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Chapter 8: Cell Division: Mitosis

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Cell Division: Mitosis

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Fitting the next phase into what we know

We've been studying cellular structure and energy usage, which we identified as one of the necessary traits of living matter. With this chapter we start a new unit on genetics and how cells pass information about their own structure to their offspring — another key trait in identifying whether a blob of matter is organic or not.

As usual, we have two types of material to understand: definitions and descriptions of objects, and definitions and descriptions of processes. Our early study of atoms and molecules, organic chemicals, and cell organelles all concentrated on the description of objects, focussing on their components, characteristics, and structure. Our more recent studies of membrane permeability, aerobic respiration, and photosynthesis involved processes describing how these objects interact and change. Processes are often more difficult to understand and remember, because they not only involve the definition of each phase in the process, but also the sequence of the phases in a specific order. Using interactive websites can help, so can drawing diagrams that identify the sequence of steps in each process.

In particular, understanding how cells divide involves recognizing objects and following processes. You will need to be able to identify structures like DNA and chromatids; you will also need to keep the phases of meiosis and mitosis in order. Doing the study guide exercises which involve labeling diagrams and creating charts should help.

So far we have discussed the components of cells: the selectively permeable membrane, rough and smooth endoplasmic reticulum, Golgi bodies, mitochondria and chloroplasts, and, at a high level, the nucleus and ribosomes. We have spent some considerable time on the processes used by cells to store and free energy for its (so far unspecified) internal processes. Now we are going to look at one of those energy-consuming processes, which also happens to be one of the characteristics of all living things: the reproduction of the cell. This section introduces one of the three fundamental themes of biology: the transmission of information. Much of what we cover in this material has been discovered or clarified during my lifetime; as theories go, all of this information is new, and based on modern techniques of microscopic observation. If you are unclear about the types of microscopes used for these pictures, you may want to review the relevant sections of Chapter 4.

Cellular Reproduction: Overview

A cell contains a lot of information about how it functions in two macromolecules, DNA and RNA (deoxyribonucleic acid and ribonucleic acid). We will discuss later how DNA and RNA work within the cell to dictate the function of ribosomes in protein synthesis, but for now, just keep in mind that the DNA and RNA, along with a lot of protein, normally exist within the cell in long strands called chromatin. A section of chromatin which controls the production of a single protein or characteristic is called a gene. Only when the cell divides does the chromatin organize itself into tightly-coiled chromosomes.

A cell reproduces in one of two ways. It can either divide and clone itself, creating two cells which are exact copies of, or genetically identical to, the original cell. Or it can divide its genetic material, mix it with the genetic material of another cell, and create one or more new cells which are a genetic mix of the original pair. In this chapter, we are concerned only with the processes of eukaryotic cells (everything except the Kingdom Monera, which contains bacteria); these processes are common to single-celled organisms like protozoa; fungi like mushrooms and slime mold; plants like ferns, pine trees, and roses; and to all animals. This commonality of process is one reason why the evolution model groups these four kingdoms together as having a common ancestor.

Asexual Reproduction: binary fission

Asexual reproduction simply means that we don't mix any genetic materials together to form the resulting genetic product: all the materials come from a single "parent" organism or cell and therefore the result is identical to the parent. This cloning method of reproduction has two major processes: mitosis, in which the nucleus material (the chromatin, containing all the genetic instructions for cell function) is duplicated and distributed to opposite sides of the cell, and cytokinesis, in which the cell actually divides by binary fission to produce two new cells, called daughter cells, each with a sufficient number and diversity of organelles to continue to function. The primary purpose of interphase is to create the general cell materials (cytosol, organelles, membranes) necessary for cell duplication, which the primary purpose of mitosis is to duplicate the information the cell needs to perform its functions so that each daughter cell has a copy.

Sexual Reproduction: mitosis

The process of mitosis occurs at a specific point in the cell's life cycle, and can be broken down into four observable phases, outlined (together with the interphase processes) in the cell life-cycle table below.

 Phase Function
 Interphase: G1 Gap Growth of cell
 Interphase: S Synthesis of DNA/RNA (chromosome duplication)
 Interphase: G2 Gap Protein synthesis
 Mitosis: Prophase Chromatin condenses and chromatids form. Mitotic spindles develop.
 Mitosis: Pro-metaphase Nuclear membrane and nucleolus break down. Microtubules attach to chromatid kinetochores and to polar regions of cells.
 Mitosis: Metaphase Chromatids are arranged along central plate between poles.
 Mitosis: Anaphase Chromatids separate and each daughter chromosome moves toward its own pole.
 Mitosis: Telophase Nucleus reforms in each daughter cell
 Cytokinesis Cell material (organelles, cytosol) divides, new wall forms between daughter cells [overlaps telophase]

During mitotic prophase, when the chromatin coils into chromosomes, normal body cells (somatic cells) wind up with two copies of each genetic strand: the chromatids of the chromosome. The members of the pair are identical, and each daughter cell gets one. If at any point during mitosis, the duplication or separation of the chromatin material is corrupted, a mutation occurs, and the most likely outcome is that the resulting cell will not be able to survive.

Cells do not arbitrarily decide "now is a good time to divide". Cell functions are all controlled by the availability of enzymes, and cell mitosis is no exception. Special proteins (cyclins) attach to and activate protein enzymes called kinases, which are then able to add phosphate ions (recall phosphorylation?) to activate yet other proteins, which then start up the mitosis process. When the cyclin breaks down, the mitosis process stops. Drugs which interrupt or enhance the performance of the cyclins can be used to promote mitosis, thus promoting growth and healing, or to stop or at least slow down the runaway mitosis which is characteristic of cancer.

For an overview, step through the process of mitosis at the Cells Alive!. Note the appearance of the cell at the prophase, metaphase, anaphase, telophase, and cytokinesis steps of the process.

For more detail, watch the steps of mitosis at John Kyrk's site.

Then watch the short movie showing cell division:

Now look at the two pictures of an onion root tip (apical meristem tissue) below. Because the apical meristem is an area of rapid grown, many cells will be undergoing mitosis at any given moment. In the first picture, see if you can identify cells actively in the process of prophase (chromosome formation from chroMatids), metaphase (chromosomes lining up on the division line), and anaphase (chromosomes migrating to the poles of the cell).


In the picture below, can you find the cell which is actively dividing in cytokinesis phase?


Sexual Reproduction: meiosis

In sexual reproduction, the genetic material of two parent cells (gametes) is halved, then mixed to produce one or more offspring cells (zygotes) whose characteristics are similar to that of one or the other parent, but not identical with either parent. The mix is unpredictable, and the zygote may develop into an organism more able or less able to cope with its environment than its parent.

Recall that during mitotic prophase, the chromatin is duplicated and coiled into homologous chromosomes, which the single parent cell divides into two sets, one for each daughter cell. The cell is called diploid. In sexual reproduction, we start with two parent cells, so the mechanism of dividing the chromosomes in the gametes must be different, or the offspring cells will wind up with double the genetic material. When gametes form, they have half the genetic material (and so are haploid cells). In a given organism, the gamete cells will have n number of chromosomes during meiosis, and the somatic cells will have 2n chromosomes during mitosis. Keep in mind that the only purpose of gamete cells is the reproduction of the organism; the process of meiosis produces haploid cells, the gametes, which must then unite in pairs to produce diploid body cells, the zygote.

Because it must both halve the cell material and mix it, the process of meiosis is more complex than mitosis. The parent cell divides twice; and each division has the same four phases as mitosis, with some key differences. The table below summarizes the phases of meiosis.

 Phase Function
Interphase DNA chromatin replicates
Prophase I Homologous chromosomes form, join as tetrads, and cross-over of genetic material occurs
Metaphase I Tetrads line up on equatorial plane for distribution
Anaphase I Chromosomes separate and move to cell poles: one pair at each pole
Telophase I Chromosomes partially unravel. Cytokinesis occurs, and two cells form and separate
Prophase II Chromosomes form again
Metaphase II Chromosomes line up on equatorial plane for distribution
Anaphase II Chromatids separate and move to cell poles: one chromatid at each pole
Telophase II Nuclei form at opposite poles of each cell. Cytokinesis occurs, and two haploid cells form from each daughter cell of telophase I.

Step through the process of meiosis at the Cells Alive! site. Note the appearance of the cell at each pass through prophase, metaphase, anaphase, and telophase steps of the process. How does the second pass compare to the first pass of each phase (e.g., how does prophase II differ from prophase I)?

For more detail, watch the steps of mitosis at John Kyrk's site.

The picture below shows cells from the reproductive organs of a whitefish. Some of the cells in this picture are undergoing different phases of meiosis. Can you find cells in prophase? metaphase? anaphase? Can you tell whether you are looking at prophase I or prophase II?


Increasing Genetic Diversity

Two mechanisms contribute to "mixing up" the genetic information passed from cell to cell through sexual reproduction.

During meiosis, chromosomes are reordered through the crossing-over process. In the joined sister chromatids of the tetrad, identical sections are cut out and switched. Because this switching involves a specific of genes that govern specific traits, each chromatid winds up with a full compliment of genes (assuming nothing goes wrong), instead of having only genes from one parent, each chromatid now has some genes from each parent.

Independent orientation applies to the inheritance possibilities for combinations of genes. When the gametes form during meiosis, each daughter gamete inherits the chromatid for one gene independently from the chromatids for any other gene. This just means than any mix-and-match pairing or set of pairings is possible.

Consider a single chromosomes A, which exists in two copies A and a in the parent cell. These are duplicated into sister chromatids AA and aa. The sister chromatids in each pair are identical, but the pairs need not be--so a single chromosome A has 2 copies of 2 different strands.

If we look at the possible combinations for 2 chromosomes, A and B, we have to consider the mix- and -match combinations for AA, aa, BB, and bb, where we can take any A-type chromatid and pair it with any b-type chromatid. We wind up with one of the following possibilities:

AB, Ab, aB, and ab.

As we will see in the next chapter, rules of inheritance can be used to predict likely outcomes for different traits, but cannot precisely predict the outcome for any single offspring.