Mitosis is the ordered division of a eukaryotic cell nucleus into two nuclei with identical sets of chromosomes. The process of mitosis, while continuous, can be viewed as a series of events (prophase, prometaphase, metaphase, anaphase, and telophase). Once two nuclei have formed, the cell divides into two cells by cytokinesis.
learning outcomes
You should be able to:
Show how packaging of DNA into chromosomes ensures that each daughter cell receives an entire set.
Explain how cell structures are involved in mitosis.
Describe cytokinesis.
Differentiate among a chromosome, a chromatid, and a daughter chromosome, and identify their roles in a dividing cell.
A chromosome is a DNA molecule in the cell, and when the cell is in mitosis the chromosome is complexed with proteins to produce a visible, condensed structure. A chromatid is a DNA molecule complexed with proteins that is the product of S phase DNA replication. Chromatids are in pairs (the two products of replication) and lie attached to one another via the centromere until anaphase of mitosis. A daughter chromosome appears in an anaphase of mitosis, and was formerly a chromatid but has now separated from its partner as it migrated to the spindle pole.
What are the various levels of “packing” by which the genetic information contained in linear DNA is condensed during prophase?
During interphase, DNA is somewhat condensed by histone proteins into nucleosomes, and these fold over one another to form chromatin fibers. During prophase, the fibers attach as loops to proteins, and these in turn loop extensively to form the chromosome.
Taxol is an anticancer drug that binds to microtubules, preventing them from adding or removing subunits and causing them to form bundles. How do you think this drug works to stop cancer? Is Taxol specific to cancer cells?
Chromosomes are attached to spindle microtubules, and molecular motors on the microtubules move the chromosomes along. In addition, spindle microtubules shorten from the poles, and this causes the attached chromosomes to move to the poles. Taxol prevents these processes, and so inhibits cell division. The drug also inhibits the division of normal cells that enter M phase.
How does cytokinesis differ in plant and animal cells?
In plant cells, a cell plate forms from Golgi vesicles and this makes a new cell wall to separate the two daughter cells. Cell membranes grow below the cell walls. In animal cells, a “purse string” of microfilaments contracts to pinch off the cell membrane, and the cells separate.
How would the cell cycle be affected if there were a non-
A nonfunctional cohesion would not allow close attachment of chromatids during cell division, and there would not be a centromere. The two chromatids would be separate and there would be no organization for kinetochore attachment. There would be ineffective segregation of one chromatid of a pair to each daughter cell.
Mitosis results in two cells that are genetically identical. But there is another eukaryotic cell division process, meiosis, that results in genetic diversity. In the next section we will discuss the roles of mitosis and meiosis in sexual reproduction, and then turn to the details of meiosis in Key Concept 11.5.