ap bio lab 3 final

7/18/2019 AP Bio Lab 3 Final

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Niteesh Settypalli

Period 5

11/06/13

Title: Mitosis and Meiosis

Abstract: In this lab, our goals were to observe mitosis in plant and animal cells, compare the

relative lengths of the stages of mitosis in cells, simulate the stages of meiosis, observe evidence

of crossing-over in meiosis, and estimate the distance of a gene locus from its centromere. For

accomplishing these goals, we examined sections of fish and onion root cells, while using

mathematics and illustrations for the simulation portions. Basically, we took sections of beet and

fish cells, examined them under a microscope, and then drew illustrations and recorded our

results. In the end, we were able to recognize the different cell stages and we also were able todetermine the relative time a cell spends in each phase.

Introduction: All new cells come from previously existing cells. New cells are formed by

karyokinesis- the process in cell division which involves replication of the cell’s nucleus and

cytokinesis-the process in cell division which involves division of the cytoplasm. Two types of

nuclear division include mitosis and meiosis. Mitosis typically results in new somatic, or body,

cells. Mitotic cell division is involved in the formation of an adult organism from a fertilized egg,

asexual reproduction, regeneration, and maintenance or repair of body parts. Meiosis results in

the formation of either gametes in animals or spores in plants. The cells formed have half the

chromosome number of the parent cell.

Mitosis is best observed in cells that are growing at a rapid pace, such as in the whitefish

blastula or onion root cell tips. The root tips contain a special growth region called the apical

meristem where the highest percentage of cells are undergoing mitosis. The whitefish blastula is

formed immediately after the egg is fertilized, a period of rapid growth and numerous cell

divisions where mitosis can be observed.

There are several stages included in before, during, and following mitosis. Interphase

occurs right before a cell enters mitosis. During interphase, the cell will have a distinct nucleus

with one or more nucleoli, which is filled with a fine network of threads of chromatin. During

interphase, DNA replication occurs. After duplication the cell is ready to begin mitosis. Prophase

is when the chromatin thickens until condensed into distinct chromosomes. The nuclear envelope

dissolves and chromosomes are in the cytoplasm. The first signs of the microtubule-containing

spindle also begin to appear. Next the cell begins metaphase. During this phase, the centromere

of each chromosome attaches to the spindle and are moved to the center of the cell. This level

position is called the metaphase plate. The chromatids separate and pull to opposite poles during



the start of anaphase. Once the two chromatids are separate, each is called a chromosome. The

last stage of mitosis is telophase. At this time, a new nuclear envelope is formed and the

chromosomes gradually uncoil, forming the fine chromatin network seen in interphase.

Cytokinesis may occur forming a cleavage furrow that will form two daughter cells when

separated.

Meiosis is more complex than mitotic stages and involves two nuclear divisions called

Meiosis I and Meiosis II. They result in the production of four haploid gametes and allow genetic

variation because of crossing over of genetic material. Prior the process, interphase replicates the

DNA. During prophase I, the first meiotic stage, homologous chromosomes move together to

form a tetrad and synapsis also begins. This is where crossing over occurs, resulting in the

recombination of genes. In Metaphase I, the tetrads move to the metaphase plate in the middle of

the cell as on mitotic metaphase. Anaphase I brings the tetrads back to their original two stranded

form and moves them to opposite poles. During Telophase I, the centriole is finished and the cell

prepares for a second division. In Meiosis II, in Prophase II, centrioles move to opposite ends of

the chromosome group. In Metaphase II, the chromosomes are centered within the center of each

daughter cell. Anaphase II involves the centromere of the chromatids separating. Telophase II

occurs when the divided chromosomes separate into different cells, known as haploid cells.

Sordaria can be used to demonstrate the results of crossing over during meiosis. It spends

most of its life haploid and only becomes diploid when the fusion of the mycelia of two different

strains results in the fusion of two different types of haploid nuclei to form a diploid nucleus.

Meiosis, followed by mitosis, in Sordaria results in the formation of eight haploid ascospores

contained within a sac called an ascus. They are contained in a perithecium, a fruiting body, until

mature enough to be released. The arrangement of spores directly reflects whether or not

crossing over occurred. If an ascus has four tan ascospores in a row and four black ascospores in

a row -4:4 arrangement, then no crossing over has taken place. If the asci has black and tan

ascospores in sets of two -2:2:2:2 arrangement, or two pairs of black ascospores and four tan

ascospores in the middle -2:4:2 arrangement, then crossing over has taken place.

Results:

Sketches:

1. Interphase

2. Prophase



3. Metaphase

4. Anaphase

5. Telophase

Analysis Questions

1. Explain why 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.

Mitosis leads to two new daughter cells because the cells become replicated and split

during cytokinesis one time. This results in two identical diploid cells. During

interphase, the cell grows, take in nutrients, and replicated its DNA in the S phase.2. How does mitosis differ in plant and animal cells? How does plant mitosis accommodate

a rigid, inflexible cell wall?

In animal cells during cytokinesis, the cell forms a cleavage furrow for the cell to

split, but in plants the form a cell plate..

3. What is the role of the centrosome (the area surrounding the centrioles)? Is it necessary

for mitosis? Defend your answer.

The centrosome is where the microtubules assemble and connect to the

chromosomes. They are necessary for mitosis, because without them, there is no

microtubule formation, and the chromosomes will not be separated.

4. Calculate the percent of cells in each phase.Percentage of cells in stage x 1,440 minutes = minutes of cell cycle spent in stage



Table 3.1

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?

The results would have been different because the count of the different phases

would have decreased drastically.

2. Based on the data in Table 3.1, what can you infer about the relative length of time anonion root tip call spends in each stage of cell division?

Prophase is the longest stage of mitosis and then going in sequential order

decreases in the length of time it takes to complete.

3. Draw and label a pie chart of the onion root tip cell cycle using the data from table

3.1.

Title:

1. List three major differences between the events of mitosis and meiosis.

Number of Cells

Field

1Field

2

Field 3 Total Percent of total

cells counted

Time in each stage

Interphase 42 36 47 125 61.27% 14 hours 42 minutes

Prophase 10 13 18 41 20.10% 4 hours 49 minutes

Metaphase 6 5 4 15 7.35% 1 hour 46 minutes

Anaphase 2 3 2 7 3.43% 49 minutes

Telophase 7 5 4 16 7.84% 1 hour 59 minutes

204



Three major differences between mitosis and meiosis are: 1) in mitosis, the

nucleolus is only divided once, while in meiosis it is divided twice. 2) Mitosis

produces two identical daughter cells, in meiosis it produces up to four different

daughter cells. 3) Synapsis and crossing over occurs in meiosis not mitosis.

2. Compare mitosis and meiosis with respect to each of the following table 3.2.:

3. How are meiosis I and meiosis II different?

Meiosis I begins with a tetrad and separates the homologous pairs. Meiosis II

separates the two sister chromatids.

4. How do oogenesis and spermatogenesis differ?Oogenesis produces egg cells and spermatogenesis produces sperm cells.

5. Why is meiosis important for sexual reproduction?

The chromosome count is reduced to haploid so it may be fertilized. It also

allows crossing over, which results in variations in organisms.

Table 3.3

Number of 4:4 Number of Asci

Showing

Crossover

Total Asci % Asci showing

crossover

divided by 2

Gene to

centromere

distanace (map

units)

59 68 127 26.8% 1

Mitosis Meiosis

Chromosome Number of Parent

Cells

46 46

Number of DNA Replication 1 2

Number of Divisions 1 2

Number of daughter cells

produced

2 4

Chromosome number ofdaughter cells

46 23

Purpose/Function To produce somatic cells To produce autosome cells



Draw a pair of chromosomes in MI and MII and show how you would get a 2:4:2

arrangement of ascospores by crossing over.

Discussion:

Other than possibly miscounting in part 3A, there is not really any source of error. From

the data collected in this experiment, it can be concluded that the mitotic stages of the

whitefish blastula and the onion root-tip can be observed with a light microscope. The

time spent in each phase of mitosis can be recorded and it is concluded that the most time

spent in a stage is in interphase. It can also be concluded that the least time spent in a

stage is in telophase. It is also understood that someone can simulate meiosis using a

chromosome simulation kit. On the last part of the lab, Lab 3B. 2, one could conclude

that more asci do not cross over than do the number of asci that do cross over.

ap bio lab 3 final

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