250-7052s mitosis and meiosis - Wikispaces. In the early stages of meiosis, ... Sordaria fimicola, an ascomycete fungus ... crossing over has occurred,

Mitosis It was discovered in 1858, by Rudolf Virchow, that new cells can only arise from previously existing cells. This is done in two ways: mitosis and meiosis. Somatic (body) cells divide exclusively through mitosis and cytokinesis, while germ cells produce gametes through meiosis. Plant cells simply enlarge, essentially by absorbing water. When they reach a certain size, they divide, forming two identical daughter cells. The various parts of the cell are divided in such a way that the new daughter cell is identical to the parent cell. Strictly speaking, mitosis implies only the division of the nucleus, and is therefore distinct from cell division, in which the cytoplasm is divided. In most organisms, cells divide by the ingrowth of the cell wall, if present, and the contraction of the cell membrane, which cuts through the spindle fibers. In land plants (bryophytes and vascular plants) and a few algae, cell division takes place when a cell plate forms. Small droplets appear across the equatorial plate of the cell and gradually fuse, forming a disc that grows outward until it reaches the wall of the dividing cell, completing the separation of the two daughter cells. The circular DNA of prokaryotes is simply replicated before division. In eukaryotes, however, the hereditary material is part of their com-plex chromosomes. Equal division of this material requires a more complicated method in order for the chromosomes to be replicated, separated, and apportioned precisely between the daughter cells. Mitosis, or nuclear division, is this complicated process that ensures the equal division of the nuclear material between the daughter cells in eukaryotic organisms. During mitosis the chromosomes appear as long, slender threads; they become shorter and shorter as their coiling tightens, and move to the center of the cell where they fully contract. They then split longitudinally into two identical halves that appear to be pulled to opposite poles of the cell by a series of microtubules. In these two genetically identical groups, the coiling of the chromo-somes relaxes again, and they are reconstituted into the nuclei of the two daughter cells. It is a continuous process that can be divided into five major phases: interphase, prophase, metaphase, anaphase, and telophase.

BACKGROUND

Copymaster. Permission granted to make unlimited copies for use in any one school building. For educational use only. Not for commercial use or resale. 1

AP Biology Mitosis and Meiosis

Lab Activity Student Study Guide

TM

© 2002 WARD’S Natural Science Establishment, Inc. All Rights Reserved

DID YOU KNOW? In 1951, American cell biolo-gist Daniel Mazia and Japa-nese biologist Katsuma Dan, were the first to isolate the cel-lular structures that are in-volved in mitosis. These struc-tures are known as the mitotic apparatus and consist of the spindle and the centrioles.

Interphase: Interphase is generally considered to be a resting phase. Interphase is made up of three phases G1, S, and G2. G1 is usually the longest phase of the cell. The cell is recovering from the cell division; it will usually double the number of organelles and in overall size, while it performs its assigned functions. The S phase is also called the synthesis phase. During this phase each chromosome is duplicated, resulting in a doubled chromosome con-sisting of two chromatids attached at the centromere. During G2 the final preparations for nuclear division occur. The cell synthesizes proteins and enzymes needed for cell division. Prophase: Just prior to mitosis, the pair of centrioles duplicates. Dur-ing prophase, the two pairs of centrioles migrate to opposite poles. Centrioles form spindle fibers, which become microtubules and even-tually attach to the centromeres. Additional fibers known as asters also radiate outward from the centrioles. The chromosomes first be-come visible, starting out as long threads in the nucleus and con-dense, becoming shorter and thicker. Each chromosome is composed of two longitudinal halves, called chromatids, that are joined in a nar-row area known as the centromere, where the chromatids are not coiled. The centromere divides the chromosomes into two arms of varying lengths. Metaphase: The spindle fibers enter the nuclear region, extend from the centrioles to the centomere, and attach at a point known as the kinetochore. Once the spindle fibers are attached, they align the cen-tromeres along the equatorial region of the nucleus known as the metaphase plate, so that the arms of the chromosomes point towards the poles of the cell. Anaphase: The centromere divides and the two chromatids separate from each other, forming two identical daughter chromosomes. The spindle fibers attach to the centromere and pull the newly-divided chromosomes towards the poles and away from the metaphase plate. The spindle fibers appear to move, but in fact, the microtubules are continuously formed at one end of the spindle fiber and then disas-sembled at the other.

2 © 2002 WARD’S Natural Science Establishment, Inc. All Rights Reserved Copymaster. Permission granted to make unlimited copies for use in any one

school building. For educational use only. Not for commercial use or resale.

Telophase: After the chromosomes reach the poles, a nuclear membrane forms around each set of daughter nuclei and the chromosomes uncoil and elongate, once again becoming invisi-ble. The spindle fibers break down and disappear. In animal cells, a cleavage furrow, an indentation in the cell membrane between the daughter nuclei begins to develop. This marks the end of mitosis. Cytokinesis: As mitosis ends, cytokinesis begins, resulting in the formation of two daughter cells. The cleaved membrane slowly draws together, forming a narrow bridge, then sepa-rates the cell into two daughter cells. The cells now enter inter-phase.

Mitosis ends when the processes are complete and the chromosomes have once more disappeared from view. The two daughter cells enter interphase. The two daughter nuclei produced are identical to one another and to the nucleus that divided to produce them. In order to investigate the process of mitosis, plant and animal tissues where cells are dividing rapidly must be examined. In animals, the most rapidly growing and dividing tissues are found in the embry-onic stages of development. Although most animal tissues continue to undergo mitosis throughout the life cycle of the organism, they do so very slowly when compared to their embryos. Some animal cells, like most plant tissues, rarely replicate after the organism reaches maturity. In plants, these tissues are primarily found in the tips of stems and roots. The root tips of plants are exceptionally good places to look for cells undergoing mitosis. Plant root tips consist of several different zones where various developmental and functional processes of the root are performed. The primary region for the formation of new cells is the apical meristem. The root cap offers protection for the rest of the root, the region of elongation is the area where the bulk of cell growth occurs, and the region of maturation is where tissue differen-tiation occurs.



Meiosis Sexual reproduction provides a mechanism to produce genetic varia-tion, since the genes of two different individuals can be arranged in various ways. This requires that the chromosome number of the par-ent cell, normally diploid, be reduced to half that, to create a haploid cell. The type of cell division resulting in a haploid parent cell is called meiosis. In meiosis, a germ cell divides into four haploid gametes. When two gametes — an egg and a sperm for most animals — combine during fertilization, forming a zygote, the diploid chromosome number is restored. Meiosis consists of one DNA replication and two nuclear divisions, meiosis I and II. This results in the formation of four daughter cells, each with only half the number of chromosomes as the parent. Genetic variability is further increased by a process called crossing over. In the early stages of meiosis, the homologous pairs of chromo-somes move close together in such a way that all four chromatids are

entwined, forming a tetrad. This proc-ess, known as synapsis, allows for the exchange of chromosome sections be-tween the homologous pairs. The example used in the investigation is Sordaria fimicola, an ascomycete fungus that is haploid for the bulk of its life cy-cle. The only diploid portion of the S. fimicola’s life cycle occurs when the nuclei of specialized hyphae cometogether. These hyphae, which belong to different strains of the species, fuse to form a zy-gote. This zygote then undergoes meio-sis to produce the haploid ascospores, yielding four haploid nuclei contained in a sac called an ascus. After meiosis, the four nuclei undergo mitosis, result-ing in the ascus containing eight haploid ascospores. Many asci form inside a fruiting body called a perithecium. One type of genetic variability in S. fimi-cola is the color of the ascospores. Most strains are the dark brown, wild-type ascospores, although there are variants. Certain strains have tan or gray asco-spores. A tan ascospore strain mated with the wild-type variety produces a series of perithecia containing asci with fou r tan and fou r wi ld- type ascospores each.



Mature Perithecium

8 Ascospores

8 Haploid Nuclei

Mitosis

Meiosis

Zygote(Diploid)

Fertilization

CellPartitioning Ascus

Spore Discharge

Ascospores

Mitosis

Mitosis

Filament

4 HaploidNuclei

Mycelium(Fungus growswithin its food)

MycelialFusion

Figure 1

DID YOU KNOW? It was German botanist Wilhelm Hofmeister, who dis-covered the regular alternation of sexual and asexual genera-tions in plants. He called this life cycle pattern involving distinct gametophytes and sporophytes: “alternation of generations”.

How these ascospores are arranged within the ascus is a direct repre-sentation of whether or not crossing-over has occurred between the centromere and the site for the gene for ascospore color. If no crossing over has occurred, the ascospores will be arranged in a 4:4 manner. If crossing over has occurred, they will occur in a 2:4:2 or 2:2:2:2 manner. By observing the ascospore arrangement, the percentage of asci ex-hibiting crossover can be determined. This frequency appears to be affected largely by the distance from the gene to the centromere. From the crossover frequency, the distance in map units from the gene for ascospore color, and the chromosome centromere, can be calculated.



MeiosisI

MeiosisII

Mitosis

or

or or or

MitosisMeiosis

IIMeiosisI

tn

tn

tn

tn

+

++

+

tn

tn

tn

tn

+

+

+

+

Formation of Non-crossover Asci

Formation of Crossover Asci

Figure 2

• Examine and compare the phases of mitosis in animal and plants cells

• Determine the relative time cells spend in each phase of mitosis • Prepare microscope slides of mitotic cells using allium root tips • Follow the processes of mitosis and meiosis in the life cycle

of Sordaria • Examine the arrangement of Sordaria ascospores microscopically

to determine the frequency of crossing over • Calculate the distance, in map units, between a specific gene and

the chromosome centromere

MATERIALS NEEDED PER GROUP Whitefish mitosis slide Onion mitosis slide Compound microscope Toluidine blue, 0.5% Hydrochloric acid, 1 M Garlic root tip Microscope slide Coverslip Compound microscope Clothespin Scalpel Pipet Bunsen burner Paper towel Microscope slides Coverslips Inoculating loop SHARED MATERIALS Sordaria demonstration cross plate



OBJECTIVES

MATERIALS

DID YOU KNOW? Garlic, also referred to as “the stinking rose”, has been used since the days of the Egyptians for medicinal purposes. Mod-ern scientific research confirms the ability of garlic to lower cholesterol and blood pres-sure, stimulate the immune system, inhibit the growth of intestinal parasites, and pro-tect against a variety of toxins.

Part I: Mitosis A. Observing Mitosis in Plant and Animal Cells 1. Observe the prepared microscope slide of onion root tip mitosis,

first at 100X, then 400X. Using the provided Plant Cell Mitosis il-lustrations as a guide, identify cells that represent each mitotic phase.

2. In the Analysis section, draw each phase of plant cell mitosis that you

see. Write a brief description of each phase below each drawing. 3. Observe the prepared microscope slide of whitefish blastula. Us-

ing the provided Animal Cell Mitosis illustrations as a guide, identify each phase of animal cell mitosis.

4. In the Analysis section, draw each phase of plant or animal cell

mitosis that you see. Write a brief description of each phase be-low each drawing.

B. Relative Lengths of Phases of Mitosis 1. Examine at least three fields of view of the apical meristem of the

onion root tip at 400X. In each view, count the number of cells in the various stages of mitosis. Record this data in Table 1 in the Analysis section.

It is recommended that the magnification of the fields of view be standardized for the entire class to ensure accu-rate and comparable data for all lab groups. A random or straight-line method may be used; always make sure that none overlap.

2. Calculate the total number of cells counted and the percentage of

total cells counted for each stage of mitosis. Record this data in Table 1 as well.

3. Assuming that it takes an average of 24 hours (1,440 minutes) for

onion root tip cells to complete the cell cycle, calculate the amount of time cells spent in each phase of the cycle. Use the for-mula provided below. Enter your results in Table 1.

Percent of cells in phase x 1,440 minutes = _________ minutes cell

spent in phase



PROCEDURE

NOTE

DID YOU KNOW? Researchers at Cornell Univer-sity have shown how tiny mo-lecular motors carrying target proteins help guide the mitotic spindle that transfers genetic material from the nucleus of a mother cell to a newly-formed daughter cell. If these molecu-lar motors fail, the spindle can-not properly orient itself with the axis of the cell and genetic material cannot be transferred.

Copymaster. Permission granted to make unlimited copies for use in any one school building. For educational use only. Not for commercial use or resale.

C. Preparation of an Garlic Root Tip Squash

As a general laboratory practice, it is recommended that you wear proper protective equipment such as gloves, safety goggles, and a lab apron to avoid staining any clothing or skin.

Due to time constraints, your instructor may have grown garlic root tips in advance. If this is the case, begin with Step 4.

1. Separate a clove of garlic into individual sections. Remove the

paper-like skin from each. Run 2-3 toothpicks crosswise through one garlic section and suspend it in a small vial or 100 ml beaker topped off with water, with the root primordia (blunt end) in the water.

The section used must have root primordia present or it will not produce root tips. The root tips will grow within 24 hours for very fresh garlic, or as long as three days for older garlic. Because Allium has a mitotic cycle of ap-proximately 12.5 hours, it is important to “plant” the garlic and harvest the root tips at approximately the same time of day in order to get the greatest percentage of mer-istematic cells undergoing mitosis.

2. Place the vial or beaker in a box or dark place until the root tips

have grown to a length of about 4 to 5 mm. It is important that the garlic grows in the dark to ensure that it produces roots rather than shoots. Several viable root tips will grow on each section of garlic.

3. Remove the garlic from the box approximately one half-hour be-

fore performing the experiment to expose the root tips to light. 4. Blot as much excess water from the root tips as possible. Any ex-

cess water on the slide will affect your results. Do not allow the root tips to dry out, however.

5. Using a scalpel or razor blade, cut off the end of one of the emer-

gent root tips; the section should be approximately 1 to 2 mm long. Place the root tip on a clean microscope slide and apply two or three drops of HCl to the root tip.

6. Holding the slide with a clothespin or forceps, pass it through the

flame of a Bunsen burner for five seconds.

Do not hold the slide over the flame.

8 © 2002 WARD’S Natural Science Establishment, Inc. All Rights Reserved

NOTE

NOTE

NOTE

DID YOU KNOW? Scientists have discovered sev-eral checkpoint mechanisms that ensure each step in the mitotic process is properly exe-cuted before the cell moves on to the next phase.


7. Without harming the root tip, blot the specimen with a paper towel to remove the excess HCl.

You may wish to touch a corner of the paper towel to the drop on the slide and allow the paper towel to soak it up. This may not remove the liquid from the slide as well as blotting, but it will not disturb the root tip.

8. Add a few drops of 0.5% aqueous toluidine blue stain, covering

the root tip.

Toluidine blue is a mild irritant. Avoid contact with skin and eyes, and do not ingest.

9. Pass the slide through the flame of a Bunsen burner for one to

two minutes. Let the slide stand for one minute.

Do not hold the slide over the flame.

10. Without disturbing the specimen, use a paper towel to remove the excess stain.

11. Add a drop of toluidine blue and cover with a coverslip. Using a

pencil eraser or other blunt instrument, gently press down on the coverslip to squash and spread out the root tip. Blot off excess stain, if any, that may have come out from under the coverslip.

12. View the slide under a microscope at 100X. Locate the apical mer-

istem. Examine the slide at 400X. Locate cells in the various stages of mitosis, and make sketches of what you find. Keep in mind that since the root tip has been squashed, the meristem may not be readily recognizable.

Part II: Meiosis To perform the first part of the meiosis lab — creating models of chromo-somes with pop beads or other items — in accordance with AP Biology re-quirements, we recommend WARD’S Chromosome Simulation Kit, avail-able separately (36 W 1602). 1. Place a drop of water on a clean slide with an inoculating loop. 2. With an inoculating loop, scrape several perithecia from the dem-

onstration cross plate. Scrape the perithecia from the interface of two crossing strains (Figure 3) close to the edge of the plate and place in the drop of water on the slide. Avoid picking up agar along with perithecia; it will interfere with results.


NOTE

NOTE

NOTE

Figure 3

DID YOU KNOW? It has been found that many human cancers are likely to stem from defective mitotic checkpoints.


3. Cover the slide with a coverslip. Using a pencil eraser or other blunt instrument, gently press down on the coverslip to squash and spread out the perithecia. The pressure should be sufficient to squeeze the asci from the perithecia, but not enough to crush the asci themselves.

It may be helpful to slide the coverslip around on top of the sample, with slight pressure, to spread out the asci and make them easier to observe. Keep in mind, however, that applying too much pressure may rupture the asci, releas-ing the individual ascospores.

4. View the slide under a microscope at 100X. Locate the asci. You

may wish to view the slide at 400X to determine the color of some ascospores. The slide preparation should show collapsed perithe-cia and asci clusters (rosettes), with mature ascospores in various arrangements. Immature ascospores will all be light colored. Since S. fimicola is homothallic, the preparation will show both hybrid and self-fertilized perithecia of both parental types. Hy-brid perithecia, however, will not occur very far from the line of contact between the two varieties. Prepare three slides to get an adequate sampling of hybrids, if possible.

5. Count approximately 50 hybrid asci from at least three fields of

view, preferably from different slides. Record this data in Table 3 in the Analysis section.

6. Use the formulas in the Analysis section to calculate the

frequency of crossing-over and the number of map units between the centromere and the gene for ascospore color.


NOTE

DID YOU KNOW? In 1903, Walter Sutton discov-ered that chromosomes con-tained genes and that their be-havior during meiosis was ran-dom, concepts that later pro-vided the basis for the Chro-mosomal Theory of Heredity.

WARD’S Name: AP Biology Lab 3 Group: Mitosis and Meiosis Date: Lab Activity

ANALYSIS



Stage: ___________ Description: _______ ____________________________________________________________________







Using the data from the table, determine the distance in map units from the gene for ascospore

color to the chromosome centromere. Using the formula provided below, calculate the percentage of asci that showed crossover. This percentage crossover must be divided by two, since only half the ascospores in each hybrid ascus are the result of crossing over. Dropping the % symbol gives you the map distance from the gene to the centromere. Record this data in Table 3.

% Crossover = # showing crossover x 100% total counted Gene Distance from Centromere = % Crossover 2

# of 4:4 # of Crossover Total % Asci Showing

Crossover Gene Distance From

Centromere

Table 3

Stage % of Total # of Cells Interphase Prophase

Metaphase Anaphase Telophase

Total

Table 2

Table 1 Cells in Each Stage of Mitosis

Stage # of Cells in

Field 1 # of Cells in

Field 2 # of Cells in

Field 3 Total # of

Cells % of Total # of

Cells Time of Each Stage (min)

Interphase

Prophase

Metaphase

Anaphase

Telophase

Total number of cells counted:____________

WARD’S Name: AP Biology Lab 3 Group: Mitosis and Meiosis Date: Lab Activity



1. Referring to the percentage of total cells counted in each phase of mitosis, determine which phase takes the longest for the cell to complete, and explain why. Sketch a pie graph of the percentage of cells in each phase to illustrate. Be sure to include a title and key for your graph.

2. What is the relationship between the processes of mitosis and cytokinesis? 3. Which of the following is significantly different between plant and animal cell mitosis?

a. metaphase b. anaphase c. cytokinesis d. prophase

ASSESSMENT



4. How does meiosis lead to genetic variability within a population? Use S. fimicola as an example. 5. How does this represent an adaptive advantage for organisms that reproduce sexually? 6. Define the following terms:

somatic cell – germ cell – chromatin – centromere – diploid – haploid – zygote –

7. Create a Venn diagram showing at least two similarities and two differences between mitosis and

meiosis.



8. Why is S. fimicola an ideal organism for the demonstration of crossing-over? 9. When eukaryotic cells undergo mitosis, it results in two identical daughter cells, each with a nu-

cleus. Prokaryotic cells are cells that do not contain a distinct nucleus. Research cell division in pro-karyotic cells and compare and contrast it to division in eukaryotic cells.

10. A classmate of yours was absent on the days the lab was performed. Come up with a list of materi-

als (excluding pen and paper) you might use to visually demonstrate the concepts of mitosis, meio-sis, and crossing-over.

Telophase Cytokinesis

Metaphase Anaphase

Prophase Interphase

Animal Cell Mitosis



Metaphase

Interphase Prophase

Anaphase

Cytokinesis Telophase

Plant Cell Mitosis



Documents

250-7052s mitosis and meiosis - Wikispaces. In the early stages of meiosis, ... Sordaria fimicola, an ascomycete fungus ... crossing over has occurred,