SURIGAO STATE COLLEGE OF TECHNOLOGY
LEARNING MODULE
Module 10
THE CELL CYCLE
Topics:
10.1. Phases of the Cell Cycle
10.2. Mitosis and Meiosis
10.3. Programmed Cell Death
10.4 Cancer
Time Frame: 4 hours
Introduction
The cell cycle is an ordered series of events involving cell growth and cell
division that produces two new daughter cells. Cells on the path to cell
division proceed through a series of precisely timed and carefully regulated stages of
growth, DNA replication, and division that produces two identical (clone) cells.
This module tackles the stages of mitosis, meosis, programmed cell death, how can
cancer occurs and the current treatments.
Objectives:
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Illustrate the phases of the cell cycle.
State and discuss the stages of mitosis and meosis;
Explain the genetics of cancer; and
Discuss the new strategies for combating cancer.
Pre-test
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Multiple Choice. Encircle the letter of the best answer.
1. A cell is going through meiosis. The sister chromatids are lined up on the
metaphase plate. What phase of meiosis is described here?
a. Metaphase I
c. Metaphase
b. Prophase II
d. Anaphase I
2. An adult organism has 60 chromosomes or 30 homologous chromosomes. 30 are
maternally derived, 30 are paternally derived. How many chromosomes are in
each cell after mitosis?
a. 60 chromosomes, 30 homologs.
b. 120 chromosomes, 60 homologs.
c. 30 chromosomes, no homologs.
d. 30 chromosomes, 60 homologs
3. An adult organism has 60 chromosomes or 30 homologous pairs of
chromosomes. 30 are maternally derived, 30 are paternally derived. How many
chromosomes are in each cell after meiosis?
a. 30 chromosomes, no homologous chromosomes.
b. 60 chromosomes, 30 homologous chromosomes.
c. 120 chromosomes, 60 homologous chromosomes.
d. 30 chromosomes, 60 homologs
4. Which of the following is mitosis not applicable?
a. Repair (of a wound) in multicellular organisms
b. Asexual reproduction in unicellular organisms
c. Development (e.g., baby in mother's womb)
d. Production of gametes
5. Which choice best describes the cell cycle?
Cells grow and develop during interphase. Cells reproduce during the
a. mitotic phase.
b. Cells grow and develop during the mitotic phase. Cells reproduce during
interphase.
c. The nucleus of a cell divides during interphase. The cytoplasm of a cell
divides during the mitotic phase.
d. The nucleus of a cell divides during the mitotic phase. The cytoplasm of a
cell divides interphase.
6. During which stage of interphase do cells perform their normal cell functions
(such as growing and making enzymes to digest your food)?
a. S stage
c. G2 stage
b. Mitosis
d. G1stage
7. Which of the following is true of crossing over?
a. Segments of DNA are traded between unrelated chromosomes.
b. Crossing over occurs more often in male gametes than female gametes.
c. Most homologue pairs do not have any crossover events
d. Most homologue pairs do not have any crossover events
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8. Which of the following explains how apoptosis is related to cancer?
a. The cues that would trigger apoptosis have failed in cancer cells.
b. Cancer cells are formed when necrosis occurs instead of apoptosis.
c. Cancer is an example of uncontrolled apoptosis.
d. Cancer cells cause excessive apoptosis in surrounding cells to make room
for more cancer cell growth.
9. Which of the following scenarios demonstrates apoptosis?
a. An immune cell self-destructs once it is no longer needed by the immune
system.
b. Muscle fibers are signaled to contract and move.
c. A severe burn causes skin cells to die.
d. All are correct responses
10. Which of the following is true of normal adult cells but NOT cancer cells?
a. Division in the presence of external growth signals
b. Contact with other cells increases likelihood of division
c. Large amount of telomerase present
d. Cell death after a finite number of cell divisions
Lesson 10.1 Phases of the Cell Cycle
The cell cycle has two major phases: interphase and the mitotic phase (Figure 86).
During interphase, the cell grows and DNA is replicated. During the mitotic phase,
the replicated DNA and cytoplasmic contents are separated, and the cell divides.
Figure 86. The Cell Cycle
(www2.le.ac.uk)
Interphase
During interphase, the cell undergoes normal growth processes while also preparing
for cell division. In order for a cell to move from interphase into the mitotic phase,
many internal and external conditions must be met. The three stages of interphase
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are called G1, S, and G2.
G1 Phase (First Gap)
The first stage of interphase is called the G1 phase (first gap) because, from a
microscopic aspect, little change is visible. However, during the G 1 stage, the cell is
quite active at the biochemical level. The cell is accumulating the building blocks of
chromosomal DNA and the associated proteins as well as accumulating sufficient
energy reserves to complete the task of replicating each chromosome in the nucleus.
S Phase (Synthesis of DNA)
Throughout interphase, nuclear DNA remains in a semi-condensed chromatin
configuration. In the S phase, DNA replication can proceed through the mechanisms
that result in the formation of identical pairs of DNA molecules—sister chromatids—
that are firmly attached to the centromeric region. The centrosome is duplicated
during the S phase. The two centrosomes will give rise to the mitotic spindle, the
apparatus that orchestrates the movement of chromosomes during mitosis. At the
center of each animal cell, the centrosomes of animal cells are associated with a pair
of rod-like objects, the centrioles, which are at right angles to each other. Centrioles
help organize cell division. Centrioles are not present in the centrosomes of other
eukaryotic species, such as plants and most fungi.
G2 Phase (Second Gap)
In the G2 phase, the cell replenishes its energy stores and synthesizes proteins
necessary for chromosome manipulation. Some cell organelles are duplicated, and
the cytoskeleton is dismantled to provide resources for the mitotic phase. There may
be additional cell growth during G2. The final preparations for the mitotic phase must
be completed before the cell is able to enter the first stage of mitosis.
The Mitotic Phase
The mitotic phase is a multistep process during which the duplicated chromosomes
are aligned, separated, and move into two new, identical daughter cells. The first
portion of the mitotic phase is called karyokinesis, or nuclear division. The second
portion of the mitotic phase, called cytokinesis, is the physical separation of the
cytoplasmic components into the two daughter cells.
Karyokinesis (Mitosis)
Karyokinesis, also known as mitosis, is divided into a series of phases—prophase,
prometaphase, metaphase, anaphase, and telophase—that result in the division of
the cell nucleus (Figure 87). Karyokinesis is also called mitosis.
Activity No 1.
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Lesson 2. Mitosis and Meiosis
What is Mitosis?
Mitosis is a type of cell division in which one cell (the mother) divides to produce
two new cells (the daughters) that are genetically identical to itself. In the context of
the cell cycle, mitosis is the part of the division process in which the DNA of the cell's
nucleus is split into two equal sets of chromosomes.
The great majority of the cell divisions that happen in your body involve mitosis.
During development and growth, mitosis populates an organism’s body with cells,
and throughout an organism’s life, it replaces old, worn-out cells with new ones. For
single-celled eukaryotes like yeast, mitotic divisions are actually a form of
reproduction, adding new individuals to the population.
In all of these cases, the “goal” of mitosis is to make sure that each daughter cell
gets a perfect, full set of chromosomes. Cells with too few or too many
chromosomes usually don’t function well: they may not survive, or they may even
cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at
random and toss it into piles for the two daughter cells. Instead, they split up their
duplicated chromosomes in a carefully organized series of steps.
Phases of mitosis
Mitosis consists of four basic phases: prophase, metaphase, anaphase, and
telophase. Some textbooks list five, breaking prophase into an early phase (called
prophase) and a late phase (called prometaphase). These phases occur in strict
sequential order, and cytokinesis - the process of dividing the cell contents to make
two new cells - starts in anaphase or telophase.
Let’s start by looking at a cell right before it begins mitosis. This cell is in interphase
(late G_22start subscript, 2, end subscript phase) and has already copied its DNA,
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so the chromosomes in the nucleus each consist of two connected copies,
called sister chromatids. You can’t see the chromosomes very clearly at this point,
because they are still in their long, stringy, decondensed form.
This animal cell has also made a copy of its centrosome, an organelle that will play
a key role in orchestrating mitosis, so there are two centrosomes. (Plant cells
generally don’t have centrosomes with centrioles, but have a different type
of microtubule organizing center that plays a similar role.)
Figure 87. Phases of Mitosis
(courses.lumenlearning.com)
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Prophase
In early prophase, the cell starts to break down some structures and build others up,
setting the stage for division of the chromosomes.
The chromosomes start to condense (making them easier to pull apart later on).
The mitotic spindle begins to form. The spindle is a structure made of microtubules,
strong fibers that are part of the cell’s “skeleton.” Its job is to organize the
chromosomes and move them around during mitosis. The spindle grows between
the centrosomes as they move apart.
The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made,
disappears. This is a sign that the nucleus is getting ready to break down.
In late prophase (sometimes also called prometaphase), the mitotic spindle begins
to capture and organize the chromosomes.
The chromosomes become even more condensed, so they are very compact.
The nuclear envelope breaks down, releasing the chromosomes.
The mitotic spindle grows more, and some of the microtubules start to “capture”
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chromosomes.
Microtubules can bind to chromosomes at the kinetochore, a patch of protein found
on the centromere of each sister chromatid. (Centromeres are the regions of DNA
where the sister chromatids are most tightly connected.)
Microtubules that bind a chromosome are called kinetochore microtubules.
Microtubules that don’t bind to kinetochores can grab on to microtubules from the
opposite pole, stabilizing the spindle. More microtubules extend from each
centrosome towards the edge of the cell, forming a structure called the aster.
Metaphase
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Chromosomes line up at the metaphase plate, under tension from the mitotic spindle.
The two sister chromatids of each chromosome are captured by microtubules from
opposite spindle poles.
In metaphase, the spindle has captured all the chromosomes and lined them up at
the middle of the cell, ready to divide.
All the chromosomes align at the metaphase plate (not a physical structure, just a
term for the plane where the chromosomes line up).
At this stage, the two kinetochores of each chromosome should be attached to
microtubules from opposite spindle poles.
Before proceeding to anaphase, the cell will check to make sure that all the
chromosomes are at the metaphase plate with their kinetochores correctly attached
to microtubules. This is called the spindle checkpoint and helps ensure that the
sister chromatids will split evenly between the two daughter cells when they separate
in the next step. If a chromosome is not properly aligned or attached, the cell will halt
division until the problem is fixed.
Anaphase
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
The sister chromatids separate from one another and are pulled towards opposite
poles of the cell. The microtubules that are not attached to chromosomes push the
two poles of the spindle apart, while the kinetochore microtubules pull the
chromosomes towards the poles.
In anaphase, the sister chromatids separate from each other and are pulled towards
opposite ends of the cell.
The protein “glue” that holds the sister chromatids together is broken down, allowing
them to separate. Each is now its own chromosome. The chromosomes of each pair
are pulled towards opposite ends of the cell.
Microtubules not attached to chromosomes elongate and push apart, separating the
poles and making the cell longer.
All of these processes are driven by motor proteins, molecular machines that can
“walk” along microtubule tracks and carry a cargo. In mitosis, motor proteins carry
chromosomes or other microtubules as they walk.
Telophase
The spindle disappears, a nuclear membrane re-forms around each set of
chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes
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also start to decondense.
In telophase, the cell is nearly done dividing, and it starts to re-establish its normal
structures as cytokinesis (division of the cell contents) takes place.
The mitotic spindle is broken down into its building blocks.
Two new nuclei form, one for each set of chromosomes. Nuclear membranes and
nucleoli reappear.
The chromosomes begin to decondense and return to their “stringy” form.
Cytokinesis in animal and plant cells.
Cytokinesis in an animal cell: an actin ring around the middle of the cell pinches
inward, creating an indentation called the cleavage furrow.
Cytokinesis in a plant cell: the cell plate forms down the middle of the cell, creating a
new wall that partitions it in two.
Cytokinesis, the division of the cytoplasm to form two new cells, overlaps with the
final stages of mitosis. It may start in either anaphase or telophase, depending on
the cell, and finishes shortly after telophase.
In animal cells, cytokinesis is contractile, pinching the cell in two like a coin purse
with a drawstring. The “drawstring” is a band of filaments made of a protein called
actin, and the pinch crease is known as the cleavage furrow. Plant cells can’t be
divided like this because they have a cell wall and are too stiff. Instead, a structure
called the cell plate forms down the middle of the cell, splitting it into two daughter
cells separated by a new wall.
When division is complete, it produces two daughter cells. Each daughter cell has a
complete set of chromosomes, identical to that of its sister (and that of the mother
cell). The daughter cells enter the cell cycle in G1.
When cytokinesis finishes, we end up with two new cells, each with a complete set of
chromosomes identical to those of the mother cell. The daughter cells can now begin
their own cellular “lives,” and – depending on what they decide to be when they grow
up – may undergo mitosis themselves, repeating the cycle.
Activity No. 2
Mitosis - Internet Lesson
In this internet lesson, you will review the steps of mitosis and view video simulations
of cell division.
Mitosis Tutorial http://www.cellsalive.com/
Click on the link to “MITOSIS” Read the text on this page and view the animation,
you can slow down the video by clicking step by step through the phases.
1. Which stage does the following occur:
Chromatin condenses into
chromosomes
___________________________________________
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Chromosomes align in center
___________________________________________
of cell.
Longest part of the cell cycle. ___________________________________________
Nuclear envelope breaks
down.
___________________________________________
Cell is cleaved into two new
daughter cells.
___________________________________________
Daughter chromosomes
arrive at the poles.
___________________________________________
Chromatids are pulled apart
___________________________________________
Watch the video carefully.
2. The colored chromosomes represent chromatids. There are two of each color
because one is an exact duplicate of the other. -How many chromosomes are visible
at the beginning of mitosis? ________________
-- How many are in each daughter cell at the end of mitosis? __________________
--The little green T shaped things on the cell are: ____________________________
-- What happens to the centrioles during mitosis? _______________________
3. Identify the stages of mitosis in these cells
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Meiosis
Meiosis, also called reduction division, division of a germ cell involving two
fissions of the nucleus and giving rise to four gametes, or sex cells, each possessing
half the number of chromosomes of the original cell.
The process of meiosis is characteristic of organisms that reproduce sexually. Such
species have in the nucleus of each cell a diploid (double) set of chromosomes,
consisting of two haploid sets (one inherited from each parent). These haploid sets
are homologous—i.e., they contain the same kinds of genes, but not necessarily in
the
same
form.
In
humans,
for
example,
each
set
of
homologous chromosomes contains a gene for blood type, but one set may have the
gene for blood type A and the other set the gene for blood type B.
To put that another way, meiosis in humans is a division process that takes us from
a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a
single set of chromosomes. In humans, the haploid cells made in meiosis are sperm
and eggs. When a sperm and an egg join in fertilization, the two haploid sets of
chromosomes form a complete diploid set: a new genome.
Prior to meiosis, each of the chromosomes in the diploid germ cell has replicated
and thus consists of a joined pair of duplicate chromatids. Meiosis begins with the
contraction of the chromosomes in the nucleus of the diploid cell. Homologous
paternal and maternal chromosomes pair up along the midline of the cell. Each pair
of chromosomes—called a tetrad, or a bivalent—consists of four chromatids. At this
point, the homologous chromosomes exchange genetic material by the process of
crossing over. The homologous pairs then separate, each pair being pulled to
opposite ends of the cell, which then pinches in half to form two daughter cells. Each
daughter cell of this first meiotic division contains a haploid set of chromosomes. The
chromosomes at this point still consist of duplicate chromatids.
In the second meiotic division, each haploid daughter cell divides. There is no further
reduction in chromosome number during this division, as it involves the separation of
each chromatid pair into two chromosomes, which are pulled to the opposite ends of
the daughter cells. Each daughter cell then divides in half, thereby producing a total
of four different haploid gametes. When two gametes unite during fertilization, each
contributes its haploid set of chromosomes to the new individual, restoring the diploid
number.
To put that another way, meiosis in humans is a division process that takes us from
a diploid cell—one with two sets of chromosomes—to haploid cells—ones with a
single set of chromosomes. In humans, the haploid cells made in meiosis are sperm
and eggs. When a sperm and an egg join in fertilization, the two haploid sets of
chromosomes form a complete diploid set: a new genome.
.
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Figure 88. Phases of Meiosis
(courses.lumenlearning.com)
Prophase I
Prophase I, the first step in meiosis I, is similar to prophase in mitosis in that the
chromosomes condense and move towards the middle of the cell. The nuclear
envelope degrades, which allows the microtubules originating from the centrioles on
either side of the cell to attach to the kinetochores in the centromeres of
each chromosome.
Unlike
in
mitosis,
the
chromosomes
pair
with
their homologous partner. This can be seen in the red and blue chromosomes that
pair together in the diagram. This step does not take place in mitosis. At the end of
prophase I and the beginning of metaphase I, homologous chromosomes are primed
for crossing-over.
Between prophase I and metaphase I, homologous chromosomes can swap parts of
themselves that house the same genes. This is called crossing-over and is
responsible for the other law of genetics, the law of independent assortment. This
law states that traits are inherited independently of each other. For traits on different
chromosomes, this is certainly true all of the time. For traits on the same
chromosome, crossing-over makes it possible for the maternal and paternal DNA to
recombine, allowing traits to be inherited in an almost infinite number of ways.
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Metaphase I
In metaphase I of meiosis I, the homologous pairs of chromosomes line up on the
metaphase plate, near the center of the cell. This step is referred to as a reductional
division. The homologous chromosomes that contain the two different alleles for
each gene are lined up to be separated. As seen in the diagram above, while the
chromosomes line up on the metaphase plate with their homologous pair, there is no
order upon which side the maternal or paternal chromosomes line up. This process
is the molecular reason behind the law of segregation.
The law of segregation tells us that each allele has the same chance of being
passed on to offspring. In metaphase I of meiosis, the alleles are separated, allowing
for this phenomenon to happen. In meiosis II, they will be separated into individual
gametes. In mitosis, all the chromosomes line up on their centromeres, and the
sister chromatids of each chromosome separate into new cells. The homologous
pairs do not pair up in mitosis, and each is split in half to leave the new cells with 2
different alleles for each gene. Even if these alleles are the same allele, they came
from a maternal and paternal source. In meiosis, the lining up of homologous
chromosomes leaves 2 alleles in the final cells, but they are on sister chromatids and
are clones of the same source of DNA.
Anaphase I
Much like anaphase of mitosis, the chromosomes are now pulled towards the
centrioles at each side of the cell. However, the centrosomes holding the sister
chromatids together do not dissolve in anaphase I of meiosis, meaning that only
homologous chromosomes are separated, not sister chromatids.
Telophase I
In telophase I, the chromosomes are pulled completely apart and new nuclear
envelopes form. The plasm membrane is separated by cytokinesis and two new cells
are effectively formed.
Results of Meiosis I
Two new cells, each haploid in their DNA, but with 2 copies, are the result of meiosis
I. Again, although there are 2 alleles for each gene, they are on
sister chromatid copies of each other. These are therefore considered haploid cells.
These cells take a short rest before entering the second division of meiosis, meiosis
II.
Phases of Meiosis II
Prophase II
Prophase II resembles prophase I. The nuclear envelopes disappear and centrioles
are formed. Microtubules extend across the cell to connect to the kinetochores of
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individual chromatids, connected by centromeres. The chromosomes begin to get
pulled toward the metaphase plate.
Metaphase II
Now resembling mitosis, the chromosomes line up with their centromeres on the
metaphase plate. One sister chromatid is on each side of the metaphase plate. At
this stage, the centromeres are still attached by the protein cohesin.
Anaphase II
The sister chromatids separate. They are now called sister chromosomes and are
pulled toward the centrioles. This separation marks the final division of the DNA.
Unlike the first division, this division is known as an equational division, because
each cell ends up with the same quantity of chromosomes as when the division
started, but with no copies.
Telophase II
As in the previous telophase I, the cell is now divided into two and the chromosomes
are on opposite ends of the cell. Cytokinesis or plasma division occurs, and new
nuclear envelopes are formed around the chromosomes.
Results of Meiosis II
At the end of meiosis II, there are 4 cells, each haploid, and each with only 1 copy of
the genome. These cells can now be developed into gametes, eggs in females and
sperm in males.
Examples of Meiosis
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Human Meiosis
Human meiosis occurs in the sex organs. Male testis produce sperm and female
ovaries produce eggs. Before these gametes are made, however, the DNA must be
reduced. Humans have 23 distinct chromosomes, existing in homologous pairs
between maternal and paternal DNA, meaning 46 chromosomes. Before meiosis,
the DNA in the cell is replicated, producing 46 chromosomes in 92 sister chromatids.
Each pair of sister chromatids has a corresponding (either maternal or paternal) set
of sister chromosomes. These pairs are known as homologous chromosomes.
During meiosis I, these homologous chromosomes line up and divide. This leaves 23
chromosomes in each cell, each chromosome consisting of sister chromatids. These
chromatids may no longer be identical, as crossing-over may have occurred during
metaphase I of meiosis I. Finally, meiosis II takes place, and the sister chromatids
are separated into individual cells. This leaves 4 cells, each with 23 chromosomes,
or 4 haploid cells.
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Fruit Flies
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Fruit flies have 4 pairs of chromosomes or 8 chromosomes in regular cells. Before
meiosis takes place, each chromosome is replicated, leaving 8 chromosomes and 16
sister chromatids. Meiosis I takes place, and there are 2 cells, each with only 4
chromosomes. Each chromosome is still made of sister chromatids, and some
crossing-over may have occurred during metaphase I. Meiosis II now takes place on
those two cells. In total, 4 cells are created, again. However, these cells have 4
chromosomes. When two gametes meet to create a new fruit fly, the resulting zygote
will have 8 chromosomes of 4 pairs of sister chromosomes, 4 coming from each
parent.
Activity No. 3
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Lesson 10.3. Programmed Cell Death
Programmed cell death (PCD; sometimes referred to as cellular suicide) is
the death of a cell as a result of events inside of a cell, such as apoptosis or
autophagy. PCD is carried out in a biological process, which usually confers
advantage during an organism's life-cycle.
You may think of it as a bad thing for cells in your body to die. In many cases, that’s
true: it’s not good for cells to die because of an injury (for example, from a scrape or
a harmful chemical). However, it’s also important that some cells of our bodies do die
– not randomly, but in a carefully controlled way.
For example, have you ever wondered how your fingers formed? It turns out that the
cells between your developing fingers were instructed to die long ago, while you
were still an embryo. If they hadn’t done so, you would have webbed hands, or
perhaps just paddles of tissue with no fingers at all.
The cells between your embryonic fingers died in a process called apoptosis, a
common form of programmed cell death. In programmed cell death, cells undergo
“cellular suicide” when they receive certain cues. Apoptosis involves the death of a
cell, but it benefits the organism as a whole (for instance, by letting fingers develop
or eliminating potential cancer cells). In this article, we’ll take a closer look at
apoptosis, seeing when it happens and why it’s important.
Apoptosis vs. necrosis
Broadly speaking, there are two ways that cells die in a multicellular organism such
as yourself:


They are killed by things that harm them (such as toxic chemicals or physical injury),
a process called necrosis.
They are triggered to undergo programmed cell death. The best-understood form of
programmed cell death is apoptosis.
Necrosis and apoptosis occur under different circumstances and involve different
steps. Simply put, necrosis is messy and causes an immune response of
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inflammation, while apoptosis is tidy and splits the cell into little parcels that can be
taken up and recycled by other cells (Figure 89).
Necrosis (the messy way)
When cells are damaged by harmful factors (such as injury or toxic chemicals), they
usually “spill their guts” as they die. Because the damaged cell’s plasma membrane
can no longer control the passage of ions and water, the cell swells up, and its
contents leak out through holes in the plasma membrane. This often causes
inflammation in the tissue surrounding the dead cell.
Figure 89. Comparison of necrosis and Apoptosis
(khanacademy.org)
Apoptosis (the tidy way)
Cells that undergo apoptosis go through a different and much more orderly process.
They shrink and develop bubble-like protrusions (technical name: “blebs”) on their
surface. The DNA in the nucleus gets chopped up into small pieces, and some
organelles of the cell, such as the endoplasmic reticulum, break down into
fragments. In the end, the entire cell splits up into small chunks, each neatly
enclosed in a package of membrane.
What happens to the chunks? They release signals that attract debris-eating
(phagocytic) immune cells, such as macrophages. Also, the fragments of the dying
cell display a lipid molecule called phosphatidylserine on their surface.
Phosphatidylserine is usually hidden on the inside of the membrane, and when it is
on the outside, it lets the phagocytes bind and "eat" the cell fragments.
Why do cells undergo apoptosis?
Many cells in the human body have the built-in ability to undergo apoptosis (in the
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same way that they have the built-in ability to copy their DNA or break down fuels).
Basically, apoptosis is a general and convenient way to remove cells that should no
longer be part of the organism.
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Some cells need to be “deleted” during development – for instance, to whittle an
intricate structure like a hand out of a larger block of tissue.
Some cells are abnormal and could hurt the rest of the organism if they survive, such
as cells with viral infections or DNA damage.
Cells in an adult organism may be eliminated to maintain balance – to make way for
new cells or remove cells needed only for temporary tasks.
Apoptosis is Part of Development
In many organisms, programmed cell death is a normal part of development. In
some cases, apoptosis during development occurs in a very predictable way: in the
worm C. elegans, 131 cells will die by apoptosis as the worm develops from a single
cell to an adult (and we know exactly which ones they are)
Apoptosis also plays a key role in human development. For instance, as we saw in
the introduction, your hand started out as a paddle-like block of tissue when you
were an embryo. The block was “carved” into fingers by apoptosis of the cells in
between the developing fingers.
Figure 90. Developing mouse paw showing apoptosis
(khanacademy.org)
Microscope images from a scientific paper, showing a developing mouse paw. The
cells between the developing digits are stained by a marker that indicates apoptotic
cells (Figure 90).
This process occurs in all sorts of vertebrate species that have finger- or toe-like
digits, and less apoptosis results in more webbing between the digits. Sometimes, if
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a small mistake happens during finger or toe development, apoptosis may be
incomplete (leading, for instance, to fused toes).
Other examples of apoptosis during normal development include the loss of a
tadpole’s tail as it turns into a frog, and the removal of unneeded neurons in as
neural circuits in the brain are “wired.”
Apoptosis can eliminate infected or cancerous cells
In some cases, a cell can pose a threat to the rest of the body if it survives. For
instance, this may be the case for cells with DNA damage, pre-cancerous cells, and
cells infected by viruses. If these cells undergo apoptosis, the threat to the rest of the
organism (such as cancer or spread of a viral infection) is removed.
When a cell’s DNA is damaged, it will typically detect the damage and try to repair it.
If the damage is beyond repair, the cell will normally send itself into apoptosis,
ensuring that it will not pass on its damaged DNA. When cells have DNA damage
but fail to undergo apoptosis, they may be on the road to cancer (Figure 91).
Figure 91. Apoptosis can remove the threat of producing cancer cells
(khanacademy.org)
Sometimes, pre-cancerous cells that have avoided internal apoptosis cues are
detected by immune cells, which try to trigger apoptosis through an external
signaling pathway. Successful cancer cells, however, manage to duck both internal
and external cues that would normally trigger apoptosis. This allows them to divide
out of control and accumulate mutations (changes in their DNA).
Apoptosis is key to immune function
Apoptosis also plays an essential role in the development and maintenance of a
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healthy immune system. When B and T cells (immune cells that bind specific
molecules) are first produced, they’re tested to see if they react against any of the
body’s own “self” components. Cells that do are eliminated right away by apoptosis.
If this process fails, self-reactive cells may be released into the body, where they can
attack tissues and cause autoimmune conditions.
Apoptosis also plays an important role in allowing the immune system to turn off its
response to a pathogen. When a pathogen is detected, the immune cells that
recognize the pathogen divide extensively, undergoing a huge increase in numbers
with the purpose of destroying the pathogen. Once the pathogen is cleared from the
body, the large numbers of pathogen-specific immune cells are no longer needed
and must be removed by apoptosis to maintain homeostasis (balance) in the
immune system.
Lesson 10.4 Cancer
What is Cancer?
Cancer is a broad term. It describes the disease that results when cellular changes
cause the uncontrolled growth and division of cells. In all types of cancer, some of
the body’s cells begin to divide without stopping and spread into surrounding tissues.
Cancer can start almost anywhere in the human body, which is made up of trillions of
cells. Normally, human cells grow and divide to form new cells as the body needs
them. When cells grow old or become damaged, they die, and new cells take their
place.
When cancer develops, however, this orderly process breaks down. As cells become
more and more abnormal, old or damaged cells survive when they should die, and
new cells form when they are not needed. These extra cells can divide without
stopping and may form growths called tumors.
Many cancers form solid tumors, which are masses of tissue. Cancers of the blood,
such as leukemias, generally do not form solid tumors. Cancerous tumors are
malignant, which means they can spread into, or invade, nearby tissues. In addition,
as these tumors grow, some cancer cells can break off and travel to distant places in
the body through the blood or the lymph system and form new tumors far from the
original tumor.
Unlike malignant tumors, benign tumors do not spread into, or invade, nearby
tissues. Benign tumors can sometimes be quite large, however. When removed, they
usually don’t grow back, whereas malignant tumors sometimes do. Unlike most
benign tumors elsewhere in the body, benign brain tumors can be life threatening.
Differences between Cancer Cells and Normal Cells
Cancer cells differ from normal cells in many ways that allow them to grow out of
control and become invasive. One important difference is that cancer cells are less
specialized than normal cells. That is, whereas normal cells mature into very distinct
cell types with specific functions, cancer cells do not. This is one reason that, unlike
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normal cells, cancer cells continue to divide without stopping.
In addition, cancer cells are able to ignore signals that normally tell cells to stop
dividing or that begin a process known as programmed cell death, or apoptosis,
which the body uses to get rid of unneeded cells.
Cancer cells may be able to influence the normal cells, molecules, and blood vessels
that surround and feed a tumor—an area known as the microenvironment. For
instance, cancer cells can induce nearby normal cells to form blood vessels that
supply tumors with oxygen and nutrients, which they need to grow. These blood
vessels also remove waste products from tumors.
Cancer cells are also often able to evade the immune system, a network of organs,
tissues, and specialized cells that protects the body from infections and other
conditions. Although the immune system normally removes damaged or abnormal
cells from the body, some cancer cells are able to “hide” from the immune system.
Tumors can also use the immune system to stay alive and grow. For example, with
the help of certain immune system cells that normally prevent a runaway immune
response, cancer cells can actually keep the immune system from killing cancer
cells.
How Cancer Arises
Figure 92. Cancer is caused by certain changes to genes, the basic physical
units of inheritance. Genes are arranged in long strands of tightly packed
DNA called chromosomes.
(Terese Winslow)
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Genetic changes that cause cancer can be inherited from our parents. They can also
arise during a person’s lifetime as a result of errors that occur as cells divide or
because of damage to DNA caused by certain environmental exposures. Cancercausing environmental exposures include substances, such as the chemicals in
tobacco smoke, and radiation, such as ultraviolet rays from the sun.
Each person’s cancer has a unique combination of genetic changes. As the cancer
continues to grow, additional changes will occur. Even within the same tumor,
different cells may have different genetic changes.
In general, cancer cells have more genetic changes, such as mutations in DNA
(Figure 92), than normal cells. Some of these changes may have nothing to do with
the cancer; they may be the result of the cancer, rather than its cause.
Fundamentals of Cancer
How Does Cancer Form?
Figure 93. Cancer Cells
(cancer.gov)
"Drivers" of Cancer
Cancer is a disease caused when cells divide uncontrollably and spread into
surrounding tissues
The genetic changes that contribute to cancer tend to affect three main types of
genes—proto-oncogenes, tumor suppressor genes, and DNA repair genes. These
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changes are sometimes called “drivers” of cancer.
Proto-oncogenes are involved in normal cell growth and division. However, when
these genes are altered in certain ways or are more active than normal, they may
become cancer-causing genes (or oncogenes), allowing cells to grow and survive
when they should not.
Tumor suppressor genes are also involved in controlling cell growth and division.
Cells with certain alterations in tumor suppressor genes may divide in an
uncontrolled manner.
DNA repair genes are involved in fixing damaged DNA. Cells with mutations in these
genes tend to develop additional mutations in other genes. Together, these
mutations may cause the cells to become cancerous.
As scientists have learned more about the molecular changes that lead to cancer,
they have found that certain mutations commonly occur in many types of cancer.
Because of this, cancers are sometimes characterized by the types of genetic
alterations that are believed to be driving them, not just by where they develop in the
body and how the cancer cells look under the microscope.
When Cancer Spreads
ENLARGE
A cancer that has spread from the place where it first started to another place in the
body is called metastatic cancer. The process by which cancer cells spread to other
parts of the body is called metastasis.
Metastatic cancer has the same name and the same type of cancer cells as the
original, or primary, cancer. For example, breast cancer that spreads to and forms a
metastatic tumor in the lung is metastatic breast cancer, not lung cancer.
Under a microscope, metastatic cancer cells generally look the same as cells of the
original cancer. Moreover, metastatic cancer cells and cells of the original cancer
usually have some molecular features in common, such as the presence of
specific chromosome changes.
Treatment may help prolong the lives of some people with metastatic cancer. In
general, though, the primary goal of treatments for metastatic cancer is to control the
growth of the cancer or to relieve symptoms caused by it. Metastatic tumors can
cause severe damage to how the body functions, and most people who die of cancer
die of metastatic disease.
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Figure 94. Metastasis of cancer cells
(cancer.gov)
In metastasis, cancer cells break away from where they first formed (primary
cancer), travel through the blood or lymph system, and form new tumors (metastatic
tumors) in other parts of the body. The metastatic cancer is the same type of cancer
as primary tumor.
Tissue Changes that Are Not Cancer
Not every change in the body’s tissues is cancer. Some tissue changes may develop
into cancer if they are not treated, however. Here are some examples of tissue
changes that are not cancer but, in some cases, are monitored:
Hyperplasia occurs when cells within a tissue divide faster than normal and extra
cells build up, or proliferate. However, the cells and the way the tissue is organized
look normal under a microscope. Hyperplasia can be caused by several factors or
conditions, including chronic irritation.
Dysplasia is a more serious condition than hyperplasia. In dysplasia, there is also a
buildup of extra cells. But the cells look abnormal and there are changes in how the
tissue is organized. In general, the more abnormal the cells and tissue look, the
greater the chance that cancer will form.
Some types of dysplasia may need to be monitored or treated. An example of
dysplasia is an abnormal mole (called a dysplastic nevus) that forms on the skin. A
dysplastic nevus can turn into melanoma, although most do not.
An even more serious condition is carcinoma in situ. Although it is sometimes called
cancer, carcinoma in situ is not cancer because the abnormal cells do not spread
beyond the original tissue. That is, they do not invade nearby tissue the way that
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cancer cells do. But, because some carcinomas in situ may become cancer, they are
usually treated.
Figure 95. Comparison of Normal cells, hyperplasia, dysplasia and cancer cells
(cancer.gov)
Normal cells may become cancer cells. Before cancer cells form in tissues of the
body, the cells go through abnormal changes called hyperplasia and dysplasia. In
hyperplasia, there is an increase in the number of cells in an organ or tissue that
appear normal under a microscope. In dysplasia, the cells look abnormal under a
microscope but are not cancer. Hyperplasia and dysplasia may or may not become
cancer (Figure 95).
Types of Cancer
There are more than 100 types of cancer. Types of cancer are usually named for the
organs or tissues where the cancers form. For example, lung cancer starts in cells of
the lung, and brain cancer starts in cells of the brain. Cancers also may be described
by the type of cell that formed them, such as an epithelial cell or a squamous cell.
Here are some categories of cancers that begin in specific types of cells:
 Carcinoma
Carcinomas are the most common type of cancer. They are formed by epithelial
cells, which are the cells that cover the inside and outside surfaces of the body.
There are many types of epithelial cells, which often have a column-like shape
when viewed under a microscope.
Carcinomas that begin in different epithelial cell types have specific names:
Adenocarcinoma is a cancer that forms in epithelial cells that produce fluids or
mucus. Tissues with this type of epithelial cell are sometimes called glandular
tissues. Most cancers of the breast, colon, and prostate are adenocarcinomas.
Basal cell carcinoma is a cancer that begins in the lower or basal (base) layer of
the epidermis, which is a person’s outer layer of skin.
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Squamous cell carcinoma is a cancer that forms in squamous cells, which are
epithelial cells that lie just beneath the outer surface of the skin. Squamous cells
also line many other organs, including the stomach, intestines, lungs, bladder, and
kidneys. Squamous cells look flat, like fish scales, when viewed under a
microscope. Squamous cell carcinomas are sometimes called epidermoid
carcinomas.
Transitional cell carcinoma is a cancer that forms in a type of epithelial tissue
called transitional epithelium, or urothelium. This tissue, which is made up of many
layers of epithelial cells that can get bigger and smaller, is found in the linings of the
bladder, ureters, and part of the kidneys (renal pelvis), and a few other organs.
Some cancers of the bladder, ureters, and kidneys are transitional cell carcinomas.

Sarcoma
Sarcomas are cancers that form in bone and soft tissues, including muscle, fat,
blood vessels, lymph vessels, and fibrous tissue (such as tendons and ligaments).
Osteosarcoma is the most common cancer of bone. The most common types of
soft tissue sarcoma are leiomyosarcoma, Kaposi sarcoma, malignant fibrous
histiocytoma, liposarcoma, and dermatofibrosarcoma protuberans.
Leukemia
Cancers that begin in the blood-forming tissue of the bone marrow are called
leukemias. These cancers do not form solid tumors. Instead, large numbers of
abnormal white blood cells (leukemia cells and leukemic blast cells) build up in the
blood and bone marrow, crowding out normal blood cells. The low level of normal
blood cells can make it harder for the body to get oxygen to its tissues, control
bleeding, or fight infections.
There are four common types of leukemia, which are grouped based on how
quickly the disease gets worse (acute or chronic) and on the type of blood cell the
cancer starts in (lymphoblastic or myeloid).

Lymphoma
Lymphoma is cancer that begins in lymphocytes (T cells or B cells). These are
disease-fighting white blood cells that are part of the immune system. In
lymphoma, abnormal lymphocytes build up in lymph nodes and lymph vessels, as
well as in other organs of the body.
There are two main types of lymphoma:
Hodgkin lymphoma – People with this disease have abnormal lymphocytes that
are called Reed-Sternberg cells. These cells usually form from B cells.
Non-Hodgkin lymphoma – This is a large group of cancers that start in
lymphocytes. The cancers can grow quickly or slowly and can form from B cells or
T cells.
 Multiple Myeloma
Multiple myeloma is cancer that begins in plasma cells, another type of immune
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cell. The abnormal plasma cells, called myeloma cells, build up in the bone
marrow and form tumors in bones all through the body. Multiple myeloma is also
called plasma cell myeloma and Kahler disease.
.
 Melanoma
Melanoma is cancer that begins in cells that become melanocytes, which are
specialized cells that make melanin (the pigment that gives skin its color). Most
melanomas form on the skin, but melanomas can also form in other pigmented
tissues, such as the eye.
 Brain and Spinal Cord Tumors
There are different types of brain and spinal cord tumors. These tumors are
named based on the type of cell in which they formed and where the tumor first
formed in the central nervous system. For example, an astrocytic tumor begins in
star-shaped brain cells called astrocytes, which help keep nerve cells healthy.
Brain tumors can be benign (not cancer) or malignant (cancer).
Other Types of Tumors
 Germ Cell Tumors
Germ cell tumors are a type of tumor that begins in the cells that give rise to
sperm or eggs. These tumors can occur almost anywhere in the body and can
be either benign or malignant.

Neuroendocrine Tumors
Neuroendocrine tumors form from cells that release hormones into the blood in
response to a signal from the nervous system. These tumors, which may make
higher-than-normal amounts of hormones, can cause many different
symptoms. Neuroendocrine tumors may be benign or malignant.

Carcinoid Tumors
Carcinoid tumors are a type of neuroendocrine tumor. They are slow-growing
tumors that are usually found in the gastrointestinal system (most often in the
rectum and small intestine). Carcinoid tumors may spread to the liver or other
sites in the body, and they may secrete substances such as serotonin or
prostaglandins, causing carcinoid syndrome.
Is Cancer Genetic?
Genetic factors can contribute to the development of cancer.
A person’s genetic code tells their cells when to divide and expire. Changes in the
genes can lead to faulty instructions, and cancer can result.
Genes also influence the cells’ production of proteins, and proteins carry many of the
instructions for cellular growth and division.
Some genes change proteins that would usually repair damaged cells. This can lead
to cancer. If a parent has these genes, they may pass on the altered instructions to
their offspring.
Some genetic changes occur after birth, and factors such as smoking and sun
exposure can increase the risk.
Other changes that can result in cancer take place in the chemical signals that
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determine how the body deploys, or “expresses” specific genes.
Finally, a person can inherit a predisposition for a type of cancer. A doctor may refer
to this as having a hereditary cancer syndrome. Inherited genetic mutations
significantly contribute to the development of 5–10 percent of cancer cases.
How do gene mutations interact with each other?
The gene mutations you're born with and those that you acquire throughout your life
work together to cause cancer.
For instance, if you've inherited a genetic mutation that predisposes you to cancer,
that doesn't mean you're certain to get cancer. Instead, you may need one or more
other gene mutations to cause cancer. Your inherited gene mutation could make you
more likely than other people to develop cancer when exposed to a certain cancercausing substance.
It's not clear just how many mutations must accumulate for cancer to form. It's likely
that this varies among cancer types.
Types of Cancer Treatments
Innovative research has fueled the development of new medications and treatment
technologies. Doctors usually prescribe treatments based on the type of cancer, its
stage at diagnosis, and the person’s overall health.
The side effects of chemotherapy include hair loss. However, advances in treatment
are improving the outlook for people with cancer.
Below are examples of approaches to cancer treatment:
 Chemotherapy aims to kill cancerous cells with medications that target
rapidly dividing cells. The drugs can also help shrink tumors, but the side
effects can be severe.
 Hormone therapy involves taking medications that change how certain
hormones work or interfere with the body’s ability to produce them. When
hormones play a significant role, as with prostate and breast cancers, this is a
common approach.
 Immunotherapy uses medications and other treatments to boost the immune
system and encourage it to fight cancerous cells. Two examples of these
treatments are checkpoint inhibitors and adoptive cell transfer.
 Precision medicine, or personalized medicine, is a newer, developing
approach. It involves using genetic testing to determine the best treatments
for a person’s particular presentation of cancer. Researchers have yet to
show that it can effectively treat all types of cancer, however.
 Radiation therapy uses high-dose radiation to kill cancerous cells. Also, a
doctor may recommend using radiation to shrink a tumor before surgery or
reduce tumor-related symptoms.
 Stem cell transplant can be especially beneficial for people with bloodrelated cancers, such as leukemia or lymphoma. It involves removing cells,
such as red or white blood cells, that chemotherapy or radiation has
destroyed. Lab technicians then strengthen the cells and put them back into
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
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the body.
Surgery is often a part of a treatment plan when a person has a cancerous
tumor. Also, a surgeon may remove lymph nodes to reduce or prevent the
disease’s spread.
Targeted therapies perform functions within cancerous cells to prevent them
from multiplying. They can also boost the immune system. Two examples of
these therapies are small-molecule drugs and monoclonal antibodies.
Biomarker Testing for Cancer Treatment
Biomarker testing is a way to look for genes, proteins, and other substances
(called biomarkers or tumor markers) that can provide information about
cancer. Biomarker testing can help you and your doctor choose a cancer
treatment.
Activity No. 4
Instruction:
Create a model that explains the relationship between the cell cycle and the
development of cancer. Your model can be an illustration, a description, a
video explanation, or a physical representation.
Self-Check
10.4
Give substantial answers to the following questions:
1. What does G0 mean in the cell cycle?
2. What is the main difference between apoptosis and
necrosis?
3. What happens with apoptotic cells?
4. Briefly describe the intrinsic and extrinsic pathways for
apoptosis.
5. Is there a vaccine for cancer?
6. What are the stages of cancer and describe each stage?
7. What are the general signs and symptoms of cancer?
8. What are some preventions tips to reduce your risk of
getting cancer?
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LOOKING BACK
Review the following concepts:

Haploid – Organism with only one copy of each gene in each cell, or gametes
with such.

Diploid – Two copies of each gene, per cell.

PolyploidDominance – Multiple (more than two) copies of each gene per cell.

Sister Chromatids – The replicated DNA that exist as a single chromosome
until separated in anaphase.

Apoptosis is a form of programmed cell death, or “cellular suicide.” It is
different from necrosis, in which cells die due to injury. Apoptosis is not the
only form of programmed cell death, but it is the form we understand best.

Apoptosis is an orderly process in which the cell’s contents break down and
are packaged into small packets of membrane for “garbage collection” by
immune cells. It contrasts with necrosis (death by injury), in which the dying
cell’s contents spill out and cause inflammation.

Apoptosis removes cells during development. It also eliminates pre-cancerous
and virus-infected cells, although “successful” cancer cells manage to escape
apoptosis so they can continue dividing. Apoptosis maintains the balance of
cells in the human body and is particularly important in the immune system.
Post test
Multiple Choice. Encircle the letter of the best answer.
1. A cell is going through meiosis. The sister chromatids are lined up on the
metaphase plate. What phase of meiosis is described here?
c. Metaphase I
c. Metaphase
d. Prophase II
d. Anaphase I
2. An adult organism has 60 chromosomes or 30 homologous chromosomes. 30 are
maternally derived, 30 are paternally derived. How many chromosomes are in
each cell after mitosis?
a. 60 chromosomes, 30 homologs.
b. 120 chromosomes, 60 homologs.
c. 30 chromosomes, no homologs.
d. 30 chromosomes, 60 homologs
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3. An adult organism has 60 chromosomes or 30 homologous pairs of
chromosomes. 30 are maternally derived, 30 are paternally derived. How many
chromosomes are in each cell after meiosis?
a. 30 chromosomes, no homologous chromosomes.
b. 60 chromosomes, 30 homologous chromosomes.
c. 120 chromosomes, 60 homologous chromosomes.
d. 30 chromosomes, 60 homologs
4. Which of the following is mitosis not applicable?
a. Repair (of a wound) in multicellular organisms
b. Asexual reproduction in unicellular organisms
c. Development (e.g., baby in mother's womb)
d. Production of gametes
5. Which choice best describes the cell cycle?
Cells grow and develop during interphase. Cells reproduce during the
a. mitotic phase.
b. Cells grow and develop during the mitotic phase. Cells reproduce during
interphase.
c. The nucleus of a cell divides during interphase. The cytoplasm of a cell
divides during the mitotic phase.
d. The nucleus of a cell divides during the mitotic phase. The cytoplasm of a
cell divides interphase.
6. During which stage of interphase do cells perform their normal cell functions (such
as growing and making enzymes to digest your food)?
a. S stage
c. G2 stage
b. Mitosis
d. G1stage
7. Which of the following is true of crossing over?
a. Segments of DNA are traded between unrelated chromosomes.
b. Crossing over occurs more often in male gametes than female gametes.
c. Most homologue pairs do not have any crossover events
d. Most homologue pairs do not have any crossover events
8. Which of the following explains how apoptosis is related to cancer?
a. The cues that would trigger apoptosis have failed in cancer cells.
b. Cancer cells are formed when necrosis occurs instead of apoptosis.
c. Cancer is an example of uncontrolled apoptosis.
d. Cancer cells cause excessive apoptosis in surrounding cells to make room
for more cancer cell growth.
9. Which of the following scenarios demonstrates apoptosis?
a. An immune cell self-destructs once it is no longer needed by the immune
system.
b. Muscle fibers are signaled to contract and move.
c. A severe burn causes skin cells to die.
d. All are correct responses
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10. Which of the following is true of normal adult cells but NOT cancer cells?
a. Division in the presence of external growth signals
b. Contact with other cells increases likelihood of division
c. Large amount of telomerase present
d. Cell death after a finite number of cell division
References
Karp, G. Karp, G.( 2013) Cell and Molecular Biology, Wiley and Sons
Pollard, T.D. and Earnshaw, W.C. 2008. Cell Biology. Saunders Elsevier, USA
Raven, J. 2018. Biology. McGraw Hill. USA.
https://courses.lumenlearning.com/biology1/chapter/the-cellcycle/#:~:text=The%20cell%20cycle%20is%20an,two%20identical%20(clone)%20ce
lls.
https://www.britannica.com/science/mitosis
https://www.britannica.com/science/meiosis-cytology
https://biologydictionary.net/meiosis/
https://www.khanacademy.org/science/biology/developmental-biology/apoptosis-indevelopment/a/apoptosis
https://www.cancer.gov/about-cancer/understanding/what-is-cancer
https://www.medicalnewstoday.com/articles/323648
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