Mastering Biology Chp. 9 HW

PART B _____ is a carcinogen that promotes colon cancer.

fat [A diet high in fat increases the risk of both colon and breast cancer.]

PART B During _____ both the contents of the nucleus and the cytoplasm are divided.

the mitotic phase [The mitotic phase encompasses both mitosis and cytokinesis.]

PART A - Comparing chromosome separation in bacteria and eukaryotes In all cells, separation of replicated chromosomes is a prerequisite for cell division. However, the mechanism of chromosome separation in bacteria is distinct from that in eukaryotes in several ways. Sort the following statements into the appropriate bin.

BACTERIA ONLY: -chromosome separation begins at the origin of replication on DNA 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 BOTH BACTERIA AND EUKARYOTES: -chromosomes replicate before cell division -replicated chromosomes separate by attaching to some other structural feature of the cell [Although the processes of chromosome separation in bacteria and eukaryotes have a common evolutionary origin, the actual mechanisms are different. Structurally, bacterial cells contain a single chromosome that is much shorter than those in eukaryotic cells, and bacterial cells lack a mitotic spindle. The bacterial chromosome does not fully condense before separation. However, the physical separation of the replicated bacterial chromosomes still involves attachment to some structure in the cell: possibly the plasma membrane at the origins of replication.]

PART C During _____ the cell grows and replicates both its organelles and its chromosomes.

interphase [These are the events of interphase.]

In some organisms, such as certain fungi and algae, cells undergo mitosis repeatedly without subsequently undergoing cytokinesis. What would result from this?

large cells containing many nuclei

Why is it difficult to observe individual chromosomes with a light microscope during interphase?

They have uncoiled to form long, thin strands. [Except during the M phase, the DNA is extended, allowing its genes to be transcribed for protein synthesis.]

Which of the following is true of benign tumors, but not malignant tumors?

They remain confined to their original site [Benign tumors can often be surgically removed because their boundaries are well defined, whereas some malignant tumors go on to metastasize.]

PART C - The roles of proteins in bacterial cell division and eukaryotic cytokinesis 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. Plant and animal cells also require cytoskeletal proteins for cytokinesis, although the roles these proteins play differ among bacteria, plants, and animals. For each space in the table, drag the appropriate label to indicate if the statement is true or false for each group of organisms.

-Cells divide by a constriction of a ring protein: True for bacteria True for animal False for plants -The presence of a cell wall prevents the cell from dividing by constriction: False for bacteria False for animals True for plant cells -Tubulin subunits or tubulin-like molecules function in the division of the cell True for bacteria False for animals True for plants [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.]

PART B - Cytokinesis in plant cells 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.

-Vesicles from the Golgi apparatus move along microtubules, coalesce at the plane of cell division, and form a cell plate -The cell plate consists of the plasma membrane and cell wall that will eventually separate the two daughter cells [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.]

PART C - Changes in DNA structure during the cell cycle 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.

1. Are sister chromatids present in all or part of this phase? S - YES G2- YES Beginning of M - YES End of M - NO 2. Is the DNA condensed in all or part of this phase? S - NO G2- NO Beginning of M - YES End of M - YES 3. Does the cell contain twice as much DNA in this phase as it did in the G1 phase? S - YES G2- YES Beginning of M - YES End of M - YES [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.]

PART A - Mitosis and cell cycle terminology As in most areas of biology, the study of mitosis and the cell cycle involves a lot of new terminology. Knowing what the different terms mean is essential to understanding and describing the processes occurring in the cell. Drag the terms on the left to correctly complete these sentences. Not all the terms will be used.

1. DNA replication produces two identical DNA molecules, called SISTER CHROMATID(S), which separate during mitosis. 2. After chromosomes condense, the CENTROMERE(S) is the region where the identical DNA molecules are most tightly attached to each other. 3. During mitosis, microtubules attach to chromosomes at the KINETOCHORE(S). 4. In dividing cells, most of the cell's growth occurs during INTERPHASE. 5. The MITOTIC SPINDLE(S) is a cell structure consisting of microtubules, which forms during early mitosis and plays a role in cell division. 6. During interphase, most of the nucleus is filled with a complex of DNA and protein in a dispersed form called CHROMATIN. 7. In most eukaryotes, division of the nucleus is followed by CYTOKINESIS, when the rest of the cell divides. 8. The CENTROSOME(S) are the organizing centers for microtubules involved in separating chromosomes during 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.]

PART C - The mitotic spindle The mitotic spindle consists of two types of microtubules: kinetochore microtubules and nonkinetochore microtubules. In animal cells, these two types of microtubules function differently in the stages of mitosis. Complete the sentences by dragging the terms to the appropriate locations. Terms may be used once, more than once, or not at all. (The mitotic spindle)

1. During prophase, the microtubules of the mitotic spindle LENGTHEN. 2. During anaphase, the nonkinetochore microtubules LENGTHEN and move past each other, and the kinetochore microtubules SHORTEN. 3. During telophase, the nonkinetochore microtubules DISASSEMBLE. [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.]

PART D - Roles of the mitotic spindle 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. Consider three questions concerning the animal cell that has been treated with the inhibitor. Drag the terms to answer the questions. Terms may be used once, more than once, or not at all. (Roles of the mitotic spindle)

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.]

PART B - Phases of the cell cycle The cell cycle represents the coordinated sequence of events in the life of a cell from its formation to its division into two daughter cells. Most of the key events of the cell cycle are restricted to a specific time within the cycle. In this exercise, you will identify when various events occur during the cell cycle. Recall that interphase consists of the G1, S, and G2 subphases, and that the M phase consists of mitosis and cytokinesis. Drag each label to the appropriate target.

A - nondividing cells exit cell cycle B - at this point cell commits to go through cell cycle. C - DNA replicates D - Two centrosomes have formed E - mitotic spindle begins to form F - cell divides forming two daughter cells. [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.]

Which of the following is found in binary fission but not in mitosis?

Duplicated chromosomes attach to the plasma membrane. [This is not true of mitosis]

PART A Which of these phases encompasses all of the stages of mitosis but no other events?

E [This is mitosis.]

PART B - Mechanisms underlying the events of mitosis Mitosis unfolds through a sequence of stages marked by specific events in the cell. The structural changes in the cell are brought about by a series of tightly coordinated underlying mechanisms. Sort each process into the appropriate bin to indicate the stage of mitosis in which it occurs. If a process occurs in more than one stage, sort it to the stage when it first occurs.

PROPHASE: -Tubulins assemble into spindle microtubules -Cohesins join sister chromatids of duplicated chromosomes PROMETAPHASE: -Microtubules attach to kinetochores METAPHASE: -Kinetochores are motionless in relation to cell poles ANAPHASE: -Kinetochores move toward poles of the cells -Cohesins break down TELOAPHASE: -Spindle microtubules disassemble [The micrographs in Part A show some of the cellular processes that occur during the stages of mitosis. -In prophase, the microtubules of the spindle apparatus begin to assemble from individual tubulin subunits. As the identical chromatids of each pair of sister chromatids condense during this stage, they are held together by cohesin proteins. -Prometaphase is marked by fragmentation of the nuclear envelope, expansion of the spindle into the nuclear region, and attachment of some spindle fibers to the chromosomes via the kinetochores. -Metaphase, marked by the alignment of chromsomes along the metaphase plate, is brought about by kinetochores aligning and then remaining motionless relative to the poles of the cell. -In anaphase, the cohesin proteins are cleaved, and the kinetochores move toward the poles of the cell, separating the sister chromatids. -As telophase proceeds, the kinetochore microtubules of the spindle disassemble. As the chromosomes reach the poles of the cell, the nuclear envelopes of the two new daughter nuclei form.]

PART A - Identifying the stages of mitosis 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.]

PART A Which of these is NOT a carcinogen?

all of the above are carcinogens (cigarette smoke, fat, UV light, testosterone) [All of these substances can cause cancer.]