Cell Cycle

 

Before one can examine the phases of mitosis or meiosis, one must first have a grasp on what a cell does to occupy its time. This is better known as the “Cell Cycle”. The cell cycle outlines how a cell occupies its time. It can be split into two overall phases. The first, and largest phase is called Interphase.

 

This is the phase that proceeds division (i.e. mitosis or meiosis) and one where the cell carries out a majority of its preparation functions. Interphase can be further broken down into sub-phases, but for the purposes of Grade 11 University Preparation, these are not needed to be known (yet). In terms of what should be known, during Interphase the cell undergoes a rapid growth, synthesis of DNA (in preparation for cell division in Mitosis), as well as other processes that prepare the cell for division. This is also the phase where the cell carries out a majority of its functions which are dependent on the type of cell in question. These include making structural proteins, transporting nutrients, eliminating waste, and controlling the sysnthesis of DNA and the replication of the genetic information in the chromosomes.

 

During interphase, the genetic material is still called chromatin. This includes the DNA molecules and associated proteins in the nucleus. The term chromatin refers to the long, thin strands of chromosomes that can be found dispersed throughout the nucleus. It is during interphase that the chromosome duplicates itself. The original and its copy are connected by a centromere. When attached, each portion of the pair are called sister chromatids. They contain identical information. The whole pair is referred to as one chromosome. This leads to the next phase, Cell Division.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This phase is where this section of this website will focus on. This is the stage where the cell undergoes division. This process is called mitosis (meiosis in sex cells) and can be further broken down into varies stages. This section of the website will seek to explore these stages in greater detail.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mitosis

           

Mitosis can be divided into four stages. It occurs in all somatic cells in the body. The four stages are called prophase, metaphase, anaphase, and telophase.

           

Prophase is the first stage of mitosis. Within prophase, the chromosomes (which are already duplicated and paired off as well as connected by centromeres) are now visible under a microscope. This is because they have shortened up and have become thick enough to see. During prophase, the cytoplasm within the cell begins to separate, moving its parts to opposite ends of the cell. At this point the centrioles, which provide a spot for the spindle fibers to attach to, are apparent. These fibers act as a guide line for the attachments and movement of the chromosomes during cell division. This is predominately what happens in animal cells. In plant cells however, we do not have centrioles, although spindle fibers are still present. The centromere that was joining the two sister chromatids anchors itself onto the spindle fibers. The nuclear membrane is also beginning to degrade to enable the separation process that is going to happen.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

Metaphase is the second stage of mitosis. Here, chromosomes (two sister chromatids attached) begin to align at the middle of the cell. This area of the cell is called the equatorial plate. They appear even darker and thicker now and are fully attached to the spindle fibers. However, the chromosomes are still entangled and can even become entangled with each other. Once all the chromosomes have aligned, the cell is ready to enter the next stage of division.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

Anaphase is the third stage of mitosis. Here, the centromeres divide and the sister chromatids, which can now be referred to as individual chromosomes, move to opposite sides of the cell. This is accomplished by the pulling apart done by the spindle fibers. This will yield the same number and type of chromosomes at each side of the cell. With the chromosomes separated to either end of the cell, anaphase is complete and the cell is ready to enter the last stage of mitosis.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

Telophase is the last stage of mitosis. Here the chromosomes, now at opposite ends of the cell, begin to lengthen. The spindle fibers dissolve and a new nuclear membrane begins to form around each mass of chromatin. This phase is quickly followed by the actual division of the cell, called cytokinesis. This is where the cytoplasm begins to divide by the formation of a furrow, which pinches the cell into two parts. In a plant cell, this process is followed by the formation of a cell wall/plate. This completes the stages of mitosis.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

At this point, your students (and yourself for the matter) may be wondering why cells need to divide at all. Is there an actual reason that cells cannot just continue to grow larger and larger? There is in fact an actual reason for this, and it’s one of math. The relationship between surface area and volume of a cell is what determines this need for division. To carry out its functions appropriately, a cell must exchange materials and wastes that go through the cell’s cytoplasm. When a cell divides, it can increase its surface area to volume ratio.

 

Meiosis

 

Now that you and your students have a good grasp on how everyday somatic (read: body) cells divide, the next cell cycle phases to move into are those that make up meiosis. The main difference between meiosis and mitosis lies in their purpose. Mitosis occurs in all somatic/body cells. Meiosis on the other hand only occurs in sec cells. It is important to reiterate this as much as possible throughout this section, otherwise when it comes to counting the number of chromosomes towards the end of the process, things can start to become confusing.

           

Another large difference lies in the process itself. While mitosis has multiple stages, they all occur in one phase. What this means is that although there are multiple steps, once cytokinesis occurs, mitosis is complete. With meiosis on the other hand, not only do we have multiple stages, our process does not end after the first physical division; we have two phases. At the end of this process we will have 4 daughter cells, each with one half the number of chromosomes as the parent cell.

 

So let’s begin. Just as in mitosis, we begin with interphase. Here the DNA is synthesized and the cell’s chromosomes duplicate. Just as with mitosis, there are other process and phases involved within interphase, but for the purposes of Grade 11, they are not needed to be covered.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

           

Prophase I ('a' above) has our chromosomes thickening and condensing. A process called synapsis occurs here. This is where a pair of homologous chromosomes line up closely together, forming what is called a tetrad. Each tetrad consists of four chromatids. Genetic recombination can also occur at this point. Centrioles, just as in mitosis, begin to move to each end of cell.

 

This leads us to metaphase I ('b' above). Here, the tetrads move towards the middle of the cell and line up along the equatorial plate. One here, the cell now moves into anaphase I ('c' above).

           

This third stage sees the chromosomes being pulled to opposite ends of the cell. Although similar to anaphase in mitosis, a large difference is present here. At this point in meiosis, the sister chromatids remain together after the homologous chromosomes have moved to opposite poles of the cell. Essentially, the cell has ‘split’ the tetrad, with two sister chromatids now at each side of the cell. The sister chromatids stay together. This brings us to fourth stage.

           

Telophase I ('d' above), and its associated cytokinesis ('e' above), first sees the spindle fibres continuing to move the homologous chromosomes to each side of the cell. At the end of this, each side of the cell will have a haploid number of chromosomes. A haploid number is half of the total number of chromosomes. This move is followed by cytokinesis which creates that furrow in the middle of the cell, separating it into two daughter cells, each with one half of the number of chromosomes of the parent. In mitosis, haploid numbers do not exist – each daughter cell contains the exact same amount of chromosomes as the parent. This completes the first phase of meiosis, and the cell is now ready to enter meiosis II.

 

The first stage of meiosis II is prophase II ('f' above). The second phase of meiosis directly echoes the stages of mitosis. The main difference is the number of chromosomes that the cell is dealing with at this point. In prophase II, the chromosomes do not replicate, but instead they move towards the center of the cell as they begin to darken and thicken.

           

The second stage of this phase is metaphase II ('g' above). Here the chromosomes line up at the equatorial plate and the sister chromatids are fully attached to the spindle fibres. They are ready to be pulled apart now.

           

The third stage, anaphase II ('h' above), has the chromatids separate and begin to move to each end of the cell. Once these chromatids have separated, just as in mitosis, each are now referred to as a full chromosome. In meiosis, they are now called daughter chromosomes. The cell begins to stretch apart at its poles, thus leading to telophase II.

           

Telophase II involves the membrane beginning to form around each group of chromosomes. Cytokinesis ('i' above) once again takes place, pinching through the middle, yielding a total of four daughter cells, each containing one half the number of chromosomes. When these sex cells unite during fertilization, the haploid cells become one diploid. If meiosis did not yield haploid numbers, offspring would then have double the amount of chromosomes needed, resulting in a whole new host of issues.

 

Misconceptions

 

Although mitosis and meiosis are fairly straightforward concepts, because there are so many steps involve, it is not unusual for students to become lost among the steps. This leads to rationalizing certain terms and outcomes that have unfortunately lead to a series of common misconceptions about cell division. The following are a few of them with their true explanations.

 

The first misconception is that meiosis results in the formation of sperm and egg cells. This misconception stems from the fact that students are told (correctly) that meiosis occurs only in sex cells. Sperm and egg are usually the associated mediums of such. However, the haploid cells that are created are not fully developed sperm and egg cells. Instead, the sex cells produced are called spermatogonia (in males) and oogonium (in females). These cells undergo their own divisions (spermatogenesis and oogenesis respectively) before yielding the traditional egg and sperm cells students are familiar with.

 

The next misconception is that chromosomes and chromatid are interchangeable terms for the same thing. This misconception most likely derives from the fact that both words are spelt similarly and either one or the other are found throughout each stage of mitosis and meiosis. Therefore, these are terms that the students will be seeing often throughout the unit. The actual fact of the matter is that chromatid is found before cell division, when the DNA replicates itself. Each chromatid is attached to each other in pairs, by a centromere. Each half of this pair is called a chromatid; one is the original and one is the copy. The total pair (i.e. both sister chromatids) as a whole are called chromosomes. In addition, (and this could be where more of the confusion is spreading from) once the chromatids are separated towards the end of each division process, the separated molecules are once again called chromosomes.

 

Another misconception is that nothing happens during interphase before cell division; it is just the resting period for the cell. This misconception stems from students not fully learning about the processes of interphase until later in their education should they pursue the sciences. In high school, interphase is reduced to a series of short explanations that amount in students thinking it is just ‘that phase before mitosis or meiosis’. Yes, students may understand that the cell prepares for division in this phase, but there is so much more. To address this misconception, teachers should of course not dive deep into the process of each phase of interphase, but instead at least label them or inform their students that so much more is happening during these phases other than the cell just ‘resting’. Interphase itself is made up of G1, S, and G2 phases. Within these phases the cell is carrying out its many duties (depending on the type of cell), going under periods of immense growth, as well as synthesizing the replicated DNA strands needed for mitosis and meiosis.

 

There is also a misconception that telophase and cytokinesis are interchangeable terms. This misconception stems from the two processes usually being clumped together (as they are in this website as well) towards the end of each phase. Although both remain important parts of the physical cell division, each accomplishes their own unique task. The best way to address this misconception is to make a point of the fact that during telophase, there are actually two nuclei in the one cell that has yet to split. When the cell furrows and does begin to physically separate, this is cytokinesis.

 

STSE

 

Mitosis and meiosis have a very special role to play when it comes to integrating with application type questions and lessons. It’s special because it is a topic that is literally happening in every student as they are being taught YET remains as abstract and out of reach for students as quantum physics. Below are several real world applications of this topic.

 

First, having the students relate the stages of mitosis to a basic understanding of how various cancers work or come to be in relation to cell division would allow them to grasp what happens when mitosis ‘goes wrong’. Many text books refer to cancer as “mitosis out of control’. This is because mitosis is controlled by various genes inside every cell. When these genes do not do their job, even in one cell, the processes that control the rate of mitosis can begin to change. Suddenly, that one cell with this defect is now creating multiple copies of itself faster than its ‘normal’ counterparts. These cells build up and eventually form masses, more commonly known as tumours, which can damage the surrounding tissues. Even worse, sometimes these cells divide so rapidly and abolish most remaining control systems placed on cells by the body and spread to other parts of the body in a process known as metastasis. Once in the new location, the cells continue to divide faster than the surrounding ones, taking up valuable resources and space from the healthy cells, and damage those tissues too. To make an even further connection for the students, time can be spent examining the way common cancer treatments work in their relation to halting/killing these rapidly dividing cells. It’s one thing to destroy a cancer cell, but how would one prevent it from coming back? What do these medicines do to the function of the DNA? What stage of mitosis can these medicines (or even the cancerous mutation for that matter) effect? These are all great guiding questions for students to help them make that

connection between something as unfortunately common as cancer, to the abstract stages of mitosis and cell division.

 

 

 

 

 

 

 

 

 

 

 

 

 

Just as cancer aimed to bring the abstract processes of cell division to a topic of interest, perhaps a more relatable way to do so would be to examine how cell division helps us in a very visible way. The most common problem every student will encounter – healing a cut or open wound. Of course, there is a lot of advanced biology at play here, especially in terms of immunology. However, this does not mean that it cannot be broken down to a level consistent with the curriculum covered in this unit. For example, a very broad connection can be made to what happens on a cellular level when one has a wound. In the case of broken skin, these cells have been damaged or even completely lost. Much like any other part of one’s body, when something is lost your body replaces it. Therefore, in the case of a skin cut, our skin cells (amongst others of course) grow and divide to try and make up for the lost cells. This is a very ‘real’ process of cell division that most students should have already had some exposure to.

 

A series of resources for these can be found on the Resources tab for this unit.