Chapter 12-AP Biology

What is mitosis? How is it different from cytokinesis?

Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis, the division of the cytoplasm.

Describe binary fission.

A method of asexual reproduction by "division in half." In prokaryotes, binary fission does not involve mitosis, but in single-celled eukaryotes that undergo binary fission, mitosis is part of the process.

What is a kinetochore?

A structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle

What is a protein kinase?

An enzyme that transfers phosphate groups from ATP to a protein, thus phosphorylating the protein.

At which end do kinetochore microtubules shorten during anaphase? Explain the Inquiry Figure that supports where this shortening occurs.

As the chromosomes moved poleward, the microtubule segments on the kinetochore side of the mark shortened, while those on the spindle pole side stayed the same length. During anaphase in this cell type, chromosome movement is correlated with kinetochore microtubules shortening at their kinetochore ends and not at their spindle pole ends. This experiment supports the hypothesis that during anaphase, a chromosome is walked along a microtubule as the microtubule depolymerizes at its kinetochore end, releasing tubulin subunits.

G1

"First Gap" The first part of interphase. If a cell receives the go-ahead signal at this checkpoint, the cell cycle will continue. It is regulated by the cyclin-CDk complexes.

G2

"Second Gap" The last part of interphase. When sufficient MPF accumulates, the G2 checkpoint is passed, and mitosis is promoted.

S

"Synthesis" When the chromosomes duplicate

chromosome

A cellular structure carrying genetic material, found in the nucleus of eukaryotic cells. Each chromosome consists of one very long DNA molecule and associated proteins. (A bacterial chromosome usually consists of a single circular DNA molecule and associated proteins. It is found in the nucleoid region, which is not membrane bounded.)

density-dependent inhibition

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. Follow-up studies revealed that the binding of a cell-surface protein to its counterpart on an adjoining cell sends a growth-inhibiting signal to both cells, preventing them from moving forward in the cell cycle, even in the presence of growth factors.

Describe cytokinesis in a plant cell.

Cytokinesis in plant cells, which have cell walls, is markedly different. There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect in the cell plate as it grows. The cell plate 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 cell plate has formed between the daughter cells.

Explain the difference between kinetochore and nonkinetechore microtubules. What is the function of each?

During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called kinetochore microtubules. Meanwhile, microtubules that do not attach to kinetochores have been elongating, and by metaphase they overlap and interact with other nonkinetochore microtubules from the opposite pole of the spindle.

What are growth factors? How does platelet-derived growth factor (PDGF) stimulate fibroblast division?

Growth factor is a protein released by certain cells that stimulates other cells to divide. Fibroblasts have PDGF receptors on their plasma membranes. The binding of PDGF molecules to these receptors triggers a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide.

What happens if all the chromosome kinetochores are not attached to spindle fibers? When this occurs, which checkpoint is not passed?

If all the chromosome kinetochores are not attached to spindle fibers, the sister chromatids remain together, delaying anaphase. When this occurs, the M phase checkpoint is not passed.

centromere

In a duplicated chromosome, the region on each sister chromatid where they are most closely attached to each other by proteins that bind to specific DNA sequences; this close attachment causes a constriction in the condensed chromosome.

Describe cytokinesis in an animal cell.

In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate. On the cytoplasmic side of the furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. The actin microfilaments interact with the myosin molecules, causing the ring to contract. The contraction of the dividing cell's ring of microfilaments is like the pulling of a drawstring. The cleavage furrow 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.

M

In the M phase, mitosis distributes the daughter chromosomes to daughter nuclei, and cytokinesis divides the cytoplasm, producing two daughter cells. The kinetochores must all be attached to spindle fibers during metaphase. This will activate an enzyme (separase), which allows the sister chromatids to separate and anaphase will proceed.

What does MPF trigger? What are some specific activities that it triggers?

MPF triggers the cell's passage past the G2 checkpoint into M phase.

chromatid

One of two identical joined copies of the original chromosome

The activity of cyclin-dependent kinases (CDks) rises and falls. Why?

The activity of CDks rises and falls with changes in the concentration of its cyclin partner, MPF.

Define chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope.

chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope.

anchorage dependence

To divide, they must be attached to a substratum, such as the inside of a culture jar or the extracellular matrix of a tissue.

How is the cell plate formed? What is the source of the material for the cell plate?

Vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate.