Case Study Conclusion: Genetic Similarities and Differences
Humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9% genetically identical to each other. But the mere 0.1% that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. These variations are passed on in a family with sexual reproduction. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as “nonself” and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient’s own cells through the same mechanism.
As you have also learned, a good HLA match is often difficult to find, even between full siblings. This is due to the genetic variations within gametes of a single person due to crossing over and random assortment. The variations are multiplied when two unique gametes fertilize. Kim has to find his match outside of his family. Every year, about 14,000 people in the United States must try to find a compatible donor from a bone marrow registry. These registries store information on HLA type from potential donors, like the woman shown above. She is swabbing her cheeks for a DNA match. It can take months to years to find a compatible match — if one is found at all.
In the meantime, Kim has to stop the production of abnormal WBCs with chemotherapy. Chemotherapy is the treatment of cancer with drugs ("anticancer drugs") that can destroy cancer cells. In current usage, the term "chemotherapy" usually refers to cytotoxic drugs which affect rapidly dividing cells in general, in contrast with targeted therapy (see below). Chemotherapy drugs interfere with cell division in various possible ways, e.g. with the duplication of DNA or the separation of newly formed chromosomes. Chemotherapy has the potential to harm healthy tissue, especially those tissues that have a high replacement rate (e.g. intestinal lining). Due to these side effects, patients may lose their hair follicles, Digestive system lining, and taste buds. These cells usually repair themselves after chemotherapy. Because some drugs work better together than alone, two or more drugs are often given at the same time. This is called "combination chemotherapy"; most chemotherapy regimens are given in a combination.
In this chapter, you learned about human sexual and asexual reproduction.
- The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. It is more complicated in eukaryotic than prokaryotic cells.
- In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. During the mitotic phase, first, the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis), which produces two daughter cells.
- Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each replicated chromosome consists of two sister chromatids, which are joined together at a centromere.
- During mitosis, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases, called prophase, metaphase, anaphase, and telophase.
- The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle at key checkpoints.
- Cancer is a disease that occurs when the cell cycle is no longer regulated, for example, because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
- In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with only one of each pair of homologous chromosomes, and the zygote is a diploid cell with two of each pair of chromosomes.
- Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).
- Meiosis is followed by gametogenesis, the process in which the haploid daughter cells change into mature gametes. Males produce gametes called sperm in a process known as spermatogenesis, and females produce gametes called eggs in the process known as oogenesis.
- Sexual reproduction produces offspring that are genetically unique. Crossing-over, independent assortment, and the random union of gametes work together to result in an amazing amount of potential genetic variation.
- Sexual reproduction has the potential to produce tremendous genetic variation in offspring.
- During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over.
- In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly.
- This variation is due to independent assortment and crossing-over during meiosis, and random union of gametes during fertilization.
- The goal of mitosis is to produce a new cell that is identical to the parent cell.
- The goal of meiosis is to produce gametes that have half the DNA of the parent cell.
- When chromosomes do not divide equally among gametes, the damaged gametes produce. This process is called nondisjunction.
- Trisomy is a state where humans have an extra autosome; they have three of a particular chromosome instead of two.
- The most common trisomy in viable births is Trisomy 21 (Down Syndrome) due to nondisjunction.
Chapter Summary Review
- What are cyclin-dependent kinases? What is their role?
- What are cell cycle checkpoints?
- What is interphase?
- Summarize each phase of the cell cycle.
- Describe the structure of a chromosome in the prophase of mitosis.
- What is cytokinesis and when does it occur?
- What is centromere?
- Describe the main steps of mitosis.
- Cells go through a series of events that include growth, DNA synthesis, and cell division. Why are these events best represented by a cycle diagram?
- Explain how the cell cycle is regulated.
- Define and explain random assortment and random fertilization.
- Why is DNA replication essential to the cell cycle?
- True or False. When a eukaryotic cell divides, the nucleus divides first in the process of mitosis.
- What happens during mitosis?
- What is meiosis?
- What is diploid? How many chromosomes are in a diploid human cell?
- What is a zygote? How does the zygote form the organism?
- What is the result of crossing-over?
- How many cell divisions occur during meiosis?
- Why are you genetically distinct?
- Describe the steps of Meiosis I and Meiosis II.
- Describe nondisjunction. List and explain some of the chromosome disorders.
- Compare and contrast mitosis and meiosis.