Knowt
Campbell Chapter 12 - The Cell Cycle
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Key Concepts 12.1, 12.2, and 12.3
Cell division is the ability of organisms to produce more of their own kind
Rudolf Virchow's concept of "Omnis cellula e cellula"
Cell division plays important roles in reproduction, development, and renewal/repair
The cell cycle and its phases
The distribution of chromosomes to daughter cells during cell division
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Chapter 12: The Cell Cycle
The cell division process is part of the cell cycle
Passing identical genetic material to cellular offspring is a crucial function of cell division
Most cell division results in genetically identical daughter cells
DNA replication and distribution in cell division
Chromosomes and their structure
Characteristics of eukaryotic chromosomes
The number of chromosomes in different eukaryotic species
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Mitosis is the division of genetic material in the nucleus
Followed immediately by cytokinesis, the division of the cytoplasm
Results in two cells that are genetically equivalent to the parent cell
From a fertilized egg, mitosis and cytokinesis produce somatic cells
Somatic cells have 23 pairs of chromosomes, one set inherited from each parent
Reproductive cells, or gametes, have half as many chromosomes as somatic cells
Human gametes have one set of 23 chromosomes
The number of chromosomes in somatic cells varies among species
Distribution of Chromosomes During Eukaryotic Cell Division:
When a cell is not dividing, each chromosome is in the form of a long, thin chromatin fiber
After DNA replication, the chromosomes condense as part of cell division
Chromosomes become densely coiled and folded, visible with a light microscope
Each duplicated chromosome consists of two sister chromatids
Sister chromatids are joined copies of the original chromosome
Sister chromatids are attached all along their lengths by protein complexes called cohesins
Each sister chromatid has a centromere, a region where it is attached most closely to its sister chromatid
Later in the cell division process, sister chromatids separate and move into two new nuclei
Once separated, sister chromatids are considered individual chromosomes
This step essentially doubles the number of chromosomes during cell division
Figure 12.4:
Shows a highly condensed, duplicated human chromosome
Chromosome duplication and condensation occur during cell division
Each duplicated chromosome consists of two sister chromatids connected by sister chromatid cohesion
Sister chromatids contain a copy of the DNA molecule
Molecular and mechanical processes separate sister chromatids into two chromosomes and distribute them to daughter cells
Figure 12.5:
Illustrates chromosome duplication and distribution during cell division
Asks how many chromatid arms the chromosome in Figure 2 has
Shows the point where one chromosome becomes two
BioFlix® Animation: Chromosome Duplication:
Provides an animation of chromosome duplication during cell division
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The cell cycle consists of four phases: G1, S, G2, and M.
G1 is the growth phase where the cell continues to grow.
S is the synthesis phase where DNA replication occurs.
G2 is the phase where the cell completes preparations for cell division.
M is the phase where the cell divides.
The duration of each phase varies in different types of cells.
The M phase occupies less than 1 hour.
The S phase occupies 10-12 hours, about half the cycle.
The G1 and G2 phases together occupy the rest of the time.
Some cells in a multicellular organism divide infrequently or not at all and spend their time in G1 or G0 phase.
Mitosis is conventionally broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.
Cytokinesis completes the mitotic phase.
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The mitotic spindle, made of microtubules, plays a crucial role in mitosis.
Meiosis is a variation of cell division that produces gametes with half the number of chromosomes as the parent cell.
Meiosis in humans occurs in special cells in the ovaries or testes.
Fertilization fuses two gametes together and restores the chromosome number.
Mitosis conserves the chromosome number in every somatic cell nucleus of the new individual.
The remainder of the chapter focuses on mitosis and the cell cycle in eukaryotes.
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Mitosis is just one part of the cell cycle, which also includes interphase.
Interphase is a much longer stage that accounts for about 90% of the cell cycle.
Interphase can be divided into three phases: G1, S, and G2.
During interphase, the cell grows by producing proteins and cytoplasmic organelles.
Mitosis and cytokinesis make up the mitotic phase of the cell cycle.
Mitosis distributes the daughter chromosomes to daughter nuclei, while cytokinesis divides the cytoplasm to produce two daughter cells.
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Prophase is the first stage of mitosis where chromatin fibers condense into discrete chromosomes.
The nucleoli disappear, and the mitotic spindle begins to form.
Prometaphase is the stage where the nuclear envelope fragments, and microtubules invade the nuclear area.
The chromosomes become even more condensed, and kinetochores form at the centromere of each chromatid.
Microtubules attach to the kinetochores and jerk the chromosomes back and forth.
Nonkinetochore microtubules interact with those from the opposite pole of the spindle, lengthening the cell.
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Main ideas:
Metaphase: centrosomes at opposite poles, chromosomes at metaphase plate, kinetochores attached to microtubules
Anaphase: cohesin proteins cleaved, sister chromatids become independent chromosomes, chromosomes move towards opposite ends of the cell
Telophase and Cytokinesis: daughter nuclei form, nuclear envelopes arise, nucleoli reappear, chromosomes become less condensed, remaining spindle microtubules depolymerized, division of cytoplasm begins
Supporting details:
Metaphase:
Centrosomes at opposite poles of the cell
Chromosomes at the metaphase plate, with centromeres at the plate
Kinetochores of sister chromatids attached to kinetochore microtubules from opposite poles
Anaphase:
Shortest stage of mitosis
Cohesin proteins cleaved, allowing sister chromatids to separate
Daughter chromosomes move towards opposite ends of the cell as kinetochore microtubules shorten
Centromeres are pulled ahead of the arms, moving at a rate of about 1 µm/min
Cell elongates as nonkinetochore microtubules lengthen
By the end of anaphase, both ends of the cell have complete collections of chromosomes
Telophase and Cytokinesis:
Two daughter nuclei form in the cell
Nuclear envelopes form from fragments of the parent cell's nuclear envelope and other portions of the endomembrane system
Nucleoli reappear
Chromosomes become less condensed
Remaining spindle microtubules are depolymerized
Mitosis is complete, resulting in the division of one nucleus into two genetically identical nuclei
Cytokinesis begins, resulting in the division of the cytoplasm
In animal cells, cytokinesis involves the formation of a cleavage furrow
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Main ideas:
Structure and function of the spindle during anaphase
Spindle microtubules elongate and shorten
Centrosomes and centrioles in animal cells
Kinetochore microtubules and movement of chromosomes
Metaphase plate and interaction of microtubules
Supporting details:
Structure and function of the spindle during anaphase:
Spindle correlates well with its function during anaphase
Anaphase begins when cohesins holding sister chromatids are cleaved by separase
Chromatids become individual chromosomes that move towards opposite ends of the cell
Spindle microtubules elongate and shorten:
Spindle microtubules elongate by incorporating more subunits of the protein tubulin
Spindle microtubules shorten by losing subunits
Centrosomes and centrioles in animal cells:
Spindle microtubules assemble at the centrosome, a subcellular region that organizes the cell's microtubules
Centrosome contains a pair of centrioles, but they are not essential for cell division
Centrioles are not present in plant cells, which still form mitotic spindles
Kinetochore microtubules and movement of chromosomes:
Each sister chromatid of a duplicated chromosome has a kinetochore
Kinetochore microtubules attach to kinetochores during prometaphase
Chromosomes move towards the pole from which the microtubules extend
Movement stops when microtubules from the opposite pole attach to the kinetochore on the other chromatid
Chromosome moves back and forth before settling midway between the two ends of the cell
Metaphase plate and interaction of microtubules:
Metaphase plate is a plane midway between the spindle's two poles
Metaphase plate is an imaginary plate, not an actual cellular structure
Microtubules that do not attach to kinetochores elongate and overlap with other nonkinetochore microtubules from the opposite pole
Asters, radial arrays of short microtubules, extend from each centrosome
Page 8: The Cell Cycle
Kinetochore microtubules function in poleward movement of chromosomes
Two mechanisms involving motor proteins are at play
Motor proteins on kinetochores "walk" chromosomes along microtubules
Microtubules depolymerize at kinetochore ends after motor proteins pass
Chromosomes are also "reeled in" by motor proteins at spindle poles
Microtubules depolymerize after passing by motor proteins at poles
Both mechanisms are used, with varying contributions among cell types
Nonkinetochore microtubules elongate the whole cell during anaphase
Responsible for elongating the cell during anaphase
Nonkinetochore microtubules from opposite poles overlap during metaphase
Motor proteins attached to microtubules walk them away from each other
Microtubules push apart, elongating the cell
Microtubules lengthen by the addition of tubulin subunits to their ends
Duplicate groups of chromosomes arrive at opposite ends of the cell
Cytokinesis in animal cells occurs through cleavage
Cleavage furrow appears near the old metaphase plate
Contractile ring of actin microfilaments and myosin molecules forms
Actin microfilaments interact with myosin, causing the ring to contract
Cleavage furrow deepens until the parent cell is pinched in two
Produces two completely separated cells with their own nucleus and cytosol
Cytokinesis in plant cells involves the formation of a cell plate
No cleavage furrow
Vesicles from the Golgi apparatus move to the middle of the cell
Vesicles coalesce to form a cell plate
Cell wall materials are deposited along the cell plate
Eventually, the cell plate becomes the new cell wall
Page 9: Inquiry - At which end do kinetochore microtubules shorten during anaphase?
Experiment by Gary Borisy and colleagues
Labeled microtubules of a pig kidney cell in early anaphase with a yellow fluorescent dye
Marked a region of the kinetochore microtubules between one spindle pole and the chromosomes
Used a laser to eliminate fluorescence from the marked region
Monitored changes in microtubule length on either side of the mark
Results of the experiment
Microtubule segments on the kinetochore side of the mark shortened
Microtubule segments on the spindle pole side stayed the same length
Conclusion of the experiment
Chromosome movement is correlated with kinetochore microtubules shortening at their kinetochore ends
Microtubules depolymerize at kinetochore ends, releasing tubulin subunits
Supports the hypothesis that chromosomes are walked along microtubules during anaphase
Page 9: Animation - Microtubule Depolymerization
Animation demonstrating the process of microtubule depolymerization during anaphase
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Cytokinesis in animal and plant cells
Animal cells: cleavage of the cell through a cleavage furrow
Plant cells: formation of a cell plate
Vesicles carrying cell wall material collect inside the cell plate
Cell plate enlarges and fuses with the plasma membrane
Two daughter cells are formed, each with its own plasma membrane
A new cell wall forms between the daughter cells
Binary fission in bacteria
Bacteria and archaea can undergo binary fission
Binary fission refers to the process of cell division in half
Bacterial binary fission does not involve mitosis
Bacterial chromosomes are circular DNA molecules
Replication of bacterial chromosomes and distribution to daughter cells is a challenge
DNA replication starts at the origin of replication
One origin moves towards the opposite end of the cell as replication continues
Cell elongates during replication
When replication is complete and the cell has doubled in size, the plasma membrane pinches inward, dividing the cell into two daughter cells
Bacterial chromosomes move similar to the movement of centromeres in eukaryotic chromosomes during anaphase of mitosis
Proteins play important roles in bacterial chromosome movement and cell division
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The evolution of mitosis
Mitosis may have evolved from simpler prokaryotic mechanisms of cell reproduction
Proteins involved in bacterial binary fission are related to eukaryotic proteins involved in mitosis
Variations of cell division exist in different organisms
Unicellular eukaryotes such as dinoflagellates, diatoms, and some yeasts have nuclear division processes that resemble ancestral mechanisms
Nuclear envelope remains intact during these types of nuclear division
Hypothesis uses currently existing species as examples and ignores potential intermediate mechanisms used by extinct species
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Question 1: How many chromosomes are shown in the illustration in Figure 12.8? Are they duplicated? How many chromatids are shown?
The illustration in Figure 12.8 shows 6 chromosomes.
The chromosomes are duplicated, as each chromosome consists of two identical chromatids.
Question 2: Compare cytokinesis in animal cells and plant cells.
In animal cells, cytokinesis occurs through the formation of a cleavage furrow, which pinches the cell membrane inward until the cell is divided into two daughter cells.
In plant cells, cytokinesis occurs through the formation of a cell plate, which is made up of vesicles containing cell wall material. The cell plate grows outward until it fuses with the existing cell wall, dividing the cell into two daughter cells.
Question 3: During which stages of the cell cycle does a chromosome consist of two identical chromatids?
A chromosome consists of two identical chromatids during the S phase of the cell cycle, when DNA replication occurs.
Question 4: Compare the roles of tubulin and actin during eukaryotic cell division with the roles of tubulin-like and actin-like proteins during bacterial binary fission.
Tubulin and actin play important roles in eukaryotic cell division. Tubulin forms microtubules, which are involved in the formation of the mitotic spindle and the separation of chromosomes. Actin is involved in cytokinesis, helping to form the cleavage furrow or cell plate.
In bacterial binary fission, tubulin-like and actin-like proteins are involved in a similar manner. They polymerize to form filaments that help in the separation of daughter chromosomes and the division of the bacterial cell.
Question 5: A kinetochore has been compared to a coupling device that connects a motor to the cargo that it moves. Explain.
A kinetochore is a protein structure located on the centromere of a chromosome. It serves as a connection point for microtubules of the mitotic spindle, similar to how a coupling device connects a motor to the cargo it moves. The kinetochore helps to move and align the chromosomes during cell division.
Question 6: What other functions do actin and tubulin carry out? Name the proteins they interact with to do so.
Actin and tubulin have other functions in addition to their roles in cell division.
Actin interacts with myosin to generate muscle contractions and is involved in cell movement and shape changes.
Tubulin interacts with various proteins to form structures like cilia and flagella, which are involved in cell motility.
Concept 12.3 The eukaryotic cell cycle is regulated by a molecular control system
The timing and rate of cell division in different parts of a plant or animal are crucial to normal growth, development, and maintenance.
The frequency of cell division varies with the type of cell.
Regulation at the molecular level controls cell cycle differences in different cell types.
Understanding the mechanisms of cell cycle regulation is important for understanding normal cell cycles and cancer cell behavior.
The Cell Cycle Control System
The cell cycle is driven by specific signaling molecules present in the cytoplasm.
Experiments with mammalian cells grown in culture provided evidence for the presence of signaling molecules.
Fusion experiments showed that the cell cycle events can be triggered in a cell by the cytoplasm of another cell in a different phase of the cell cycle.
The cell cycle control system is a cyclically operating set of molecules that directs the sequential events of the cell cycle.
Mechanisms of cell division in several groups of organisms
Different organisms have different mechanisms of cell division.
Bacteria use binary fission, where the daughter chromosomes move to opposite ends of the cell.
Diatoms and some yeasts have a spindle formed within the nucleus, and the nuclear envelope remains intact during cell division.
Most eukaryotes, including plants and animals, have a spindle that forms outside the nucleus, and the nuclear envelope breaks down during mitosis.
Dinoflagellates have chromosomes that attach to the intact nuclear envelope, and microtubules pass through the nucleus inside cytoplasmic tunnels.
Video: Nuclear Envelope Breakdown and Formation During Mitosis in C. elegans, a Eukaryote
The video demonstrates the breakdown and formation of the nuclear envelope during mitosis in the eukaryote C. elegans.
Page 12: The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases
Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the sequential events of the cell cycle.
Regulatory molecules are mainly proteins of two types: protein kinases and cyclins.
Protein kinases activate or inactivate other proteins by phosphorylating them.
Cyclins are proteins that fluctuate in concentration throughout the cell cycle.
Cyclin-dependent kinases (Cdks) are kinases that are attached to cyclins to be active.
The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner.
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers the cell's passage into the M phase.
Cell Cycle Checkpoints
The cell cycle control system has checkpoints where stop and go-ahead signals can regulate the cycle.
Three important checkpoints are found in the G1, G2, and M phases.
The cell cycle is regulated at these checkpoints by both internal and external signals.
Page 15: Inquiry Do molecular signals in the cytoplasm regulate the cell cycle? experiment
Researchers at the University of Colorado conducted an experiment to investigate whether cytoplasmic molecules control the progression of the cell cycle.
They induced cultured mammalian cells at different phases of the cell cycle to fuse.
When a cell in the S phase was fused with a cell in G1, the G1 nucleus immediately entered the S phase and DNA was synthesized.
When a cell in the M phase was fused with a cell in G1, the G1 nucleus immediately began mitosis, even though the chromosomes had not been duplicated.
Conclusion
The results suggest that molecules present in the cytoplasm during the S or M phase control the progression to those phases.
Animation: Control of the Cell Cycle
The animation provides a visual representation of the control of the cell cycle.
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MPF helps switch itself off during anaphase by initiating a process that leads to the destruction of its own cyclin
The noncyclin part of MPF, the Cdk, persists in the cell, inactive until it becomes part of MPF again by associating with new cyclin molecules synthesized during the S and G2 phases of the next round of the cycle
The fluctuating activities of different cyclin-Cdk complexes are important in controlling all stages of the cell cycle and give go-ahead signals at some checkpoints
MPF controls the cell's passage through the G2 checkpoint
The activity of cyclin-Cdk protein complexes regulates cell behavior at the G1 checkpoint
Animal cells have at least three Cdk proteins and several different cyclins that operate at the G1 checkpoint
Stop and Go Signs: Internal and External Signals at the Checkpoints:
Animal cells have built-in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals
Signals at checkpoints come from cellular surveillance mechanisms inside the cell and from outside the cell
Three important checkpoints are the G1, G2, and M phases
The G1 checkpoint is the most important, if a cell receives a go-ahead signal at this checkpoint, it will usually complete the cell cycle and divide
If a cell does not receive a go-ahead signal at the G1 checkpoint, it may exit the cycle and enter the G0 phase
Most cells of the human body are in the G0 phase
Mature nerve cells and muscle cells never divide, while other cells like liver cells can be "called back" from the G0 phase to the cell cycle by external cues
Molecular mechanisms that help regulate the cell cycle:
The pathways that link signals originating inside and outside the cell with the responses by cyclin-dependent kinases and other proteins are still being studied
An internal signal occurs at the M checkpoint, where anaphase does not begin until all the chromosomes are properly attached to the spindle
Unattached kinetochores to spindle microtubules delay anaphase, and only when all kinetochores are properly attached does the appropriate regulatory protein complex become activated
MPF acts as a kinase and indirectly activates other kinases
MPF causes phosphorylation of various proteins of the nuclear lamina, promoting fragmentation of the nuclear envelope during prometaphase
MPF contributes to molecular events required for chromosome condensation and spindle formation during prophase
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External factors can influence cell division
Cells fail to divide if an essential nutrient is lacking in the culture medium
Most types of mammalian cells divide in culture only if the growth medium includes specific growth factors
Platelet-derived growth factor (PDGF) stimulates fibroblast division
PDGF molecules bind to receptor tyrosine kinases on the plasma membranes of cells
This triggers a signal transduction pathway that allows cells to pass the G1 checkpoint and divide
PDGF is released by platelets in the vicinity of an injury, promoting fibroblast proliferation to help heal the wound
Density-dependent inhibition and anchorage dependence regulate cell division
Crowded cells stop dividing (density-dependent inhibition)
Cells must be attached to a substratum to divide (anchorage dependence)
Cell-surface protein binding between adjacent cells sends a signal that inhibits cell division
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Cancer cells exhibit loss of cell cycle controls
They do not stop dividing when growth factors are depleted
Cancer cells can go on dividing indefinitely and do not exhibit density-dependent inhibition or anchorage dependence
Differences between normal cells and cancer cells
Cancer cells stop dividing at random points in the cell cycle, rather than at the normal checkpoints
Cancer cells may have abnormalities in the signaling pathway or the cell cycle control system
Mutations in genes can alter the function of protein products, resulting in faulty cell cycle control
Page 16: The Cell Cycle and Malignant Tumors
Malignant tumors show changes in cells beyond excessive proliferation
Unusual numbers of chromosomes may be present
Debate on whether this is a cause or effect of tumor-related changes
Altered metabolism and loss of constructive function
Abnormal changes on cell surface cause cancer cells to lose attachments to neighboring cells and extracellular matrix
Spread of cancer cells into nearby tissues
Secretion of signaling molecules that cause blood vessels to grow toward the tumor
Some tumor cells separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body
Formation of new tumors in other parts of the body is called metastasis
Treatment options for localized tumors and metastatic tumors
Localized tumors can be treated with high-energy radiation that damages DNA in cancer cells more than normal cells
Majority of cancer cells have lost the ability to repair DNA damage
Metastatic tumors are treated with chemotherapy
Chemotherapeutic drugs interfere with specific steps in the cell cycle
Example: Taxol freezes the mitotic spindle, preventing cell division and leading to destruction of actively dividing cells
Side effects of chemotherapy due to effects on normal cells that divide frequently
Nausea, hair loss, susceptibility to infection are common side effects
Cancer Cell Behavior and Metastasis
Cancer cells invade neighboring tissue
Tumor grows from a single cancer cell
Cancer cells spread through lymph and blood vessels to other parts of the body
A small percentage of cancer cells may metastasize to another part of the body
Figure 12.20: Growth and Metastasis of a Malignant Breast Tumor
Genetic and cellular changes contribute to a tumor becoming malignant
Malignant tumor cells grow uncontrollably and can spread to neighboring tissues and other parts of the body
Metastasis is the spread of cancer cells beyond their original site
Cell Transformation and Abnormal Cell Behavior
Cells in culture can divide indefinitely if given a continual supply of nutrients
Example: HeLa cells, derived from a tumor removed from Henrietta Lacks in 1951
Transformation causes cells to behave like cancer cells
Normal mammalian cells in culture divide only about 20 to 50 times before stopping and dying
Cancer cells evade normal controls that trigger apoptosis when something is wrong
Abnormal cell behavior can be catastrophic in the body
Benign Tumors vs Malignant Tumors
Single cell in tissue undergoes steps to convert to a cancer cell
Immune system recognizes and destroys abnormal cells, but some may evade destruction
Proliferation of abnormal cells leads to the formation of a tumor
Benign tumors remain at the original site and can be removed by surgery
Malignant tumors include cells that can spread to new tissues and impair organ functions
Malignant tumors are also called transformed cells
Individual with a malignant tumor is said to have cancer
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Medical treatments for cancer aim to block the cell cycle of cancerous tumor cells
Cell cycle inhibitors derived from human umbilical cord stem cells can be used as potential treatments
A study was conducted using human glioblastoma cells to determine where in the cell cycle the inhibitor blocks the division of cancer cells
The cells were treated with a fluorescent chemical and run through a flow cytometer to analyze the DNA content
The data was plotted in histograms to compare the control sample and the treated sample
Interpreting Histograms
The histograms show the distribution of cells based on the amount of fluorescence, which indirectly represents the relative amount of DNA per cell
The x-axis represents the amount of fluorescence per cell
In the control sample histogram:
The first peak (region A) represents cells with a lower amount of DNA per cell
The second peak (region C) represents cells with a higher amount of DNA per cell
In the control sample histogram:
The population of cells in region A is in the G1 phase of the cell cycle
The population of cells in region C is in the G2 phase of the cell cycle
The S phase population of cells does not show a distinct peak in the histogram
The treated sample histogram shows the effect of growing cancer cells alongside human umbilical cord stem cells that produce the potential inhibitor
The histogram should be labeled with the cell cycle phases
The phase with the greatest number of cells in the treated sample should be identified
The distribution of cells among G1, S, and G2 phases in the control and treated samples should be compared
A mechanism by which the stem cell-derived inhibitor might arrest the cancer cell cycle at a specific stage should be proposed
Concept Check 12.3
Figure 12.14 shows nuclei resulting from experiment 2 containing different amounts of DNA because different treatments were applied to the cells, affecting DNA replication.
MpF allows a cell to pass the G2 phase checkpoint and enter mitosis by activating cyclin-dependent kinases (CDKs) that phosphorylate target proteins involved in mitosis.
Receptor tyrosine kinases and intracellular receptors can function in triggering cell division by activating signaling pathways that lead to cell cycle progression.
Interview with Bruce Alberts: Cancer control and careers in science
Bruce Alberts discusses the importance of understanding the molecular basis of cancer for developing effective treatments
He emphasizes the need for interdisciplinary collaboration in cancer research
Alberts highlights the role of basic research in uncovering fundamental mechanisms of cancer
BBC Video: Are Fruit Flies the key in the Fight Against Cancer?
The video explores how fruit flies are used as model organisms in cancer research
Fruit flies share many genes and biological processes with humans, making them valuable for studying cancer
Researchers use fruit flies to identify genes involved in cancer development and test potential treatments
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The mitotic spindle, made up of microtubules, controls chromosome movement during mitosis.
In animal cells, it arises from the centrosomes and includes spindle microtubules and asters.
Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate.
After sister chromatids separate, motor proteins move them along kinetochore microtubules toward opposite ends of the cell.
The cell elongates when motor proteins push nonkinetochore microtubules from opposite poles away from each other.
Mitosis is usually followed by cytokinesis.
Animal cells carry out cytokinesis by cleavage.
Plant cells form a cell plate.
During binary fission in bacteria, the chromosome replicates and the daughter chromosomes actively move apart.
Some of the proteins involved in bacterial binary fission are related to eukaryotic actin and tubulin.
Mitosis likely evolved from prokaryotic cell division.
Certain unicellular eukaryotes exhibit mechanisms of cell division that may be similar to those of ancestors of existing eukaryotes.
Such mechanisms might represent intermediate steps in the evolution of mitosis.
Chromosomes exist as single DNA molecules in the S phase of interphase and the stages of mitosis.
ConCept 12.3 the eukaryotic cell cycle is regulated by a molecular control system (pp. 244–250):
Signaling molecules present in the cytoplasm regulate progress through the cell cycle.
The cell cycle control system is molecularly based.
Cyclic changes in regulatory proteins work as a cell cycle clock.
The key molecules are cyclins and cyclin-dependent kinases (Cdks).
The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received.
Important checkpoints occur in G1, G2, and M phases.
Both internal signals and external signals control the cell cycle checkpoints via signal transduction pathways.
Most cells exhibit density-dependent inhibition of cell division as well as anchorage dependence.
Cancer cells elude normal cell cycle regulation and divide unchecked, forming tumors.
Malignant tumors invade nearby tissues and can undergo metastasis, exporting cancer cells to other sites, where they may form secondary tumors.
Recent cell cycle and cell signaling research, and new techniques for sequencing DNA, have led to improved cancer treatments.
test youR unDeRstAnDInG level 1: Knowledge/Comprehension:
Through a microscope, you can see a cell plate beginning to develop across the middle of a cell and nuclei forming on either side of the cell plate.
This cell is most likely a plant cell in the process of cytokinesis.
suMMARy oF Key ConCepts:
Unicellular organisms reproduce by cell division; multicellular organisms depend on cell division for their development from a fertilized egg and for growth and repair.
Cell division is part of the cell cycle, an ordered sequence of events in the life of a cell.
ConCept 12.1 Most cell division results in genetically identical daughter cells (pp. 235–237):
The genetic material (DNA) of a cell—its genome—is partitioned among chromosomes.
Each eukaryotic chromosome consists of one DNA molecule associated with many proteins.
Together, the complex of DNA and associated proteins is called chromatin.
The chromatin of a chromosome exists in different states of condensation at different times.
In animals, gametes have one set of chromosomes and somatic cells have two sets.
Cells replicate their genetic material before they divide, each daughter cell receiving a copy of the DNA.
Prior to cell division, chromosomes are duplicated.
Each one then consists of two identical sister chromatids joined along their lengths by sister chromatid cohesion and held most tightly together at a constricted region at the centromeres.
When this cohesion is broken, the chromatids separate during cell division, becoming the chromosomes of the daughter cells.
Eukaryotic cell division consists of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).
ConCept 12.2 the mitotic phase alternates with interphase in the cell cycle (pp. 237–244):
Between divisions, a cell is in interphase: the G1, S, and G2 phases.
The cell grows throughout interphase, with DNA being replicated only during the synthesis (S) phase.
Mitosis and cytokinesis make up the mitotic (M) phase