Ap Biology Lab 3

AP BIOLOGY LAB 3 MITOSIS & MEIOSIS

INTRODUCTION: All new cells come from previously existing cells. New cells are formed by the process of cell division, which involves both division of the cell’s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis). There are two types of nuclear division, mitosis and meiosis. Mitosis typically results in new somatic (body) cells. Formation of an adult organism from a fertilized egg, asexual reproduction, regeneration and maintenance or repair of body parts are accomplished through mitotic cell division. You will study mitosis in Exercise 3A. Meiosis results in the formation of either gametes (in animals) or spores (in plants). These cells have half the chromosome number of the parent cell.

Where does one find cells undergoing mitosis? Plants and animals differ in this respect. In higher plants, the process of forming new cells is restricted to special growth regions called meristems. These regions usually occur at the tips of stems or roots. In animals, cell division occurs anywhere new cells are formed or as new cells replace old ones. However, some tissues in both plants and animals rarely divide once the organism is mature.

To study the stages of mitosis, you need to look for tissues where there are many cells in the process of mitosis. This restricts your search to the tips of growing plants, such as the onion root tip, or, in the case of animals, to developing embryos, such as the whitefish blastula.

EXERCISE 3.1a Roots consist of different regions (see the figure 3.1a). The root cap functions in protection. The apical meristem (Figure 3.1b) is the region that contains the highest percentage of cells undergoing mitosis. The region of elongation is the area in which growth occurs. The region of maturation is where root hairs develop and where cells differentiate to become xylem, phloem, and other tissues.

Figure 3.1a Figure 3.1b MATERIALS: Microscope Prepared onion root tip slides Onion Root Tip pictures

PART I: OBSERVATION OF PREPARED ONION ROOT TIP SLIDES PROCEDURE: 1. With your microscope locate the region of rapidly dividing cells at the tip of the prepared onion root tip slide. After locating the cells under low power, switch to high power. 2. Locate cells that appear to be in various stages of mitosis, using the diagram given in your notes as your guide. 3. Answer the following questions concerning each stage of mitosis, and draw pictures of each stage in your lab book.

INTERPHASE: In the microscope, locate a few cells that are in interphase. The non-dividing cell is in the stage called interphase. The nucleus may have one or more dark-stained nucleoli and is filled with a fine network of threads, the chromatin. During interphase, DNA replication occurs. Draw what you see in your lab book. Then answer questions 1-5 using your text, notes and what you are observing through the microscope.

1. Describe the contents of the nucleus during interphase. 2. Are the nucleolus and nuclear membrane present in the cell during interphase? 3. Are distinct rod-shaped chromosomes easily observed in the nucleus during interphase? 4. What term is used to describe DNA during interphase? 5. List the 3 stages of interphase and tell what happens during each. (See your text) a. b. c.

PROPHASE: In the microscope, locate a few cells that are in prophase. The first sign of division occurs in prophase. There is a thickening of the chromatin threads, (as they wind around histones to form nucleosomes, and then supercoil) which continues until it is evident that the chromatin has condensed into chromosomes. Each chromosome at this point consists of two chromatids joined at a centromere. As prophase contines, the chromatids continue to shorten and thicken. In late prophase, the nuclear envelope and nucleolus are no longer visible, and the chromosomes are free in the cytoplasm. Just before this time, the first sign of a spindle appears in the cytoplasm; the spindle apparatus is made up of microtubules, and it is thought that these microtubules may pull the chromosomes toward the poles of the spindle where the two daughter nuclei will eventually form. Draw prophase in your lab book, then answer questions 6-8 using what you see in the microscope, your text and notes for references.

6. How are chromosomes formed during prophase? 7. What changes have occurred to the nucleolus and nuclear membrane from interphase to prophase? 8. Why can chromosomes be seen now, if they couldn't be seen during interphase?

METAPHASE: In the microscope, locate a few cells that are in metaphase. At metaphase the chromosomes have moved to the middle of the spindle. One particular portion of each chromosome, the centromere, attaches to the spindle at the kinetochore. The centromeres of all the chromosomes lie at about the same level of the spindle, on a plane called the metaphase plate. At metaphase you should be able to observe the chromatids of at least some of the chromosomes. Draw them in your lab book then answer questions 9-12 using what you see in the microscope and your notes for references.

9. Describe where the chromosomes are now located in relation to the cell. 10. At this point, can you actually see any evidence that the DNA has replicated? 11. What are the fibers called that are visible during this phase?

12. What term is used to describe the structure where each of these fibers attaches to a chromosome? ANAPHASE: In the microscope, locate a few cells that are in anaphase. At the beginning of anaphase, the spindle fibers which are attached to the kinetochore (they are actually called kinetochore fibers) pull in opposite directions, breaking the centromere. Each kinetochore fiber then shortens, dragging its sister chromatid to the opposite poles of the cell. Once the two chromatids separate, each is called a chromosome. They continue their poleward movement until they form two compact clumps, one at each pole. Draw them in you lab book then answer questions 13-14 using what you see in the microscope, your text and your notes for references.

13. In metaphase, chromosome pairs were lined up along the cell's center. Describe what is occurring to each chromosome pair during anaphase. 14. Toward what area of the cell are the chromosomes being directed? 15. What structure is responsible for the movement of chromosomes during this phase?

TELOPHASE: In the microscope, locate a few cells that are in telophase. Telophase is the last stage of division, and is marked by a pronounced condensation of the chromosomes, followed by the formation of a new nuclear envelope around each group of chromosomes. The chromosomes gradually uncoil to form the fine chromatin network seen in interphase, and the nucleoli and nuclear envelope reappear. Cytokinesis may begin at this point. This is the division of the cytoplasm into two cells. In plants, a new cell wall is laid down between the daughter cells (this is called a cell plate) In animal cells, the old cell will pinch off in the middle along a cleavage furrow to form two new daughter cells. Draw them in you lab book then answer questions 16-18 using what you see in the microscope, the text and your notes for references.

16. What cell parts begin to reappear during this phase? 17. What cell parts disappear during this phase? 18. Describe the location of the chromosomes now compared to where they were during metaphase.

Analysis Questions:

1. The term "mitosis" comes from the Greek word meaning "thread". Explain why this word was chosen to describe nuclear division.

2. Explain how mitosis leads to two daughter cells, each of which is diploid and genetically identical to the original cell. What activities are going on in the cell during interphase?

3. How does mitosis differ in plant and animal cells? How does plant mitosis accommodate a rigid, inflexible cell wall?

4. What is the role of the centrosome (the area surrounding the centrioles)? Is it necessary to for mitosis? Defend your answer.

PART II: TIME FOR CELL REPLICATION

To estimate the relative length of time that a cell spends in the various stages of cell division, you will examine pictures of the meristematic region of an onion root tip. The length of the cell cycle is approximately 24 hours for cells in actively dividing onion root tips.

PROCEDURE: While it may be hard to imaging that you can estimate how much time a cell spends in each phase of cell division from a picture, yet this is precisely what you will do in this part of the lab. Since you are working with a picture, you cannot get any information about how long it takes a cell to divide. What you CAN determine is how many cells are in each phase. From this, you can infer the percentage of time each cell spends in each phase.

1. Starting with the bottom picture, determine which phase of the cell cycle each cell is in. This is best done in pairs. The partner observing the cells, calls out the phase of each cell while the other partner records. Then switch so the recorder becomes the observer and vice versa. You will need to continue until you have identified 200 cells. If you run out of cells in the bottom picture, move up to the top picture and count until you reach 200. (This is 200 total, not 200 cells each!!!)

2. Record your data in Table 1.

3. Calculate the percentage of cells in each phase, and record that number in Table 1 also.

Consider that it takes, on average, 24 hours (or 1,440 minutes) for onion root tip cells to complete the cell cycle. You can calculate the amount of time spent in each phase of the cell cycle from the percentage of cells in that stage.

TABLE 1

Picture 1 Picture 2 Total Percent of Time in Total Cells Each Stage Counted Interphase

Prophase

Metaphase

Anaphase

Telophase

Total Cells Counted

ANALYSIS: 1. 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?

2. Based on the data in Table 1, what can you infer about the relative length of time an onion root tip cell spends in each stage of cell division?

3. Draw and label below a pie chart of the onion root tip cell cycle, using the data from Table 1.

Title: