Lab+3+Mitosis+and+Meiosis

AP Lab 3A: Observing Cells in Mitosis

OBJECTIVES : Upon completing this activity you will be able to: • Identify the stages of mitosis • Determine the time cells spend in each stage of mitosis • Distinguish between mitosis in animal cells and plant cells

Today you’ll be observing cells under the microscope that are undergoing mitosis. You will also be countingthe number of cells in each stage of mitosis so that you can calculate the amount of time a cell spends in eachstage. Part one : Observing Animal Cells in Mitosis The specimen of whitefish you will be using comes from a ** blastula **. A blastula is a rapidly dividing hollow ball of cells that is formed during a process called // blastulation //. The blastula forms about a week after fertilization of the ovum takes place and is made up of approximately 100 cells. You will observe the blastula of a whitefish, which is a common specimen used for the study of cell division. Materials : Whitefish cells in mitosis slideMicroscope Procedure : 1. Using a microscope, look at a slide of whitefish cells in mitosis at 400x. 2. In your lab notebook, draw a section of the slide that fills the field of view with cells. Find and label cells inthe following stages of mitosis: a. Interphase b. Prophase c. Metaphase d. Anaphase e. Telophase

Part Two : Calculating Time Spent in Each Phase of Mitosis In plants, cell division for growth is restricted to growing tips called ** meristematic ** tissue. These are located at the tips of every stem and root. This is unlike mitosis in a growing animal where cell divisions can occur all over the body. The diagram below illustrates the position and appearance of developing and growing cells in a plant root. Similar zones of development occur in the growing stem tips, which may give rise to specialized structures such as leaves and flowers. Roots consist of different regions. The ** root cap ** functions in protection. The ** zone of cell division ** is where mitosis is actively occurring. The ** zone of elongation ** is the area in which growth occurs (cells get bigger). The ** zone of specialization ** is where root hairs develop and where cells differentiate into specialized tissues.

Procedure : 1. Get 4 ** different **// Allium // cards from your teacher. 2. For each card, you need to count the number of individual cells you see in each phase of mitosis. Record this information in the data chart provided. 3. Once you have finished recording data, you need to calculate the following things: a. Total number of cells in each phase b. Average number of cells in each phase c. Percent of time spent in each phase. Assume that // Allium // cells divide once daily. 4. You will need to make ** appropriate ** graphs of each of these data sets in your lab notebook. ** Hint ** : Look closely at your data table to determine what graphs you need to produce. 5. Return your // Allium // cards to your teacher.


 * Name || Interphase || Prophase || Metaphase || Anaphase || Telophase ||
 * BATRA_APBIO || 163 || 29 || 13 || 5 || 4 ||
 * BURBY_APBIO || 157 || 27 || 18 || 15 || 8 ||
 * CLAYTOR_APBIO || 180 || 43 || 12 || 9 || 10 ||
 * COHEN_APBIO || 121 || 62 || 12 || 4 || 12 ||
 * CROSBY_APBIO || 158 || 36 || 7 || 9 || 10 ||
 * GOSCIAK_APBIO || 230 || 27 || 10 || 7 || 6 ||
 * GUBSER_APBIO || 166 || 86 || 6 || 8 || 9 ||
 * HARRIS_APBIO || 206 || 14 || 9 || 8 || 14 ||
 * HEINERT_APBIO || 157 || 20 || 12 || 6 || 3 ||
 * HERNANDEZ_APBIO || 160 || 25 || 12 || 4 || 9 ||
 * PATEL_APBIO || 172 || 34 || 13 || 8 || 7 ||
 * PROROK_APBIO || 139 || 27 || 11 || 9 || 6 ||
 * ROSKOPH_APBIO || 107 || 18 || 8 || 9 || 7 ||
 * SERIO_APBIO || 115 || 33 || 7 || 6 || 7 ||
 * SIMPSON_APBIO || 125 || 71 || 8 || 7 || 4 ||
 * SMITH_APBIO ||  ||   ||   ||   ||   ||
 * VARGO_APBIO || 207 || 23 || 6 || 5 || 4 ||

Lab Questions: Answer these in your lab notebook. 1. What differences did you notice between the animal cell mitosis and the plant cell mitosis? 2. Describe the behavior of the chromosomes during each of the stages of mitosis. 3. Based on the data you collected, what can you infer about the relative length of time an onion root-tip cell spends in each stage of cell division? 4. Why do you think onion root tip and whitefish blastula are chosen as mitosis study organisms?

__ Background __
 * Crossing Over during Meiosis in //Sordaria// **

Meiosis reduces the chromosome number in half during the formation of gametes in animals and spores in plants. During the first meiotic "reduction division," the chromosomal pairs are partitioned so that each gamete (or spore) contains one of each chromosomal pair (haploid). When haploid gametes unite during fertilization, the resulting zygote is diploid, having received one chromosome of each pair from both parents. Meiosis involves two successive nuclear divisions that produce four haploid cells. The first division (meiosis I) is the reduction division. The second division (meiosis II) separates the duplicate chromatids.

Meiosis cell division produces cells that are different from the original cell, increasing genetic variation in the population. Each diploid cell undergoing meiosis can produce 2 n different chromosomal combinations, where **n** is the haploid number. In humans, n = 23. Thus humans can produce 2 23 or over eight million different combinations.

In addition, meiosis increases variation because during meiosis I, each pair of chromosomes comes together in a process known as **synapsis**. Chromatids of homologous chromosomes may exchange parts in a process called **crossing over**. You can estimate the relative distance between two genes on a given chromosome by calculating the percentage of crossing over that takes place between them.

//Sordaria fimicola// is an ascomycete fungus that can be used to demonstrate the results of crossing over during meiosis. //Sordaria// is a haploid organism for most of its life cycle. It become diploid only when the fusion of the mycelia (filametlike groups of cells) of two different strains results in the fusion of the two different types of haploid nuclei to form a diploid nucleus. The diploid nucleus must then undergo meiosis to resume its haploid state.

Meiosis followed by mitosis, in //Sordaria// results in the formation of eight haploid **ascospores** contained within a sac called an **ascus** (plural, **asci**). Many asci are contained within a fruiting body called a **perithecium**. When the ascospores are mature, the ascus ruptures, releasing the spores. Each spore can develop into a new haploid fungus. The life cycle of //Sordaria// //fimicola// is shown in Figure 1:

Figure 1.

To observe crossing over in //Sordaria//, one must make hybrids between wild-type and mutant strains. Wild-type //Sordaria// have black ascospores (+), One mutant strain has tan spores (tn). When mycelia of these two different strains come together and undergo meiosis, the asci that develop will contain four black ascospores and four tan ascospores (4:4 pattern). The arrangement of the spores directly reflects whether or not crossing over has occurred. In figure 2, no crossing over has occurred.

Figure 2.

If crossing over has occurred, the pattern of the ascospores will be changed. The patterns may be a 2:2:2:2 or 2:4:2. Figure 3 shows the result of a crossing over between the centromere of the chromosome and the gene for ascospore color.

Figure 3.

__ Procedure __

1. Use the prepared slide of //Sordaria// to look for the ascospores.

2. View the slide using the 10X objective. Locate a group of hybrid asci (those containing both tan and black ascospores). Count at least 50 hybrid asci and enter your data into Table 2.

Table 2.
 * 4:4 Asci || 2:2:2:2 or 2:4:2 Asci || Total Asci ||

__ For the Lab report __ 1. Using your data in Table 2, determine the distance between the gene for spore color and the centromere. Calculate the percent of crossovers by dividing the number of crossover asci (2:2:2:2 or 2:4:2) by the total number of asci and multiplying this answer X 100.

The percentage of crossover is __.__

2. Calculate the genetic map distance between the centromere and the spore color gene by dividing the percent of crossover asci by 2. The percent of crossover asci is divided by 2 because only half of the spores in each ascus are the result of crossing over.

The map distance is ___.

3. Discuss why meiosis is important for sexual reproduction