Bowie- Biology 12
Understanding Mitosis & Meiosis
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Mitosis & Meiosis Notes
Name: ___________________________________________________
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It is estimated that the 100 trillion cells that make up the
human body began from a single fertilized egg.
Tissue growth requires cell division.
Cell division is also required to repair and replace cells as they become old or
damaged.
Not all cells divide at the same rates. Red blood cells are replaced at a rate of one
million every second. Brain cells replace themselves only about once every 60 years.
To this point scientists still do not know why.
A single cell (the parent cell) gives rise to two daughter cells (this name does not
imply gender) through the process of mitosis.
The cell's genetic material (DNA) is contained within the nucleus. These molecules
are found in structures called chromosomes.
Human cells contain 46 chromosomes each (except the sex cells; eggs and sperm, which contain only
half - 23 chromosomes. We will talk about these later).
To prepare for mitosis, the cell must replicate each DNA molecule. This doubling of the DNA allows each
daughter cell to have identical genetic material.
Every cell in your body contains your full genetic code (the full 46 chromosomes); except the sex cells
(as mentioned earlier) However, every different type of cell has a different shape, function and life span.
What enables cells with identical DNA to carry out different functions and have different shapes remains
a question for study.
The Cell Cycle
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Date: ____________________________
The sequence from one cell division to the next is called the
cell cycle.
We think of it as occurring in phases, but really it is one
continuous process.
The stages of mitosis take up only a small portion of the cell
cycle. Most of the cell's life is spent in the interphase stage.
Interphase is marked by rapid growth, replication of
chromosomes, another period of growth and preparation for
further divisions. It is 3/4 of the circle chart shown here (G1, S
& G2).
Let's look at each stage of the cell cycle.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Interphase
 During interphase the cell grows, makes structural proteins that repair damaged parts, transport
nutrients to where they are needed, eliminate waste, and prepare
themselves for mitosis by building proteins.
 The most important process during this phase is the synthesis
and replication of DNA.
 During interphase, the genetic material is called chromatin.
 In this form, all the DNA appears in the form of long, thin strands
dispersed throughout the nucleus in a tangled
mass.
 During interphase, each chromosome duplicates
itself and becomes attached to attached to its
duplicate by a structure called a centromere.
 While they are attached together, the original
chromosome and its duplicate are called sister
chromatids.
 Since sister chromatids contain identical DNA,
the pair, attached at the centromere, is
considered to be one chromosome.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Prophase
 Prophase is the first phase of mitosis.
 The chromosomes in the nucleus become visible under a microscope as
they shorten and thicken.
 In animal cells, centrioles are small structures in the
cytoplasm that separate and move to the 2 poles of
the cell.
 Centrioles provide and attachment site for the
spindle fibres.
 Spindle fibers serve as guide wires for the
attachment and movement of the chromosomes
during cell division.
 The centrioles and the spindle fibers, together, form
the spindle apparatus.
 Most plants don't have centrioles, but they still form
spindle fibers.
 The spindle fibers attach to the centromeres of the chromosomes.
 During interphase the nuclear membrane dissolves to allow the chromosomes to separate and move
through the cytoplasm (in a later phase).
Bowie- Biology 12
Understanding Mitosis & Meiosis
Metaphase
 The 2nd phase of mitosis is metaphase.
 Chromosomes composed of sister chromatids
move toward the center of the cell and line up
along the equatorial plate.
 The chromosomes appear as dark, thick
filamentous structures that are attached to the
spindle fibres.
 It is still difficult to count the chromosomes
during this time as they are generally
entangled. The chromatids can become
intertwined during metaphase.
Anaphase
 Anaphase is the 3rd phase of mitosis.
 The centromeres divide and the sister
chromatids, now referred to as
chromosomes, move to opposite poles of
the cell.
 Usually the separation occurs smoothly
and allows the two ends of the cell to
have identical copies of the DNA.
 Occasionally segment of the chromatids
will break apart and may reattach.
Telophase
 The last phase of mitosis is telophase.
 Chromosomes reach the opposite poles
of the cell and begin to lengthen.
 The spindle fibers dissolve and a
nuclear membrane forms around each
mass of chromatin.
 Telophase is followed by cytokinesis,
the division of the cytoplasm.
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Bowie- Biology 12
Understanding Mitosis & Meiosis
Cytokinesis
 Cytokinesis is the division of the cytoplasm and the
organelles.
 Cytokinesis is quite distinct from nuclear division.
 In an animal cell, a furrow develops, pinching off the
cell into two parts.
 In plant cells, the separation is accomplished by a cell
plate that forms between the 2 chromatin masses. The
cell plate will develop into a new cell wall, eventually
sealing off the contents of the new cells from each other.
 This is the end of cell division.
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Bowie- Biology 12
Understanding Mitosis & Meiosis
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Bowie- Biology 12
Label the diagram of mitosis:
Understanding Mitosis & Meiosis
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Bowie- Biology 12
Understanding Mitosis & Meiosis
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Meiosis
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Meiosis is the process by which sex cells, called gametes, are formed.
It involves 2 stages of cell division that are somewhat similar to the phases in mitosis.
In mitosis, the number of chromosomes in the same in the parent and daughter cells.
In meiosis, the number of chromosomes in the daughter cells is 1/2 the number of chromosomes in the
parent cell.
In humans, a parent cell will contain 46 chromosomes, the daughter cells after meiosis contain 23
chromosomes.
The number of chromosomes in the gamete is called the haploid chromosomes number, or n.
The number of chromosomes in the other body cells is twice the haploid number and is called the
diploid number, or 2n.
During reproduction, the embryo will obtain 1/2 its genes from its mother and 1/2 from its father. The
type of chromosomes you receive from each parent is the same. For example, although you may have
the same eye colour as your father, you also receive a gene for eye colour from your mother (you just
happen to exhibit your father's eye colour).
The paired chromosomes are called homologous chromosomes because they are similar in shape, size
and gene arrangement.
The genes in homologous chromosomes deal with the same traits.
Every human cell in the body (except in the sex cells), contain 23 pairs of homologous chromosomes for
a total of 46 chromosomes.
During fertilization, a haploid (n=23) sperm cell units with a haploid (n = 23) egg cell to produce a diploid
(2n = 46) zygote. The fertilized cell is called a zygote until it begins to divide, at which point it is called an
embryo.
Stages of Meiosis
 Meiosis takes place in 2 nuclear divisions.
 Meiosis I is often called reduction division because the
diploid (2n) chromosome number is reduced to the haploid,
or n, chromosomes number.
 Meiosis II is when the 2 chromatids separate.
 As with mitosis, DNA replication takes place before the cell
division phase.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Meiosis I
 The stages of meiosis have the same names as mitosis, but happens
in two phases.
 During prophase I, the nuclear membrane begins to dissolve, the
centriole splits and its parts move to opposite poles within the cell,
and the spindle fibres are formed.
 The chromosomes come together in homologous pairs. Each
chromosome of the pair is a homologue and is composed of a pair of
sister chromatids. The whole structure is referred to as a tetrad
because each pair is composed of four chromatids.
 This process is referred to as synapsis.
 As the chromosomes synapse, they often intertwine. Sometimes the intertwined chromatids from
different homologue break and exchange segments or undergo crossing over.
 Crossing over enables the exchange of genetic material between homologous pairs of chromosomes.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Metaphase I
 In metaphase I, the homologous chromosomes attach themselves to the spindle fibers and line up along
the equatorial plate.
Anaphase I
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In anaphase I, the homologous chromosomes move toward opposite poles.
The process is known segregation.
At this point of meiosis, reduction division occurs.
One member of each homologous pair will be found in each of the new cells.
Telophase I
 During telophase I, a membrane begins to form around each nucleus. However, unlike in mitosis, the
chromosomes in the two nuclei are not identical.
 Each daughter nuclei contains one member of the chromosome pair.
 Although homologous chromosomes are similar, they are not identical. They do not carry the exact
same information.
 The cells are now ready to begin the second stage of meiosis.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Meiosis II
 During meiosis II pairs of chromatids will separate and move to opposite poles.
 Note that unlike in mitosis and meiosis I, there is no replication of the chromosomes prior to meiosis II.
Prophase II
 Prophase II signals the beginning of the 2nd division.
 During this stage, the nuclear membrane dissolves and the spindle fibers begin to form.
Metaphase II
 The chromosomes, each with two chromatids, lines up along the equatorial plate.
 The chromatids remain pinned together by the centromere.
Anaphase II
 This begins with the breaking of the attachment between the two chromatids and by their movement to
the opposite poles.
 This stage ends when the nuclear membrane begins to form around the chromatids, now referred to a
chromosomes.
Telophase II
 In this stage, the 2nd nuclear division is completed and then the 2nd division of cytoplasm takes place.
 Four haploid daughter cells are produced from each meiotic division.
Bowie- Biology 12
Meiosis in Review
Understanding Mitosis & Meiosis
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Bowie- Biology 12
Understanding Mitosis & Meiosis
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Reproduction and Cell Division
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Plants and animals grow through mitotic division and form sex cells (called gametes) by meiosis.
In animals, meiosis takes place in the testes of males to produce sperm and in the ovaries of females to produce
egg cells.
Two haploid (n) gametes (23 chromosomes each) unite to form a diploid (2n) zygote (fertilized egg that contains
46 chromosomes).
The newly fertilized egg (now called a zygote) begins to divide via mitosis. At this point, each cell division
produces 2 diploid (2n) cells - all with 46 chromosomes each.
As the zygote continues to divide, cells begin to specialize into all the particular types of cells in the body (e.g.
brain cells, skin cells, muscle cells, cardiac cells, blood cells, etc.).
Specialization produces all the cells of the body (known as somatic cells) and the gamete-producing cells (the
cells that make sperm and eggs). The gamete-producing cells undergo meiosis to produce haploid cells (gametes).
The formation of sex cells during meiosis is known as gametogenesis.
Gametogenesis is not exactly the same for males and females.
In females, the cytoplasm in the 4 daughter cells (gametes) does not divide equally.
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One of the daughter cells, called the ootid, receives most of the cytoplasm.
 The other 3 daughter cells, called polar bodies, die. Their cellular material gets absorbed into the body's
other cells.
 The cells that produce ovum in females are called oocytes. Only 1 ovum (egg cell) is produced from
meiosis.
 The ovum contains a large amount of cytoplasm, organelles and nutrients (compared to the sperm cells).
This will be helpful should the egg become fertilized.
 During the reproductive lifetime, a female will release one ovum per month from puberty (10 - 14 years
old) until menopause (between 40-55 years old).
In males, the 4 "daughter" cells share equal division of the cytoplasm so that they are all the same size.
 The cells that produce sperm in males are known as spermatocytes.
 As the job of sperm is to "swim" its way to the egg, they have relatively little cytoplasm (compared to the
egg), have a flagellum and streamline shape to enhance ease of movement.
 Overall, males produce many, many more sperm (1 billion per day) than females do eggs (400-500 eggs
available to be released over the reproductive lifetime).
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Let's Check Our Understanding
1)
A muscle cell of a mouse contains 22 chromosomes. Based on this information, how many
chromosomes are there in the following types of mouse cells:
A)
Daughter muscle cell formed from mitosis: ________________________
B)
Egg cell: _________________________
C)
Fertilized egg cell: ______________________________
2)
When meiosis occurs in females, the cytoplasm is not divided equally among the resulting four cells.
Explain why.
3)
Draw a diagram of meiosis to show how a female child is produced from the union of a sperm and an
egg.
Sex Cells
 The chromosomes of males and females
differ in one major way:
 Females have 2 identical sized X
chromosomes. Males possess one large
X chromosome and one smaller, hookshaped Y chromosomes. These are
known as the sex chromosomes.
 The other chromosomes that are NOT
sex chromosomes are known as
autosomes.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Abnormal Meiosis (Nondisjunction)
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Like any other process, mistakes can occur during meiosis.
Nondisjunction occurs when two homologous chromosomes move to the same pole during meiosis.
The result is that one daughter cell will be missing a chromosome while the other daughter cell will have one
extra chromosome.
Any cell that has too many or too few chromosomes will not function as it should.
Nondisjunction can happen in mitosis as well, but it has much more drastic effects during the formation of sex
cells.
In humans, nondisjunction produces gametes with 22 or 24 chromosomes.
If a gamete (sperm or egg) containing 24 chromosomes unites with a gamete (sperm or egg) that has the correct
number (23) of chromosomes, the zygote will have 47 rather than 46 chromosomes.
The zygote will have 3 copies rather than the normal pair. This is known as trisomy.
If the cell is missing one of the chromosomes, it will only have one copy, rather than the pair. This is known as
monosomy.
This extra chromosome or missing chromosome condition will be passed on to each cell of the forming embryo
through mitotic division.
Nondisjunction Disorders
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Chromosomes can be arranged into homologous pairs and
arranged from largest to smallest in order to analyze the
chromosome pairs. The sex chromosomes are always placed last.
This type of chart is known as a karyotype.
People who have one extra chromosome 21, called Trisomy 21, have a genetic
condition called Down syndrome.
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Turner syndrome is a monosomic disorder resulting from
nondisjunction in the sex cells. This nondisjunction produced a
female with a single X chromosome. The baby will appear female,
but do not usually develop sexually, and tend to be short and
have thick, widened necks. Most fetuses are miscarried before
the 20th week of pregnancy.
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Klinefelter syndrome is caused by a nondisjunction that results in the child inheriting two X
chromosomes and a single Y chromosome. Therefore, they have XXY. The child appears to be male
at birth; however, as he enters sexual maturity, he begins producing high levels of female sex
hormones. Males with Klinefelter syndrome are sterile.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Cloning
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Cloning is a process of forming identical offspring from a single cell.
Since the offspring arises from a single cell, it will be identical (or almost identical) to the parent cell.
This occurs because there is no mixing of two parental sets of DNA.
Cloning is referred to as asexual reproduction (because it does not involve the combining of two
different sets of DNA).
Research into cloning and advancing techniques has led to the field of genetic engineering. Genetic
engineering is when new genetic material is produced by substituting or altering existing genetic
material. This technique is known as recombinant DNA.
In animal cloning, a nucleus from an embryo in its first stages of division can be inserting into an egg of
another animal (whose nucleus has been removed, called enucleated) and the egg will develop into a
clone of the animal from which it was removed.
This only works if the nucleus is taken from an early stage blastula cell or an undifferentiated cell. If it is
taken from a later stage, development will not occur due to the original cell’s “specialization”.
Specialization turns off some of the genes that allow cell division. In mammals, cells must be taken
before the 8 cell stage of embryonic development. After this point, the cells have specialized too much
and their ability to stimulate cell division is lost.
A nucleus that can bring a cell from egg to adult is referred to as totipotent.
Bowie- Biology 12
Understanding Mitosis & Meiosis
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Cancer
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Cancer is a broad group of
diseases associated with the
uncontrolled growth and division
of cells.
Normally, there is a lot of cell to
cell communication designed to
ensure that the rate of cell division
is balanced in order to replace old
or damaged cells and to respond
as needed to situations requiring
greater cell division. Normal cells
usually don’t divide in isolation (if
removed from the body and
placed in a petri dish). Cancer
cells, on the other hand will divide in isolation.
Normal cells also usually have a strong attachment for one another. Cancer cells do not have this
strong attachment. Therefore, it is easier from them to break apart and travel to other parts of the
body (known as metastasis). This can make it hard to locate and control the cancerous cells.
Cancerous tumours that are actively growing are known as malignant. Tumours that are not
cancerous or actively growing are known as benign.