Lab #7: Mitosis and Meiosis
Objectives:
1. To begin to understand Reproduction and Life Cycles.
2. To get hands-on experience with mitosis.
3. To get hands-on experience with meiosis.
4. To compare experimental vs. observational studies.
As we discussed in class, biological growth is a multi-dimensional phenomena.
Basically, the basic unit of life, the cell, must duplicate itself. One "mother" cell must
become two "daughter" cells, through the process of Cell Division, mostly MITOSIS.
This is a completely asexual event and involves replication of the nucleus and its
constituents and dividing up of the cytoplasm, to make two identical cells.....
..............
..........MEIOSIS, on the other hand,
is involved only in sexual reproduction, and is the main source of genetic variation. In
meiosis, cells with a full compliment of chromosomes divide to yield cells with one-half
as many chromosomes. These cells become the "Egg" and the "Sperm" that unite later
in a process known as FERTILIZATION.
Mitosis and meiosis also allow us to see science from a non-experimental
perspective. Sometimes, science just records what it observes. Cells in actively
growing regions of plants (meristematic regions), such as roots and shoots tips, have
cells that are easily stained to show the process of mitosis. From these cells we can
see cells actively dividing and at different steps in the process. The process of
mitosis is ubiquitous in all diploid organisms. Often times, original, detailed
descriptions of observations of processes have been the basis of future, experimental
work. The observational descriptions we make today about mitosis and meiosis will
allow us to develop hypothesis about genetic relationships in the next few labs.
Actually it will be very important that you do not lose track of these basic principles
of heredity for all the kernels of corn that you count.
1
Lab #7: Mitosis and Meiosis
II Forming
Hypotheses
Recall the Experiment
Cycle or Scientific
Process:
1. Make observations about the natural world.
2. Ask questions about those observations.
3. Formulate a reasonable testable hypothesis to explain observations.
4. Create, execute, and replicate experiments testing the hypothesis and generating
results.
5. Analyze results and draw inferences. This stimulates further inquiry. The cycle
begins anew.
Observation: Descriptions of the steps in mitosis and meisos.
Question1: What is the process by which organisms grow and replace cells?
Question 2: What is the process by which
organisms produce gametes or sexually
reproducing cells?
Hypothesis 1: The process of mitosis is
composed of five steps that replicate the
chromosomes and then divides the
genetic material equally between two daughter cells.
Hypothesis 2: The two stages of meiosis replicate and reduce the number of
chromosomes to produce four cells.
Experiment: Observing and describing the process of mitosis and meiosis in several
sections of onion cells (mitosis) and a fungus (meiosis).
Results: Record your diagrams and descriptions in your lab book
2
I. Background Material
A. Cell Cycle
As a plant grows, new cells and nuclei arise from preexisting cells, Mitosis.
Mitosis occurs predominantly in meristematic regions of the plant: the root and stem
tips, vascular and cork cambium, and in organs in early stages of growth. The two
stages of mitosis are generally recognized as:
a. Mitosis - division of the nucleus into two nuclei;
b. Cytokinesis - division of the cytoplasm and building of a new cell wall
laid down between the new cells.
In actively dividing cells, the term Cell Cycle is used to describe the life history
of the cell, as it is made during mitosis, enlarges and develops, and then itself divides
into two new cells. In other words, repeated cellular divisions are separated by
periods of time during which growth and preparation for the next division occur.
During the "S" or synthesis phase of the Cell Cycle DNA is replicated. Since
DNA is the "stuff" of which CHROMOSOMES are made, this replication results in the
doubling of all chromosomes. This is necessary because during mitosis the
chromosomes of one nucleus are divided between the two new nuclei.
PART 1 - "Mitosis in African Blood Lily".
This part of the lab will be devoted to looking at the Mitotic phase of the cell
cycle. Although mitosis is often divided into phases, this only a pedagogical (teaching)
tool. Prophase, prometaphase, metaphase, anaphase, and telophase are merely markers
or frozen images (freeze frames) in a CONTINUOUS PROCESS. ................... To
underscore this CONCEPT (mitosis is a continuous process) watch the short film of
actual mitosis in Haemanthus.
3
PART 2 - MITOSIS IN ONION (Allium cepa) ROOT TIPS
Obtain a prepared slide of onion root tips in longitudinal section. Under low
power, note the location of the apical meristem in relationship to cap and more mature
regions. Under higher power try to find cells in various stages of mitosis.
Now you are going to look at fresh tissue squashes of onion roots to find your
own mitotic stages. Get a fresh onion root from the container and observe it under
the dissecting 'scope. Next, using a new razor blade, cut away and save the first 1
millimeter of the root tip containing the apical meristem. Place the tip on a glass slide
and add a few drops of acid-alcohol. The acid-alcohol dissolves the pectin that holds
the root cells together. Add more acid-alcohol, if necessary, to keep the mass of
tissue moist.
After 5- 10 minutes, blot away the excess liquid and add a single drop of acetocarmine stain and work this into the tissue using two probes. This takes time and
patience, but soon the tissue will be a mass of bits. Finally, place a glass cover-slip
over the cells and squash with a medium downward pressure, as directed by your TA.
Observe your squash under the microscope and search carefully for a region
of mitotic activity. Again, this takes much patience, work and luck. Draw what
you see in your lab books.
Find a region filled with actively dividing cells. Count and record the number
of cells in each phase. What do the numbers mean for mitosis and the cell cycle?
Using the books available in lab, look at as series of pictures of the
Phases of Mitosis. What is happening to chromosomes during the
process of Mitosis?
Interphase:
Prophase:
Prometaphase:
Metaphase:
Anaphase:
Telophase:
4
PART 3 THE CIRCLE OF LIFE - MEIOSIS AND
FERTILIZATION
Two critical events occur in the life history of all sexually reproducing
organisms: meiosis and fertilization. During the interphase preceding meiosis, the
chromosomal material; (DNA) is replicated. Then, during meiosis, the nucleus
undergoes two divisions, one of which is a reduction division. By a precise mechanism,
meiosis produces four daughter nuclei, each with one-half the number of
chromosomes, and thus one-half as much DNA, as the parent nucleus. Whereas the
parent nucleus is diploid (2n), each of the daughter nuclei is haploid (n). In diploid
cells, the chromosomes are present in matched pairs called homologous chromosomes.
Each parent contributes one member of each pair during sexual reproduction. In the
reduction division of meiosis, the pairs of homologous chromosomes are separated.
Haploid cells, therefore, contain only one member of each homologous pair of
chromosomes. See your text for details.
Look at as series of pictures of the Phases of Meiosis in the various biology
books in the lab. What is happening to chromosomes during the process of
Meiosis?
In the lab various colors of clay are available. Demonstrate the steps of meiosis to
your TA, using two sets of homologous chromosomes, moving the clay models through
the phases of Meiosis I and Meiosis II, first without crossing over and secondly with
crossing over. Be especially attentive to Metaphase I, crossing over, and independent
assortment. (Use different colors of clay to represent the two sets of homologous
chromosomes).
.
5
II. CROSSING OVER
In this part of lab you will look at genetic segregation and the frequency of
crossing over in a fungus. By understanding the process of crossing over, you can
tie together the principles of DNA structure with inheritance of characters
(Mendelian genetics).
GENETIC SEGREGATION AND CROSSING OVER
In meiosis, genetic recombination may occur as a result of the exchange of genetic
material between homologous chromosomes during the process of crossing over.
Crossing over occurs during prophase I, when homologous chromosomes synapse.
While they are joined in this complex, nonsister chromatids may break at
corresponding points and exchange parts. A point at which they temporarily joined
as a result of this exchange is called a chiasma.
Fungal
Filaments
Figure 1
Sordaria fimicola is a saprotrophic fungus that
occurs on dung and decaying plant matter. It
belongs to the class Ascomycetes, a group
characterized by saclike sporangia called asci
(singular, ascus), each of which, at maturity,
contains eight linearly arranged haploid (n)
ascospores (see figure 1). The asci (about 20) are
grouped together within a structure called the
perithecium. It is the dark brown perithecium on
the agar plate that you can observe with the naked
eye. When the ascospores within a given ascus are
mature, the ascus elongates and penetrates into
the neck of the perithecium. Upon reaching the top
of the neck, the tip of the ascus breaks open and
spore discharge occurs. The spent ascus quickly
shrivels up, and another one begins to elongate.
7
Genetics of Sordaria fimicola
Sordaria fimicola spends most of its life as a haploid mycelium, a mass of cells
arranged in filaments. When conditions are favorable, cells of filaments from two
different mating types fuse. Ultimately, the nuclei fuse and 2n zygotes are
produced (see figure 2). Each 2n zygote undergoes meiosis, and the resulting cells
(ascospores) remain aligned. The position of an ascospore within the ascus depends
on the orientation of separating chromosomes on the equatorial plane of meiosis I.
After meiosis, each resulting ascospore divides once by mitosis, resulting in eight
ascospores per ascus. This unique sequence of events means that it is easy to
detect the occurrence of crossing over involving chromatids carrying alleles that
encode for color of spores and mycelia.
Figure 2
Ascus
If two mating types of Sordaria, one with black spores and the other with tan
spores, are grown on the same petri dish, mycelia from the two may grow together,
and certain cells may fuse. Nuclei from two fused cells then fuse, and the
resulting zygote contains one chromosome carrying the allele for black spores and
another carrying the allele for tan spores. After meiosis I and II take place, one
mitosis follows, and the result is eight ascospores in one ascus: four black spores
and four tan spores. If no crossover events occur, the two genes will segregate
during meiosis I and produced a 4:4 arrangement of ascospores. If a crossover
event does occur, the two genes will not segregate until meiosis II resulting in a
2:2:2:2 or 2:4:2 sequence of ascospores (Figure 3).
7
Map Distance: The genes on a chromosome are situated at specific distances from
the centromere. For any given gene, that distance remains constant. In the case
of a gene located close to the centromere, few if any, crossovers are to be
expected in the gene-to-centromere interval. If the gene is farther removed from
the centromere, a greater number of crossovers are to be expected between it
and the centromere. In other words, the greater the distance of the gene from
the centromere, the greater the frequency of crossovers in the gene-tocentromere interval. Since the frequency of crossovers in the gene-to-centromere
interval is a function of the distance of the gene to centromere, the crossover
frequency can be used to determine how far from the centromere a gene is
located.
Map distances for haploid organisms can be calculated. Map distances are
estimated on the basis of the frequency of second division (MII) segregation as
follows. First, the total number of second division (MII) segregation asci are
determine. This number is them divided by the total number of bicolored asci that
were counted. The resulting number is multiplied by 100 to obtain the percentage
of second division segregation. This percentage of crossovers must be divided in
half in order to obtain the map distance between the gene locus and the
centromere. Division by 2 is necessary because only two of the four chromatids of
the tetrad are involved in crossing over. The value obtained by dividing % MII by 2
(the relative distance between the locus and the centromere) is expressed in
arbitrary units called map units.
8
Analysis of Hybrid Perithecia
1. From your instructor obtain the following cross plate:
Wild-type (+) by tan spored mutant (t)
2. Working under a dissecting microscope and using a dissecting needle, transfer
four or five perithecia from the cross plate into a drop of distilled water on a
clean glass slide. Select perithecia from those regions of the cross plate where
the mycelia of the two strains are in contact. Cover the perithecia with a
coverslip.
3. Tap gently on the coverslip with the blunt end of a dissecting needle to break
open the perithecia and release the asci. Do not move the coverslip.
4. Examine your preparation under the compound microscope for the presence of
hybrid perithecia. If no hybrid perithecia are present, make a new slide.
5. If hybrid perithecia are present, remove the slide from the microscope. Place a
piece of blotting paper over the coverslip and, while holding the coverslip in
position with one hand, apply gentle pressure to the coverslip with the thumb of
your free hand.
6. Using the compound microscope, locate a cluster (rosette) of hybrid asci and
count the number of first and second division segregation asci (MI and MII).
Record the data in Table 1.
7. When all of the asci in a given cluster have been counted, locate another cluster
of hybrid asci and again count the number of first and second division
segregation asci, entering the data in Table 1. When all of the asci on the slide
have been counted, prepare a new slide and again count the hybrid asci. Count
200 bi-colored asci.
8. Using your data, determine the percent of second division segregation of asci
(% MII).
9
Sordaria
Laboratory
Number of
Number of Meiosis Total
Meiosis I Asci II Asci 2:2:2:2 or Number
4:4
2:4:2
of Asci
Worksheet
Gene
to
% Meiosis II
Centromere
Asci / Total #
Distance(%
asci counted)
Meiosis II / 2)
1. How many different arrangements did you find? __________________
2. What were the different arrangements? To answer this question list the
black spores as + and the tan (mutant) spores as t. For example, no crossing
over would be recorded as ++++ t t t t
3. How do you explain the perithecia in which the asci contained either eight
black or eight tan ascospores?
4. Were any other ascospore arrangements observed other than the ones
previously mentioned? If so, how might these be explained?
10
11