Simplified Module 3

In
GENERAL BIOLOGY 1
QUARTER 1
CELL: THE BASIC UNIT OF LIFE

 Lesson 5 – Cell Cycle

a. Mitosis
b. Meiosis

 Competencies:
1. Characterize the phases of the cell cycle and their control points
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2. Describe the stages of mitosis/meiosis given 2n=6
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3. Explain the significance or applications of mitosis/meiosis
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4. Identify disorders and diseases that result from the malfunction of the cell
during the cell cycle
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Lesson 5 – Cell Cycle

What’s In

In lesson 4, you have learned about the classification of different cell types and
some cell modifications that lead to adaptation to carry out specialized
functions.
In this next topic, you will learn on the phases of cell cycle and their control
points, stages of mitosis/meiosis, comparison and their role in the cell division
cycle.

What I Need to Know

One of the distinct characteristics of living things is being able to

preserve themselves. Cells need to undergo cycles as part of their growth
and to repair or replace damaged parts. Cell cycle enables a living thing to
continue its existence by multiplying itself in controlled and systematic
processes. This lesson will enhance your understanding on cell cycle. This will
provide learners with the concepts on the different stages of cell cycle and
the two types of cell division: mitosis and meiosis and explain their
significance on an organism.
What’s New

Direction: Label the diagram below with the following labels:

Anaphase Metaphase
Cell division (M Phase) Prophase
Cytokinesis Telophase
G1 –cell grows Interphase
G2- prepares for mitosis Mitosis
S-DNA replication

The Cell Cycle Coloring Worksheet

Then on the diagram, lightly color the G1 phase light GREEN, the S
phase dark BLUE, the G2 phase light BLUE, and the stages of mitosis
VIOLET. Color the arrows indicating all of the interphases in BLUE. Color the
part of the arrow indicating mitosis PURPLE and the part of the arrow
indicating cytokinesis light VIOLET.

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What Is It

Cell Division—involves the distribution of identical genetic material or DNA

to two daughter cells. What is most remarkable is the fidelity with which the
DNA is passed along, without dilution or error, from one generation to the
next. Cell Division functions in reproduction, growth, and repair.

Core Concepts:
• All organisms consist of cells and arise from preexisting cells.
• Mitosis is the process by which new cells are generated.
• Meiosis is the process by which gametes are generated for reproduction.
• The Cell Cycle represents all phases in the life of a cell. • DNA replication
(S phase) must precede mitosis so that all daughter cells receive the same
complement of chromosomes as the parent cell.
• The gap phases separate mitosis from S phase. This is the time when
molecular signals mediate the switch in cellular activity.
• Mitosis involves the separation of copied chromosomes into separate
cells.
• Unregulated cell division can lead to cancer.
• Cell cycle checkpoints normally ensure that DNA replication and mitosis
occur only when conditions are favorable and the process is working
correctly.

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 • Mutations in genes that encode cell cycle proteins can lead to
unregulated growth, resulting in tumor formation and ultimately invasion of
cancerous cells to other organs.

The Cell Cycle control system is driven by a built-in clock that can be adjusted
by external stimuli (i.e., chemical messages).
Checkpoint—a critical control point in the Cell Cycle where ‗stop‘ and ‗go-
ahead‘ signals can regulate the cell cycle.
• Animal cells have built-in ‗stop‘ signals that halt the cell cycles and
checkpoints until overridden by ‗go-ahead‘ signals. • Three major
checkpoints are found in the G1, G2, and M phases of the Cell Cycle.

The G1 Checkpoint—the Restriction Point

• The G1 checkpoint ensures that the cell is large enough to divide and that
enough nutrients are available to support the resulting daughter cells.
• If a cell receives a ‗go-ahead‘ signal at the G1 checkpoint, it will usually
continue with the Cell Cycle. • If the cell does not receive the ‗go-ahead‘
signal, it will exit the Cell Cycle and switch to a non-dividing state called
G0.
• Most cells in the human body are in the G0 phase.
The G2 Checkpoint—ensures that DNA replication in S phase has been
successfully completed.
The Metaphase Checkpoint—ensures that all of the chromosomes are
attached to the mitotic spindle by a kinetochore.
Kinase—a protein which activates or deactivates another protein by
phosphorylating them. Kinases give the ‗go-ahead‘ signals at the G1 and G2
checkpoints. The kinases that drive these checkpoints must themselves be
activated.
• The activating molecule is a cyclin, a protein that derives its name from
its cyclically fluctuating concentration in the cell. Because of this
requirement, these kinases are called cyclin-dependent kinases or CDKs.
• Cyclins accumulate during the G1, S, and G2 phases of the Cell Cycle.
• By the G2 checkpoint, enough cyclin is available to form MPF complexes
(aggregations of CDK and cyclin) which initiate mitosis. • MPF functions by
phosphorylating key proteins in the mitotic sequence.
• Later in mitosis, MPF switches itself off by initiating a process which leads
to the destruction of cyclin. • CDK, the non-cyclin part of MPF, persists in
the cell as an inactive form until it associates with new cyclin molecules
synthesized during the interphase of the next round of the Cell Cycle.

Discuss the stages of mitosis and meiosis.

Mitosis (apparent division)—is nuclear division; the process by which the

nucleus divides to produce two new nuclei. Mitosis results in two daughter
cells that are genetically identical to each other and to the parental cell from

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which they came. Cytokinesis—is the division of the cytoplasm. Both
mitosis and cytokinesis last for around one to two hours.
Prophase—is the preparatory stage, during prophase, centrioles move toward
opposite sides of the nucleus.

• The initially indistinct chromosomes begin to condense into visible

threads.
• Chromosomes first become visible during early prophase as long, thin,
and intertwined filaments but by late prophase, chromosomes are more
compacted and can be clearly discerned as much shorter and rod-like
structures.
• As the chromosomes become more distinct, the nucleoli also become
more distinct. By the end of prophase, the nucleoli become less distinct,
often disappearing altogether.

Metaphase—is when chromosomes become arranged so that their

centromeres become aligned in one place, halfway between the two spindle
poles. The long axes of the chromosomes are 90 degrees to the spindle axis.
The plane of alignment is called the metaphase plate.

Anaphase—is initiated by the separation of sister chromatids at their junction

point at the centromere. The daughter chromosomes then move toward the
poles.

Telophase—is when daughter chromosomes complete their migration to the

poles. The two sets of progeny chromosomes are assembled into two-groups
at opposite ends of the cell. The chromosomes uncoil and assume their
extended form during interphase. A nuclear membrane then forms around
each chromosome group and the spindle microtubules disappear. Soon, the
nucleolus reforms.
Meiosis—reduces the amount of genetic information. While mitosis in diploid
cells produces daughter cells with a full diploid complement, meiosis
produces haploid gametes or spores with only one set of chromosomes.
During sexual reproduction, gametes combine in fertilization to reconstitute
the diploid complement found in parental cells. The process involves two
successive divisions of a diploid nucleus.

First Meiotic Division The first meiotic division results in reducing the number
of chromosomes (reduction division). In most cases, the division is
accompanied by cytokinesis.

Prophase I—has been subdivided into five substages: leptonema, zygonema,

pachynema, diplonema, and diakinesis.

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 • Leptonema—Replicated chromosomes have coiled and are already
visible. The number of chromosomes present is the same as the number
in the diploid cell.
• Zygonema—Homologue chromosomes begin to pair and twist around
each other in a highly specific manner. The pairing is called synapsis. And
because the pair consists of four chromatids it is referred to as bivalent
tetrad.
• Pachynema—Chromosomes become much shorter and thicker. A
form of physical exchange between homologues takes place at specific
regions. The process of physical exchange of a chromosome region is
called crossing-over. Through the mechanism of crossing-over, the parts
of the homologous chromosomes are recombined (genetic
recombination).
• Diplonema—The two pairs of sister chromatids begin to separate
from each other. It is at this point where crossing-over is shown to have
taken place. The area of contact between two non-sister chromatids,
called chiasma, become evident.
• Diakinesis—The four chromatids of each tetrad are even more
condensed and the chiasma often terminalize or move down the
chromatids to the ends. This delays the separation of homologous
chromosomes.

In addition, the nucleoli disappear, and the nuclear membrane begins to break
down.

Metaphase I—The spindle apparatus is completely formed and the

microtubules are attached to the centromere regions of the homologues. The
synapsed tetrads are found aligned at the metaphase plate (the equatorial
plane of the cell) instead of only replicated chromosomes.

Anaphase I—Chromosomes in each tetrad separate and migrate toward the

opposite poles. The sister chromatids (dyads) remain attached at their
respective centromere regions.

Telophase I—The dyads complete their migration to the poles. New nuclear
membranes may form. In most species, cytokinesis follows, producing two
daughter cells. Each has a nucleus containing only one set of chromosomes
(haploid level) in a replicated form.

Second Meiotic Division The events in the second meiotic division are quite
similar to mitotic division. The difference lies, however, in the number of
chromosomes that each daughter cell receives. While the original
chromosome number is maintained in mitosis, the number is reduced to half
in meiosis.

Prophase II—The dyads contract.

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Metaphase II—The centromeres are directed to the equatorial plate and
then divide. Anaphase II—The sister chromatids (monads) move away from
each other and migrate to the opposite poles of the spindle fiber.
Telophase II—The monads are at the poles, forming two groups of
chromosomes. A nuclear membrane forms around each set of chromosomes
and cytokinesis follows. The chromosomes uncoil and extend.

Cytokinesis—The telophase stage of mitosis is accompanied by cytokinesis.

The two nuclei are compartmentalized into separate daughter cells and
complete the mitotic cell division process. In animal cells, cytokinesis occurs
by the formation of a constriction in the middle of the cell until two daughter
cells are formed. The constriction is often called cleavage, or cell furrow.
However, in most plant cells this constriction is not evident. Instead, a new
cell membrane and cell wall are assembled between the two nuclei to form a
cell plate. Each side of the cell plate is coated with a cell wall that eventually
forms the two progeny cells.

Table 1: Comparison of Mitosis and Meiosis

(Source:http://courses.washington.edu/bot113/spring/WebReadings/
PdfReadings/TA
BLE_COMPARING_MITOSIS_AND.pdf)

Disorders and Diseases

• incorrect DNA copy (e.g., cancer)

• chromosomes are attached to string-like spindles and begin to move to
the middle of the cell (e.g., Down Syndrome, Alzheimer‘s, and Leukemia)

Other chromosome abnormalities:

• arise from errors in meiosis, usually meiosis I;

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 • occur more often during egg formation (90% of the time) than during
sperm formation; • become more frequent as a woman ages.
• Aneuploidy—is the gain or loss of whole chromosomes. It is the most
common chromosome abnormality. It is caused by non-disjunction, the
failure of chromosomes to correctly separate:
• homologues during meiosis I or
• sister chromatids during meiosis II

What’s More

Direction: Complete the chart by noting what occurs in each phase of the cell
cycle.

Gap O (GO)
Interphase
Gap 1 (G1)

S Phase

Gap 2 (G2)

Prophase
Mitosis or M
Phase Metaphase

Anaphase

Telophase

Cytokinesis C

What I Have Learned

Direction: The diagram below shows cells in various phases of the cell cycle.
Note the cells are not arranged in the order in which the cell cycle occurs.
Use the diagram to answer questions 1-6. Write you answer in CAPITAL
letters.

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 1. ________Interphase (G2) 4. _________Metaphase
2. ________Prophase 5. _________Anaphase
3. ________Prometaphase 6. _________Telophase &
Cytokinesis

What I Can Do

Direction: Gene mutations in a cell can result in uncontrolled cell division,

called cancer. Exposure of cells to certain chemicals and radiation increases
mutations and thus increases the chance of cancer. Research on the causes
of cancers and disorders/diseases that result from the malfunction of the cell
during the cell cycle and answer the following questions.

1. Define cancer
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________.

2. What are the causes of cancer?

_______________________________________________________________
_______________________________________________________________
_______________________________________________________________.

Summary

The importance of cell cycle is very evident that the growth and
sustainability of multicellular organisms depend on this process. Cells that
are damaged and lost will be replenished when cells divide. Errors in mitosis
lead to an incorrect copy of the DNA which may produce deadly functional

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consequences depending on the error. The positive correlation with the
malfunction of these processes to the onset of major diseases such as
cancer, stroke, atherosclerosis, inflammation, and some neurodegenerative
disorders in increasingly proven in various studies.

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Assessment: (Post-Test)

Direction: Select the letter of your choice. Write it in CAPITAL letters. Your
answers should be written on a separate sheet of paper.

__1. Which level of organization is the basic unit of life?

A. Cell
B. Tissue
C. Organ
D. System
__2. Which of the following is NOT a postulate of a
unified cell theory?
A. All living things are composed of cells
B. Cells are the basic unit of life
C. All cells undergo complete development
D. All new cells arise from existing cells
__3. Who coined the term cell for the box like structure he observed when
viewing cork tissue?
A. Matthias Schleiden
B. Theodor Schwann
C. Rudolf Virchow
D. Robert Hooke
__4. In many cells, the structure that controls the cell activities is the
_____________.
A. Cell Membrane
B. Organelle
C. Nucleolus
D. Nucleus
__5. Which part of the cell serves as venue for cellular respiration and is known
as the powerhouse of the cell?
A. Nucleolus
B. Chromosome
C. Mitochondrion
D. Nucleus
__6. Which type of tissue would be found in the epidermis and form the lining of
internal organs such as the intestines?
A. Nervous tissue
B. Muscular tissue
C. Connective tissue
D. Epithelial tissue
__7. The process by which the nucleus divides to produce two new nuclei that
results in two daughter cells that are genetically identical to each other and to
the parental cell from which they came.
A. Meiosis
B. Interphase
C. Mitosis
D. Cytokinesis

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__8. A type of passive transport which relies on carrier proteins in order for the
substances to move down their concentration gradient.
A. Active transport
B. Facilitated diffusion
C. Osmosis
D. Sodium-potassium pump
__9. Which of the following is an example of passive transport which occurs
when particles move from an area of higher concentration to an area of lower
concentration?
A. Phagocytosis
B. Pinocytosis
C. Diffusion
D. Osmosis
__10. This process utilizes additional metabolic energy against the
concentration gradient to move molecules across the membrane from a region
of lower concentration to a region of higher concentration.
A. Active Transport
B. Passive Transport
C. Osmosis
D. Exocytosis

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 KEY ANSWERS

LESSON 5 CELL CYCLE

What’s New
The Cell Cycle Coloring Worksheet

What’s More

Gap O (GO)
Interphase Gap 1 (G1)
S Phase
Gap 2 (G2)
Prophase
Mitosis or M Metaphase
Phase Anaphase
Telophase
Cytokinesis C

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What I Have Learned

1. D
2. A
3. F
4. C
5. E
6. B

What I can do

1. Cancer is a term for diseases in which abnormal cells divide without

control and can invade nearby tissues.
2. Cancer is caused by accumulated damage to genes. Such changes
may be due to chance or to exposure to a cancer causing substances.
Risk Factors:
1) Biological or internal factors such as age, gender, inherited genetic
defects and skin types
2) Environmental exposure
3. Occupational risk factors including carcinogens such as chemicals,
radioactive materials and asbestos
4. Lifestyle-related factors like tobacco, alcohol, UV radiation in sunlight,
food-related such as nitrites and Poly aromatic hydrocarbon

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