The Cell Cycle and Cell Division Ch 7. Chapter 7 The Cell Cycle and Cell Division Key Concepts 7.1 Different Life Cycles Use Different Modes of Cell Reproduction

The Cell Cycle and Cell Division

Ch 7

Chapter 7 The Cell Cycle and Cell Division

• Key Concepts• 7.1 Different Life Cycles Use Different

Modes of Cell Reproduction• 7.2 Both Binary Fission and Mitosis

Produce Genetically Identical Cells• 7.3 Cell Reproduction Is Under Precise

Control


• 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

• 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Chapter 7 Opening Question

How does infection with HPV result in uncontrolled cell reproduction?

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction• The lifespan of an organism is linked to

cell reproduction, or cell division: a parent cell duplicates its genetic material and then divides into two similar cells.

• Cell division is important in growth and repair of multicellular organisms and the reproduction of all organisms.

Figure 7.1 The Importance of Cell Division

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction

• Organisms have two basic strategies for reproducing themselves:

– Asexual reproduction

– Sexual reproduction


• Asexual reproduction

– The offspring are clones—genetically identical to the parent

– Any genetic variations are due to mutations (changes in DNA sequences due to environmental factors or copying errors)


• Single-celled prokaryotes usually reproduce by binary fission

• Single-celled eukaryotes can reproduce by mitosis and cytokinesis

• Many multicellular eukaryotes can also reproduce by asexual means

Figure 7.2 Asexual Reproduction on a Large Scale-Aspens in Utah


• Sexual reproduction

– Involves fusion of gametes

– Results in offspring with genetic variation

– Gametes form by meiosis—a process of cell division that reduces genetic material by half


• DNA in eukaryotic cells is organized into chromosomes.

• Somatic cells: body cells not specialized for reproduction

• Each somatic cell contains two sets of chromosomes that occur in homologous pairs.– One homolog came from the female

parent and one from the male parent and have corresponding genetic information.

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction• Gametes have only one set of

chromosomes—one homolog from each pair.

– They are haploid; number of chromosomes = n

• Fertilization: two haploid gametes fuse to form a zygote

– They are diploid; number of chromosome in zygote = 2n

Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction• All sexual life cycles involve meiosis:

– Gametes may develop immediately after meiosis

– Or each haploid cell may develop into a haploid organism (haploid stage of the life cycle) that eventually produces gametes by mitosis

• Fertilization results in a zygote and begins the diploid stage of the life cycle.

Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)




• The essence of sexual reproduction is:

– Random selection of half the diploid chromosome set to form a haploid gamete

– Followed by fusion of haploid gametes from separate parents to make a diploid cell

• This results in shuffling of genetic information in a population, and no two individuals have exactly the same genetic makeup.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells

• Four events in cell division:

• Reproductive signals initiate cell division

• DNA replication

• DNA segregation—distribution of the DNA into the two new cells

• Cytokinesis—division of the cytoplasm and separation of the two new cells

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Prokaryotes divide by binary fission:

results in reproduction of the entire organism.

• Reproductive signals may be environmental factors such as nutrient availability.


• Replication:

• Most prokaryotes have one circular chromosome with two important regions:

– ori—where replication starts

– ter—where replication ends

• Replication occurs as the DNA is threaded through a “replication complex” of proteins at the center of the cell.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Segregation:

• As replication proceeds, the ori complexes move to opposite ends of the cell.

• DNA sequences adjacent to the ori region actively bind proteins for the segregation, using ATP.

• An actin-like protein provides a filament along which ori and other proteins move.

Figure 7.4 Prokaryotic Cell Division: Binary Fission

Cytokinesis

• Cytokinesis:

• After chromosome segregation, the cell membrane pinches in by contraction of a ring of protein fibers under the surface.

• As the membrane pinches in, new cell wall materials are deposited, resulting in separation of the two cells.

Cell Division related to entire organism function

• Eukaryotic cells divide by mitosis followed by cytokinesis.

– Reproductive signals are usually related to functions of the entire organism, not the environment of a single cell.

– Most cells in a multicellular organism are specialized and do not divide.


– Replication of each chromosome occurs as they are threaded through replication complexes.

• DNA replication only occurs during a specific stage of the cell cycle.


– In segregation, one copy of each chromosome ends up in each of the two new cells.

• More complex than in prokaryotes: eukaryotes have a nuclear envelope, and there are multiple chromosomes.

– Cytokinesis in plant cells (which have cell walls) is different than in animal cells (no cell walls).


• In mitosis, one nucleus produces two daughter nuclei, each containing the same number of chromosomes as the parent nucleus.

• Mitosis is continuous, but it is convenient to subdivide it into phases.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 12.11

Evidence of development of mitosis from binary fission


• The cell cycle is the period from one cell division to the next, divided into stages in eukaryotes.

• M phase: Mitosis (segregation of chromosomes into two new nuclei), followed by cytokinesis.

• Interphase: cell nucleus is visible and cell functions occur, including DNA replication.

• The cell cycle appears to be driven by specific chemical signals in the cytoplasm.– Fusion of an S phase and a G1 phase cell,

induces the G1 nucleus to start S phase.– Fusion of a cell in mitosis with one in

interphase induces the second cell to enter mitosis.

A molecular control system drives the cell cycle

Fig. 12.12

• The distinct events of the cell cycle are directed by a distinct cell cycle control system.– These molecules trigger and coordinate key

events in the cell cycle. – The control cycle has

a built-in clock, but it is also regulated by external adjustments and internal controls.

Fig. 12.13

• A checkpoint in the cell cycle is a critical control point where stop and go signals regulate the cycle.– Many signals registered at checkpoints

come from cellular surveillance mechanisms .

– These indicate whether key cellular processes have been completed correctly.

– Checkpoint also register signals from outside the cell.

• Three major checkpoints are found in the G1, G2, and M phases.


• For many cells, the G1 checkpoint, the restriction point in mammalian cells, is the most important.– If the cells receives a go-ahead signal, it

usually completes the cell cycle and divides.

– If it does not receive a go-ahead signal, the cell exits the cycle and switches to a nondividing state, the G0 phase.• Most human cells are in this phase.• Liver cells can be “called back” to the cell cycle

by external cues (growth factors), but highly specialized nerve and muscle cells never divide.

• Rhythmic fluctuations in the abundance and activity of control molecules pace the cell cycle.– Some molecules are protein kinases that

activate or deactivate other proteins by phosphorylating them.

• The levels of these kinases are present in constant amounts, but these kinases require a second protein, a cyclin, to become activated.– Level of cyclin proteins fluctuate cyclically.– The complex of kinases and cyclin forms

cyclin-dependent kinases (Cdks).

• Cyclin levels rise sharply throughout interphase, then fall abruptly during mitosis.

• Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration.

Fig. 12.14a

• MPF (“maturation-promoting factor” or “M-phase-promoting-factor”) triggers the cell’s passage past the G2 checkpoint to the M phase.– MPF promotes mitosis by phosphorylating

a variety of other protein kinases.– MPF stimulates

fragmentation of the nuclear envelope.

– It also triggers the breakdown of cyclin, dropping cyclin and MPF levels during mitosis and inactivating MPF. Fig. 12.14b

• The key G1 checkpoint is regulated by at least three Cdk proteins and several cyclins.

• Similar mechanisms are also involved in driving the cell cycle past the M phase checkpoint.

• While research scientists know that active Cdks function by phosphorylating proteins, the identity of all these proteins is still under investigation.

• Scientists do not yet know what Cdks actually do in most cases.

• Some steps in the signaling pathways that regulate the cell cycle are clear.– Some signals originate inside the cell, others

outside.

Internal and external cues help regulate the cell cycle

• The M phase checkpoint ensures that all the chromosomes are properly attached to the spindle at the metaphase plate before anaphase.– This ensures that daughter cells do not end

up with missing or extra chromosomes.• A signal to delay anaphase originates at

kinetochores that have not yet attached to spindle microtubules.– This keeps the anaphase-promoting

complex (APC) in an inactive state.– When all kinetochores are attached, the

APC activates, triggering breakdown of cyclin and inactivation of proteins uniting sister chromatids together.

• A variety of external chemical and physical factors can influence cell division.

• Particularly important for mammalian cells are growth factors, proteins released by one group of cells that stimulate other cells to divide.– For example, platelet-derived growth factors

(PDGF), produced by platelet blood cells, bind to tyrosine-kinase receptors of fibroblasts, a type of connective tissue cell.

– This triggers a signal-transduction pathway that leads to cell division.

• Each cell type probably responds specifically to a certain growth factor or combination of factors.

• The role of PDGF is easily seen in cell culture.– Fibroblasts in culture will only divide in the

presence of medium that also contains PDGF.

Fig. 12.15

• In a living organism, platelets release PDGF in the vicinity of an injury.

• The resulting proliferation of fibroblasts help heal the wound.

• Growth factors appear to be a key in density-dependent inhibition of cell division.– Cultured cells normally

divide until they form a single layer on the inner surface of the culture container.

– If a gap is created, the cells will grow to fill the gap.

– At high densities, the amount of growth factors and nutrients is insuffi-cient to allow continued cell growth.

Fig. 12.16a

• Most animal cells also exhibit anchorage dependence for cell division.– To divide they must be anchored to a

substratum, typically the extracellular matrix of a tissue.

– Control appears to be mediated by connections between the extracellular matrix and plasma membrane proteins and cytoskeletal elements.

• Cancer cells are free of both density-dependent inhibition and anchorage dependence.

Fig. 12.16b

• Cancer cells divide excessively and invade other tissues because they are free of the body’s control mechanisms. – Cancer cells do not stop dividing when

growth factors are depleted either because they manufacture their own, have an abnormality in the signaling pathway, or have a problem in the cell cycle control system.

• If and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle.

Cancer cells have escaped from cell cycle controls

• Cancer cell may divide indefinitely if they have a continual supply of nutrients.– In contrast, nearly all mammalian cells

divide 20 to 50 times under culture conditions before they stop, age, and die.

– Cancer cells may be “immortal”.• Cells (HeLa) from a tumor removed from a

woman (Henrietta Lacks) in 1951 are still reproducing in culture.


• The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell.– Normally, the immune system recognizes

and destroys transformed cells.– However, cells that evade destruction

proliferate to form a tumor, a mass of abnormal cells.

• If the abnormal cells remain at the originating site, the lump is called a benign tumor. – Most do not cause serious problems and

can be removed by surgery.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• In a malignant tumor, the cells leave the original site to impair the functions of one or more organs.– This typically fits the colloquial definition of

cancer.– In addition to chromosomal and metabolic

abnormalities, cancer cells often lose attachment to nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors in a event called metastasis.



Fig. 12.17

• Treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs.– These treatments target actively dividing

cells.• Researchers are beginning to

understand how a normal cell is transformed into a cancer cell.– The causes are diverse.– However, cellular transformation always

involves the alteration of genes that influence the cell cycle control system.


Figure 7.5 The Phases of the Eukaryotic Cell Cycle


Interphase has three subphases:

• G1 (Gap 1)—variable, may last a long time

• S phase (synthesis)—DNA is replicated

• G2 (Gap 2)—the cell prepares for mitosis; synthesizes microtubules for segregating chromosomes


• Prophase: three structures appear

– Condensed chromosomes

– Reoriented centrosomes

– Spindle


• Even during interphase, DNA is packaged by winding around specific proteins, and other proteins coat the DNA coils.

• In prophase, the chromosomes become much more tightly coiled and condensed.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• After replication, each chromosome has

two DNA molecules called sister chromatids, joined at a region called the centromere.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Karyotype: the condensed chromosomes

for a given organism can be distinguished by their sizes and centromere positions


• Karyotype analysis was used to identify and classify organisms, but DNA sequencing is more commonly used today.

• Karotype analysis is still used to identify chromosome abnormalities.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• The centrosome determines orientation of

the spindle.– Consists of two centrioles—hollow

tubes formed by microtubules.• The centrosome is duplicated during S

phase; centrosomes move towards opposite sides of the nucleus at the G2–M transition.

• Centrosome position determines the plane of cell division—important in the development of multicellular organisms.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Centrosomes serve as poles toward which

the chromosomes move.

• The spindle forms between the poles from microtubules:

– Polar microtubules overlap in the middle region of the cell and keep the poles apart.

– Astral microtubules interact with proteins attached to the cell membrane; also help keep the poles apart.


– Kinetochore microtubules attach to kinetochores on the chromatid centromeres. • Sister chromatids attach to

kinetochore microtubules from opposite sides so that the two chromatids will move to opposite poles.

• Sister chromatids become daughter chromatids after separation.


• Prometaphase: the nuclear envelope breaks down and chromatids attach to the kinetochore microtubules.

• Metaphase: the chromosomes line up at the midline of the cell.

• Anaphase: the chromatids separate, and daughter chromosomes move toward the poles.

Figure 7.6 The Phases of Mitosis (1)

Figure 7.6 The Phases of Mitosis (2)

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Two mechanisms move the chromosomes

to opposite poles:

– Kinetochores have molecular motor proteins (kinesin and dynein), which move the chromosomes along the microtubules.

– The kinetochore microtubules shorten from the poles, drawing the chromosomes toward the poles.

• Mitosis is a continuum of changes.– For description, mitosis is usually broken

into five subphases: • prophase, • prometaphase, • metaphase, • anaphase, and • telophase.


• By late interphase, the chromosomes have been duplicated but are loosely packed.

• The centrosomes have been duplicated and begin to organize microtubules into an aster (“star”).


Fig. 12.5a

• In prophase, the chromosomes are tightly coiled, with sister chromatids joined together.

• The nucleoli disappear.• The mitotic spindle begins

to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell.


Fig. 12.5b

• During prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes.

• Microtubules from one pole attach to one of two kinetochores, special regions of the centromere, while microtubules from the other pole attach to the other kinetochore.


Fig. 12.5c

• The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase.


Fig. 12.5d

• At anaphase, the centromeres divide, separating the sister chromatids.

• Each is now pulled toward the pole to which it is attached by spindle fibers.

• By the end, the two poles have equivalent collections of chromosomes.


Fig. 12.5e

• At telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other.

• Two nuclei begin for form, surrounded by the fragments of the parent’s nuclear envelope.

• Chromatin becomes less tightly coiled.

• Cytokinesis, division of the cytoplasm, begins.


Fig. 12.5f

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Telophase: nuclear envelopes form around

each set of chromosomes and nucleoli appear, and the spindle breaks down and chromosomes become less compact.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Cytokinesis:

• In animal cells, the cell membrane pinches in between the nuclei.

• A contractile ring of actin and myosin microfilaments forms on the inner surface of the cell membrane; the two proteins produce a contraction to pinch the cell in two.

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• In plant cells, vesicles from the Golgi

apparatus appear along the plane of cell division.

• The vesicles fuse to form a new cell membrane.

• Contents of vesicles also contribute to forming the cell plate—the beginning of the new cell wall.

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• After cytokinesis, each daughter cell

contains all of the components of a complete cell.

• Chromosomes are precisely distributed.

• The orientation of cell division is important to development, but there does not appear to be a precise mechanism for distribution of the cytoplasmic contents.

Table 7.1

Concept 7.3 Cell Reproduction Is Under Precise Control• Cell reproduction must be under precise

control.

• If single-celled organisms had no control over reproduction, they would soon overrun the environment and starve to death.

• In multicellular organisms, cell reproduction must be controlled to maintain body form and function.

Concept 7.3 Cell Reproduction Is Under Precise Control• Prokaryotic cells divide in response to

environmental conditions.

• In eukaryotes, cell division is related to the needs of the entire organism.

• Mammals produce growth factors that stimulate cell division and differentiation.

– Example: platelets in the blood secrete growth factors that stimulate cells to divide to heal wounds.

Concept 7.3 Cell Reproduction Is Under Precise Control• Progression through the eukaryotic cell

cycle is tightly regulated.

• The G1–S transition is called R, the restriction point.

• Passing this point usually means the cell will proceed with the cell cycle and divide.

Figure 7.8 The Eukaryotic Cell Cycle

Concept 7.3 Cell Reproduction Is Under Precise Control• Specific substances trigger the transition

from one phase to another.

• The first evidence for these substances came from cell fusion experiments.

• Fusion of mammalian cells at G1 and S phases showed that a cell in S phase produces a substance that activates DNA replication.

Figure 7.9 Regulation of the Cell Cycle (Part 1)



• The cell cycle appears to be driven by specific chemical signals in the cytoplasm.– Fusion of an S phase and a G1 phase cell,

induces the G1 nucleus to start S phase.– Fusion of a cell in mitosis with one in

interphase induces the second cell to enter mitosis.

A molecular control system drives the cell cycle

Fig. 12.12

Concept 7.3 Cell Reproduction Is Under Precise Control• The trigger substances turned out to be

protein kinases: cyclin-dependent kinases (CDKs).

• They catalyze phosphorylation of proteins that regulate the cell cycle and are activated by binding to cyclin, which exposes the active site (allosteric regulation).

Concept 7.3 Cell Reproduction Is Under Precise Control• CDKs function at cell cycle checkpoints:

– G1 checkpoint is triggered by DNA damage.

– S checkpoint is triggered by incomplete replication or DNA damage.

– G2 checkpoint is triggered by DNA damage.

– M checkpoint is triggered by a chromosome that fails to attach to the spindle.

Concept 7.3 Cell Reproduction Is Under Precise Control• Each CDK has a cyclin to activate it, which

is made only at the right time.

• After the CDK acts, the cyclin is broken down by a protease.

• Synthesis and breakdown of cyclins is important in controlling the cell cycle.

• Cyclins are synthesized in response to various signals, such as growth factors.

Figure 7.10 Cyclins Are Transient in the Cell Cycle

• Rhythmic fluctuations in the abundance and activity of control molecules pace the cell cycle.– Some molecules are protein kinases that

activate or deactivate other proteins by phosphorylating them.

• The levels of these kinases are present in constant amounts, but these kinases require a second protein, a cyclin, to become activated.– Level of cyclin proteins fluctuate cyclically.– The complex of kinases and cyclin forms

cyclin-dependent kinases (Cdks).

• Cyclin levels rise sharply throughout interphase, then fall abruptly during mitosis.

• Peaks in the activity of one cyclin-Cdk complex, MPF, correspond to peaks in cyclin concentration.

Fig. 12.14a

• MPF (“maturation-promoting factor” or “M-phase-promoting-factor”) triggers the cell’s passage past the G2 checkpoint to the M phase.– MPF promotes mitosis by phosphorylating

a variety of other protein kinases.– MPF stimulates

fragmentation of the nuclear envelope.

– It also triggers the breakdown of cyclin, dropping cyclin and MPF levels during mitosis and inactivating MPF.

Fig. 12.14b


• 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

• 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells• Two mechanisms move the chromosomes

to opposite poles:

– Kinetochores have molecular motor proteins (kinesin and dynein), which move the chromosomes along the microtubules.

– The kinetochore microtubules shorten from the poles, drawing the chromosomes toward the poles.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity

• Meiosis consists of two nuclear divisions but DNA is replicated only once.

• The haploid cells produced by meiosis are genetically different from one another and from the parent cell.

Figure 7.11 Mitosis and Meiosis: A Comparison (Part 1)

Figure 7.11 Mitosis and Meiosis: A Comparison (Part 2)

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • The function of meiosis is to:

– Reduce the chromosome number from diploid to haploid

– Ensure that each haploid cell has a complete set of chromosomes

– Generate diversity among the products

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • Meiosis I

• Homologous chromosomes come together and line up along their entire lengths.

• The homologous chromosome pairs separate, but individual chromosomes made up of two sister chromatids remain together.

Figure 7.12 Meiosis: Generating Haploid Cells (1)

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • Meiosis I is preceded by an S phase during

which DNA is replicated.

• Each chromosome then consists of two sister chromatids.

• At the end of meiosis I, two nuclei form, each with half the original chromosomes (one member of each homologous pair).

• The centromeres did not separate, so each chromosome is still two sister chromatids.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • Meiosis II

• Not preceded by DNA replication

• Sister chromatids separate

• End result: four haploid cells that are not genetically identical

Figure 7.12 Meiosis: Generating Haploid Cells (2)


• Shuffling of genetic material during meiosis occurs by two processes:

– Crossing over

• In prophase I homologous chromosomes (synapsis) and the four chromatids form a tetrad, or bivalent.


• The homologs seem to repel each other at the centromeres but remain attached at chiasmata.


• Genetic material is exchanged between nonsister chromatids at the chiasmata.

• Any of the four chromatids in the tetrad can participate, and a single chromatid can exchange material at more than one point.

• Crossing over results in recombinant chromatids and increases genetic variability of the products.

Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes


• Prophase I may last a long time.

–Human males: prophase I lasts about 1 week, and 1 month for entire meiotic cycle

–Human females: prophase I begins before birth, meiosis continues up to decades later during the monthly ovarian cycle and is completed only after fertilization.


– Independent assortment

• At anaphase I, it is a matter of chance which member of a homologous pair goes to which daughter cell.

• The greater the number of chromosomes, the greater the potential for genetic diversity.

• In humans, 223 (8,388,608) different combinations of maternal and paternal chromosomes can be produced.


Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • Meiosis is complex, and errors can occur.

• Nondisjunction

• Homologous pair fails to separate at anaphase I

• Sister chromatids fail to separate at anaphase II

• Both result in aneuploidy—an abnormal number of chromosomes.

Figure 7.14 Nondisjunction Leads to Aneuploidy

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • Most human embryos from aneuploid

zygotes do not survive. Many miscarriages are due to this.

• The most common human aneuploidy is trisomy 16.

• Trisomy 21 (Down syndrome) is one of the few aneuploidies that allow survival.


• Polyploidy

• Sometimes, organisms with triploid (3n), tetraploid (4n), and even higher numbers can form.

• This can occur through an extra round of DNA replication before meiosis, or lack of spindle formation in meiosis II.

• Polyploidy occurs naturally in some species and can be desirable in plants.


• Translocation

• Crossing over between non-homologous chromosomes in meiosis I

• Location of genes relative to other DNA sequences is important, and translocations can have profound effects on gene expression.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • A translocation that occurs in humans

between chromosomes 9 and 22 can result in a form of leukemia.

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

• Cells can die in one of two ways:

• In necrosis, the cell is damaged or starved for oxygen or nutrients. The cell swells and bursts.

• Cell contents are released to the extracellular environment and can cause inflammation.


• Apoptosis is genetically programmed cell death. Two possible reasons:

• The cell is no longer needed (e.g., the connective tissue between the fingers of a fetus)

• Old cells are prone to genetic damage that can lead to cancer—especially true of epithelial cells that die after days or weeks

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms • Events of apoptosis:

• Cell detaches from its neighbors

• DNA is cut into small fragments

• Membranous lobes (“blebs”) form and break into fragments

• Surrounding living cells usually ingest remains of the dead cell by phagocytosis

Figure 7.15 Apoptosis: Programmed Cell Death


• Plants use apoptosis in the hypersensitive response.

• They protect themselves from disease by undergoing apoptosis at the site of infection by a fungus or bacterium, preventing spread to other parts of the plant.

Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms • Programmed cell death is controlled by

signals:

– Internal signals may be linked to cell age or damaged DNA.

– Both internal and external signals lead to activation of caspases, which hydrolyze target proteins in a cascade of events.

• The cell dies as caspases hydrolyze proteins of the nuclear envelope, nucleosomes, and cell membrane.

Answer to Opening Question

• Human papilloma virus (HPV) stimulates the cell cycle when it infects the cervix.

• Two proteins regulate the cell cycle:

– Oncogene proteins are mutated positive regulators of the cell cycle—in cancer cells they are overactive or present in excess.


– Tumor suppressors are negative regulators of the cell cycle, but are inactive in cancer cells.

• Example: RB blocks the cell cycle at R. HPV causes synthesis of E7 protein, which fits into the protein-binding site of RB, thereby inactivating it.

Figure 7.16 Molecular Changes Regulate the Cell Cycle in Cancer Cells


• Chemotherapy drugs stop cell division by targeting cell cycle events.

• Some drugs block DNA replication; others damage DNA, stopping cells at G2; and still others prevent normal functioning of the mitotic spindle.

• Unfortunately, these drugs also act on normal cells and are toxic to rapidly dividing cells in the intestines, skin, and bone marrow.


• Research into more specific chemotherapy drugs is ongoing.

– Example: a drug has been identified that affects the protein produced as a result of the translocation between chromosomes 9 and 22.

– It has been successful at treating leukemia caused by this translocation.

• Treatments for metastasizing cancers include high-energy radiation and chemotherapy with toxic drugs.– These treatments target actively dividing

cells.• Researchers are beginning to understand how

a normal cell is transformed into a cancer cell.– The causes are diverse.– However, cellular transformation always

involves the alteration of genes that influence the cell cycle control system.


Documents

The Cell Cycle and Cell Division Ch 7. Chapter 7 The Cell Cycle and Cell Division Key Concepts 7.1 Different Life Cycles Use Different Modes of Cell Reproduction