Introduction and Learning Objectives
All species - bacteria, tree frogs, fungi, people - grow and reproduce. For growth and reproduction to occur, cells must divide. In order for cells to give rise to new cells, all genetic information and cellular contents must be replicated. This tutorial and the next will focus on the cell division events that occur in eukaryotic cells - mitosis and meiosis. This tutorial will focus on mitosis and by the end of this tutorial you should have a basic working understanding of:
- The role of mitosis in the life cycle of single-celled and multicellular organisms
- The phases of the cell cycle
- The major chromosomal events of each stage of mitosis
- Chromosome movement during the cell cycle
- define diploid versus haploid cells
- summarize the role of mitosis in eukaryotes
- be able to diagram and label the stages of mitosis
- be able to explain why mitosis results in daughter cells that are genetically identical to the parent cell
- explain the differences between karyokinesis and cytokinesis
The Cell Cycle
Cell division is a precisely regulated process. Although mitosis is a process by which eukaryotic cell division occurs (remember, eukaryotic cells can also divide via meiosis - a process you will learn about in the next tutorial), many events need to take place prior to the physical separation of a mother cell into two daughter cells. Mitosis is a highly orchestrated process, with many checkpoints that insure events occur in the proper sequence. However, cells only divide at certain times and in specific situations.
Figure 1. Simplified Diagram of an Animal Cell. (Click to enlarge)
All of the components of a cell must be replicated prior to cell division (only a fraction of the components are shown in this figure of an animal cell (Fig. 1)). The replication of new components is highly regulated. Only if the cell receives the proper signal (the nature of which is dependent on the particular cell and the environment in which it exists), will events be set in motion that lead to cell division. As long as the signal is present, the cells will continue to divide in a cyclical process known as the cell cycle.
The cell cycle (Fig. 2) is a continuous process, but to make it easier to study it can be broken down into four phases. The M phase is the mitotic phase. The other three phases are collectively known as interphase. The three phases of interphase following mitosis are: the the G1 growth phase, the S phase or synthesis phase, which is when DNA is replicated, and the G2 growth phase. Although the synthesis of nuclear DNA is restricted to the S phase, replication of cellular components can occur throughout the cell cycle (mostly being limited to G1, S, and G2).
Figure 2. The Cell Cycle. (Click to enlarge)
Introduction to Mitosis
Single-celled organisms use mitosis to reproduce while multi-cellular organisms use mitosis to grow (this allows the organism to get larger while the cells remain small and thus preserve their surface area to volume ratio). The dynamic process of mitosis takes many eukaryotic cells about 1-3 hours to complete. During this time, the cell completes a number of stages. Often the process is simplified and drawn as discrete steps, but it is important to remember that these steps represent landmarks in a continuous process.
The chromosomes that were replicated during the S phase are partitioned so that each new daughter cell has a complete set of chromosomes. Karyokinesis is defined as the separation of the chromosomes. The organelles and other cytoplasmic components that were replicated during G1, S, and G2 are partitioned in a process known as cytokinesis.
To determine which stage of cell division is occurring during mitosis, one observes the behavior of the nucleus. The stages of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase.
Major Events of Mitosis: Prophase
Figure 3. The Organization of DNA in Chromosomes. (Click to enlarge)
The first phase of mitosis, prophase, is marked by the condensation of chromosomes (Fig. 3). During interphase, DNA is replicated and the replicated chromosomes remain relatively stretched out. It is difficult to visualize individual chromosomes prior to prophase using standard microscopy. This changes once the cell progresses into G2.
The chromosomes begin to condense at some point during G2. In this complex process, each replicated elongated chromosome becomes supercoiled and, as a result, becomes considerably shorter and more tightly packed (the replicated chromosome shown in Fig. 3). Each replicated chromosome is only a few microns long, and by the end of prophase appears as two replicated, identical chromatids attached at the centromere (a condensed area found on all eukaryotic chromosomes). It is important to consider the significance of chromosome condensation; a highly compacted chromosome is easier to move than a stretched-out one. Moreover, chromosomes will physically be moved around in the cell later. If the DNA were stretched out, it would be subject to physical shearing. In the condensed form, it is less prone to physical damage.
Major Events of Mitosis: Prometaphase and Metaphase
As the chromosomes condense, the nuclear envelope begins to disassemble (in most eukaryotes). Dissolution of the nuclear envelope signifies further progress in mitosis, and this landmark is used to identify a cell that has successfully begun prometaphase.
The transition into metaphase (Fig. 4)represents an intricate cellular event whereby all of the chromosomes move into a line (the metaphase plate).
It is not understood how the cell "knows" when the chromosomes are precisely lined up. However, it is known that this is an important "checkpoint" and that the cell will not proceed to the next step until all of the chromosomes are properly positioned. The jostling of chromosomes can take a good deal of time. Therefore, this stage takes the longest time in most cells.
Figure 4. Metaphase. (Click to enlarge)
Major Events of Mitosis: Anaphase
Each chromatid has a centromere; therefore, in metaphase, the back-and-forth jostling results in chromosomes that are not only lined up in a single plane, but each sister chromatid is aligned opposite one another. This arrangement is well suited for accurate partitioning of the chromatid.
Once the cell senses proper alignment along the metaphase plate, the replicated chromatids separate rapidly, signifying anaphase. Two notable things happen during anaphase: first, the centromeres that hold the chromatids together dissolve, separating the chromatids from each other; and second, the newly freed chromatids (now properly called chromosomes) move rapidly toward the poles. Figure 5 shows a cell in anaphase. The separated chromatids (now chromosomes) are stained blue and the fibers that pull the chromosomes to each pole of the cell are stained green.
Figure 5. A cell during anaphase (Click to enlarge). From Wikipedia Commons.
Major Events of Mitosis: Telophase and Cytokinesis
The replicated chromosomes (formally called chromatids) are at opposite poles at the end of anaphase. Both halves of the cell contain equal numbers and kinds of chromosomes. At this point, the nuclear membranes of the daughter cells begin to reform around the new chromosomes located at either end of the cell. At the same time, the chromosomes begin to decondense. The decondensation of chromosomes and nuclear envelope reformation characterizes the telophase (Fig. 6) stage of mitosis. Note that these two new nuclei contain identical genetic material. For instance, in a human cell there are 46 chromosomes: 23 of paternal origin and 23 of maternal origin. Telophase marks the last part of karyokinesis (the division of genetic material).
At the same time that nuclear membranes are reforming during telophase, something remarkable is happening to the other cellular components. Remember, the other organelles have already replicated on their own. They are also being separated into the two ends of the cell. The cell then closes off the center of the parent cell, thereby forming two new daughter cells. As previously mentioned, the process of partitioning the parental cytoplasm (including organelles) is called cytokinesis. Remember, both cytokinesis and karyokinesis are parts of mitosis.
Figure 6. Telophase. (Click to enlarge)
Cell division includes the division of chromosomes (karyokinesis), as well as the division of the cytoplasm (cytokinesis). Reformation of the nuclear envelope around the daughter cell chromosomes marks the completion of karyokinesis. However, cell division is not complete until the cytoplasm has divided during cytokinesis (which typically begins during telophase).
The process of karyokinesis has been conserved throughout evolution. However, cytokinesis can be accomplished in more than one way. Cells are different and these differences put constraints on cytoplasmic partitioning. Two examples are plants and animals; figure 7 summarizes these differences.
Figure 5. Telophase and Cytokinesis in Plant Cells vs. Animal Cells. (Click to enlarge)
Plant cells have cell membranes and rigid cell walls, however, animal cells have only cell membranes and are much more flexible. Therefore, these two types of cells have different mechanisms for cytokinesis.
Animal cells begin cytokinesis when the cell membrane pinches inward. This is called furrowing because a cleavage furrow forms between the two halves of the cell. The furrow gets deeper and deeper as the cytoplasm separates more and more. This process almost looks like someone is pulling a string tighter and tighter around the cell, to the point of splitting it in two. The cell membrane finally seals off and the original parental cell becomes two separate daughter cells.
Plant cells are not able to undergo the same process because they have a rigid, inflexible cell wall. These cells divide by forming a new piece of cell wall in their center. Vesicles deposit wall-building materials along the central area of the cell. Typically, this is the same plane where the chromosomes were lined up previously during metaphase. The new cell wall begins to form and the cell membrane is extended. The two daughter cells are eventually sealed off from one another by a new cell wall.
Other types of cytokinesis are also known, but will not be covered here. In all cases, however, the cytokinetic process is adapted to the particular character of the cell.
All eukaryotes have linear chromosomes, and partitioning the chromosomes equally into daughter cells is a carefully orchestrated process.
Mitosis insures that each sister chromatid goes into one daughter cell as the cell divides. During this continuous process, there are landmarks that mark the progression of mitosis. These landmarks are the so-called "stages" of mitosis (prophase, prometaphase, metaphase, anaphase and telophase). Be sure you know the major criteria that demarcate one stage from the other.
After reading this tutorial, you should have a working knowledge of the following terms:
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