You should have a working knowledge of the following terms:
Introduction and Goals
In order for cells to give rise to new cells, all genetic information and cellular contents must be replicated. You previously learned how DNA is replicated. Now you will learn how this newly replicated parental DNA is precisely partitioned and how cytoplasmic contents are divided into two daughter cells. This tutorial will focus on the cell division events that occur in eukaryotic cells; namely, mitosis. By the end of this tutorial you should have a basic working understanding of:
- The phases of the cell cycle
- The major events of each stage of mitosis
- The role of microtubules in cell division
- Chromosome movement during the cell cycle
- Regulation of the cell cycle
- Which parts of the cell cycle encompass karyokinesis versus cytokinesis
- Some variations of karyokinesis
The Cell Cycle
Cell division is a precisely regulated process. Although mitosis is the process by which cell division occurs, 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). 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 two new daughter cells will continue to replicate themselves in a cyclical process known as the cell cycle.
The cell cycle 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
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 the necessary nuclear genetic information. Karyokinesis is defined as the separation of 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. Each stage will be discussed next. Nuclear structure and function will be further addressed in tutorial 26.
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. 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 (depicted in this figure). Each replicated chromosome is only a few microns long, and by the end of prophase appears as two replicated chromatids attached at the centromere. 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.
While the chromosomes are condensing within the nucleus, the microtubules that will form the mitotic scaffolding are also forming around the nuclear envelope. The two poles of the future mitotic spindle can be seen by the end of prophase. These poles (centrosomes) are visible with appropriate microscopy techniques.
Major Events of Mitosis: Prometaphase and Metaphase
As the chromosomes condense and the mitotic spindle begins to form, 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. Nuclear membrane breakdown during prometaphase allows the microtubules that make up the mitotic spindle to enter the area where the condensed chromosomes are located.
As the spindle microtubules (also called spindle fibers) begin attaching to the condensed chromosomes, a complex interaction begins between the mitotic spindle and the chromosomes. The chromosomes begin to jostle back and forth; the microtubules push and pull each of the attached chromosomes until the net forces between all chromosomes are equal. This scenario marks the transition into metaphase and represents an intricate cellular event whereby all of the chromosomes appear in a line (the metaphase plate) because the balance of forces drives the collection of all replicated and condensed chromosomes into a single plane in preparation for their segregation.
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.
Major Events of Mitosis: Anaphase
Spindle microtubules typically attach to replicated chromosomes at the centromere. Importantly, 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. Three things happen during anaphase: first, the centromeres that hold the chromatids together dissolve, separating the pairs from each other; second, the newly freed chromatids (now properly called chromosomes) move rapidly toward the spindle poles (centrosomes); and third, the spindle poles move apart. Anaphase is a rapid process (in some cells it is completed within 5 minutes). The sister chromatids are pulled apart by molecular motors that associate with the spindle fibers during anaphase.
Major Events of Mitosis: Telophase and Cytokinesis
The replicated chromosomes (formally called chromatids) are at opposite ends of the spindle 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 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.
How Does Mitosis Happen?
Mitosis involves the highly specific and orchestrated movement of chromosomes. This physical movement is performed by small molecular motors located within the mitotic spindle and in association with centromeric areas of the chromosomes. These motors use ATP as an energy source and transport their chromosomal cargo along the microtubules.
Spindle fibers radiate out from two centrosomes that flank either side of the nucleus (which will soon divide). The centrosomes are the organizers of the spindle fibers. The spindle fibers radiate out from the centrosome at the beginning of prophase. They form a structure that stretches all the way across the condensed chromosomes at metaphase, then they shorten as mitosis nears its completion.
The spindle fibers are made up of microtubules. Microtubules are just what they sound like, tiny tubes. They are composed mostly of a polymer protein called tubulin. Remember, a polymer is a large molecule made up of many, very similar, smaller molecules. Many molecules of tubulin join together to form this long hollow tube (microtubule). This process is something similar to fishing, where the centrosomes cast their microtubules attempting to latch onto a chromosome. If the microtubule comes up short, it rapidly depolymerizes and the centrosome sends out another microtubule.
Figure 4. Metaphase. (Click to enlarge)
The microtubules extend outward from the centrosome and intersect one another toward the center of the condensed chromosomes. Some of the microtubules from one centrosome reach out and attach to microtubules from the other centrosome. These microtubules help to push the spindle poles apart in anaphase. Other microtubules attach to the sister chromatids. The sister chromatids are attached to one another at the centromere. The structure at the point where they attach is termed the kinetochore. Those microtubules that attach to the kinetochore are referred to as kinetochore microtubules.
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. That is, every eukaryote uses microtubules and molecular motors to move chromosomes. However, cytokinesis can be accomplished in more than one way. Cells are different and these differences put constraints on cytoplasmic partitioning. Two examples (using plants and animals) will be discussed here, however, other cytokinetic avenues exist. The figure below 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.
Variations on Karyokinesis
So far we have covered cell division in eukaryotes. You may be wondering if prokaryotes (e.g., E.coli) divide in the same way. Remember, prokaryotes only have one large circular chromosome; rather than the multiple linear chromosomes possessed by eukaryotes. When bacteria divide, the replicated chromosomes attach to different parts of the plasma membrane. The cell then elongates and separates the chromosomes in one action. When the cell furrows, the two chromosomes (and other cell contents) are split into the two new cells (recall, binary fission). The process of binary fission is not limited to bacteria. Mitochondria and chloroplasts also divide in this manner, which is one piece of evidence that supports their prokaryotic origins.
Not all eukaryotes divide in the same way, and the differences may provide clues as to how mitosis evolved (as depicted in this hypothetical scenario). For example, the nuclear membranes of dinoflagellates (eukaryotes to be discussed in tutorial 27) do not disappear during prometaphase. Instead tunnels, containing microtubules, run through the prometaphase nucleus. The chromosomes attach to the intact nuclear membrane and the microtubules pull the chromosomes and attached nuclear membrane to either end of the cell during anaphase. Similarly, the nuclear membranes in diatoms (eukaryotes to be discussed in tutorial 30) do not break down either. Here the spindle forms inside the nucleus. The spindle then pulls the replicated chromosomes apart and the nucleus pinches in two.
Figure 6. Hypothetical Evolution of Cell Division. (Click to enlarge)
Regulation of the Cell Cycle
This circular diagram depicts the events that occur in a cell that is in a constant state of division. However, few cells exist in this state. For instance, nerve cells are not duplicated after a certain point in development, which is why spinal cord injuries and nervous system diseases are so devastating. Conversely, blood cells are constantly replaced from bone marrow cells, which divide frequently.
Figure 2. The Cell Cycle. (Click to enlarge)
There are many things that control when, and if, a cell will divide. One example is nutrient availability. A cell must replicate DNA (and its various organelles) before it can divide, and there are many ways that a cell can monitor its nutritional status; if nutrients are insufficient, it typically will not divide. Other factors that can control the rate or timing of cell division are hormones, growth factors, and a variety of environmental cues (e.g., light).
When a cell is not dividing, it is typically arrested in a special subset of the G1 phase of the cell cycle known as quiescence, or the G0 phase. If the conditions are right and the cell receives the proper signals to divide, the cell moves through G1 and into S phase. Once the cell begins this process it is committed to dividing and many regulatory molecules within the cell take over. These regulatory molecules have been conserved throughout evolution. This means that all organisms (e.g., yeast, trees, cats, humans, etc.) share the same basic molecular machinery.
Figure 7. Cell Cycle Regulatory Proteins. (Click to enlarge)
These regulatory proteins are involved in a complex set of reactions that must work properly or the cell cycle would get out of control. The most prominent molecule involved in cell cycle control is cyclin-dependent kinase (CDK). The presence of "kinase" in the name reveals that this enzyme will phosphorylate something. The targets of phosphorylation include some of the proteins within the nuclear envelope and spindle fibers. Kinase activity depends on the concentration of another protein, cyclin, present in the cell. Because kinase activity is dependent on cyclin, the enzyme is known as a cyclin-dependent kinase. The cyclin-CDK complex is also referred to as MPF (maturation promoting factor). To remember its role in the cell, one can also think of it as M phase promoting factor.
Mitosis was examined in this tutorial. All eukaryotes have multiple chromosomes, and partitioning the chromosomes equally into daughter cells is a complicated event. In order for this partitioning to occur, eukaryotes have a spindle apparatus. (Although it may appear slightly different, depending on the species, it is involved in moving chromosomes.)
The spindle apparatus' purpose is to insure that each sister chromatid goes into one daughter cell as the cell divides. This process of division in eukaryotes is known as mitosis. 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.
Available data indicate that very early in the course of evolution, eukaryotes developed a cell control mechanism. That is, if one examines the "cell cycle" control machinery among all eukaryotes, one finds the same major molecular players; this is an additional piece of evidence that points to the common ancestry of all eukaryotes. The cell cycle can be described in terms of M phase (mitosis), S phase (DNA synthesis), and two transitional stages (designated G1 and G2). In order for a eukaryotic cell to enter and be maintained within the cell cycle, one or more signals need to be present. Although the actual control molecules are conserved, a variety of external signals can trigger their activity. In the case of a protist, nutrient availability is typically required; in the absence of a suitable signal, the cell cycle machinery shuts down and the cell does not divide, entering the G0 phase of quiescence (or synthesize new DNA).
Central to the cell cycle machinery is a set of enzymes (protein kinases) that act to add a phosphate group onto other proteins. A common theme in eukaryotic cellular regulation is the phosphorylation of proteins. There are many different types of kinases and they can act at various times in the life of the cell to affect a number of different processes. Important to our discussion on cell cycle control is the subset of protein kinases termed cyclin-dependent kinases or CDKs. They increase their activity in response to external signals (e.g., nutrient availability) and trigger the cell's mitotic machinery and DNA synthesis machinery to become active by joining with cyclin to form the MPF (maturation promoting factor) complex. As one might expect, many of the components of the cell's mitotic machinery are sensitive to phosphorylation. If one takes a close look at the cyclin-dependent kinase activity during the cell cycle, one will find that it is low in G1, then gradually climbs through S phase, and peaks in mitosis. Its activity rapidly falls around the M/G1 boundary.
As one considers the process of mitosis, keep in mind that the structural and functional changes are regulated by the cell cycle machinery. For example, the nuclear envelope breaks down in prophase because its proteins are being phosphorylated, and the control of the mitotic spindle is also brought about via the phosphorylation of specific molecules. One can think of cyclin-dependent kinase as the conductor of the orchestra; the instrumental players have diverse roles that act to insure that the cell's genetic material is accurately partitioned into two new daughter cells, each containing the same genetic material as its mother.