Chapter 12 Text

What best distinguishes living things from nonliving matter?
What is it called?
The continuity of life is based on reproduction of cells, or __.

__ plays several important roles in the life of an organism. When a unicellular organism, such as an amoeba, divides and forms duplicate offspring, the division of one cell reproduces an entire organism. __ on a larger scale can produce __ from some multicellular organisms. It also enables sexually reproduciing organisms to develop from a single cell, a fertilized egg- or __. And after an organism is fully grown, it continues to function in __ and __, replacing cells that die from normal wear and tear or accidents.

The cell division process is an integral part of the __, the life of a cell from the time it is first formed from a dividing parent cell until its own division into two cells. Passing __ to cellular offspring is a crucial function of cell division.

Most cell division involves the distribution of identical genetic material-__- to two __ cells. (The special type of cell division that produces __ and __ results in daughter cells that are not genetically identical.)
A dividing cell duplicates its __, allocates the two copies to opposite ends of the cell, and only then splits into __ cells.

a cell's endowment of DNA, its genetic information: __
The overall length of DNA in a __ is enormous. Yet before the cell can divide to form genetically identical __ cells, all of this DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete __.

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into __.
The nuclei of human __ cells (all body cells except the reproductive cells) each contain __ chromosomes made up of two sets of __, one set inherited from each parent.
Reproductive cells, or __-sperm and eggs- have half as many chromosomes as __ cells, or one set of __ chromosomes in humans.

Eukaryotic chromosomes are made of __, a complex of DNA and associated protein molecules. Each single chromosome contains one very long, linear DNA molecule that carries several hundred to a few thousand __, the units that specify an organism's inherited traits. The associated proteins maintain the structiure of the __ and help control the activity of the __.

When a cell is not dividing, it is a __. After DNA duplication, the __ condense: Each __ becomes densely coiled and folded, making the __ much shorter and thick.

Each duplicated chromosome has two __.The two __, each containing an identical DNA molecule are initially attached all along their lengths by adhesive protein complexes called __; this attachment is known as __.

In its condensed form, the duplicated chromosome has a narrow "waist" at the __, a specialized region where the two chromatids are most closely attached. The part of a chromatid on either side of the __ is referred to as an __ of the chromatid. Later in the cell division process, the two __ of each duplicated chromosome separate and move into two new __, one forming at each end of the cell.

Once the sister chromatids separate, they are considered individual chromosomes. Thus, each new nucleus receives a collection of chromosomes identical to that of the parent cell.
__, the division of the mucleus, is usually followed immediately by __, the division of the cytoplasm. There are now two cells, each the genetic equivalent of the parent cell.

You produce __ cells by mitosis.
You prduce __ by a variation of cell division called __, which yields nonidentical daughter cells that have only one set of chromosomes, thus half as many chromosomes as the parent cell.

Meiosis occurs in the __. In each generation of humans, meiosis reduces the chromosome number from __ to __.
__ fuses two __ together and returms the chromosome number to 46, and mitosis conserves that number in every __ cell nucleus of the new individual.

Mitosis is just one part of the cell cycle. In fact, the __, which includes both __ and __, is usually the shortest part of the cell cycle. Mitotic cell division alternates with a much longer stage called __, which often accounts for about __ of the cycle. It is during __ that the cell grows and copies its chromosomes in preparation for cell division.

Interphase c an be divided into subphases: the __ (first gap), the __ (synthesis) and the __ (second gap). During all three subphases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are only duplicated during the __. Thus a cell grows (__), continues to grow as it copies its chromosomes (_), grows more as it completes preparationf for cell division (_), and divides (_). The daughter cells may then repeat the cycle.

What is the longest phase of interphase? most variable in length?
Mitosis is conventionally broken down into five stages: __,__,__,__, +__. Overlapping with the latter stages of mitosis, __ completes the mitotic phase.

Many of the events of mitosis depend on the __, which begins to form in the cytoplsm during __. This structure consists of fibers made of microtubules and associated proteins. WHile the __ assembles, the other microtubules of the cytoskeleton partially disassemble, probably providing the material used to construct the spindle. The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein __ and shorten (depolymerize) by losing subunits.

In animal cells, the assenbly of spindle microtubules starts at the __, a subcellular region comtaining material that functions throughout the cell cycle to organize the cell's microtubules (also called __). A pair of __ is located at the center of the __, but they are not essential for cell division: If the __ are destroyed with a laser microbeam, a spindle nevertheless forms during mitososis. In fact, __ are not even present in plant cells, which do form __.

During __ in animal cells, the single __ replicates, forming two __, which remain together near the nucleus. The two __ move apart during __ and __ of mitosis as spindle microtubules grow out from them. By the end of __, the two __, one at each pole of the spindle, are at opposite ends of the cell. An __, a radial array of short microtubules, extends from each __. The spindle includes the __, the spindle microtubules and the __.

Each of the two sister chromatids of a replicated chromosome has a __, a structure of proteins associated with specific sections of chromosomal DNA at the centromere. The chromosome' s two __ face in opposite directions. During __, some of the spindle microtubules attach to the __; these are called __. When one of a chromosome's __ is "captured" by microtubules, the chromosome begins to move toward the pole from which those mictrotubules extend. However, this movement is checked as soon as microtubules from the opposite pole attach to the other __. Next, the chromosome moves first in one direction, then the other, back and forth, finally settling midway between the two ends of the cell.

At __, the centromeres of all the duplicated chromosomes are on a plane midway between the spindle's two ples. This imaginary plane is called the __ of the cell. Meanwhile, microtubules that do not attach to the __ have been elongating, and by __ they overlap and interact with other __ microtubules from the opposite pole of the spindle. (These are sometimes called "_" microtubules.) By __, the microtubules of the __ have also grownand are in contact with the plasma membrane. The spindle is now complete.

__ commenses suddenly when the __ holding together the sister chromatids of each chromosome are cleaved by enzymes. One the chromatids become sparate, full-fledged chromosomes, they move toward opposite ends of the cell.

Two mechanisms are in play when discussing how kinetochore microtubules function in the poleward movement of chromosomes., both involving __. A clever experiment carried out in Gary Borisy's lab at the U of Wisconsin in 1987 suggested that motor proteins on the __ "walk" the chromosomes along the microtubules which __ at their __ ends after the motor proteins have passed (referred to as the __ mechanism). However, other researchers, working with different cell types or cells from other species, have shown that chromosomes are "reeled in" by motor proteins at the spindle poles and that the microtubules __ after they pass by these motor proteins. The general consensus now is that the relative contributions of these two mechanisms vary among cell type.

What is the function of the nonkinetochore microtubles?
Nonkinetochore microtubules from opposite poles overlap each other extensively during__. During __, the region of overlap is reduced as motor proteins attached to the microtubules walk them away from one another, using energy from __. As the microtubules push apart from each other, their spindle poles are pushed apart, elongating the cell. At the same time, the microtbules lengthen somewhat by the addition of __ subunits to their overlapping ends. As a result, the microtubules continue to overlap.

At the end of __, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell. Nuclei re-form during __. __ generally begins during __ or __, and the spindle eventually disassembles.

In animal cells, __ occurs by a process known as __. The first sign of this is the appearance of a __, a shallow groove in the cell surface near the old __. On the cytoplasmic side of the furrow is a contractile ring of __microfilaments associated with molecules of the protein __. The __ microfilaments interact with the __ molecues, causing the ring to contract. The __ 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.

__ in plant cells is different. There is no __. Instead, during __, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a __. Cell wall materials carried in the vesicles collect in the __ as it grows. The __ 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. Meanwhile, a new cell wall arising from the contents of the __ is formed between the daughter cells.

The __ of single-celled eukaryotes includes mitosis and occurs by a type of cell division called __, meaning division in half. Prokaryotes also reproduce by this, but they're process does not involve mitosis. In bacteria, most genes are carried on a single __ that consists of a circular DNA molecule and associated proteins. Although bacteria are smaller and simpler than eukaryotic cells, the challenge of replicating their __ in an orderly fashion and distributing the copies equally to two daughter cells is still formidable.

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 __ 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 grows inward, dividing the parent E. coli cell into two __cells. Each cell inherits a complete __.

The movement of bacterial chromosomes is reminiscent of the poleward movements of the __ regions of eukaryotic chromosomes during __ of mitosis, but bacteria don't have visible __ or even microtubules. In most bacterial species studied, the two origins of replication end up at opposite ends of the cell or in some other very specific location, possibly anchored there by one or more proteins. Several proteins have been identified that play important roles: One resembling eukaryotic __ may function in bacterial chromosome movement during cell division, and another that is related to __ may help separate the two bacterial daughter cells.

As eukaryotes evolved, along with their larger __ nuclear envelopes, the ancestral process of __ somehow gave rise to mitosis. Possible intermediate stages are represented by two unusual types of nuclear division found today in certain unicellular eukaryotes. These two examples 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 __, replicated chromosomes are attached to the nuclear envelope and separate as the nucleus elongates prior to dividing. In __ and __, a spindle within the nucleus sparates the chromosomes. In most eukaryotic cells, the nuclear envelope breaks down and a spindle separates the chromosomes.

The __ and __ of cell division in dif. parts of a plant o animal are crucial to normal growth, development, and maintenance. The frequency of cell division varies with the type of cell. These cell cycle differences result from __ at the molecular level.

What controls the cell cycle?

The sequential events of the cell cycle are directed by a distinct __, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle. It proceeds on its own. However, the cell cycle is regulated at certain __ by both internal and external signals.

A __ in the cell cycle is a control point where stop and go-head signals can regulate the cycle. Anmal cells generally have built-in stop signals that halt the cell cycle at __ until overridden by go-ahead signals. Many signals registered at __ come from cellular surveillance mechanisms inside the cell; the signals report whether crucial cellular processes that should have occurred by that point have in fact been completed correctly and thus whether or not the cell cycle should proceed. __ also register signals from outside the cell. Three major ones are found in the __,__, and __ phases.

For many cells, the __ checkpoint- dubbed the "restriction point" in mammalian cells- seems to be the most important. If a cell receives a go-ahead signal at the __ checkpoint, it will usually complete the __, __, __, and __ phases and divide. If it does not receive a go-ahead signal at that point, it will exit the cycle, switching into a nonviding state called the __.

Most cells of the human body are actually in the __. Mature nerve cells and muscle cells never divide. Other cells, such as liver cells, can be "called back" from the __ phase to the cell cycle by external cues, such as growth factors released during injury.

__ in the abundance and __of cell cycle control molecules pace the sequential events of the cell cycle. These regulatory molecules are mainly proteins of two types: __ and __. Particular __ give the go-ahead signals at the __ and __ checkpoints.

Many of the kinases that drive the cell cycle are actually present at a constant concentration in the growing cell, but much of the time they are in an __ form. To be __, such a kinase must be attached to a __, a protein that gets its name from its cyclically fluctuating concentration in the cell. Because of this requirement, these kinases are called __.

The activity of a __ rises and falls with changes in the concentration of its __ partner.
The cyclin-Cdk complex that was discovered first in frog eggs: __
The peaks of __ correspond to the peaks of __ concentration.
The __ level rises during the __ and __ phases and then falls abruptly during the __ phase.

The initials MPF stand for __, but we can think of MPF as __ because it triggers the cell's passage past the __ checkpoint into the __ phase. When __ that accumulate during __ associate with __ molecules, the resulting MPF complex phosphorylates a variety of proteins, initiating mitosis.

__ acts both directly as a kinase and indirectly by activating other kinases. There is also evidence that __ contributes to molecular events required for chromosome condensation and spindle formation during __.

During __, MPF helps switch itself off by initiating a process that leads to the destruction of its own __. The noncyclin part of MPF, the __, persists in the cell in inactive form until it associates with new __ molecules synthesized during the __ and __ of the next round of the cycle.

An example of an internal signal occurs at the __ phase checkpint. __, the separation of sister chromatids, does not begin until all the chromosomes are properly attached to the spindle at the __. Researchers have learned that as long as some __ are unattached to spindle microtubules, the sister chromatids remain together, delaying __. Only when the __ of all the chromosomes are attached to the spindle does the appropriate regulatory protein become activated. Once activated, the protein sets off a chain of molecular events that ultimately results in the enzymatic cleavage of __, allowing the sister chromatids to separate. This mechanism ensures that daughter cells do not end up with missing or extra chromosomes.

Chemical and Physical External Factors:
- cells fail to divide if an essential __ is lacking in the culture medium.
Most types of mammalian cells divide in culture only if the growth medium includes specific __, a protein released by certain cells that stimulates other cells to divide.

Adding __, which is made by blood cell fragments called platelets, causes cells of fibroblasts - a type of connective tiessue cell-to proliferate.

The effect of an external physical factor on cell division is clearly seen in __, 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. If some cells are removed, those bordering the open space begin dividing again and continue until the vacancy is filled. Recent studies have revealed that the binding of a __ protein to its counterpart on an adjoining cell sends a __ to both cells, preventing them from moving forward in the cell cycle, even in the presence of __.

Most animal cells also exhibit _. To divide, they must be attached to a __, such as the inside of a culture jar or the extracellular matrix of a tissue. Experiments suggest that like cell density, __ is signaled to the cell cycle control system via pathways involving plasma membrane proteins and elements of the cytoskeleton linked to them.

__ cells exhibit neither density-dependent inhibition nor anchorage dependence. They do not heed the normal signals that regulate the cell cycle. They divide xcessively and invade other tissues. If unchecked, they can kill the organism. In addition to their lack of __ and __ , they do not stop dividing when growth factors are depleted.

If and when they stop dividing, __ cells do so at random points in the cycle, rather than at normal __. MOreover, these cells can go on dividing indefinitely in culture if they are given a continual supply of nutrients; in essence, they are "_".
A striking example is a cell line that has been reproducing in culture since 1951. Cells of this line are called __ because their original source was a tumor removed from a woman named Henrietta Lacks. By contrast, nearly all normal mammalian cells growing in culture divide only about 20 to 50x before they stop dividing, age and die.

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 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 othwerwise normal tissue.

If the abnormal cells remain at the original site, the lump is called a __. Most __ do not cause serious problems and can be completely removed by surgery. In contrast, a __ becomes invasive enough to impair the functions of one or more organs. An individula with a __ is said to have cancer.

How are the cells of malignant tumors abnormal, aside from their excessive proliferation?

Abnormal changes on the cell surface cause cancer cells to lose attachments to neighboring cells and the __, which allows them to spread into nearby tissues. Cancer cells may also secrete __ that cause blood vessels to grow toward the tumor. 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 location distant from their original site is called __.

A tumor that is localized can be treated with __, which damages DNA in cancer cells much more than it does in normal cells, apparently because the majority of cancer cells have lost the ability to repair such damage. To treat known or suspected metastatic tumors, __ is used, inwhich drugs that are toxic to actively dividing cells are administered through the circulatory system.

Though the causes of cancer are diverse, __ always involves the alteration of genes that somehow influence the cell cycle control system.