kernels.
Click on image to enlarge.
Assessing Your Learning
Compose answers to the questions below in Microsoft Word and save the file as a backup copy in the event that a technical problem is encountered while attempting to submit the assignment. Make sure to run a spell check. Copy the answer for the first question from Microsoft Word by simultaneously holding down the Ctrl and A keys to select the text, and then simultaneously holding down the Ctrl and C keys to copy it. Then, click the link on the Lab Preview Page to open up the online submit form for the laboratory. Paste the answer for the first question into the online dialog box by inserting the cursor in the box and simultaneously holding down the Ctrl and V keys. The answer should now appear in the box. Repeat this process for each question. Review all work to make sure that all questions have been completely answered and then click on the Submit button at the bottom of the page.
LAB 6
1. Why are spindle fibers important for mitosis? (5 points)
2. State the four bases that make up DNA. (4 points)
a.
b.
c.
d.
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3. What are the two base pairs? (2 points)
a.
b.
4. Answer the following questions:
a. Define the term crossing over. (3 points)
b. Explain why crossing over is important in meiosis. (3 points)
5. What are the two main differences between mitosis and meiosis? (4 points)
a.
b.
6. Answer the following questions:
a. Explain the difference between mitosis and cytokinesis. (3 points)
b. When does mitosis occur during the cell cycle? (1 point)
c. When does cytokinesis occur during the cell cycle? (1 point)
7. Explain the differences that occur during cytokinesis of plant and animal cells. (5 points)
8. Refer to the images below, labeled A through E. Each image details a stage of the cell cycle for a cell undergoing mitosis. Place the images in correct order by placing the letters in the correct sequence, according to the stages of mitosis. (5 points)
a. Interphase
b. Prophase
c. Metaphase
d. Anaphase
e. Telophase/cytokinesis
9. Refer to the image below. What stage of mitosis is the cell below undergoing? (1 point)
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10. Refer to the images below. Place the cells identified with the letters A through E in correct order for a cell undergoing mitosis. (5 points)
a. Interphase
b. Prophase
c. Metaphase
d. Anaphase
e. Telophase/cytokinesis
11. Refer to the data on the corn kernel color frequency from Part III of the lab. (Remember there were four possible types for this part of the lab.)
a. What was the phenotypic frequency from Step 1? (2 points)
b. What was the phenotypic frequency from Step 2? (2 points)
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c. What was the phenotypic frequency from Step 3? (2 points)
12. Recall from the background information that purple corn kernels are dominant and yellow kernels are recessive. The second ear of corn was the result of crossing two heterozygous ears of male purple corn (Pp x Pp). This is represented by the Punnett square below. Complete the Punnett square by writing the correct letters that correspond to each number indicated in the table. (4 points)
P p
P 1 2
p 3 4
13. Once the Punnett square for Question 12 is complete, calculate the ratio of purple and yellow kernels (recall that if the dominant trait is present, it will be expressed).
a. What is the ratio of purple to yellow kernels based on the Punnett square? (5 points)
b. How did this compare to the ratio obtained from counting the corn kernels for ear number two in part III of the lab? (5 points)
14. Recall from the background information that purple kernels are dominant and yellow kernels are recessive. Also recall that smooth kernels are dominant and wrinkled kernels are recessive. The third corn ear was the result of crossing a male ear of corn with the following gametes, PpSs, with a female ear of corn with the same gametes, PpSs. This is represented by the Punnett square below. Complete the Punnett square by writing the correct letters that correspond to each number indicated in the table (for example, PPSS or ppss). (8 points)
PS Ps pS ps
PS 1 2 3 4
Ps 5 6 7 8
pS 9 10 11 12
ps 13 14 15 16
15. Once the Punnett square for Question 14 is complete, calculate the ratio of corn kernel varieties (recall that if the dominant trait is present, it will be expressed).
a. What is the ratio of all kernel varieties based on the Punnett square? (5 points)
b. How does this compare to the ratio obtained from counting the corn kernels? (5 points)
16. (Application) How might the information gained from this lab pertaining to mitosis and meiosis be useful to a student employed in a healthcare related profession? (20 points)
Have You Met The Objectives For This Lesson?
10/11/13 9:54 PMBIO156 – Lab 6
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