Science 102 Spring Semester 2006: Lab 3
Mitosis, Meiosis, and Genetics
Mitosis
Sexually-reproducing, multicellular organisms begin life as a single cell, the
fertilized egg. This cell, the zygote, through the process of mitosis (nuclear division),
and cytokinesis (cytoplasmic division), becomes two daughter cells, which in turn
become four cells, eight, sixteen, thirty-two, etc. as each cell continues to divide. At a
particular point in this process of development (or embryogenesis), cells begin to
differentiate into different types of cells that will perform specialized roles in the
functioning of the whole organism. The information that controls the behavior and
differentiation of each cell is carried in the chromosomes. In lab 1, you saw darkstaining nuclei in cells. These nuclei stain darkly because the chromosomes in the cells
were ‘unwound’, forming diffuse chromatin in the nucleus. This is the way that the
chromosomes are when they are actively functioning to direct the cell’s function. When
cells reproduce, this diffuse chromatin becomes reduced into discrete bodies
(chromosomes) through a complex coiling process. Chromosomes are the ‘packages’ in
which genetic information is passed during cell division. Chromosomes typically occur
in homologous pairs (in diploid organisms). The number of pairs of chromosomes and
the size and shape of each pair are characteristic and constant for a given kind of
organism. Humans, for example, have 23 pairs of homologous chromosomes in each of
their cells (except for egg and sperm cells), while the fruit fly, Drosophila melanogaster,
has 4 pairs.
Prior to cell division, a cell synthesizes a duplicate of each chromosome.
Duplicate chromosomes remain attached to one another in a region called the
centromere until the actual division process occurs. While attached to one another, each
duplicate is called a chromatid, and the two together - sister chromatids. It is important
to remember that, although each chromosome is at this point doubled, consisting of two
sister chromatids connected at the centromere, they still represent only ONE
CHROMOSOME. They will not become separate chromosomes until they have
separated from each other.
Mitosis (nuclear division) consists of the separation of the sister chromatids. As
they separate, the cytoplasm with its constituent organelles is divided approximately
equally between the daughter cells, a process called cytokinesis. The outcome of mitotic
cell division is two identical cells with identical sets of chromosomes. Mitosis and
cytokinesis are key processes in the growth and development of an organism. They are
also the means by which wounds are repaired and dead cells are replaced.
Details of Mitotic Cell Division
Mitosis is part of the normal cell cycle. There are three general portions of the
cell cycle: Interphase, Mitosis, and Cytokinesis.
Interphase
In interphase, the cell is actively growing, respiring, and synthesizing DNA,
RNA, and proteins. Nuclear material is surrounded by a membrane. The dark-staining
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nucleoli are visible within the nucleus. Collectively, the chromosome mass is called
chromatin. The chromosomes are not individually distinguishable because they are
uncoiled into long, thin strands and appear only as dark granules within the nucleus.
Chromosomes and organelles are duplicated during interphase.
Mitosis
The phases of mitosis are: Prophase, Metaphase, Anaphase, and Telophase.
 Prophase - The nuclear membrane and nucleoli break down. Chromosomes
appear first as thin threads, and later as thicker, distinct bodies as they coil up.
At this point, each chromosome contains two DNA molecules organized as
sister chromatids held together by a centromere. The spindle, a structure
composed of microtubules, forms, although it may not be easily visible until
the next stage. In animal cells, the formation of the spindle appears to be
associated in some way with the centrioles, which move in opposite
directions. The centrioles may have something to do with organizing the
spindle. However, a spindle forms in a dividing plant cell, yet plant cells lack
centrioles. In animal cells, additional microtubules radiate from the centrioles
and form a star-shaped aster. Centrioles and asters have not been found in the
cells of higher plants.
 Metaphase - The chromosomes move to the equatorial plane of the cell, and
line up along the equator. It is useful to think of the chromosomes lining up
end-to-end in metaphase of mitosis, although it doesn’t actually happen this
way.
 Anaphase - The centromeres holding sister chromatids together split apart,
and sister chromatids move away from each other towards opposite ends of
the cell.
 Telophase - Nuclear division is completed in telophase. The chromosomes
are now at each end of the cell. New nuclear membranes begin to form
around each group of chromosomes, and the chromosomes begin to uncoil. It
is often difficult to distinguish late anaphase from early telophase. It should
be remembered that mitosis is a continuous process (not stopping between
each phase) and as such, each phase gradually grades into the next.
Cytokinesis
Cytokinesis usually begins during telophase. Cytokinesis is the division of the
cytoplasm into two separate cells. In animal cells, the first sign of this splitting of
cytoplasm is the formation of a crease around the cell, a cleavage furrow. As
cytokinesis continues, the furrow becomes deeper and deeper, finally pinching the parent
cell into two daughter cells.
In plant cells, division of the cytoplasm results from the formation of a cell plate
across the center of the cell. This cell plate forms internally in the cell as the result of
Golgi vesicles lining up and fusing.
Once mitosis and cytokinesis are complete, the daughter cells may again enter
interphase of the cell cycle.
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Observing mitosis and cytokinesis in plant cells: Onion root tip.
Obtain a slide of an onion root tip. Examine it with the scanning objective lens,
and locate the region of active cell division just behind the dead cells at the tip of the
root. Focus on this region, and switch to the medium-power objective. Focus again and
switch to the high-power objective lens. Find cells in each of the following stages:
Interphase, Prophase, Metaphase, Anaphase, Telophase, Early Cytokinesis. Draw
representative cells on the paper provided, labeling all structures.
Because all the cells on your slide were stopped at once, the relative numbers of
cells in each phase of the cell cycle provide a rough estimate of the relative duration of
each phase. After you have drawn cells in each phase and are familiar with their
appearance, count the number of cells in each of the six stages above that appear in ONE
field of view under the high-power objective (40X). Record your data below.
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How many cells are in interphase?
How many cells are in prophase?
How many cells are in metaphase?
How many cells are in anaphase?
How many cells are in telophase?
How many cells are in early cytokinesis?
____________
____________
____________
____________
____________
____________
 Which is the longest portion of the cell cycle?
 Which portions of the cell cycle are very brief?
Observing mitosis and cytokinesis in animal cells: Whitefish Blastula
The most convenient source of actively dividing cells in animals is the early
embryo, where cells are large and divide rapidly with a short interphase. In the blastula
(an early embryonic stage), a large percentage of cells will be dividing at any given time.
By examining a cross section of a whitefish blastula, you should be able to locate many
dividing cells in various stages of mitosis and cytokinesis.
Obtain a slide of a whitefish blastula. Find a cross section using the scanning
objective, switch to the 10X objective, focus, and switch to the 40X objective. Find the
six stages of the cell cycle above, and draw and label what you see in each stage on the
paper provided.
Although the behavior of the chromosomes is the same as in plant cells, several
distinctive animal structures should be visible:

Centrioles - small dots at the poles around which the microtubules of the spindle
seem to organize.

Asters - a star-like array of microtubules surrounding each centriole pair.

Cleavage Furrow - a constriction near the center of the dividing cell.
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Mitosis in onion root tip
All drawings are done at total magnification = 400X
Label (where present): cytoplasm, nucleus, nucleolus (nucleoli), chromatin, spindle,
chromosomes, cell plate, daughter cells.
Interphase
Anaphase
Prophase
Telophase
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Metaphase
Recent Cytokinesis
Mitosis in whitefish blastula
All drawings are done at total magnification = 400X
Label (where present): cytoplasm, nucleus, nucleolus (nucleoli), chromatin, spindle,
chromosomes, cleavage furrow, daughter cells.
Interphase
Anaphase
Prophase
Telophase
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Metaphase
Recent Cytokinesis
Meiosis
Meiosis takes place in all organisms that reproduce sexually. Meiosis is
necessary in sexually reproducing organisms because it prevents the chromosome number
from doubling with every generation when fertilization occurs. Meiosis is the process by
which the gametes (sperm and eggs) are produced. In animals, it only occurs in the
gonads, while in plants it only occurs in structures called sporangia.
Meiosis consists of two nuclear divisions, meiosis I and II, with an atypical
interphase between the divisions during which cells do not grow or synthesize DNA.
This means that meiosis I and II result in a sum total of four cells from each parent cell.
Each new cell contains half the number of diploid chromosomes, one chromosome from
each homologous pair. Recall that cells with only one of each homologous pair of
chromosomes are called ‘haploid’ cells.
Meiosis I
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Prophase I - Chromosomes coil and condense as they do in mitosis. The
nuclear envelope and nucleoli break down. However, in meiosis, there is
another essential event in prophase I. Homologous chromosomes, each
composed of two sister chromatids, come together and coil around each
other. This is called synapsis. While synapsed, the pair of chromosomes
are called a tetrad. During synapsis, chromatids of homologous
chromosomes exchange or ‘trade’ pieces of DNA. This exchange of
genetic material is called crossing over or recombination.
Metaphase I - Chromosomes move to the equatorial region of the cell as
tetrads. In other words, the homologous pair goes to the center of the cell
together. It is useful to think of the pair of them lying there side-by-side
(as opposed to end-to-end like in mitosis).
Anaphase I - Each chromosome (consisting of two sister chromatids held
together at the centromere) separates from its homologue and one
homologue of each pair moves toward each pole.
Telophase I - The homologous chromosomes have aggregated at opposite
poles. Cytokinesis begins and divides the original cell into two haploid
daughter cells.
It is by the end of Meiosis I that the resulting two cells have reduced their chromosome
number by half. Although each chromosome in each cell still consists of two sister
chromatids connected by a centromere, there are half as many chromosomes as in the
original parent cell.
Meiosis II
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
Interphase - no DNA synthesis occurs
Prophase II - No synapsis or crossing over occurs because the cell
contains no homologous pairs to cross over with. Chromosomes
condense.
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Metaphase II - The chromosomes move to the midline of the cell again.
This time, they line up just like in mitosis. You can imagine them as
being end-to-end.
Anaphase II - Each centromere splits, and the single-chromatid
chromosomes thus formed move away from each other toward opposite
ends of the spindle.
Telophase II - the chromosomes begin to unwind, cytokinesis begins, and
the nuclear membrane re-forms.
EXAMINE THE POSTER OF MEIOSIS.
The Principles of Genetics
The phenotype of an organism (the way it looks or behaves, or its physiology) is
in large part determined by the genes it carries (its genotype). Most organisms are
diploid, so that most carry two copies of each chromosome (a homologous pair). One
chromosome of a homologous pair comes from the mother, and one comes from the
father. In humans, there are 23 pairs of homologous chromosomes. We each received
chromosome numbers 1 through 23 from our mother, and 1 through 23 from our father.
The 2 chromosomes designated number 1 are a homologous pair, the 2 chromosomes
designated number 2 are a homologous pair, and so on.
Each member of a homologous pair carries the same genes. For example, the
gene for the p53 protein is on chromosome 17. We each have received a copy of the p53
gene from each of our parents, on chromosome 17. However, we may not have received
exactly the same form of the gene, or allele, from each parent. For instance, in the case
of Li Fraumeni syndrome, the allele the patient received from one parent is mutated so
that the p53 protein transcribed from that DNA does not function correctly. Thus,
because we are diploid, we each carry two copies of every gene (except those on the sex
chromosomes). The two copies may be exactly the same (i.e. the same alleles), or they
may be different. For instance, we may have received the allele for blue eyes from one
parent and the allele for brown eyes from the other parent.
When the two alleles are the same (eg. both for blue eyes), the genotype is said to
be homozygous. When the two alleles are different (eg. one for blue eyes and one for
brown), the genotype is said to be heterozygous. When the genotype is heterozygous,
often only one allele of the two is expressed in the phenotype. This allele is said to be
dominant, while the other is recessive. In the case of eye color, the brown allele is
dominant and the blue allele is recessive. Thus, an individual heterozygous for eye color
(as in our example above) would show a brown-eyed phenotype. In the case of recessive
and dominant alleles, the only time the phenotype associated with the recessive allele is
expressed is when the individual has two copies of the recessive allele (homozygous
recessive). Dominant alleles are usually represented by an uppercase letter (eg. B for
brown eyes), and recessive alleles are usually represented by the lower case letter (b).
For this example, there are three possible genotypes: BB, Bb, and bb. However, because
of dominance, there are only two possible phenotypes: Brown eyes (genotypes BB and
Bb), and blue eyes (genotype bb).
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For most traits, there are at least two alleles. The paired alleles are separated
(along with the chromosomes that they reside on) during meiosis I. Half of the gametes
formed will receive a copy of one allele, while the other half will receive the other. If the
genotypes of the parents are known, all possible combinations of alleles (the offspring
genotypes) can be determined. However, it is important to realize which sperm will join
with a particular egg is purely a matter of chance. Because of this chance component, the
rules of probability play heavily in genetics and inheritance.
Punnett Square
A useful tool for predicting all the possible combinations and the expected ratios
of offspring genotypes (when the genotypes of the parents are known) is the Punnett
square. To construct a Punnett square, a 2 by 2 table is made, as below. The female
gametes are listed across the top of the square, and the male gametes are listed along the
left side.
Example:
Suppose a couple have a child. The mother has blue eyes and the father has
brown eyes. We know the genotype of the mother since the blue eye allele is recessive.
The mother’s genotype is __________.
However, since the brown allele is dominant to the blue allele, we cannot be sure
of the father’s genotype. He is either __________ or __________.
The alleles of an individual separate during meiosis so that each gamete receives
just one of the alleles.
The eggs of the mother therefore receive either __________ or __________.
If the father is homozygous, the sperm receive either __________ or __________.
However, if the father is heterozygous the sperm receive either __________ or
__________.
Below, fill out the punnett squares for the cross above, for both paternal genotypic
possibilities:
Homozygous Father:
Gametes
Possible genotypes of the offspring are __________________________.
Heterozygous Father:
Gametes
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Possible genotypes of the offspring are ___________________________, in the
ratio _________________________.
Human Genetic Traits
Several human phenotypic traits are inherited in a simple Mendelian manner. For
the following traits, determine your own phenotype, then determine your possible
genotype. Record your own data in the table. We will then tabulate all the data for the
class.
1. PTC Tasting
Place a piece of PTC (phenylthiocarbamide) paper on your tongue. If it tastes
bitter, you carry a dominant allele, T. However, if you cannot taste the paper, you are
homozygous recessive for the trait.
2. Tongue Rolling
If you have the ability to roll your tongue into a tube-like shape, you have a
dominant allele, R. If you are unable to roll your tongue you are homozygous recessive
for the trait.
3. Widow’s Peak
If you have a pointed frontal hairline, you have a Widow’s peak. This is a
dominant trait. If however, you have a straight hairline you are homozygous recessive
(ww) for the trait.
4. Earlobe Attachment
If your earlobes are free, you have a dominant allele (F). If you have attached
earlobes, you are homozygous recessive for the trait.
5. Bent Little Finger
Hold out one hand with the fingers together. If the end of your little finger bends
inward, you have the dominant allele for this trait (L). If your little finger is straight, you
are homozygous recessive.
6. Hitchhiker’s Thumb
Hold your hand out and make a fist with the thumb extended. If the tip of the
thumb bends backward so that it is at or near a right angle to the rest of the thumb, you
are homozygous recessive (hh). If the tip of the thumb extends at an angle of less than 90
degrees, then you have the dominant allele for the trait.
7. Mid-digital Hair
The presence of hair on the middle segment of the digits is a dominant condition
(M). Even the slightest amount of hair qualifies as dominant. If you do not have any hair
on the middle segment, you are homozygous recessive for this trait.
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8. Interlocking Fingers
Clasp your hands together. If your left thumb is crossed over your right, you have
a dominant allele (C). If your right thumb is on top of your left you are homozygous
recessive for the trait.
9. Eye Color
If you have blue or gray eyes, you lack pigment on the outer layer of your iris and
are homozygous recessive for this trait (bb). If you have any color other than blue or
gray, you have a dominant allele. The exact color of a non-blue eyed individual is
controlled by other genes.
TRAIT
OWN
PHENOTYPE
OWN
GENOTYPE
# IN CLASS DOMINANT
# IN CLASS RECESSIVE
PTC Tasting
Tongue Rolling
Widow’s Peak
Earlobe
Attachment
Bent Little
Finger
Hitchhiker’s
Thumb
Mid-digital
Hair
Interlocking
Fingers
Eye Color
Once you have completed the table we will demonstrate genetic individuality. All
PTC tasters will stand on one side of the room and non-tasters at the other side. Then,
each group will further divide, according to tongue rolling. Continue dividing for each
trait until each person stands alone. How many traits did we have to consider before you
stood alone?
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