Ch. 12 The Cell Cycle Dynamic Study Module Flashcards

The DNA was replicated during the S phase of interphase, which occurs between the two G phases.

Ex.

The DNA was replicated during the S phase of interphase, which occurs between the two G phases.

Interphase can be divided into subphases: the G₁ phase (“first gap”), the S phase (“synthesis”), and the G₂ phase (“second gap”). The G phases were misnamed as “gaps” when they were first observed because the cells appeared inactive, but we now know that intense metabolic activity and growth occur throughout interphase. In fact, during all three subphases of interphase, a cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. Duplication of the chromosomes, which is crucial for the eventual division of the cell, occurs entirely during the S phase. Thus, a cell grows (G₁), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for cell division (G₂), and divides (M). The daughter cells may then repeat the cycle.

DNA replication does not occur during the G₂ phase of interphase.

DNA replication does not occur during the G₁ phase of interphase.

DNA replication occurs during the S phase of interphase, not before interphase.

DNA replication occurs during the S phase of interphase. A mutation is unlikely to replicate the entire genome of a cell.

DNA replication occurs during the S phase of interphase, not after the G₂ phase.

the beginning of the formation of a spindle apparatus

Ex.

One event occurring during prophase is the beginning of the formation of a spindle apparatus.

Mitosis includes five phases during which a variety of events occur.

During prophase, chromatin condenses into distinct chromosomes, the nucleolus disappears, and the mitotic spindle apparatus begins to form. In addition, centrosomes move away from each other during prophase, apparently propelled along the surface of the nucleus by the lengthening bundles of microtubules between them. This structure is responsible for maneuvering the chromosomes around the cell during mitosis.

In prometaphase, the nuclear envelope fragments and the kinetochores attach to microtubules. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell.

Anaphase begins when the paired centromeres of each chromosome separate, liberating the sister chromatids, which begin moving toward opposite poles of the cell.

In telophase, a new nuclear envelope begins to synthesize, the mitotic spindle breaks down, the chromatin uncoils, and cytokinesis begins.

Telophase

Ex.

In terms of the nuclear envelope, telophase is essentially the opposite of prometaphase.

Mitosis includes five phases during which a variety of events occur.

During prophase, chromatin condenses into distinct chromosomes, the nucleolus disappears, and the mitotic spindle begins to form. This structure is responsible for maneuvering the chromosomes around the cell during mitosis.

In prometaphase, which is essentially the opposite of telophase, the nuclear envelope fragments and the kinetochores attach to microtubules. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell.

In anaphase, the mitotic spindle separates the sister chromatids and moves them to opposite ends of the cell.

In telophase, fragments of the nuclear envelope begin to reassemble, the mitotic spindle breaks down, the chromatin uncoils, and cytokinesis begins.

During interphase, the chromatin cannot be seen in a cell and the spindle has not begun to form yet.

The S phase is part of interphase and not part of mitosis.

dispersed in the nucleus as long strands of chromatin

Ex.

During interphase, the genetic material of a typical eukaryotic cell is dispersed in the nucleus as long strands of chromatin.

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. Mitosis is one part of the cell cycle. The mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. During interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division. Interphase can be divided into subphases: the G₁ phase (“first gap”), the S phase (“synthesis”), and the G₂ phase (“second gap”). During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. However, chromosomes are duplicated only during the S phase, which occurs between the two G phases.

During interphase, the chromosomes cannot be distinguished individually under a light microscope because they are still in the form of loosely packed chromatin fibers. In addition, the nucleus is still intact during interphase, with the genetic material inside.

The chromosomes do not condense until prophase of mitosis, when the spindle fibers attach to them. Unlike RNA and proteins, the DNA does not typically leave the nucleus.

it had formed a cell plate

Ex.

You would know a dividing cell was a plant cell rather than an animal cell if you saw that it had formed a cell plate.

In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and share of cytosol, organelles, and other subcellular structures.

Cytokinesis in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect in the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell. Two daughter cells result, each with its own plasma membrane. Both plant and animal cells have microtubules, but plant cells lack centrioles. At this point in mitosis, the nucleoli have dispersed and are no longer visible.

All of the listed responses are correct.

Ex.

All of the listed responses are correct.

Mitosis includes five phases during which a variety of events occur.

During prophase, chromatin condenses into distinct chromosomes, the nucleolus disappears, and the mitotic spindle apparatus begins to form. In addition, centrosomes move away from each other during prophase, apparently propelled along the surface of the nucleus by the lengthening bundles of microtubules between them. This structure is responsible for maneuvering the chromosomes around the cell during mitosis.

In prometaphase, the nuclear envelope fragments and the kinetochores attach to microtubules. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell.

In anaphase, the centrioles are at opposite ends of the cell. The paired centromeres of each chromosome are separated by spindle microtubules, liberating the sister chromatids, which begin moving toward opposite poles of the cell.

In telophase, a new nuclear envelope begins to synthesize, the mitotic spindle breaks down, the chromatin uncoils, and cytokinesis begins.

a centromere

Ex.

The region of a chromosome holding the two double strands of replicated DNA together is called a centromere.

Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion. Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA sequences and gives the condensed, duplicated chromosome a narrow “waist.”

Chromatin is a complex of DNA and protein.

Centrioles are components of centrosomes from which microtubules called asters radiate that are involved in the division of chromosomes during cell division.

A chromatid is a duplicated chromosome.

The nuclear envelope fragments in most eukaryotes but remains intact in diatoms and dinoflagellates.

Ex.

The nuclear envelope fragments in most eukaryotes but remains intact in diatoms and dinoflagellates.

As eukaryotes with nuclear envelopes and larger genomes evolved, the ancestral process of binary fission, seen today in bacteria, somehow gave rise to mitosis. Variations on cell division exist in different groups of organisms. These variant processes may be similar to mechanisms used by ancestral species and thus may resemble steps in the evolution of mitosis from a binary fission-like process presumably carried out by very early bacteria. Possible intermediate stages are suggested by two unusual types of nuclear division found today in certain unicellular eukaryotes—dinoflagellates, diatoms, and some yeasts. These two modes of nuclear division are thought to be cases where ancestral mechanisms have remained relatively unchanged over evolutionary time. In both types, the nuclear envelope remains intact, in contrast to what happens in most eukaryotic cells.

In most eukaryotes, the nuclear envelope fragments during mitosis. This does not occur in diatoms and dinoflagellates.

Microtubules facilitate the movement of chromosomes in mitosis in all eukaryotes.

Chromosomes are duplicated prior to mitosis in all eukaryotes.

Sister chromatids separate during mitosis in diatoms and dinoflagellates.

are genetically identical to the parent cell (assuming no mutation has occurred)

Ex.

The function of the mitotic cell cycle is to produce daughter cells that are genetically identical to the parent cell (assuming no mutation has occurred).

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. Passing identical genetic material to cellular offspring is a crucial function of cell division. A cell’s endowment of DNA, its genetic information, is called its genome. Before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied, or replicated, and then the two copies must be separated so that each daughter cell ends up with a complete genome. Cell division involves the distribution of identical genetic material (DNA) and same number of chromosomes to two daughter chromosomes. Both daughter cells of a mitotic division have complete sets of maternal and paternal chromosomes, not chromatids, identical to those of the parent cell.

identical copies of each other if they are part of the same chromosome

Ex.

Chromatids are identical copies of each other if they are part of the same chromosome.

During interphase of the cell cycle, all of a eukaryotic cell’s chromosomes are duplicated. Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. The two chromatids, each containing an identical DNA molecule, are initially attached all along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion. Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA sequences and gives the condensed, duplicated chromosome a narrow “waist.”

Centrioles do not hold chromatids together. Centrioles are involved in distributing chromosomes to daughter cells during mitosis.

Bacterial cells have a single, circular chromosome, whereas chromatids are found only in eukaryotic cells.

Cytokinesis

Ex.

In animal cell mitosis, the cleavage furrow forms during the cytokinesis stage of the cell cycle.

In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract. The contraction of the dividing cell’s ring of microfilaments is like the pulling of a drawstring. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and share of cytosol, organelles, and other subcellular structures.

Mitosis includes five phases during which a variety of events occur. During prophase, chromatin condenses into distinct chromosomes, the nucleolus disappears, and the mitotic spindle apparatus begins to form. In addition, centrosomes move away from each other during prophase, apparently propelled along the surface of the nucleus by the lengthening bundles of microtubules between them. This structure is responsible for maneuvering the chromosomes around the cell during mitosis. In prometaphase, the nuclear envelope fragments and the kinetochores attach to microtubules. During metaphase, the mitotic spindle aligns the chromosomes at the metaphase plate, a region along the equator of the cell. In anaphase, the centrioles are at opposite ends of the cell. The paired centromeres of each chromosome are separated by spindle microtubules, liberating the sister chromatids, which begin moving toward opposite poles of the cell. In telophase, a new nuclear envelope begins to synthesize, the mitotic spindle breaks down, the chromatin uncoils, and cytokinesis begins.

The G₁ phase is the part of interphase during which cell growth occurs. This phase also contains a cell cycle checkpoint that can determine if a cell continues on to the S phase.

20

Ex.

The cell will contain 20 chromosomes at the end of mitosis.

When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber. After DNA replication, however, the chromosomes condense as a part of cell division: Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and so thick that we can see them with a light microscope. Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell. Once the sister chromatids separate, they are no longer called sister chromatids but are considered individual chromosomes. The number of chromosomes in a cell at the end of mitosis is half as many as the number of sister chromatids the cell had at the beginning of mitosis.

The mitotic spindle breaks down.

Ex.

The mitotic spindle breaks down does not occur during prophase of mitosis. This occurs during telophase, not prophase.

A number of events occur during prophase of mitosis. The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion). The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.

The mitotic spindle forms during prophase of mitosis.

The chromosomes condense during prophase of mitosis.

The nucleoli disappear during prophase of mitosis.

The centrosomes move away from each other during prophase of mitosis.

Each duplicated chromosome appears as two identical sister chromatids during prophase of mitosis.

Binary fission involves the replication and division of a single chromosome, whereas mitosis involves the division of multiple, replicated chromosomes.

Ex.

Binary fission involves the replication and division of a single chromosome, whereas mitosis involves the division of multiple, replicated chromosomes.

Prokaryotes (bacteria and archaea) can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. The term binary fission, meaning “division in half,” refers to this process and to the asexual reproduction in prokaryotes. In E. coli, the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. Using the techniques of modern DNA technology to tag the origins of replication with molecules that glow green in fluorescence microscopy, researchers have directly observed the movement of bacterial chromosomes. This movement is reminiscent of the poleward movements of the centromere regions of eukaryotic chromosomes during anaphase of mitosis, but bacteria don’t have visible mitotic spindles or even microtubules.

There are many differences between binary fission and mitosis.

Binary fission involves the division of a single chromosome, and mitosis involves the division of multiple chromosomes.

A cell plate forms across the middle of a plant cell, not a bacterial cell.

Binary fission does not involve microtubules, but mitosis does.

The cell contains more than one nucleus.

Ex.

The cell contains more than one nucleus describes a cell that undergoes mitosis but not cytokinesis.

In the cells of some organisms, mitosis may occur without cytokinesis. In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract. The contraction of the dividing cell’s ring of microfilaments is like the pulling of a drawstring. The cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and its own share of cytosol, organelles, and other subcellular structures.

The cell would have one nucleus if cytokinesis occurred.

Any cell that has undergone mitosis still has at least one nucleus.

There is not enough information to determine if transformation of the cell occurred.

Since the cell went through mitosis, it had to go through interphase.

It cannot be determined if the cell died.

Cells of benign tumors do not metastasize; those of malignant tumors do.

Ex.

The difference between a benign tumor and a malignant tumor is that cells of benign tumors do not metastasize while those of malignant tumors do.

Cancer cells do not heed the normal signals that regulate the cell cycle. They divide excessively and invade other tissues. If unchecked, they can kill the organism. The abnormal behavior of cancer cells can be catastrophic when it occurs in the body. The problem begins when a single cell in a tissue undergoes transformation, the process that converts a normal cell to a cancer cell.

The body’s immune system normally recognizes a transformed cell as an insurgent and destroys it. However, if the cell evades destruction, it may proliferate and form a tumor, a mass of abnormal cells within otherwise normal tissue. The abnormal cells may remain at the original site if they have too few genetic and cellular changes to survive at another site. In that case, the tumor is called a benign tumor. Most benign tumors do not cause serious problems or spread to other parts of the body, and can be completely removed by surgery. However, benign tumors can occasionally lead to health problems.

In contrast, a malignant tumor includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs. Sometimes a few cells from a malignant tumor may separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body. There, they may proliferate and form a new tumor. This spread of cancer cells to locations distant from their original site is called metastasis.

division of the cytoplasm

Ex.

Cytokinesis refers to division of the cytoplasm.

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. Mitosis is just one part of the cell cycle. In fact, the mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. During interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division. Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm. One cell has become two, each the genetic equivalent of the parent cell.

Cytokinesis does not involve the division of chromosomes, nor does it involve any cellular movement.

All of the listed responses are part of the mitotic spindle.

Ex.

All of the listed responses are part of the mitotic spindle.

Many of the events of mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This structure consists of fibers made of microtubules and associated proteins. In animal cells, the assembly of spindle microtubules starts at the centrosome, a subcellular region containing material that functions throughout the cell cycle to organize the cell’s microtubules. An aster, a radial array of short microtubules, extends from each centrosome. The spindle includes the centrosomes, the spindle microtubules, and the asters. Each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure made up of proteins that have assembled on specific sections of DNA at each centromere. In a dividing animal cell, the nonkinetochore microtubules are responsible for elongating the whole cell during anaphase. Nonkinetochore microtubules from opposite poles overlap each other extensively during metaphase.

regulate the cell cycle through a variety of stop and go signals

Ex.

Checkpoints in the cell cycle control system regulate the cell cycle through a variety of stop and go signals.

The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both trigger and coordinate key events in the cell cycle. The cell cycle control system has been compared to the control device of an automatic washing machine. Like the washer’s timing device, the cell cycle control system proceeds on its own, according to a built-in clock. However, just as a washer’s cycle is subject to both internal control (such as the sensor that detects when the tub is filled with water) and external adjustment (such as when someone starts or stops the machine), the cell cycle is regulated at certain checkpoints by both internal and external signals that stop or restart the cycle. A checkpoint is a control point in the cell cycle 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 control system regulates whether cell division proceeds.

The cell cycle control system can initiate or stop cell division.

The cell cycle control system regulates cell division.

The cell cycle control system does not function properly in cancer cells.

Previous

The chromosomes align along the metaphase plate of the cell.

Ex.

The chromosomes align along the metaphase plate of the cell during metaphase of mitosis.

A variety of events occur during metaphase of mitosis. The centrosomes are now at opposite poles of the cell. The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

The nuclear envelope disappears during prometaphase of mitosis.

The chromosomes condense during prophase of mitosis.

The sister chromatids are pulled apart toward opposite sides of the cell during anaphase of mitosis.

The nuclear envelope forms again during telophase of mitosis.

The mitotic spindle forms during prophase of mitosis.

Two distinct daughter nuclei form in the cell.

Ex.

During telophase of mitosis, two distinct daughter nuclei form in the cell.

A number of events occur during telophase. In addition to two daughter nuclei forming in the cell: Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. Nucleoli reappear. The chromosomes become less condensed. Any remaining spindle microtubules are depolymerized. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

The nuclear envelope fragments during prometaphase of mitosis.

The chromosomes align on the metaphase plate during metaphase of mitosis.

The sister chromatids separate during anaphase of mitosis.

The chromosomes condense during prophase of mitosis.

DNA replicates during interphase.

The nuclear envelope fragments.

Ex.

The nuclear envelope fragments during prometaphase of mitosis.

A number of events occur during prometaphase of mitosis. In addition to the nuclear envelope fragmenting: The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes have become even more condensed. Each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure at the centromere. Some of the microtubules attach to the kinetochores, becoming “kinetochore microtubules,” which jerk the chromosomes back and forth. Nonkinetochore microtubules interact with those from the opposite pole of the spindle.

The centrosomes move away from each other during prophase of mitosis.

The chromosomes align along the metaphase plate of the cell during metaphase of mitosis.

The mitotic spindle forms during prophase of mitosis.

The nuclear envelope forms again during telophase of mitosis.

The sister chromatids are pulled toward opposite sides of the cell during anaphase of mitosis.

the S phase of interphase in both somatic and reproductive cells

Ex.

DNA replication occurs in the S phase of interphase in both somatic and reproductive cells.

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. Mitosis is one part of the cell cycle. The mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. Mitotic cell division alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. During interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division. Interphase can be divided into subphases: the G₁ phase (“first gap”), the S phase (“synthesis”), and the G₂ phase (“second gap”). During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. However, chromosomes are duplicated only during the S phase, which occurs between the two G phases.

Chromosomes are duplicated only during the S phase ("S" stands for synthesis of DNA) of interphase of the cell cycle. During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

Interphase occurs prior to meiosis and mitosis.

The sister chromatids are pulled apart toward opposite sides of the cell.

Ex.

The sister chromatids are pulled apart toward opposite sides of the cell during anaphase of mitosis.

A variety of events occur during anaphase of mitosis. Anaphase is the shortest stage of mitosis, often lasting only a few minutes. Anaphase begins when the cohesion proteins are cleaved. This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes a full-fledged chromosome. The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 μm/min). The cell elongates as the nonkinetochore microtubules lengthen. By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes.

The chromosomes align along the metaphase plate of the cell during metaphase of mitosis.

The nuclear envelope fragments during prometaphase of mitosis.

Spindle fibers attach to the centromeres of the chromosomes during prometaphase of mitosis.

The nuclear envelope forms again during telophase of mitosis.

The mitotic spindle forms during prophase of mitosis.

the origins of replication move apart

Ex.

During binary fission in a bacterium the origins of replication move apart.

Prokaryotes can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. The term binary fission, meaning “division in half,” refers to this process. The process in eukaryotes involves mitosis, whereas that in prokaryotes does not. In bacteria, most genes are carried on a single bacterial chromosome that consists of a circular DNA molecule and associated proteins. The process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. As the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. While the chromosome is replicating, the cell elongates. When replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent cell into two daughter cells. In this way, each cell inherits a complete genome.

Bacteria lack a spindle apparatus and centrioles. In addition, at the origin of replication, the circular DNA molecule is replicated, forming two, not four, identical copies of the bacterial chromosome.

Separation of the sister chromatids

Ex.

Separation of the sister chromatids does not occur during interphase. Instead, this process occurs during anaphase of mitosis.

Mitosis is just one part of the cell cycle. In fact, the mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest part of the cell cycle. The mitotic phase alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle. Interphase can be divided into subphases: the G₁ phase (“first gap”), the S phase (“synthesis”), and the G₂ phase (“second gap”). The G phases were misnamed as “gaps” when they were first observed because the cells appeared inactive, but we now know that intense metabolic activity and growth occur throughout interphase. In fact, during all three subphases of interphase, a cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. Duplication of the chromosomes, which is crucial for the eventual division of the cell, occurs entirely during the S phase. Thus, a cell grows (G₁), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for cell division (G₂), and divides (M). The daughter cells may then repeat the cycle.

Duplication of the chromosomes occurs during the S phase of interphase.

Growth of the cell occurs during the G₁ and G₂ phases of interphase.

Production of new mitochondria occurs during all three phases of interphase.

Protein production occurs during all three phases of interphase.

Endoplasmic reticulum production occurs during all three phases of interphase.

chromatin

Ex.

The complex of DNA and protein that makes up a eukaryotic chromosome is properly called chromatin.

DNA molecules are packaged into structures called chromosomes, so named because they take up certain dyes used in microscopy (from the Greek chroma, color, and soma, body). Each eukaryotic chromosome consists of one very long, linear DNA molecule associated with many proteins. Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin. The DNA-protein complex called chromatin is organized into a long, thin fiber.

A centromere is the narrow region of the chromosome where the two duplicated chromosomes, sister chromatids, attach to one another.

A chromoplast is a pigment-storing organelle and the centrosome is the organelle from which microtubules originate.

metastasis

Ex.

The spread of cancer cells to other locations in the body is known as metastasis.

Cancer cells do not heed the normal signals that regulate the cell cycle. In culture, they do not stop dividing when growth factors are depleted. If and when they stop dividing, cancer cells do so at random points in the cell cycle rather than at the normal checkpoints. Cells in culture that acquire the ability to divide indefinitely are said to have undergone transformation, the process that causes them to behave like cancer cells. The abnormal behavior of cancer cells can be catastrophic when it occurs in the body. The problem begins when a single cell in a tissue undergoes the first changes of the multistep process that converts a normal cell to a cancer cell. Such a cell often has altered proteins on its surface, and the body’s immune system normally recognizes the cell as “nonself”—an insurgent—and destroys it. However, if the cell evades destruction, it may proliferate and form a tumor, a mass of abnormal cells within otherwise normal tissue. The abnormal cells may remain at the original site if they have too few genetic and cellular changes to survive at another site. In that case, the tumor is called a benign tumor. Most benign tumors do not cause serious problems and can be removed by surgery. In contrast, a malignant tumor includes cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs; these cells are also considered transformed cells. A few tumor cells may separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body. There, they may proliferate and form a new tumor. This spread of cancer cells to locations distant from their original site is also called metastasis.

Chemotherapy is a type of cancer treatment.

Density-dependent inhibition is the effect of an external physical factor on cell division.

Transformation is the process that causes normal cells to act like cancer cells.

A benign tumor is one that does not spread to other parts of the body.

do not exhibit density-dependent inhibition

Ex.

Observations of cancer cells in culture support the hypothesis that cancer cells do not exhibit density-dependent inhibition.

The effect of an external physical factor on cell division is clearly seen in density-dependent inhibition, a phenomenon in which crowded cells stop dividing. As first observed many years ago, cultured cells normally divide until they form a single layer of cells on the inner surface of the culture container, at which point the cells stop dividing. The binding of a cell surface protein to its counterpart on an adjoining cell sends a growth-inhibiting signal to both cells, preventing them from moving forward in the cell cycle.

Cancer cells do not heed the normal signals that regulate the cell cycle. They divide excessively and invade other tissues. Consequently, they do not exhibit density-dependent inhibition. Because cancer cells divide uncontrollably, they do not produce molecules that inhibit the growth factors required for cell division. Cancer cells are characterized by the lack of anchorage dependence.

Cells in the G0 phase have assumed a nondividing state, which does not describe cancer cells.

92

Ex.

After interphase and just prior to mitosis, a human somatic cell would have 92 chromatids.

When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber. After DNA replication, however, the chromosomes condense as a part of cell division: Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and so thick that we can see them with a light microscope. Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome.