chapter 12 mastering bio

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.

Spindle fibers attach to kinetochores during

Attachment of spindle fibers to kinetochores is one of the events of prometaphase

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.

Bacteria only

Chromosome separation begins at the origin of replication on DNA.

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.

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.

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

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

These are the events of interphase.

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.

Both bacteria and eukaryotes

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

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.

Which of the following statements are true of cytokinesis in plant cells?

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.

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

During _____ both the contents of the nucleus and the cytoplasm are divided

The mitotic phase encompasses both mitosis and cytokinesis.

Chromosomes become visible during

prophase

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.

Centromeres divide and sister chromatids become full-fledged chromosomes during

anaphase

The centrosome(s)

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

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

chromatin

In most eukaryotes, division of the nucleus is followed by

cytokinesis, when the rest of the cell divides.

During telophase, the nonkinetochore microtubules

disassemble

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

interphase

Nucleoli are present during

interphase

The mitotic spindle(s)

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

After chromosomes condense, the centromere(s)

is the region where the identical DNA molecules are most tightly attached to each other.

During mitosis, microtubules attach to chromosomes at the

kinetochore(s)

During prophase, the microtubules of the mitotic spindle

lengthen

During anaphase, the nonkinetochore microtubules lengthen and move past each other, and the kinetochore microtubules

shorten

DNA replication produces two identical DNA molecules, called

sister chromatids, which separate during mitosis.

Cytokinesis often, but not always, accompanies

telophase

During prophase a homologous pair of chromosomes consists of

two chromosomes and four chromatids