Download Cells, Mitosis and Meiosis Lab

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Nuclear Division & Inheritance
Introduction
Genetics is the branch of science that studies the nature of inheritance. Just as life on
Earth is organized and can be studied on a hierarchy of levels (see, Community Ecology),
so, too, can genetics be studied on a number of levels. As you study genetics, try to ask
yourself what level of organization is being examined: molecular, cellular, species, or
population? In today’s laboratory you will first examine genetics at the cellular level;
you will specifically study the movement of chromosomes during nuclear and cell
division. You will also see a video which examines some of the problems that arise in
humans when there are mistakes in the proper movement of chromosomes during
nuclear and cell division. At the species level of organization, you will explore some of
the human traits which are determined by specific areas of DNA (referred to as
“alleles”) on the chromosomes. DNA, the molecular genetic material, will be explored in
further detail in a subsequent laboratory. Genetics at the level of the population is the
foundation of evolution, which you will study in the final laboratory.
The purpose of this lab is to become familiar with the two processes of nuclear division, mitosis and
meiosis, and to understand how genetic information is passed from cell-to-cell and from generation-togeneration. Mitosis and meiosis are types of nuclear division. Mitosis is the process of nuclear division
that carries the genetic information contained in strands of DNA to new cells during an organism’s growth
or during asexual reproduction. Cancer, a prevalent disease in our society, is the result of uncontrolled
mitosis. It’s important for you to study the process of mitosis so that you can understand the attempts of
scientists to cure cancer. Meiosis is the process of nuclear division that carries the genetic information to
sex cells or gametes for the purpose of sexual reproduction. An important result of meiosis is genetic
diversity, which is fundamental to the survival of species and to evolution. Cytokinesis describes the
division of the cytoplasm that usually accompanies nuclear division. Thus, the division of the cell is
actually two events: either mitosis or meiosis, and cytokinesis.
Mitosis: The Basis of Growth & Asexual Reproduction
All organisms produce new cells by cell division. When a cell divides, the information contained in the
genetic material (the DNA) must be duplicated and distributed equally to the two resulting daughter cells.
In eukaryotes (cells that have a nucleus), this process of nuclear division is called mitosis. In unicellular
organisms, mitotic cell division is a means of asexual reproduction. The result of asexual reproduction is
a clone of the parent cell since the genetic information contained in the daughter cells is exactly the same
as that contained in the parent cell. For multicellular organisms, which all begin life as a single cell,
mitotic cell division is involved in growth and repair. Since the strands of DNA are replicated prior to
each mitotic division, each new cell produced contains the same genetic information as the initial cell that
began the organism’s life. Think about this for your own body!
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Q. Would you expect that the genetic information in your toes is any different from the DNA in your heart
or your brain? Explain your answer.
In eukaryotic cells the DNA is carried on several nuclear structures called chromosomes. Every organism
has a chromosome number characteristic of its species. For example, mosquitoes have 6 chromosomes,
corn has 20, cats have 38, humans have 46, and dogs have 78. In most animals and flowering plants,
chromosomes can be matched up in pairs. For example, humans have 46 chromosomes, which occur as 23
chromosome pairs. Mosquitoes have 3 chromosome pairs. The members of a pair are called homologues
or homologous chromosomes. A cell in which all the chromosomes are paired is described as diploid or
2n (for humans 2n = 46). In contrast, many algae, fungi, mosses, and some social insects contain cells in
which the chromosomes do not occur as pairs. Such cells are described as haploid or 1n.
Chromosomes are quite small and usually appear as a diffuse mass, called chromatin, in the nucleus.
Individual chromosomes can be observed under the light microscope only right before and during cell
division when they condense or coil up into tight, short threads. Before this condensing, each chromosome
has duplicated itself, so that just before nuclear and cell division, there are 2 identical copies of the original
chromosome. These duplicate chromosomes are called sister chromatids and they are attached to each
other at a region called the centromere.
Scientists have divided the process of mitosis into 4 phases for ease of study and communication. These
phases are: prophase, metaphase, anaphase, and telophase. The phase inbetween the mitotic stages of
cell division is called interphase. It is during interphase that DNA is duplicated. It is important to keep in
mind, however, that there are no pauses between designated phases and that the life of a cell is always
dynamic, never static!
1. Interphase

no nuclear or cell division

cell growth and duplication of genetic material is occurring

chromosomes are not visible but the cell nucleus and nucleolus are
2. Prophase

duplicated chromosomes condense and become visible as sister chromatids joined at the centromere

nuclear membrane disintegrates; neither the nucleus nor the nucleolus is distinct

spindle apparatus of the cell is formed
3. Metaphase

chromosomes are aligned along the cell’s equator at their centromeres

spindle fibers are visible
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4. Anaphase

migration of the chromosomes: centromeres split and move along the spindle fibers towards opposite
poles, pulling the sister chromatid
5. Telophase

chromosomes are aggregated at the poles and begin to thin out and extend in length

new nuclear membrane forms; nucleolus and nucleus begin to reappear

spindle disintegrates

cytoplasm divides (cytokinesis occurs); daughter cells begin to form
Mechanics of Mitosis
Use the various instructional materials available to you in the lab (video: Mechanics of Mitosis; posters;
Photo Atlas of Biology; models of plant mitosis) to answer the following questions:
Q. How many daugher cells are produced from one parent cell that undergoes mitosis?
Q. If the parent cell contains 4 chromosomes, how many chromosomes will be in each daughter cell after
mitosis is complete?
Q. Are the daugher cells haploid (1n) or diploid (2n)?
Q. What are two functions of mitosis for an organism?
Growth and Mitosis in Onion Root Tips
Plant root tips are areas of active growth, and thus, mitosis.
A) Select a prepared slide of an onion (Allium) root tip to find cells in all phases of mitosis.
Examine the slide first under a low power objective (10X) of the microscope.
Q. Cells undergoing active growth occur at the tip of the root in the layers beneath the epidermis (the
root’s “skin”). Why is it advantageous for the plant to have the epidermis cover the area of mitotic
division?
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Q. Where on your body would you expect mitosis to be frequently occurring? Why?
B) Now examine the slide with the high power objective (40X) to observe the details of each
phase of mitosis. Find interphase and all four phases of mitosis. Make drawings of each phase on a
separate sheet of paper, and label all the terms that are in bold type below. Make sure that you label
each phase and write down the total magnification at which you are making your observations.
1) Interphase
Search for a cell in which the nucleus and nucleolus are visible but in which the
chromosomes are not distinct.
Q. During interphase, what is happening to the DNA?
2) Prophase
Search for a cell in which the chromosomes are visible as tangled strands in the nucleus.
Each chromosome is actually composed of a pair of sister chromatids, although you will probably not be
able to see them with the 40X objective.
Q. What is the name of the region where the pair of sister chromatids are joined?
Q. Is the DNA in one of the sister chromatids identical to the DNA in its sister, or is it different? Explain
your answer.
3) Metaphase Find cells in which the chromosomes have lined up across the equator, at the center of the
cell. The chromosomes are attached by their centromeres to the spindle fibers which extend to the poles of
the cell. The centromeres may not be visible in your prepared slide.
Q. What specific part of the chromosome is aligned along the equatorial plate?
4) Anaphase Find cells in which the two chromatids from each chromosome are being pulled apart at the
centromeres. The centromere of each chromosome splits lengthwise, and is pulled apart by the spindle
fiber from the equator to the opposite poles of the cell.
Q. How many chromosomes are now in the cell?
5) Telophase
Locate cells which show completely separated chromosome groups. Some will show the
nuclear membrane reforming. At this phase, the beginning of cytokinesis can be observed. Label the
daughter cells.
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Meiosis: The Basis of Genetic Inheritance and Sexual Reproduction
In diploid organisms which reproduce sexually, cells in the sexual organs undergo meiosis to form sex cells
(gametes) which have only half the number of chromosomes of body (somatic) cells. That is, gametes
have only one chromosome from each homologous pair and are haploid, or 1n. Meiosis is the cell division
process in which the number of chromosomes is halved as homologues are separated, and gametes are
formed. Fusion of gametes (fertilization) produces a new cell called a zygote with the total number (2n)
of chromosomes characteristic of that species. Meiosis and fertilization are the basis of genetic
inheritance and sexual reproduction.
The process of meiosis and sex cell maturation in males is called spermatogenesis (“creation of sperm”);
in females it is called oogenesis (“creation of eggs”). Meiosis is a somewhat more complicated process than
mitosis, and includes two rounds of chromosome separation and cell division. We will not overwhelm you
with the details of the process. Instead, look at the drawing of Animal Meiosis. Note that the first meiotic
division results in the separation of the members of a homologous pair. (Homologous pairs do NOT
separate from each other in mitosis.) The second meiotic division results in sister chromatids separating
from each other. The net result of both meiotic divisions is four cells, each of which has half as many
chromosomes as the original parent cell. Moreover, because the homologous pairs in the first division, and
the sister chromatids in the second division, separate independently of each other, the resulting 4 cells are
NOT identical to each other. This genetic variability of gametes helps ensure the creation of a unique new
individual after fertilization occurs.
Mechanics of Meiosis
Use the various instructional materials available to you in the lab (video: Mechanics of Meiosis; posters;
Photo Atlas of Biology) to answer the following questions:
Q. How many daughter cells are produced from one parent cell that undergoes meiosis?
Q. If the parent cell contains 4 chromosomes, how many chromosomes will be in each daughter cell after
meiosis is complete?
Q. Are the daughter cells haploid (1n) or diploid (2n)?
Q. Are the daughter cells genetically identical to the parent cell (i.e., clones), or are the daughter cells
genetically different?
Q. Where does meiosis occur in a woman? Where does it occur in a man?
Q. What process restores the diploid number of chromosomes for the next generation?
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Meiosis and Gamete Formation in Lily Anthers
Meiosis occurs in the ovaries and anthers of flowering plants. The products of meiosis in the anther (the
male sexual organ of the plant) are the male gametes (sperm) of flowering plants. The sperm are contained
in the pollen of the plant. The products of meiosis in the ovary (the female sexual organ of the plant) are the
female gametes (eggs).
Look at the display of compound light microscopes showing the progressive phases of meiosis, and answer
the following questions. In each field of view, there will be a number of cells undergoing meiosis; the
nuclei of these cells will be in different phases of meiosis. Look at the cell at the tip of the pointer to view
the labelled phase; if the microscope is not equipped with a pointer, look at the cell in the center of the field
of view to see the labelled phase. PLEASE DO NOT MOVE THE SLIDES OUT OF POSITION. USE
ONLY THE FINE FOCUS ADJUSTMENT.
First Meiotic Division
Prophase I: The nuclear membrane has disintegrated and the chromosomes are visible. At this stage, the
chromosomes are present as homologues (pairs)
Metaphase I: The chromosomes are arranged along the equator. What is the name of the array of fibers by
which they are attached to the poles of the cell?
Anaphase I: The members of each pair are separating from each other, resulting in two distinct groups of
chromosomes moving towards opposite poles of the cell.
Telophase I: The two groups of chromosomes have aggregated at the poles. A cell wall
is forming which will divide the cytoplasm into two cells, each with a group of
chromosomes. Although you cannot see it, each chromosome consists of two sister
chromatids held together at the centromere.
Second Meiotic Division
Prophase II: Just as in mitotic prophase and in prophase I of meiosis, the chromosomes are not very
distinct again at this stage. Is this cell diploid or haploid at this point? Explain your answer.
Metaphase II: The chromosomes are again aligned along an equator, and a spindle connects the sister
chromatids to opposite poles. What is the name of the area on the chromatids to which the spindle fibers are
attached?
Anaphase II: The sister chromatids are now moving to opposite poles.
Telophase II: A cell wall is forming between the separated groups of sister chromatids. They are losing
their identity as distinct chromosomes and becoming uncondensed chromatin again. Because of the plane
of sectioning used to prepare this slide, you may only be able to see 2 cells. However, remember that 4
daughter cells have been produced from each original parent cell. Is the genetic material within each
daughter cell the same or different from the parent cell?
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Genes and Alleles
Chromosomes are made up of genes. A given gene may exist in several alternative forms; each form is
called an allele. During the process of fertilization, each parent will contribute to the offspring (the zygote
at this point) one allele for each gene. Therefore, each individual has two alleles for each gene. The alleles
that each parent will contribute to the offspring are determined during the processes of meiosis and
fertilization. Following fertilization, if one allele masks the expression of another allele, we say that this
allele is dominant over the other allele. The allele that is masked is called the recessive allele.
Example: To illustrate the concept of different alleles (i.e., forms of genes), we will examine a simplified
version of the eye color character in humans. We will explain eye color as if only 1 gene is responsible;
however, geneticists now know that there are at least 2 genes involved in eye color inheritance. For our
example, the allele for brown eye color is dominant over the allele for blue eye color. If an individual has
one blue eye color allele and one brown eye color allele, he or she will have brown eyes. Since this
individual has two different alleles for the same character (eye color in this example), we say that his/her
genotype is heterozygous. We can symbolize this heterozygous genotype with the letters “Bb”, where the
capital “B” symbolizes the allele for the dominant trait (brown eye color) and the “b” symbolizes the allele
for the recessive trait (blue eye color).
When an individual has two similar alleles for the same trait, the genotype is homozygous. There are two
possible homozygous genotypes. One is homozygous dominant (“BB” for our eye color example). The
other is homozygous recessive (“bb”). Note that a person with brown eye color could have either of two
possible genotypes, “BB” or “Bb”. There is only one possible genotype for a person with blue eye color,
“bb”. The phenotype is the physical expression of the genetic makeup. The phenotype for “BB” or “Bb” is
brown eye color. The phenotype for “bb” is blue eye color.
Human Genetics
Difference exist among humans in a number of easily observed traits. Your Teaching Assistant will choose
several of the following traits and help you identify which form you possess. Make a table on the following
page, including the traits and the number of students in your class showing the different forms of each trait.
Be sure to provide a concise title for the table.
Tongue Rolling: Tongue rollers carry a dominant gene R.
Widow’s Peak: A dominant gene W causes the hairline to form a distinct downward point in the center of
the forehead. Baldness will mask the expression of this gene.
Earlobe Attachment: The inheritance of a dominant gene E results in the free or unattached earlobe. If the
lobe is attached directly to the head, the individual is homozygous recessive for the attached allele “e”.
PTC Tasting: Phenylthiocarbamide (PTC) is a harmless chemical in small quantities. To individuals with
the dominant gene T, it tastes undesirably bitter. Individuals who cannot taste PTC are homozygous
recessive for the “t” allele.
Hitchhiker’s Thumb: Some individuals can bend the last joint of the thumb backwards at about a 45degree angle. These individuals are homozygous for a recessive gene, but there is considerable variation in
the expression of the gene. We shall consider those who cannot bend at least one thumb backwards about
45 degrees as carrying a dominant gene.
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Bent Little Finger: A dominant gene causes the terminal bone of the little finger to angle toward the fourth
(ring) finger. Individuals whose little fingers are straight possess the homozygous recessive condition.
Mid-digital Hair: The presence of hair on the middle segment of the fingers is caused by a dominant gene
M. The homozygous recessive condition results in the lack or absence of hair on the middle segments.
Facial Dimples: The inheritance of cheek dimples is controlled by a dominant gene D. The homozygous
recessive condition lacks the ability to express facial dimples. To determine whether or not you have
dimples, smile!
Hallux Length: Individuals whose big toe (hallux) is shorter in comparison to the second toe possess a
dominant gene for this character. The inheritance of the homozygous recessive condition results in the big
toe being longer than or equal in length to the second toe.
Index Finger Length: Place your hand flat on a table and determine if your index (second) finger is short
in relationship to the length of your fourth (ring) finger. The gene for short second finger is sex-influenced
in its expression, being dominant in males and recessive in females.
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Heredity, Health, and Genetic Disorders
Follow along with the video entitled “Heredity, Health, and Genetic Disorders, Part I: The Causes of
Genetic Disorders” to answer the following questions:
1. How many pairs of chromosomes are contained in each human body cell? How many genes do
scientists estimate are located on those chromosomes?
2. What do most genes contain instructions for building? What functions do these perform?
3. How many pairs of genes are thought to code for human skin color?
4. The different forms of genes are called ______________.
5. Alleles are thought to arise by the process called ____________________.
6. While some mutations have contributed to healthy human diversity, most mutations are
_____________.
7. Two examples of diseases caused by recessive alleles are:
8. Diseases caused by recessive genes are relatively (rare, common, lethal) (circle one) because they are
masked by the dominant allele.
9. In contrast to diseases caused by recessive alleles, an example of a disease caused by a dominant allele
is _________________________.
10. In humans, 22 of the 23 chromosome pairs are called _______________________.
11. In humans, the 23rd pair of chromosomes are called _________________________.
12. A normal human female has 2 ______ sex chromosomes, while a normal human male has an _____
and a ________ chromosome.
13. Most genes on the sex chromosomes are located on the _____ chromosome. Very few genes are
located on the ____ chromosome.
14. An example of a disease caused by an allele carried on the X-chromosome is:
15. Two examples of polygenic diseases are __________________ and ____________________.
16. One of the most common human chromosome disorders, which involves having an extra 21 st
chromosome, is called _________________________.
17. Turner’s Syndrome in females is a result of ______________________________________.
18. An extra X chromosome in males results in __________________________, the symptoms of which
are usually treatable through ________________ and __________________.
19. Most of us carry _________ lethal recessive genes.
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