MITOSIS, MEIOSIS, EARLY DEVELOPMENT, AND LIFE CYCLES
Ruth Logan, 2003
MITOSIS
We will study Mitosis and Meiosis by comparing slides of tissues undergoing these
processes to the drawings and photographs in your text. We will also make models to
illustrate the processes.
1.
ALLIUM root tip:
Notice that this slide contains 2 or 3 slices cut longitudinally through the very tip of a
growing onion root. The root cap on the rounded end of the root is a layer of dead cells
that protects the growing root as it pushes its way through the soil. Immediately behind
the root cap is the region of mitosis (more properly called the apical meristem) where
rapid cell division is occurring. This is the portion of the root that you will study in detail
while looking for cells in each of the phases of mitosis. The older parts of the root above
the meristem are developing into functional plant tissues. First the cells elongate, then
they differentiate and mature into specific plant tissues. After maturing, most plant cells
do not divide or grow again. Plant growth only occurs in specialized regions known as
meristems. Identify these regions on the onion root tips on your slide before you go on.
Examine the meristematic region of your root tip sections and identify cells in each of the
phases of mitosis. Make several sketches of cells in each phase so you will be able
to recognize early, middle, and late portions of each phase, and variations that occur
among cells. Remember that the process is continuous and try to get a feeling for the
continuity between the phases. And look at your lab partners’ slides to increase your
ability to recognize these phases in many different cells.
The onion has 16 chromosomes per cell. The phases of mitosis that you are required to
be able to recognize are: interphase, prophase (several stages), metaphase, anaphase,
and telophase (several stages). Also be able to identify spindle fibers and cell plates.
2. Whitefish blastula:
This slide contains several sections of a young whitefish embryo that is growing rapidly so
the cells undergo mitosis frequently. Therefore many of the cells ought to have been
caught in a stage of mitosis when this slide was prepared and fixed. In the onion root tip,
all the cells were oriented the same way so that the viewer always sees mitosis from the
side. Whitefish blastula cells are not so cooperative. They are turned in all directions so
that you can be viewing the mitotic figure from the side, from the end, or from some other
angle. Also since these cells have been sliced, some part of the mitotic apparatus may be
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missing. Be careful in making your observations and drawings to take both of these
possibilities into account.
Study this slide to identify the phases of mitosis and structures associated with the
process. Animal cells have two structures that are not visible in plant cells: asters and
centrioles. In addition, animal cells divide by cytokinesis rather than through the formation
of a cell plate.
3. Models
Pop beads are available in the lab. Use these to represent chromosomes and
demonstrate mitosis with them. Move the "chromosomes" through all the phases and
positions of mitosis and attempt to get a feeling for the "flow" of the process. In addition,
use this opportunity to familiarize yourself with centromeres, chromatids, and centrioles,
and review the cell cycle for the relative timing of growth, replication of chromosomes,
nuclear division, and cell division.
MEIOSIS
1. Models
Use the pop beads, pipe cleaners and colored threads available in the lab to make a
model of the process of meiosis. Compare the steps in this process to those in mitosis
carefully. What is similar and different about each part? Pass the "chromosomes”
through all phases until you are sure you understand all of them. Pay special attention to
chromosome number and structure at each stage in the process.
2. Spermatogenesis: Grasshopper Testes:
Although the stages in spermatogenesis have specific names, each represents a portion
of meiosis. It is possible to find all the stages on a section of a grasshopper testis
seminiferous tubule because cells occur in groups (clumps) that develop synchronously.
Follow these stages by observing successive clumps as they travel down the tubule.
Whenever meiotic figures are visible (supercoiled chromosomes), attempt to distinguish
whether they are tetrads or dyads.
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3. Mammalian Testes:
In the mammalian testes sections on this slide, look for a round cross-section of a
seminiferous tubule. The outer cells in this tubule are primordial germ cells and
spermatogonia, and meiosis progresses as the cells move toward the lumen (central
opening) of the tubule. Near the center there should be some spermatids and perhaps
spermatazoa. Compare each stage seen here with the comparable stage in the arthropod
above.
4. Ascaris oviduct and uterus:
Ascaris meiosis occurs after a sperm has penetrated the oogonium, so the sperm
pronucleus is present in the cell all during the process of oogenesis. Fertilization occurs
rather far up in the oviduct, and oogenesis proceeds as the cell travels down the oviduct
into the uterus. These slides contain two sections of these tubules, one each for primary
and secondary oocytes. (Slicing may have removed part of the cell you are examining).
The 2N number of chromosomes in Ascaris is 4. Attempt to identify tetrads, dyads, and
polar bodies. Try to tell whether each cell is turned sideways or endwise to your viewing
angle. Note that fertilized cells develop shells around them.
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FERTILIZATION AND DEVELOPMENT
Mitosis in action
Introduction
Development occurs in both plant and animal embryos, but while there are obvious
differences in form, the basic principles and mechanisms are thought to be the same. We
will examine examples of fertilization and early embryology in animals in order to examine
the processes of early cleavage, differentiation (cell specialization), and development of
the vertebrate body plan.
Unfortunately fertilization and development do not lend themselves easily to an
experimental approach in a student lab. Embryos die easily even under carefully
controlled conditions, and the techniques of handling them are both tricky and time
consuming. Therefore, the basic methods of this experiment will be observational. Living
material will be used to study fertilization in sea urchins. Then professionally prepared
slides of starfish and frog embryos will be studied. Careful drawings of each stage and
structure should be made and labeled to insure complete observation.
I. Fertilization: Sea Urchin gametes
A sea urchin in reproductive season will release its gametes if treated with 0.5 M KCl.
Samples of sea urchin eggs and sperm obtained in this manner will be available. Place a
sample of the eggs in a well slide obtained from the lecture bench. Place a cover slip on
the slide and look at the eggs under low power of your compound microscope. Note
especially the outer edges of each cell, and draw one.
Next add a sample of sperm to the well of your slide and replace it quickly on the
microscope slide. Sperm usually swarm around eggs in great numbers. Watch to see if
you can spot a sperm penetrating an egg. When fertilization occurs, a vitellin membrane
separates from the egg oolemma and pops out around the egg. This is thought to prevent
the penetration of additional sperm.
The zygote will begin preparing for the first cleavage division but it may take an hour or
more. Keep the slide near you on your lab bench and look again now and then. DON'T
keep the slide on the microscope with the light shining on it since that will cause the fluid
to heat up and the embryos to die. With care and luck, you may observe two or three
cleavage divisions by the end of the class period. (You can also check a beaker of
fertilized eggs that your instructor has prepared. Embryos survive longer in an open
beaker that is kept cool.)
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II. Early Starfish Embryology: (3 slides)
The zygote (fertilized egg) of a starfish undergoes cleavage just as the sea urchin zygote
did. (However, the vitellin membrane is not often intact on prepared slides: it usually gets
broken in the preparation process.) The embryo becomes a 2-cell stage, then a 4-cell
stage, an 8-cell stage and so on. The ball of cells that gradually appears is called a
morula. Each cell is now much smaller than the original zygote because division has
been occuring without growth. The entire morula is about the same size as the zygote.
Identify and draw each of these stages in embryology from your early cleavage slide. In
particular, note the individual cells (blastomeres) and the grooves between them
(cleavage furrows).
Gradually the embryo's cells spread apart to form a hollow sphere called a blastula. The
overall size of the blastula is still about the same size as the zygote, so each individual
cell is much smaller than an egg cell. So far, no net growth has occurred in the embryo;
the primary activity has been cell division. The blastula appears to be dense around the
edges, but less dense in the center due to its hollow nature. The cavity within the blastula
is called the blastocoel, and the single layer of cells on the surface of the sphere is called
the blastoderm or protoderm. It is an undifferentiated tissue layer. Some of the
blastulae on this slide may appear slightly flattened on one end indicating that these are
older blastulae, and that gastrulation (formation of the gastrula) is beginning. Notice the
change in cell shape in the flattened region. Draw and label one or two blastulae.
The starfish gastrula is formed by invagination (in-bending of cells) on one end of the
blastula. As a result of this process, the blastocoel is gradually obliterated and a new
cavity, the gastrocoel (or archenteron or primitive gut) is formed. This new cavity,
unlike the blastocoel, communicates with the exterior by way of an opening, the
blastopore, which will eventually become the anus of the animal. Pouches are produced
in the archenteron that will eventually separate and take up a position in the old blastocoel
to form the mesoderm layers. At this time, the three primary tissue layers of the organism
have become distinct and can be identified: the ectoderm makes up the outer
covering of the embryo and will become its skin layer. The endoderm lines the
archenteron and will become the lining of the gut (digestive tract) of the animal. The
mesoderm is the mass of cells resulting from the out pocketing of the archenteron and
will form most of the internal organs of the animal (circulatory system, reproductive
system, locomotor, and support systems, etc.). The cavities of the mesodermal masses
are called the coelom which will become the body cavity of the adult. Draw one or two
gastrulas and identify as many of these structures as possible.
III. Development of a body plan in the Frog: (2 slides)
Frog embryos are always sliced into thin sections for viewing because they are so large
and yolky that they are not otherwise light transparent. Examine the cross-section slides
of a frog embryo in the neural plate, neural fold, and neural tube stages. Identify the three
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tissue layers (ectoderm, mesoderm, and endoderm) that correspond to the regions you
recognized in a starfish embryo.
The endoderm consists of a single cell layer dorsal to the archenteron but contains many
more cells in its ventral side since it contains a large food supply to support the growing
embryo. Notice that these cells are large, have a “sloppy” shape, and do not contain
pigment.
The mesoderm is separated into distinct regions on the dorsal side but forms sheets on
the lateral sides that eventually meet in the mid-ventral line. Its cells are smaller and
neater than endoderm cells. It has differentiated into several regions:
1. Cordamesoderm, Notochord--in circular cross section of the embryo, the
middorsal section of the mesoderm (the chordamesoderm) differentiates into the
notochord of the animal. This is the first definite structure to appear in a vertebrate
embryo and provides it with some structural support. It is especially important
because it soon induces the formation of the neural tube in the ectoderm directly
above it. Later it is squeezed out of existence by the developing vertebrae (bones)
so that only scars remain in the adult.
2. Dorsal mesoderm, Somites -- on each side of the notochord, a blocky mass of
mesoderm forms into sections (somites, segments) that extend from anterior to
posterior down the length of the embryo. These somites will differentiate into the
segmented parts of the frog, especially the vertebrae. Look for somites on the
models of embryos available in lab to see how they are arranged in the length of
the body.
3. Intermediate mesoderm-- a thin and narrow strip of tissue connects the somite
with lateral plate mesoderm. Later, this strip will differentiate into tubules that
serve first kidneys and later testes. Intermediate mesoderm is nearly always
impossible to see on these slides. If it is apparent, it will be located at the dorsal
“corners” of the embryo cross-section.
4. Lateral Plate Mesoderm--the big sheets of mesoderm that form most of the body
tube and extend from intermediate mesoderm to meet in the mid-ventral line are
called lateral plate mesoderm. Eventually this sheet splits to form two layers (but
these may not yet have formed in the embryo on your slide):
a. somatic mesoderm-- the outer wall of lateral plate mesoderm that lies just
under the ectoderm. --(DO NOT CONFUSE WITH “SOMITE")-b. splanchnic mesoderm-- the inner wall of lateral plate mesoderm that lies
next to the endoderm.
c. coelom--the space, or body cavity, formed between the somatic and
splanchnic layers.
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The ectoderm of vertebrates lies around the outside of the spherical embryo as a single
layer of small, pigmented cells. It forms the skin AND the dorsal, hollow central nervous
system. In fact, the notochord tissue induces neural tube differentiation, so only
ectoderm immediately above notochord becomes brain and spinal cord. The process of
neurulation is basically an example of invagination: first a thickening of a longitudinal
mass of ectoderm cells occurs to form the neural plate. Gradually these cells bow inward
starting at the center of the strip, so two long neural folds are formed with the neural
groove between them. Eventually, invagination is completed so that the folds join to form
the neural tube, and the groove becomes the neurocoel or neural canal. The neural
tube will differentiate and give rise to the brain, spinal cord, and other neural structures.
In this study of the frog embryo, it is useful to develop a three dimensional vision of the
body plan so as to understand the three-layered torpedo design of a vertebrate. Refer
frequently to the models available in the lab and attempt to visualize the entire embryo
rather than just the cross-sections you are viewing.
IV. Comparison of Chick whole mount embryo to frog embryo
Chick embryos can be peeled away from the yolk so can be mounted on slides as
complete specimen. Use the slide of a 33-hour chick whole mount to observe a threedimensional embryo, and compare it to the frog slices you have been studying. Especially
note the arrangement and organization of the notochord, the neural folds/tube, and the
somites.
LIFE CYCLES
Each living organism progresses through life by growing, reproducing, and dying. These
changes and stages are summarized in the life cycle of the organism. As the generalized
life cycle shows, each involves at least the following processes:
a.
Many mitotic divisions of cells that usually result in growth of haploid or diploid
multicellular organism stages, and that MAY result in asexual reproduction in
haploid stages.
b.
A single meiotic division of a diploid cell stage that reduces the chromosome
number to haploid and produces a gamete (which will undergo fertilization) or a
spore (which will undergo mitosis),
c.
A single fertilization/syngamy step that, in fusing together the cells (fertilization)
and nuclei (syngamy) of two haploid cells, produces a diploid cell and reverses
the effect of meiosis.
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Specific species emphasize one phase of the life cycle more than another and illustrate
some of the variety possible in the roles for mitosis. In evolution, the haplontic life cycles,
those that emphasize the haploid phase by carrying out most of the life processes including mitosis - in that phase, are thought to be oldest. Some protists, especially
including the plant-like algae, exhibit this style of life cycle.
Diploid Stage
Fertilization/
Syngamy
Meiosis
Haploid Stage
Later mitosis of diploid cells evolved, the diploid phase appeared as a multicellular stage,
and gradually it became more and more important as the organism that performs the life
processes of the species. Most members of the Plant Kingdom illustrate various stages in
the evolution of this diplo-haplontic style.
Diploid Stage
Meiosis
Fertilization/
Syngamy
Haploid Stage
Finally the haploid stage became much the smaller and less long-lived part of the life
cycle. It was reduced to a single cell, a gamete that did not undergo division in that phase
at all. Virtually all modern members of the Animal Kingdom exhibit this Diplontic life
cycle.
Diploid Stage
Fertilization/
Syngamy
Meiosis
Haploid Stage
SPECIFIC LIFE CYCLES
The first four life cycles presented below are for “plants” (although three are plant-like
Protists). Specimen of these are available in lab so that you observe some stages of
these life cycles. Pay attention to the ploidy (chromosome number - haploid or diploid) of
each stage and the type of division process that produced it.
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I. Chlamydomonas -- a small single-celled aquatic Protist that has a chloroplast and two
flagella
The life cycle of Chlamydomonas is an extreme example of a haplontic, or haploiddominant, life cycle. The haploid single-celled organisms are much more frequently
present in the environment than the diploid resting spore stage. The haploid free-living
organism is common in ponds and other bodies of water throughout most of the year
where it builds population size by asexual reproduction involving only mitosis. The diploid
resting spore stage is formed by fertilization/syngamy of two individuals when living
conditions are not good for the haploid free-swimming organism, usually in winter or dry
periods. The resulting diploid spore is dormant metabolically. It never divides by mitosis
and can only undergo meiosis when good living conditions are restored. The products of
meiosis are four new haploid Chlamydomonas individuals.
Make a wet mount of Chlamydomonas by placing a couple of drops of the culture on a
microscope slide. Expect to see the tiny single cells swimming very quickly across your
field of view. Attempt to see the chloroplast (green) and the two flagella, used at the front
of the organism in the direction of locomotion. If you see quiet green spheres, two
individuals may have mated to form a resting spore. Sometimes you can catch
Chlamydomonas in the act of mating. Two would be spinning around each other with
their flagellae intertwined.
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II. Ulothrix (a filamentous green algae, a plant-like Protistan)
This life cycle is also extremely haplontic, but now there is a many-celled structure in the
haploid phase. Although the cells divide by mitosis, they don’t separate; instead they cling
together in a linear array that is called a filament. There is some controversy about
whether this is a multicellular structure (one organism) or a colony of many organisms
because the cells are very little differentiated from each other.
An organism at this stage in a life cycle is called a gametophyte because it (infrequently)
produces gametes (by mitosis, since its cells are haploid). The filament cells that do this
are called gametangia. The more major roles of mitosis in the haploid portion of this life
cycle, however, are growth (development of a spore into a multicellular organism by
repeated mitotic divisions) and the old role of asexual reproduction. Gametophyte cells
that produce spores (called zoospores because they are flagellated; haplospores because
they are haploid) by mitosis are called sporangia. They look very much like gametangia,
so most people distinguish them by observing to see whether the product cells undergo
mitosis (spores) or fertilization (gametes).
Obtain one or two filaments of the haploid stage of Ulothrix and make a wet mount slide.
Under high power of the compound microscope, note the shape of the chloroplast (cup
shaped) and look for zoosporangia and gametangia (they are not always present).
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III. Ulva (a green algae, a plant-like Protistan)
This life cycle is diplo-haplontic: it has equal haploid and diploid stages, so the life cycle is
sometimes said to exhibit “alternation of generations”. The gametophyte (haploid gameteproducing plant) looks just like the sporophyte (diploid spore-producing plant). This life
cycle has added a new role for mitosis, that of division of diploid cells, and it produces a
multicellular organism in the diploid phase. There must be an evolutionary advantage to
diploidy because many species of Ulva have lost the ability to reproduce asexually in the
haploid phase.
Obtain a small portion of a preserved Ulva for observation. Can you tell whether you have
the haploid or diploid phase before you?
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IV. Fern (a Plant)
This life cycle is also diplo-haplontic and is the most complex of the life cycles assigned.
Although both phases of the life cycle contain multicellular organisms, they do not look
alike. The diploid organism is "dominant” in that it lives much longer, is much larger than
the haploid gametophyte, and is more frequently encountered in the environment. These
organisms have obligate sexual reproduction. (That is, ferns cannot reproduce asexually
by mitosis.)
This organism shows much more highly differentiated tissues than any of the previous
ones (so is unambiguously classified as a plant). Its short-lived adult gametophyte plant
(called a Thalus or a Prothalus) has reproductive organs (archegonia and antheridia)
and some additional differentiated cells such as rhizoids and gametes. The long-lived
adult diploid sporophyte stage has many differentiated tissues, including sporangia, roots,
stems, leaves, and vascular (circulatory) tissue.
Study the sporophyte plant in the laboratory paying special attention one of the sori.
Remove a sorus from the underside of a leaf and transfer it to a microscope slide to
examine it under the microscope. You should see many sporangia attached by stalks to
a common substratum. Focus on a single sporangium; note its annulus (the cells forming
the outer ring of the sporangium) and the dark haploid spores inside it. If possible,
observe a sporangium springing open and hurling its spores into the environment.
Study one of the plastic mounts available in lab that show the fern life cycle, and learn to
recognize all stages.
Study 3 slides to see fern gametophytes (Prothallia) with:
1. Mature antheridia containing sperm,
2. Mature archegonia containing eggs, and
3. A young sporling with new leaf and root growing out of the old gametophyte
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The following animal life cycles are included because they have been observed in other
exercises in this book. Animals in general have an extreme diplontic (diploid-dominant)
life cycle with no mitosis in the haploid phase (meiosis produces gametes directly instead
of producing spores and a gametophyte in the haploid phase).
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V. Sea Urchins or Starfish (members of the phylum Echinodermata)
These animals have the classic animal-style life cycle. The diploid adult organism (most
have separate sexes) produces haploid gametes by meiosis, sheds them into the
environment where fertilization occurs, and the diploid animals grow by mitosis.
Adult male
Adult female
Zygote
Fertilization/
Syngamy
Meiosis
Egg and Sperm
VI. Ascaris (a nematode worm usually parasitic in the intestines of a mammal)
Ascaris would have the classic diplontic life cycle characteristic of most animals except
that fertilization and meiosis occur in an overlapping manner, an adaptation to their
environment. The drawing below expands just this part of the life cycle.
The reproductive cell initially produced by the female is an oogonium, a diploid cell ready
to undergo meiosis, rather than a mature egg. Meiosis starts in the oogonium AFTER it
has been fertilized by a haploid sperm (produced by usual spermatogenesis in a male).
The sperm pronucleus stays quietly in cytoplasm of the cell while the oogonium nucleus
undergoes meiosis near an edge and sheds two polar bodies. Syngamy (joining of the
nuclei) occurs on the metaphase plate during the first mitotic (growth) division of the new
diploid organism.
Adult Male
Spermatogonium
Primary spermatocyte
Secondary spermatocyte
Spermatid
Sperm
Adult Female
Fertilization
Syngamy
Zygote
Larva
Adult
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Oogonium
Primary oocyte
Secondary oocyte
Ootid
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VII. Human (phylum Chordata, sub-phylum Vertebrata, and class Mammalia)
This life cycle also differs from the classic diplontic style only by overlap of meiosis and
fertilization. The female of this species carries primary oocytes in her ovaries from the
time she herself is an embryo. The first meiotic division occurs just before ovulation so
the cell produced is a secondary oocyte. After fertilization by a haploid sperm, the cell
undergoes the second meiotic division and sheds a polar body. Syngamy occurs on the
metaphase plate during the first mitotic (growth) division of the new diploid human.
Adult Male
Adult Female
Spermatogonium
Primary spermatocyte
Secondary spermatocyte
Spermatid
Sperm
Oogonium
Primary oocyte
Secondary oocyte
Ootid
Fertilization
Syngamy
Zygote
Embryo
Adult
Evolution of Life Cycles
Compare all of the life cycles you have studied, assume that they are presented in the
order that they evolved, and deduce a concept of the trends of evolution in life cycles from
them. Consider such comparisons as: role of mitosis, differentiation of gametes,
differentiation of organs, roles of spores, importance of diploid phase, size of haploid and
diploid phase organisms, and any other characteristics that seem to have changed over
the evolutionary span represented by these organisms.
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