Biology 123 Chapter 12 (Mastering)

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)

Comparing chromosome separation in bacteria and eukaryotes The division of any cell, prokaryotic or eukaryotic, requires that the genetic information in each of the parent cell's chromosomes be faithfully copied and distributed into each daughter cell. Only then does the cell physically divide. Because of their divergent evolutionary paths, bacteria, plants, and animals have somewhat different mechanisms of chromosome separation and cell division. 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.

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.

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

chromatin

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.

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.

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 M cytokinesis S G2

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

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.

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.

Mitosis and the Cell Cycle (BioFlix tutorial)

Mitosis is the part of eukaryotic cell division in which the nucleus divides and distributes the chromosomes to the two daughter nuclei, ensuring that each daughter cell receives chromosomes identical to those of the parent cell. Mitosis is just one part of the cell cycle, the overall sequence of events from the formation of a cell by cell division to the cell's own division into two new daughter cells. In this tutorial, you will review key terms associated with mitosis and the cell cycle, cellular processes that occur during the main phases of the cell cycle, and changes in DNA structure during the cell cycle. Before beginning this tutorial, watch the Mitosis animation. Pay particular attention to the structural changes that occur in the DNA before and during cell division.

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.

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.

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.

Roles of the mitotic spindle (continued)

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.

Mitosis and cell terminology: Key structures involved in mitosis of an animal cell (labeled)

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.

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.

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.

The mitotic spindle (continued)

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.

The roles of proteins in bacterial cell division and eukaryotic cytokinesis (continued) Plant and animal cells also require cytoskeletal proteins for cytokinesis, although the roles these proteins play differ among bacteria, plants, and animals.

The physical division of one cell into two during cell division is common to all types of cell. In all cases, proteins related t 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 into 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? See Concept 13.2 (Page)

They need both if they are producing plant gametes. 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.

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. The plasma membrane of the parent cell grows inward, eventually joining with the cell plate. After chromosome separation is complete, a network of microfilaments forms near the plane of cell division that will separate the two new cells. 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.

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.

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)

Cytochalasin B is a chemical that disrupts microfilament formation. How would this interfere with cell division? See Concept 12.2 (Page) DNA replication formation of the cell plate cleavage binary fission formation of the mitotic spindle

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

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

cytokinesis

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

interphase

During mitosis, microtubules attach to chromosomes at the _______.

kinetochore(s)