mastering biology ch12

tubulin

protein that makes up microtubules

motor proteins

A protein that interacts with cytoskeletal elements and other cell components, producing movement of the whole cell or parts of the cell.

Consider an animal cell in which motor proteins in the kinetochores normally pull the chromosomes along the kinetochore microtubules during mitosis. Suppose, however, that during metaphase, this cell was treated with an inhibitor that blocks the function of the motor proteins in the kinetochore, but allows the kinetochore to remain attached to the spindle. The inhibitor has no effect on any other mitotic process, including the function of the nonkinetochore microtubules.

1. Will this cell elongate during mitosis? yes 2. Will the sister chromatids separate from each other? yes 3. Will the chromosomes move to the poles of the cell? no The inhibitor does not affect the cleavage of cohesins (the proteins that hold the sister chromatids together), the attachment of the chromosomes to the kinetochore microtubules, or the elongation of the cell due to the nonkinetochore microtubules. The inhibitor only affects the motor protein that pulls the chromosome along the kinetochore microtubule in anaphase. Thus, in the treated cell, the sister chromatids can still separate at the beginning of anaphase due to the fact that the cell is elongating (the centrosomes at the poles of the cell are moving farther apart) and the kinetochore microtubules still connect the chromosomes to the centrosomes. However, because the chromosomes cannot move along the kinetochore microtubules, they will never reach the poles of the cell.

The stages of mitosis were originally defined by cellular features observable through a light microscope. The six micrographs below show animal cells (lung cells from a newt) during the five stages of mitosis, plus cytokinesis. (Note that interphase is not represented in these micrographs.) In these images, the chromosomes have been stained blue, microtubules green, and microfilaments red. Drag each micrograph to the target that indicates the stage of mitosis or cytokinesis it shows.

As these six micrographs demonstrate, cellular events observable by light microscopy can be used to define the six stages of mitosis and cytokinesis. However, deciphering which stage is which in real cells can be much more challenging than in the drawings of idealized cells you see in your textbook. Thus, it is important to carefully observe the completeness of the mitotic spindle and the location of the chromosomes, as well as how condensed the chromosomes are.

The condensation state of DNA throughout the cell cycle opened hint

As you can observe in micrographs of cells undergoing mitosis, the chromosomes condense at the beginning of the M phase. Because the chromosomes are at least a thousand times longer than the cell, condensation greatly facilitates separation of the sister chromatids into the daughter cells during mitosis. The chromosomes remain condensed until telophase, when they begin to de-condense. But why doesn't the DNA remain condensed throughout interphase too? One important event that occurs during the S phase of interphase is the replication of the DNA. Replication requires that the DNA be in its uncondensed form. In addition, transcription of genes for protein synthesis also requires that the DNA be in an uncondensed form.

Eukaryotes only

Before separation, duplicated chromosomes condense. Nuclear envelope fragments, permitting chromosome separation. Two copies of the duplicated chromosome are attached at their centromeres before separating.

anaphase

Cell elongates between poles Sister chromatids separate. During anaphase, the two sister chromatids of each pair separate and move toward the poles of the cell. In animal cells, this process is typically accompanied by elongation of the cell between the poles.

prophase

Centrosomes begin to move apart. Spindle begins to form. During prophase, the mitotic spindle begins to form; the centrosomes begin to move toward the poles of the cell; the chromosomes begin to condense and first become visible with a light microscope.

Role of checkpoints in the cell cycle opened hint

Checkpoints are control points in the cell cycle where "stop and go" signals regulate whether or not a cell continues to the next part of the cycle. For example, cells that pass through the G1 checkpoint usually complete the cell cycle and divide. If a cell does not pass through the G1 checkpoint, it exits the cell cycle and enters a nondividing state called the G0 phase.

bacteria only

Chromosome separation begins at the origin of replication on DNA.

both bacteria and eukaryotes

Chromosomes replicate before cell division. Replicated chromosomes separate by attaching to some other structural feature of the cell.

What distinguishes the G2 phase from the S phase? opened hint Once the cell passes through the G1 checkpoint, it enters the S phase, followed by the G2 phase. Which two statements correctly describe the processes that occur in the S and G2 phases?

DNA replication occurs in the S phase. Centrosome replication occurs in the G2 phase.

Cells will usually divide if they receive the proper signal at a checkpoint in which phase of the cell cycle? See Concept 12.3 (Page)

G1 In mammalian cells, this checkpoint is termed the restriction point.

Consider a cell in which chromosome movement along the mitotic spindle is accomplished by motor proteins in the kinetochores. Drag the labels onto the flowchart to show the correct sequence of events in kinetochore function during mitosis.

In cells where kinetochores are responsible for chromosome movement, motor proteins in the kinetochores move the sister chromatids into alignment along the metaphase plate. A "tug-of-war" between the spindle fiber attachment points moves each pair of sister chromatids first toward one pole and then toward the other until the kinetochores are positioned on the metaphase plate. After the sister chromatids detach from each other at the beginning of anaphase, motor proteins in the kinetochores pull the individual chromosomes toward the poles of the cell, eventually forming two new daughter nuclei.

During mitosis, what is the sequence of events involving kinetochores?

Kinetochores are assemblies of proteins that function in attaching the chromosomes to the mitotic spindle. By the end of prometaphase, each sister chromatid of a duplicated chromosome has a kinetochore located at its centromere. In some types of cells, the kinetochore contains motor proteins that are responsible for moving chromosomes along the spindle microtubules (see diagram below). In other types of cells, the chromosomes are pulled toward the poles by motor proteins associated with the centrosomes. In these cells, the kinetochore only attaches the chromosome to the mitotic spindle.

phases of cell cycle

Many organisms contain cells that do not normally divide. These cells exit the cell cycle before the G1 checkpoint. Once a cell passes the G1 checkpoint, it usually completes the cell cycle--that is, it divides. The first step in preparing for division is to replicate the cell's DNA in the S phase. In the G2 phase, the centrosome replicates. In early M phase, the centrosomes move away from each other toward the poles of the cell, in the process organizing the formation of the mitotic spindle. At the end of the M phase when mitosis is complete, the cell divides (cytokinesis), forming two genetically identical daughter cells.

Chromosome position in metaphase

Metaphase is characterized by the alignment of the chromosomes along the metaphase plate, a plane equidistant from the poles of the spindle. Although metaphase is often illustrated with all the chromosomes neatly aligned along the metaphase plate, this is not always the case. In real cells, the arms of each chromosome may extend some distance away from the metaphase plate, and only the centromere of each chromosome may be aligned on the metaphase plate.

What role, if any, do either microtubules or microfilaments play in cytokinesis in plant cells? opened hint Both microtubules and microfilaments are parts of the cytoskeleton and are important in many cellular processes, including motion and determination of cell shape. Which of the following statements correctly describes the role of these cytoskeletal elements in plant cytokinesis?

Microtubules guide Golgi-derived vesicles to the middle of the cell where they form the cell plate.

telophase

Nuclear envelopes begin to form. Chromosomes begin de-condensing. During telophase, the chromosomes reach the poles, the spindle disassembles, the chromosomes become less condensed, and the nuclear envelopes of the daughter nuclei form.

Changes in DNA content during the cell cycle opened hint

Once a cell passes the G1 checkpoint, its DNA is replicated during the S phase of interphase. Replication means that an exact copy of the DNA in each chromosome is made, thus doubling the cell's DNA content. Only once cytokinesis is completed at the end of telophase does the cell's DNA content return to the level found in G1 cells.

The mitotic spindle is the machinery that guides the separation of chromosomes in anaphase.

Prior to metaphase, the mitotic spindle is constructed by lengthening microtubules that extend from each centrosome. In metaphase, the kinetochore microtubules have attached each pair of sister chromatids, and the nonkinetochore microtubules overlap extensively at the metaphase plate. During anaphase, the kinetochore microtubules shorten as the chromosomes move toward the poles of the cell. At the same time, the nonkinetochore microtubules lengthen and push past each other, elongating the cell. By the end of telophase, all the microtubules associated with the mitotic spindle have disassembled.

When are sister chromatids present? opened hint Sister chromatids play an essential role in ensuring that each daughter cell receives genetic material that is identical to that which was present in the original parent cell. Which statement about sister chromatids is correct?

Sister chromatids first form in the S phase, and are present until they separate in early anaphase. Sister chromatids form when the DNA molecule in each chromosome is duplicated during the S phase. Until the sister chromatids separate in early anaphase, they are joined at one or more regions along their length. Once the sister chromatids separate, each chromatid becomes a full-fledged chromosome.

As the chromosomes of a parent cell are duplicated and distributed to the two daughter cells during cell division, the structure of the chromosomes changes. Answer the three questions for each phase of the cell cycle by dragging the yes and no labels to the appropriate locations in the table. Note: Assume that by the end of the M phase, the parent cell has not yet divided to form two daughter cells.

Sister chromatids form when DNA replicates in the S phase. The sister chromatids become individual chromosomes once they separate in early anaphase. Similarly, the cellular DNA content doubles in the S phase when the DNA replicates. However, the cell's DNA content does not return to its normal (undoubled) levels until after cytokinesis is complete and two daughter cells have formed. The condensation state of the DNA is not related to the presence or absence of sister chromatids. The DNA condenses in prophase and remains condensed until after the sister chromatids separate and the new daughter cells begin to form. In late telophase/cytokinesis, the emphasis shifts to cell growth and DNA replication for the next cell cycle. For these processes to occur, the DNA needs to be de-condensed so it is accessible to the cellular machinery involved in transcription.

pro-metaphase

Spindle fibers penetrate nuclear region. Centrosomes are on opposite sides of nucleus During pro-metaphase: the nuclear envelope fragments; the mitotic spindle is completed--the centrosomes are on opposite sides of the nucleus and the spindle fibers penetrate the nuclear region; some of the spindle microtubules attach to the now completely condensed chromosomes.

Cytokinesis in animal cells is accomplished by constriction of the cell along the plane of cell division (formation of a cleavage furrow). In plant cells, which have cell walls, a completely different mechanism of cytokinesis has evolved. Which of the following statements are true of cytokinesis in plant cells? Select the two that apply.

The cell plate consists of the plasma membrane and cell wall that will eventually separate the two daughter cells. Vesicles from the Golgi apparatus move along microtubules, coalesce at the plane of cell division, and form a cell plate. In plant cell division, after chromosome separation, the microtubules of the mitotic spindle reorganize into a network that guides vesicles derived from the Golgi apparatus to the plane of cell division. These vesicles begin to fuse, forming the cell plate. As more vesicles are added to the cell plate, it grows outward, eventually fusing with the parent cell plasma membrane. Membrane from the vesicles forms the new plasma membrane for each daughter cell. At the same time, materials that were enclosed in the vesicles form the new cell wall between the new plasma membranes of the daughter cells.

The roles of proteins in bacterial cell division and eukaryotic cytokinesis Part complete The division of a bacterial cell into two daughter cells (called binary fission) is accomplished by a protein called FtsZ. FtsZ is very similar to the tubulin subunits that form microtubules in eukaryotes. After the replicated bacterial chromosomes have moved to opposite ends of the cell, a ring of FtsZ proteins forms inside the plasma membrane in the region where the cell will divide. As the FtsZ ring constricts, the plasma membrane and bacterial cell wall fold in and eventually separate into two cells.

The physical division of one cell into two during cell division is common to all types of cells. In all cases, proteins related to the cytoskeleton play some critical role. However, the mechanism by which division occurs depends on whether a rigid cell wall is present. In bacteria and animal cells, which do not have a rigid cell wall, division occurs by constriction of a ring of proteins (microtubule-like proteins in bacteria and microfilaments in animal cells) that pinches the cell in two. In plants, which do have a rigid cell wall, microtubules guide the aggregation of Golgi-derived vesicles to form the cell plate, which eventually forms the new cell wall and plasma membrane between the daughter cells.

Why do some species employ both mitosis and meiosis, whereas other species use only mitosis? Consider the kinds of cell reproduction different species need.

They need both if they are producing plant gametes. Correct. The production of gametes in plants is a function of both mitosis and meiosis. The form of cell division that produces animal gametes is meiosis, not mitosis.

After chromosomes condense, the---- is the region where the identical DNA molecules are most tightly attached to each other.

centromere(s)

The -----are the organizing centers for microtubules involved in separating chromosomes during mitosis.

centrosome(s)

During interphase, most of the nucleus is filled with a complex of DNA and protein in a dispersed form called ------

chromatin.

Cytochalasin B is a chemical that disrupts microfilament formation. How would this interfere with cell division? See Concept 12.2 (Page)

cleavage In animal cells, the cleavage furrow forms when a ring of microfilaments contracts, causing the parent cell to be pinched in two.

In a stage prior to metaphase,----are involved in holding sister chromatids together. In a stage after metaphase, cleavage of these proteins permits the sister chromatids to separate.

cohesin(s) Cohesins hold the sister chromatids of a duplicated chromosome together early in mitosis. Cleavage of the cohesins at the beginning of anaphase allows the sister chromatids to separate.

7. In most eukaryotes, division of the nucleus is followed by -----when the rest of the cell divides.

cytokinesis

What processes occur during the M phase of the cell cycle? opened hint The M (mitotic) phase begins when the cell passes through the G2 checkpoint. It is typically the shortest, but most complex, phase of the cell cycle. Which three of the following processes occur during the M phase of the cell cycle?

cytokinesis separation of sister chromatids formation of the mitotic spindle

in dividing cells, most of the cell's growth occurs during -----

interphase.

During mitosis, microtubules attach to chromosomes at the-----

kinetochore(s).

The----- is a cell structure consisting of microtubules, which forms during early mitosis and plays a role in cell division.

mitotic spindle(s)

DNA replication produces two identical DNA molecules, called, --which separate during mitosis.

sister chromatid(s)

key structures of mitosis ^

the key structures involved in mitosis are labeled in this diagram of an animal cell that shows the two sister chromatids of each duplicated chromosome beginning to attach to the mitotic spindle by means of their kinetochores. The centrosomes anchor the mitotic spindle at opposite ends of the cell.

During prophase

the microtubules of the mitotic spindle lengthen

during telophase

the nonkinetochore microtubules disassemble.

during anaphase

the nonkinetochore microtubules lengthen and move past each other, and the kinetochore microtubules shorten.

In a stage prior to metaphase, the mitotic spindle is assembled from individual----molecules. At a stage after metaphase, the mitotic spindle disassembles.

tubulin(s) Microtubules assemble from tubulin subunits early in mitosis, forming the mitotic spindle. Late in mitosis when separation of the chromosomes is complete, the mitotic spindle disassembles, releasing free tubulin molecules.

cytokinesis

two separate daughter cells form During cytokinesis, the cytoplasm divides and two separate daughter cells form.