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C.P. Biology: Chapter 8: Mitosis and Meiosis
Review and Practice on the Cell Cycle and Cell Reproduction: Mitosis and Meiosis
The Cell Cycle
All organisms are composed of cells. Organisms may be either single celled, as found in the Kingdoms of
Archaebacteria and Eubacteria and Protista; or multicellular, as found in the Kingdoms Fungi, Plantae and
Animalia. While all of the organisms found on Earth are quite diverse, they all have several characteristics
in common:
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the ability to metabolize, or maintain the biochemical processes necessary for life
the ability to grow and evolve
the ability to reproduce
All of the organisms in the Kingdom Archaebacteria and Eubacteria are prokaryotic bacteria. These single
celled organisms have no nucleus and no complex membrane bound organelles. They reproduce by an
asexual process called binary fission. Binary fission means literally “splitting into two,” and involves no
genetic recombination. Thus, this process results in the formation of identical cells, or clones. Because this
is true for all bacteria you may wonder how different strains of bacteria arise. For example, how does
non-pathogenic Escherichia coli (E. coli), identical to that found in your gut somehow produce killer E. coli
strain O157:H7, which caused many deaths in the western United States in 1993 and continues to pose a
threat? The many variations within a given species of bacteria arise from mutation of its DNA by either
mutagenic agents such as UV light or by viral infection of the bacterial cell.
While mutagenic agents and viruses can also alter eukaryotic cells, the effect is generally not seen in
the offspring. The primary force contributing to genetic diversity in the multicellular eukaryotes is sexual
reproduction. Sexual reproduction involves the recombination of genetic material between two different
individuals of the same species to produce a genetically unique offspring. For example, you obtained one
half of your genetic information (DNA) from your mother and one half from your father-thus you are a
combination of your two parents and quite genetically distinct from all other humans (unless you have an
identical twin). You came into being when two haploid gametes (sperm and egg) fused and became a
single diploid cell, called a zygote.
Since you started as a single cell, how did you become the complex multicellular organism you are today?
The zygote’s first job is to divide and produce many identical cells, through the processes of mitosis and
cytokinesis. Mitosis involves the division of the nucleus, and cytokinesis involves the division of the
cytoplasm. This results in the formation of two identical cells from one cell.
How does mitosis and cytokinesis occur, and what does the cell do between these division events? Both
of these processes occur only after cell growth, metabolism, and DNA duplication within the nucleus of
the cell. Cells within a multicellular organism have a distinct life span, which is called the cell cycle.
Consider the cell cycle as the life span of a single cell. The cell is “born” after cell division, it grows and
metabolizes in a cell cycle phase called G1 phase, and then it prepares to reproduce. First, it must
duplicate all of its genetic information, or DNA, in a cell cycle phase called S phase. Then it must spend
some time metabolizing all the necessary materials that it needs to split into two cells during the phase
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called G2 phase. Finally the nucleus of the cell, which now has double the amount of its DNA, goes
through nuclear division. This nuclear division is called mitosis. When mitosis is almost complete, the cell
cytoplasm must split. The formation of new cell wall or membrane then results in two identical cells.
Each type of cell, whether epithelial, cardiac, nerve, or some other type, has its own distinct cell cycle, so
the time spent in each phase may vary between different cell types. However, all eukaryotic cells in
multicellular organisms have the same generalized cell cycle containing G1, S, G2, Mitosis, and Cytokinesis
phases.
Figure 1. Diagram of a generalized cell cycle
Note: Normal cells in most of the organs and tissues of your body have only so many rounds of replication
and mitosis that they may complete; exceptions may include skin, blood and liver cells. Cells that lose this
control over cell division through some sort of mutation can replicate and divide wildly causing tumors
and/or cancer.
Practice Round One
A. Describe the type of reproduction organisms in the Kingdoms Archaebacteria and Eubacteria
demonstrate.
B. Contrast the main mechanism for the genetic diversity in prokaryotes and in multicellular eukaryotes.
C. In your own words, describe a generalized cell cycle and describe what occurs in each phase.
Homologous Chromosomes, Genes, and DNA
Before we can discuss the process of mitosis or meiosis, we must first discuss chromosomes. Almost
everyone in the media talks about chromosomes, DNA, and genes as if they were all the same entity.
While they are related, one must realize the relationship between these terms to gain a real
understanding of how the most basic of life processes occur. Chromosomes are highly condensed (tightly
wound), huge biomolecules, composed of DNA, protein, and some RNA. Some organisms contain only one
unpaired chromosome that is not contained in a nuclear envelope. All of these organisms are called
prokaryotes.
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Eukaryotes, by contrast, especially in multicellular organisms, have several chromosomes per cell. These
chromosomes exist as paired, or homologous, chromosomes. The homologous chromosomes condense
and pair during the mitosis stage of the life cycle of all eukaryotes. The reason these chromosomes are
paired is that half of an organism's chromosomes came from the paternal gamete and the other half came
from the maternal gamete.
Let’s use human beings again as an example. Humans have 46 individual chromosomes in every cell of
their body during mitosis, except for the gametes (sex cells like eggs or sperm) and mature red blood cells.
However, there are only 24 different human chromosomes. How can this be?
Humans acquire a set of 22 chromosomes and one X chromosome from their mother for a total of 23, and
a set of 22 chromosomes and one X or Y chromosome from their father to make up the 46 chromosomes.
Does the mother’s chromosome number five differ from the father’s chromosome number five? Yes! The
two chromosomes do differ in their specific information. While both homologous chromosomes contain
the same type of information located at the same exact place, they each have a different version of the
information. The chromosome from the one parent may contain information for blue eye color, and the
chromosome from the other parent may contain information for brown eye color. While the specific DNA
sequence information may vary from individual to individual, chromosome number five in humans
encodes for the same type of product. The pairs of homologous chromosomes contain redundant
information so that you have two copies of each chromosome, one from Mom and one from Dad. In this
way, you may exhibit traits from both parents.
As you probably know, human males have two different sex-determining chromosomes: one X and one Y.
These are not homologous, but contain very different information. Females humans have two X
chromosomes. So besides the regular 22 chromosomes, there are also two different sex chromosomes,
for a total of 24 different chromosomes in humans.
How exactly do genes relate to chromosomes? First you must realize that DNA, the essential molecule of
the chromosome, contains all the information of the cell for the manufacture of all proteins necessary to
maintain and perpetuate life. A gene is a segment of DNA that encodes for one polypeptide, or molecule
of protein. The color of your eyes results from the production of specific polypeptides, which come
together to form a protein that exhibits physical properties that makes them appear to have a certain
color. These polypeptides are coded by genes or a set of genes. The genes are DNA sequences contained
in a chromosome.
There are many genes on a single chromosome. Note that much of the DNA in a chromosome does not
encode for a polypeptide and is not gene DNA. The function of non-genic DNA is not well understood,
although some parts of this DNA are known to regulate the expression of genes.
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Figure 2. The relationship between DNA, genes, and chromosomes
Practice Round Two
A. What do the processes of Mitosis and Cytokinesis accomplish for the cell?
B. How do chromosomes, genes, and DNA relate to each other?
C. What is the difference between homologous and identical chromosomes?
D. Where did you get your pair of Chromosome Number two?
E. What is the ultimate function of DNA?
F. If humans have 23 pairs or 46 chromosomes, how can they have only 24 different chromosome types?
Mitosis: Division of the Cell Nucleus
Now to discuss how the process of mitosis occurs in a cell. It is very important to remember that the cell
must have duplicated all of its DNA in S phase of the cell cycle, before mitosis can occur.
If an organism has three pairs of homologous chromosomes, how many individual chromosomes does it
have? How many chromosomes does this same cell have after S phase of the cell cycle?
Let’s discuss the terms diploid and haploid. Any organism or cell that has paired, homologous
chromosomes is diploid. If a cell or organism has only one chromosome of each type it is considered
haploid. Ploidy does not deal with the numbers of chromosomes, but instead deals with having paired or
unpaired chromosomes. Most cells in your body are diploid and contain two of each type of
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chromosome. Gametes, or sex cells, only contain one set of chromosomes, so they contain half the diploid
number. Gametes are considered haploid cells, having only one of each of the homologous chromosomes.
Now to answer the above questions: an organism having three pairs of homologous chromosomes has six
individual chromosomes. After S phase when the DNA replicates, the cell contains 12 individual
chromosomes. Now the cell can form two nuclei, each having one pair of each type of chromosome, and
the cell can split to form two identical cells.
Mitosis occurs in several stages. Remember that these stages are part of a very rapid, continuous process
and are not discrete events as might seem from their descriptions. It is important to learn the names of
each stage and to be able to describe the cellular processes that occur.
Figure 3. Representation of a cell going through mitosis
The most important information to learn about Mitosis is the position and number of chromosomes in
each of the stages of Mitosis. You must be able to answer a question similar to the following example:
Diagram a cell that has 2 pairs of homologous chromosomes in each of the stages of mitosis.
Prophase: The first stage of Mitosis when chromosomes condense and the nuclear membrane disappears.
The chromosomes that have duplicated in S phase attach to their duplicate in pairs at a central point
called a centromere. At this stage the attached chromosomes are called sister chromatids. Sister
chromatids are identical and are very different from homologous chromosomes. All these sisters are
identical twins. Each homologous chromosome has been duplicated, and now the two duplicates are
attached to one another to form sister chromatids.
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Metaphase: The stage at which the chromosomes line up at the equator of the cell and spindle fibers
attach to the centromere of each chromosome. The spindle fibers, made of protein subunits, shorten and
start to pull the chromosomes to opposite sides of the cell.
Anaphase: The stage at which sister chromatids (identical duplicates) are separated and pulled to
opposite poles of the cell.
Telophase: The stage at which the cell membrane or cell wall forms in the center of the cell.
Cytokinesis: The stage where the cytoplasm and organelles are divided between the two cells and the
chromosomes decondense, forming two diploid, identical cells.
Figure 4. Diploid cell containing one pair of homologous chromosomes before S phase of the cell cycle
This is what a normal diploid cell would look like if chromosomes were condensed before S phase
occurred (they aren’t actually condensed until prophase) and what both cells should look like immediately
following telophase, before the chromosomes decondense.
Practice Round Three
A. Explain the difference between haploid cells and diploid cells and give an example of each.
B. Diagram a cell which has two pairs of homologous chromosomes in all the stages of Mitosis.
C. Diagram a cell which has 3 pairs of homologous chromosomes in the following stages of Mitosis:
prophase and anaphase.
Meiosis: the Formation of Gametes
We have previously discussed the concept of sexual reproduction. Generally, in most organisms the
chromosome content of the cell must be diploid, or 2n, where n represents the haploid number of
chromosomes. In other words, cells usually do not tolerate more than two homologous chromosomes in
the cell at one time. For example, if a human zygote were formed by the fusion of two diploid cells, rather
than two haploid gamete cells, the zygote would have 92 chromosomes. That translates to four sets of the
22 chromosomes and four sex chromosomes. You can see that this would soon become a huge problem
for the cell!
To avoid these problems, most organisms donate only one half of a set of chromosomes during sexual
recombination. The resulting zygote will therefore have exactly the same number of chromosomes as the
parents. The process by which the cell accomplishes this feat is called meiosis. Meiosis is a type of cell
division that occurs only in diploid cells destined to become gametes. No other type of cell undergoes
Meiosis.
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Before meiosis begins, the diploid cell goes through the entire cell cycle including S phase, just exactly as a
cell does before Mitosis occurs. Since the cell now has double the number of chromosomes of the original
diploid cell, it must go through two rounds of division to become haploid.
Example: A cell that contains two pairs of homologous chromosomes, or four chromosomes in all, goes
through S phase before Meiosis and now has four pairs of homologous chromosomes, or eight
chromosomes in all. The first meiotic division results in two cells, each having two pairs of homologous
chromosomes. The second meiotic division, undergone by both cells, results in four cells each having two
chromosomes, one of each homologous pair.
The process of Meiosis has many similarities to Mitosis. You should write a brief description for each of
the following phases of Meiosis.
I. Meiosis I
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
II. Cytokinesis
III. Meiosis II
a. Prophase II
b. Metaphase II
c. Anaphase II
d. Telophase II
IV. Cytokinesis
Figure 5. Meiosis I
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Figure 6. Meiosis II
The most important information for you to know about Meiosis is the position and number of
chromosomes in each of the stages. You must be able to answer a question similar to the following
example: Please diagram a cell which has two pairs of homologous chromosomes in each of the stages
of Meiosis.
Prophase I: The first stage of Meiosis when chromosomes condense and the nuclear membrane
disappears. The chromosomes that have duplicated in S phase attach to their duplicate in pairs at a
central point called a centromere. At this stage the attached chromosomes are called sister chromatids.
Sister chromatids are identical and are very different from homologous chromosomes. All these sisters
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are identical twins. Each homologous chromosome has been duplicated, and now the two duplicates are
attached to one another to form sister chromatids.
Metaphase I: The stage at which the chromosomes line up at the equator of the cell and spindle fibers
attach to the centromere of each chromosome. The spindle fibers, made of protein subunits, shorten and
start to pull the chromosomes to opposite sides of the cell.
Anaphase I: The stage at which sister chromatids (identical duplicates) stay together and are pulled to
opposite poles of the cell.
Telophase I: The stage at which the cell membrane or cell wall forms in the center of the cell.
Cytokinesis: The stage where the cytoplasm and organelles are divided between the two cells and the
chromosomes decondense, forming two diploid, identical cells.
And now for Meiosis II...
Prophase II: The stage of Meiosis II when chromosomes condense and the nuclear membrane disappears.
The chromosomes have not been duplicated, and the cell is still only diploid.
Metaphase II: The stage at which the chromosomes line up at the equator of the cell and spindle fibers
attach to the centromere of each attached sister chromatid.
Anaphase II: The stage at which sister chromatids separate and are pulled to opposite poles of the cell.
Telophase II: The stage at which the cell membrane or cell wall forms in the center of the cell.
Cytokinesis: The stage where the cytoplasm and organelles are divided between the two cells and the
chromosomes decondense, forming two haploid gamete cells. The gamete has one half of the information
of a diploid cell. Each of the four gametes produced has one of each homologous chromosome, in
contrast to a diploid cell that has homologous pairs of chromosomes.
Post Test on the Cell, Cycle, Mitosis and Meiosis
1. What different outcomes occur in Mitosis and Meiosis?
2. What part of Meiosis is similar to Mitosis? Why?
3. What is the first major difference in chromosome partitioning between Mitosis and Meiosis I?
4. Define: homologous chromosomes, sister chromatids, genes, haploid, diploid, gamete, and zygote.
5. Outline the cell cycle and define each phase.
6. What do chromosomes consist of?
7. Draw diagrams of a cell containing three pairs of homologous chromosomes in the following stages of
Mitosis and Meiosis: Telophase of Mitosis, Anaphase I of Meiosis, Metaphase II of Meiosis.
© copyright by Gretchen Kirchner 1996, 2001
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