Chapter 12 & 13 Portfolio
Explain the events of all stages of the cell cycle.
The cell cycle is divided into the mitotic (M) phase and interphase. The mitotic phase is includes mitosis, the division of the genetic material in the nucleus, and cytokinesis, the division of the cytoplasm. Interphase is divided into three phases: first gap, synthesis, and second gap. In first gap, the cell synthesizes protein to grow larger as it prepares to divide into two daughter cells which are genetically and physically equivalent to this parent cell. In synthesis, the cell continues to grow, but this is the only one of the three subphases where the copying of chromosomes takes place. Duplicating the genetic material in the nucleus exactly allows the cell to split into two cells with identical sets of chromosomes. Lastly, in second gap, the cell makes final preparations for cell division by checking the number of chromosomes and organelles. It also continues to synthesize RNA and proteins. In preparation for mitosis, the second gap also includes the beginning formation of the mitotic spindle. The actual division of the cell takes a short period of time in comparison to the length of time for all of interphase.
The cell cycle is divided into the mitotic (M) phase and interphase. The mitotic phase is includes mitosis, the division of the genetic material in the nucleus, and cytokinesis, the division of the cytoplasm. Interphase is divided into three phases: first gap, synthesis, and second gap. In first gap, the cell synthesizes protein to grow larger as it prepares to divide into two daughter cells which are genetically and physically equivalent to this parent cell. In synthesis, the cell continues to grow, but this is the only one of the three subphases where the copying of chromosomes takes place. Duplicating the genetic material in the nucleus exactly allows the cell to split into two cells with identical sets of chromosomes. Lastly, in second gap, the cell makes final preparations for cell division by checking the number of chromosomes and organelles. It also continues to synthesize RNA and proteins. In preparation for mitosis, the second gap also includes the beginning formation of the mitotic spindle. The actual division of the cell takes a short period of time in comparison to the length of time for all of interphase.
Figure 12.6 from the Chapter 12: The Cell Cycle Lecture Presentation for Campbell Biology by Nicole Tunbridge and Kathleen Fitzpatrick, Pearson Education, Inc.
Explain the events of all stages of mitosis and track chromosome and chromatid number through all stages of mitosis.
Mitosis is composed of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. The last coincides with cytokinesis in order to complete the process.
Prophase is the phase in which the formerly uncondensed mass of chromosomes becomes more tightly coiled. The fibers condense into chromosomes, two sister chromatids per chromosome which are joined at their centromeres. In this stage, too, the nucleolus of the cell disappears. Centromeres begin to move to opposite sides of the cell by being pushed by the microtubules of the forming mitotic spindle. This figure shows the presence of six chromosomes, or twelve chromatids, in this phase.
Prometaphase is the phase in which the nuclear envelope disappears so that microtubules of the mitotic spindle may invade the newly available area and attach to the kinetochores of the chromosomes. The kinetochores are the specialized protein structures at the centromeres that appear when the chromosomes become even more condensed. Microtubules that are not connected to kinetochores continue to grow past one another. The number of chromosomes remains the same.
Metaphase is the phase in which chromosomes align at the metaphase plate, an invisible line equidistant to the centrosomes at the poles. The sister chromatids of each chromosomes are held by microtubules coming from opposite poles. The number of chromosomes remains at six, or with twelve chromatids.
Anaphase is the phase in which the proteins holding the chromatids together are cleaved. The chromatids split apart, each becoming a chromosome and thus doubling the original number of chromosomes. The number of chromosomes increases to twelve. The kinetochore microtubules attaching each chromosome to a centromere at one of the poles then shorten. Motor proteins walk chromosomes along the microtubules, dissolving the kinetochore ends as they move along. While the kinetochore microtubules shorten, the nonkinetochore microtubules lengthen to expand the cell.
In telophase, the nuclear envelope reforms from the fragments of the parent cell’s nuclear envelope. Two nuclei form, one for each daughter cell. The six chromosomes for each of the two cells become less condensed. Spindle microtubules depolymerize. To round out the process, cytokinesis occurs to pinch the cell in two in animal cells (cleavage furrow) or divide the parent cell into two in plant cells (cell plate).
Photo credit to http://publications.nigms.nih.gov/insidethecell/ch4_phases_allbig.html
Compare the process of mitosis in plant-like and animal-like cells.
Compare the process of mitosis in plant-like and animal-like cells.
While the most noticeable difference between plant and animal cells can be observed in cytokinesis (to be later explained), there are some small differences between the two in the five other phases which make up the mitotic process. For one thing, plant cells lack the centrioles which are a part of the centrosomes at opposite poles in animal cells. Thus, they are unable to form the mitotic spindles which have microtubules extending from the centrosomes. The importance of microtubules in the latter parts of mitosis in animal cells is not seen at all in plant cells.
In terms of cytokinesis, an animal cell splits into two by cleavage. A cleavage furrow, a groove near the older metaphase plate, takes form. A ring of actin microfilaments on the inside of the furrow contracts when it interacts with myosin molecules. The actin microfilaments are pulled together tighter and tighter until the one parent cell is pinched entirely in two. A plant cell undergoes cytokinesis without changing its shape dramatically. Vesicles carrying materials from the Golgi apparatus will move to the middle of the parent cell. The material combines to form a cell plate which grows until it fuses with the plasma membrane. There are now two daughter cells with their own plasma membranes. A cell wall will also form around the plasma membranes in the former place of the cell plate.
Figure 12.10 from the Chapter 12: The Cell Cycle Lecture Presentation for Campbell Biology by Nicole Tunbridge and Kathleen Fitzpatrick, Pearson Education, Inc.
Explain how cell division is controlled in cells, using examples like MPF and PDGF.
Cell division is controlled by changing the concentrations of protein kinases and their corresponding cyclin. Cyclin-dependent kinases become active when they are attached to cyclin. The image below shows the activity of MPF in response to the changing concentration of cyclin.
Photo credit to https://www.studyblue.com/notes/note/n/module-2-cancer/deck/10767963 and Pearson Education, Inc.
MPF is a type of Cdk that can be considered an “M-phase-promoting factor” because it pushes the cell into the mitotic phase. The MPF complex composed of cyclin and Cdk molecule is able to phosphorylate a variety of proteins. MPF does a number of things in mitosis: it helps fragment the nuclear envelope, condense chromosomes, and form the mitotic spindle. Because the activity of MPF depends on the cyclin concentration and it is always there, MPF can control its own activity by destroying its own cyclin and waiting for new molecules to be synthesized. There are other similar Cdk proteins in cells that function similarly to MPF by pushing the cell into various phases. Cell behavior at checkpoints is monitored by these complexes.
Justify the effects of a change in the cell cycle mitosis and/or meiosis will have on chromosome structure, gamete viability, genetic diversity, and evolution.
Changes in the cell cycles of mitosis and meiosis would mean resulting daughter cells of either processes would have the wrong number of chromosomes, either too many or too few. Some chromatids would be unpaired in mitosis or there would be full chromosomes present in the last daughter cells of meiosis, for example. The gametes produced from meiosis would then be less viable because not all of the chromatids are unpaired, ready to be combined with the genes of another gamete reproductive cell. However, these mutations, though perhaps not always positive, might introduce new characteristics to a population.
Below is an image of nondisjunction, or the process by which meiosis does not split up all homologous pairs and all daughter cells receive the wrong number of chromatids.
Photo credit to http://faculty.fmcc.suny.edu/mcdarby/majors101book/chapter_03-chemistry/06-DNA_to_Proteins.htm
Explain the events of all stages of meiosis.
The whole process of meiosis is divided into two parts: meiosis I and meiosis II. Meiosis I divides one diploid parent cell into two haploid daughter cells through the steps of prophase, metaphase, anaphase, telophase, and cytokinesis. As such, these four steps are titled metaphase I, anaphase I, telophase I, and cytokinesis I. The steps of meiosis I function similarly to those of mitosis except for the main feature of duplicated homologous chromosomes being separated from each other after aligning along the metaphase plate instead of individual chromosomes. In this part of meiosis, too, genes can switch between chromatids to yield recombinant chromosomes. Crossing over occurs early in prophase I. Interphase does not take place between meiosis I and meiosis II because chromosomes do not need to be duplicated for the production of four haploid daughter cells; to do so would defeat the purpose of splitting up the parent cell’s chromatids into four equal parts.
The whole process of meiosis is divided into two parts: meiosis I and meiosis II. Meiosis I divides one diploid parent cell into two haploid daughter cells through the steps of prophase, metaphase, anaphase, telophase, and cytokinesis. As such, these four steps are titled metaphase I, anaphase I, telophase I, and cytokinesis I. The steps of meiosis I function similarly to those of mitosis except for the main feature of duplicated homologous chromosomes being separated from each other after aligning along the metaphase plate instead of individual chromosomes. In this part of meiosis, too, genes can switch between chromatids to yield recombinant chromosomes. Crossing over occurs early in prophase I. Interphase does not take place between meiosis I and meiosis II because chromosomes do not need to be duplicated for the production of four haploid daughter cells; to do so would defeat the purpose of splitting up the parent cell’s chromatids into four equal parts.
In meiosis II, the four steps are repeated and are named accordingly with the subscript II. This process is very similar to mitosis because the sister chromatids are pulled from one another.
Figure 13.8 from Campbell Biology: Tenth Edition, Pearson Education, Inc.
Compare the process of meiosis to the process of mitosis.
Mitosis will produce daughter cells which are identical to the parent cell while meiosis will produce daughter cells with half the number of chromosomes of the original cell. The one diploid cell becomes four haploid cells. Because this is sexual rather than asexual reproduction, the chromosomes of each daughter cell from mitosis will differ from the genetic makeup of the other daughter cells, whether that is through independent assortment or crossing over. Meiosis also involves two cell divisions instead of one.
Mitosis will produce daughter cells which are identical to the parent cell while meiosis will produce daughter cells with half the number of chromosomes of the original cell. The one diploid cell becomes four haploid cells. Because this is sexual rather than asexual reproduction, the chromosomes of each daughter cell from mitosis will differ from the genetic makeup of the other daughter cells, whether that is through independent assortment or crossing over. Meiosis also involves two cell divisions instead of one.
In observing the different phases of meiosis and mitosis, we can easily see that in metaphase, sister chromatids remain attached to one another in the former process but will be pulled to opposite poles in the other. The mitotic spindle functions similarly in both processes. However, in meiosis, homologous chromosomes bound as chiasmata will line up along the metaphase plate. Chromosomes that have crossed-over genes will be “pulled,” one to each opposite pole. In mitosis, single chromosomes will line along the metaphase plate but the sister chromatids will be pulled apart.
Figure 13-9a from the Chapter 12: The Cell Cycle Lecture Presentation for Campbell Biology by Nicole Tunbridge and Kathleen Fitzpatrick, Pearson Education, Inc.
How can recombination during meiosis be explained?
Recombinant chromosomes form during the prophase I stage of meiosis. The two chromosomes of a homologous pair align loosely along their length before the DNA of their chromatids is broken up by specific proteins at certain points. During synapsis, the tying of homologous chromosomes to one another, the broken ends of the DNA breaks are joined to breaks of the nonsister chromatids. A chromatid from the paternal cell will be tied to a portion of a chromatid from the maternal cell, and vice versa.
Recombinant chromosomes form during the prophase I stage of meiosis. The two chromosomes of a homologous pair align loosely along their length before the DNA of their chromatids is broken up by specific proteins at certain points. During synapsis, the tying of homologous chromosomes to one another, the broken ends of the DNA breaks are joined to breaks of the nonsister chromatids. A chromatid from the paternal cell will be tied to a portion of a chromatid from the maternal cell, and vice versa.
Figure 13.9 from Campbell Biology: Tenth Edition, Pearson Education, Inc.
Explain how the processes of meiosis increase genetic variation in a population.
The process of meiosis has three ways of increasing genetic variation in a population: independent assortment of chromosomes, crossing over, and random fertilization. Meiosis is sexual reproduction via the combination of chromosomes from two parent gametes, one female (egg cell) and one male (sperm cell). The possible chromosome combinations are in the trillions because these three ways of creating different physical characteristics in the organisms are so effective.
The independent assortment of chromosomes is the way chromosomes line up along the metaphase plate during metaphase I and metaphase II. The arrangement of the homologous pairs of chromosomes along the metaphase plate determines which daughter cell receives which combination of chromatids. The daughter cell into which each pair is sorted is not affected by where the other pairs are sorted. This applies to the end of meiosis I as well as the end of meiosis II. The number of possible combinations from independent can be calculated by 2^n, n being the haploid number. With n=23 in human reproductive cells, there are 8.4 million combinations of chromosomes.
Crossing over is the re-arrangement of genes between homologous chromosomes to produce recombinant chromosomes. The male parent cell has some genes from the female parent cell, and the female cell has the corresponding number of genes from the male cell. The resulting four daughter cells at the very end of meiosis are unique because the female parent and male parent genes are so subject to being combined.
Random fertilization means any sperm cell can fertilize any female egg cell. Accounting for the 8.4 million possible combinations of chromosomes from just one gamete, the fertilized zygote has 70.56 million combinations. In terms of the “big picture,” when there are 7 billion people on Earth--roughly half of them male, half of them female--there are infinitely many combinations of chromatids, and therefore, physical characteristics to be seen in a human.
Photo credit to Department of Biology at the Memorial University of Newfoundland, Principles of Cell Biology (BIOL2060) and Pearson Education, Inc.
