Q1. Explain the regulation of the cell cycle
Every cell is regulated by particular checkpoints to ensure that the daughter cells are exact duplications. This is to help guarantee that cell mutations are not passed on to the next generation of cells. In eukaryotic cells, there are three main checkpoints. These are points at which cell replication can be halted until certain conditions are met (Boundless, 2016). The first checkpoint is near the end of the G1 phase. It is here that DNA gets evaluated and checked for damage. If damage has occurred, it either gets corrected or gets prevented from moving on to the next phase (Boundless, 2016). The next checkpoint occurs at the G2 phase. This regulator evaluates all the chromosomes to ensure they have been replicated properly and that the new DNA is correct and without mistakes or damage (Boundless, 2016). Again, it either gets corrected or it is prevented from entering mitosis. The final checkpoint is the M-point near the end of the metaphase stage of mitosis. It is here that sister chromatids are evaluated to ensure they are attached properly to spindle microtubules before they are allowed to move on to anaphase (Boundless, 2016).
Q2. Classify the different phases of mitosis
There are four basic phases of mitosis: prophase, metaphase, anaphase and telophase (Phases of Mitosis, n.d.). Just before mitosis, the cell is in interphase, where it has already copied its DNA. It then enters what is known as early prophase (Phases of Mitosis, n.d.). It is here that the chromosomes begin to condense, which makes them easier to be pulled apart. The mitotic spindle starts forming between the centrosomes as they move apart. It is also in this phase that the nucleus disappears. The next is late prophase. By this phase, the chromosomes have stopped condensing and the envelope of the nucleus break down, releasing the chromosomes (Phases of Mitosis, n.d.). In metaphase, the spindle has gathered the chromosomes and has now lined them up in the middle of the cell, ready to divide. Anaphase begins now in which the chromatids are now pulled apart towards opposite ends of the cell. Finally, telophase occurs in which the cell begins to form new internal parts such as the nucleus and becomes two new daughter cells (Phases of Mitosis, n.d.).
Q3. Analyze the loss of cell cycle controls in cancer cells
As discussed above, there are regulation mechanisms by which a cell is either approved for replication or programmed to die. Sometimes, these checkpoints lose their sensitivity and damaged or mutated cells are allowed to replicate (Collins, Jacks & Pavletich, 1997). These mutated cells tend to divide very rapidly and can lead to malignant tumors (Chow, 2010) and have the ability to spread through the body. The abnormal cell growth can be caused by what are known as oncogenes (Chow, 2010). These are the proteins involved in cell regulation and sometimes they become altered which not only creates abnormal protein products but allows cell division to occur too rapidly which contributes to tumor growth (Chow, 2010). These oncogenes can also prevent a damaged cell from eliminating itself, which causes replication of that mutated cell (Chow, 2010).
Q4. Differentiate between asexual and sexual reproduction
In asexual reproduction there is no need to have two of the species. There is only one parent. Each cell receives the exact same genetic material as it divides which makes it an exact copy of the parent cell. Since the daughter cell is an exact replica of the parent, it is called a clone (The Two Methods, n.d.). In sexual reproduction both male and female sex cells are produced. In this case, the number of chromosomes are halved for each cell so that it guarantees a random combination of genes (The Two Methods, n.d.). This ensures variation in offspring. The major differences in the two types of reproduction are as follows:
Asexual – only one parent
Genetically identical offspring
Division by mitosis
It’s a quick process
Large number of offspring
Sexual – Two parents
Genetically different
Division by meiosis, resulting in gametes
Produces variation
Slower process
Small number of offspring
Q5. Classify the different phases of meiosis
In meiosis, cell division occurs twice, to produce four cells instead of two. Similar to the stages of mitosis, meiosis involves prophase, metaphase, anaphase and telophase, but with each of those stages occurring twice. Beginning with interphase, the cell copies its chromosomes and each chromosome has sister chromatids held together by a centromere. In prophase 1, homologous chromosomes come together which forms a tetrad (The Phases of Meiosis, n.d.). Exchanging of genetic material can occur, which is known as crossing over (The Phases of Meiosis, n.d.). This results in new combinations of alleles. In metaphase 1, the centromeres attach to a spindle fiber which pull the tetrads to the middle of the cell. In anaphase 1, the chromosomes now separate and move to opposite ends of the cell but the centromeres remain intact (The Phases of Meiosis, n.d.). In telophase 1, the spindle breaks down which allows the chromosomes to uncoil and the cytoplasm divides, resulting in two new cells. These cells only have half the genetic material since there is only one homologous chromosome from each pair (The Phases of Meiosis, n.d.). In prophase 2, a spindle forms in each new cell and attach to the chromosomes. In metaphase 2, the chromosomes are lined up at the centre of the cell. In anaphase 2, the centromere splits and the chromosomes are again pulled to opposite ends of the cell. Finally, in telophase 2, the spindles break down, the cytoplasm divides and new nuclei are formed (The Phases of Meiosis, n.d.). We now have four haploid cells.
Q6. How is the cell cycle normally controlled and what happens to allow cancer to occur?
As mentioned previously, the cell cycle is controlled and regulated by particular checkpoints. These checkpoints ensure that the cell is not damaged or mutated. If it is, the cell is either repaired or programmed for cell death (apoptosis). Sometimes, these checkpoints lose their sensitivity and allow these mutated cells to replicate. Oncogenes can also be responsible for the growth of cancer cells. These proteins can sometimes have mutations that are not corrected, which then produce altered protein products. These altered proteins allow the cells to divide too rapidly which can lead to tumors. Benign tumors are masses of these cells that do not spread. They remain unchanged and in one spot (Nordqvist, 2015). Malignant tumors are tumors full of abnormal cells that multiply at a very fast rate and infect other cells. These are cancerous cells that do spread throughout the body and can lead to multiple cancers and death (Nordqvist, 2015).
Q7. Differences between plant and human reproduction
Humans carry two samples of each chromosome (diploid). Plants carry one (haploid). The process that leads to the formation of haploid cells to diploid cells is called meiosis, which is what humans use to reproduce. In human fertilization, one haploid male sex cell fuses with a haploid female sex cell, resulting in a diploid zygote (Life Cycles, n.d.). In plants, there is a middle stage. In this stage, meiosis begins with diploid cells, leading to the formation of haploid spores (sporophytes). These spores develop into a gametophyte which will produce reproductive cells at a later time (Life Cycles, n.d.) This is the alternation of generation. In other words, the adult generation produces spores then the spore generation produces the sex cells (Alternation of Generations, n.d.). The sporophyte is the major stage of a plant’s life cycle.
Q8. Describe asexual and sexual reproduction.
Asexual reproduction occurs in plants but can also occur in some animal species. In plants, new cells are formed that are an exact replica of the parent cell (clone). Some plants use their stems for reproduction, others use their leaves or roots (Asexual Reproduction, 2015). Stems will often arch over and plant themselves in the dirt, producing new daughter plants (Asexual Reproduction, 2015). With leaves, mitosis produces tiny buds that fall off and plant themselves (Asexual Reproduction, 2015). With the roots, some plants will produce new stems from the original root (Asexual Reproduction, 2015). In animals, asexual reproduction usually results in a bud that grows on the body of the parent. Sometimes these buds break away and become independent. Other time, they remain attached to the parent and become colonies (Asexual Reproduction, 2015). In a process known as fragmentation, some species grow and break apart into many pieces and each piece develops into a new creature (Asexual Reproduction, 2015). In another process known as parthenogenesis, a female will produce eggs that will develop into young without fertilization (Asexual Reproduction, 2015).
Sexual reproduction requires two parents who produce offspring that has genetic material from each parents. This results in new genetic combinations. Two sex cells (gametes) fuse together to form a zygote (Bailey, n.d.). The gametes are produced by meiosis. There is internal and external fertilization. Internal occurs inside the female animal whereas external occurs outside the body (Bailey, n.d.).
Q9. Choose one or two organisms that can reproduce both ways.
Jellyfish are a species that can reproduce both ways. The asexual reproduction allows them to make many copies which makes it easier for them to find a mate than only one copy would (Asexual or sexual, n.d.). The ones that sexually reproduce can find a mate and produce offspring that have different traits from the parents. These differences can give advantages to the offspring that the parents didn’t have (Asexual or sexual, n.d.). Yeast is another organism that reproduces both ways. Asexually it uses a budding process. Sometimes it also reproduces sexually. It does this in order to produce genetic variation and to ensure survival of the species (Yeast Reproduction, n.d.). When environmental conditions are stressful, yeast will go through meiosis and release haploid spores. These spores get released when that stress is gone (Yeast Reproduction, n.d.). During sexual reproduction, the haploid spores fuse together to create a diploid zygote (Yeast Reproduction, n.d.).
References
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Bailey, R. (2016, June 23). Sexual Reproduction. Retrieved from About Education: http://biology.about.com/od/basicgenetics/a/aa062708a.htm
Boundless Biology. (2016 ). Retrieved from Boundless: https://www.boundless.com/biology/textbooks/boundless-biology-textbook/cell-reproduction-10/control-of-the-cell-cycle-89/regulation-of-the-cell-cycle-at-internal-checkpoints-398-11625
Chow, A. Y. (2010). Cell cycle control by oncogenes and tumor suppressors: Driving the transformation of normal cell into cancerous cells. Nature Education, 7-10.
Collins, K., Jacks, T., & Pavletich, P. N. (1994). The cell cycle and cancer. PNAS, 2776-2778.
Nordqvist, C. (2015, December 10). Tumors: Benign, premalignant and malignant. Retrieved from Medical News Today: http://www.medicalnewstoday.com/articles/249141.php?page=2
Phases of Mitosis. (2016). Retrieved from Khan Academy: https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/phases-of-mitosis
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Virtual classroom biology. (2013, September 10). Retrieved from Radboud University Nijmegen: http://www.vcbio.science.ru.nl/en/virtuallessons/pollenreproduction/
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