Cell Cycle
Cell Division
GenBio
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LEARNING OBJECTIVES:
The learners will:
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Demonstrate an understanding of the cell cycle and cell division (i.e., mitosis and meiosis).
Identify and differentiate the phases of the cell cycle and their control points
describe and differentiate the stages of mitosis and meiosis given 2n=6
discuss and demonstrate crossing over and recombination in meiosis
identify disorders and diseases that result from malfunctions in the cell during the cell cycle
DISCUSSION
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:
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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.
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.
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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
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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.
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Cell Cycle
Cell Division
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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.
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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.
Stages of Mitosis and Meiosis
A. 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 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.
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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.
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Cell Cycle
Cell Division
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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.
B. 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.
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Cell Cycle
Cell Division
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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.
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.
Meiosis
Mitosis
1. Requires two nuclear divisions
1. Requires one nuclear division
2. Chromosomes synapse and cross over
2. Chromosomes do not synapse nor cross over
3. Centromeres survive Anaphase I
3. Centromeres dissolve in mitotic anaphase
4. Halves chromosome number
4. Preserves chromosome number
5. Produces four daughter nuclei
5. Produces two daughter nuclei
6. Produces daughter cells genetically different 6. Produces daughter cells genetically identical to
from parent and each other
parent and to each other
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Cell Cycle
Cell Division
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7. Used only for sexual reproduction
7. Used for asexual reproduction and
growth
Table 1: Comparison of Mitosis and Meiosis
Meiosis I compared to Mitosis
Meiosis II compared to Mitosis
Meiosis I
Mitosis
Meiosis II
Mitosis
Prophase I
Prophase
Prophase II
Prophase
Pairing of homologous
chromosomes
Metaphase I
No pairing of
chromosomes
Metaphase
No pairing of
chromosomes
Metaphase II
No pairing of
chromosomes
Metaphase
Diploid number
of
duplicated
chromosomes at
metaphase plate
Anaphase
Sister chromatids
separate
becoming
daughter
chromosomes
that move to the
poles
Telophase
Bivalents at metaphase Duplicated
plate
chromosomes at
metaphase plate
Anaphase I
Anaphase
Haploid number
of
duplicated
chromosomes at
metaphase plate
Anaphase II
Homologues of each
bivalent separate and
duplicated
chromosomes move to
poles
Sister
chromatids
separate, becoming
daughter
chromosomes
that
move to the poles
Sister
chromatids
separate, becoming
daughter
chromosomes
that
move to the poles
Telophase I
Telophase
Telophase II
Two haploid daughter Two
diploid
cells not identical to daughter cells,
the parent cell
identical to the
parent cell
Table 2: Meiosis compared to Mitosis
Four
haploid Two
diploid
daughter cells not daughter
cells,
genetically identical identical to the
parent cell
Disorders and diseases that result from the malfunction of the cell during the cell cycle
• 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:
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arise from errors in meiosis, usually meiosis I;
occur more often during egg formation (90% of the time) than during sperm formation;
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Cell Cycle
Cell Division
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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
SUMMARY
GLOSSARY
anaphase
third stage of mitosis (and meiosis), during which sister chromatids separate into two new nuclear
regions of a dividing cell
cell cycle
life cycle of a single cell, from its birth until its division into two new daughter cells
centromere
region of attachment for two sister chromatids
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Cell Cycle
Cell Division
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centrosome
cellular structure that organizes microtubules during cell division
checkpoint
progress point in the cell cycle during which certain conditions must be met in order for the cell
to proceed to a subsequence phase
cleavage furrow
contractile ring that forms around a cell during cytokinesis that pinches the cell into two halves
cyclin
one of a group of proteins that function in the progression of the cell cycle
cyclin-dependent kinase (CDK)
one of a group of enzymes associated with cyclins that help them perform their functions
cytokinesis
final stage in cell division, where the cytoplasm divides to form two separate daughter cells
diploid
condition marked by the presence of a double complement of genetic material (two sets of
chromosomes, one set inherited from each of two parents)
G0 phase
phase of the cell cycle, usually entered from the G1 phase; characterized by long or permanent
periods where the cell does not move forward into the DNA synthesis phase
G1 phase
first phase of the cell cycle, after a new cell is born
G2 phase
third phase of the cell cycle, after the DNA synthesis phase
homologous
describes two copies of the same chromosome (not identical), one inherited from each parent
interphase
entire life cycle of a cell, excluding mitosis
kinetochore
region of a centromere where microtubules attach to a pair of sister chromatids
metaphase
second stage of mitosis (and meiosis), characterized by the linear alignment of sister chromatids
in the center of the cell
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Cell Division
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metaphase plate
linear alignment of sister chromatids in the center of the cell, which takes place during metaphase
mitosis
division of genetic material, during which the cell nucleus breaks down and two new, fully
functional, nuclei are formed
mitotic phase
phase of the cell cycle in which a cell undergoes mitosis
mitotic spindle
network of microtubules, originating from centrioles, that arranges and pulls apart chromosomes
during mitosis
prophase
first stage of mitosis (and meiosis), characterized by breakdown of the nuclear envelope and
condensing of the chromatin to form chromosomes
S phase
stage of the cell cycle during which DNA replication occurs
sister chromatid
one of a pair of identical chromosomes, formed during DNA replication
somatic cell
all cells of the body excluding gamete cells
telophase
final stage of mitosis (and meiosis), preceding cytokinesis, characterized by the formation of two
new daughter nuclei
EVALUATION
Multiple Choices:
1. The cytoplasmic division in plants must be facilitated by a mechanism called
a. cleavage furrow formation
b. spindle equator formation
c. spindle plate formation
d. cell plate formation
e. a & d
2. What is a major feature of prophase in mitosis?
a. Chromosomes condense
b. Crossing-over between homologous chromosomes
c. Nuclear envelope breaks up
d. a & c
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Cell Division
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e. All of the above
3. In which stage in interphase does the DNA replication take place?
a. G1
b. G2
c. S
d. G1 & S
e. G1 & G2
4. When all chromosomes line up at the spindle equator, it is referred as the
stage(s) of
a. Metaphase
b. Anaphase
c. Anaphase I
d. Anaphase II
e. None of the above
5. The cells that make up our body, and usually have a diploid chromosome number, are known as
a. Sex cells
b. Germ cells
c. Diploid cells
d. Prokaryotic cells
e. Somatic cells
6. Meiosis only occurs in
a. Somatic cells
b. Germ cells
c. Animal cells
d. Plant cells
e. Prokaryotic cells
7. Sister chromatids break apart in which of the following stages?
a. Metaphase
b. Anaphase
c. Anaphase I
d. Anaphase II
e. a & d
8. A different molecular form of the same gene is known as a/an
a. phenotype
b. genotype
c. allele
d. homologous pair
e. sister chromatids pair
9.Crossing- over can be found in the stage of
a. Prophase I
b. Prophase II
c. Interphase
d. Anaphase I
e. Anaphase II
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10. Which of the following organelles is/are important during the stage of metaphase?
a. Mitochondrion
b. Ribosome
c. Centrioles
d. Golgi Apparatus
e. All of the above
11. Which of the following process will result in a reduction of the chromosome number by half?
a. Mitosis
b. Meiosis
c. Cytoplasmic division
d. DNA replication
e. Fertilization
12. The gamete usually has a ___________ chromosome number.
a. haploid
b. diploid
c. triploid
d. tetraploid
e. polyploidy
13. Which of the following best describe the term “crossing over”?
a. An exchange of information between two sister chromatids
b. A molecular interaction between two sister chromatids
c. A molecular interaction between two non-sister chromatids
d. A separation of two sister chromatids
e. None of the above
14. There are DNA replications during the stage(s) of
a. interphase prior to mitosis
b. interphase prior to meiosis I
c. interphase between meiosis I and meiosis II
d. a & b
e. All of the above
15. Sexual reproduction requires all of the following except
a. meiosis
b. mitosis
c. gamete formation
d. fertilization
e. None of the above
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form without the consent of DGMHS is punishable by existing laws of the Republic of the Philippines.