Honors Biology: Unit 10 - Cell Cycle Check & Challenges

Pg. 216:

1. How does the importance of cell division differ in unicellular and multicellular eukaryotes?

The importance of cell division differs in unicellular and multicellular eukaryotes. Unicellular eukaryotes, like yeast and Amoeba cells, divide to produce two identical organisms. On the contrary, multicellular eukaryotes, like plant and animal cells, develop from a single fertilized egg cell, and many types of cells make up the organisms’ tissues, indicating that organs are developed through many cycles of division, beginning with that singular egg cell.

2. What is the major event in M phase? S phase? G0 phase?

The major event in the Mitosis (M) phase is the division of the nucleus; individual chromosomes separate and are distributed to two new daughter cells. The major event in the Synthesis (S) phase is the cell replicating its own DNA; the DNA of each chromosome replicates to form a new, identical set of chromosomes, which also doubles the nucleus’s gene number; each new daughter cell should receive the exact same copy of its parent’s genetic information. The major event in the Gap 0 (G0) phase is to keep cells metabolically active and specialized to perform certain tasks necessary to survival, just not cell division. It serves as a stopping point within G1 and acts as a resting state until a signal is received from the cell’s surroundings stimulating cell division.

3. What determines whether a cell stays in interphase or divides?

The restriction point (R) determines whether a cell stays in interphase or divides. It is known as the point of no return because when a cell passes through this landmark, it cannot return to G1 or G0 without completing a full cell cycle and dividing.

4. What are some factors that might influence the cell cycle?

There are quite a few factors that might influence the cell cycle. They include the presence or absence of chemical signals, probably hormones, damage to parts of the cell or chromosomes, if the cell doesn’t need to grow or repair itself anymore, DNA replication errors (causing mutations), etc.

5. Explain why DNA replication is important in cell division.

DNA replication is important in cell division. It is such a crucial process because the two new daughter cells created must have the exact same genetic information (DNA copies) as the parent cell. Without DNA replication, the genetic information would not be able to be passed on to the two new cells; cell division wouldn’t be able to take place, leading to cell death.

Pg. 225:

1. What is the difference between mitosis and mitotic cell division?

There is a difference between mitosis and mitotic cell division. First of all, mitosis is technically the division of a cell’s nucleus into two new, identical nuclei; it ensures that each new daughter cell receives the exact same copy of each chromosome. It is the asexual reproduction of somatic cells. Meanwhile, mitotic cell division simply refers to the overall process of creating a new cell with the same, identical DNA as the parent cell.

2. What is the difference between chromosomes and sister chromatids in the M phase?

There is a difference between chromosomes and sister chromatids in the Mitosis (M) phase. The sister chromatids are the two copies of each chromosome made during the Synthesis (S) phase, and are now ready to be separated and delivered to the new nuclei. Chromosome segregation occurs when the sister chromatids separate, so each new nucleus receives one copy of each chromosome. The chromatid is known as one half of the replicated chromosome.

3. How do the chromosomes move during mitosis?

Chromosomes move during mitosis by having microtubules form spindle fibers that push them to opposite sides of the cell, called spindle poles, which form around centrioles, if they are there. The sister chromatids are drawn to the opposite poles, then they are moved back to line up in the center of the cell during metaphase, forming the metaphase plate.

4. Briefly describe the major events of mitosis.

The major events of mitosis can be summarized as PMAT. Prophase is chromosome condensation and the formation of spindle fibers drawing the sister chromatids to opposite poles. Metaphase is the lining up of the replicated chromosomes down the middle, forming a metaphase plate. Anaphase is when the spindle fibers pull the sister chromatids apart. Telophase occurs when two new nuclear membranes form, the spindle fibers disappear, the chromosomes uncoil, and the cell begins to split. Cytokinesis occurs later, with the division of the cytoplasm (cell); it is technically not mitosis.

Pg. 222:

1. What are the three different roles of DNA polymerase?

There are three different roles of DNA polymerase. First, it adds the DNA nucleotides to the end of the RNA primer. Then, the enzyme helps catalyze the formation of a new strand of DNA to match and pair up with the old strand. Lastly, it proofreads its own work to ensure no mistakes were made. A new nitrogen base pair will be produced to fix the problem, if there is one. The sugar phosphate bond is sealed off.

2. Explain how DNA synthesis can proceed in both directions from a replication origin, even though DNA polymerase can synthesize DNA only in one direction.

DNA synthesis can proceed in both directions from a replication origin, even though DNA polymerase can synthesize DNA in only one direction. This is possible because the double helix, a sugar phosphate chain, is antiparallel; thus, synthesis of new DNA can happen in both directions because of the replisomes. Meanwhile, DNA polymerase can only extend a primer in one direction. DNA synthesis on the lagging strand is short and unconnected, so other enzymes follow the replisome to fill gaps on the lagging strand and join together the short segments.

3. How is the number of replication errors in cells kept to a minimum?

The number of replication errors in cells is kept to a minimum through the fact that the DNA polymerase proofreads its own work. After adding a new nucleotide to the growing chain, it checks to see if the resulting base pair is correct. If not, the enzyme will pause to remove the faulty nucleotide and replace it with the correct one. This way, most of the replication errors are repaired.

4. How does the cell repair damaged DNA?

The cell can repair damaged DNA or mutated DNA by the process of excision repair. The enzyme recognizes the mismatch of base pairs, breaks the sugar-phosphate bonds, and removes the damaged DNA. DNA polymerase then fills in the deleted DNA sequence (a replacement is synthesized), and another enzyme forms the sugar-phosphate bonds between the replacement piece and the neighboring nucleotides.

5. How are histones involved in gene expression?

Histones are involved in gene expression because nucleosomes are wrapped around histone proteins. This tightly-packed structure can turn off certain sections of DNA by excluding the enzymes involved in gene expression. The enzymes can modify the histones bound to specific genes, loosening the chromatin structure and turning on/activating the DNA. This way, different cell types within the same organism can activate specific gene groups while keeping other groups silent, which means those are not expressed.

Pg. 229:

1. Explain why neither cyclins nor kinases alone can cause a cell to progress through the cell cycle.

Neither cyclins nor kinases alone can cause a cell to progress through the cell cycle. They need each other to perform correctly. For a kinase to be active, it must be bound to the appropriate cyclin. As the amount of a particular cyclin rises, more kinases are activated. In turn, the kinases activate various enzymes needed to allow the cell cycle to progress by taking a phosphate out of an ATP molecule (transferring it to the enzyme in order to activate it).

2. How do controls on the cell cycle protect multicellular organisms from accumulating large numbers of damaged or defective cells?

The controls on the cell cycle protect multicellular organisms from accumulating large numbers of damaged or defective cells. There are several checkpoints during the cell cycle that make sure everything has gone according to plan; they consist of proteins that detect mistakes and quickly halt the cell cycle until repairs are made, so as not to cause further damage. This is known as cell-cycle arrest. Additionally, different proteins activate enzymes that prevent the progression of the damaged part to the next phase of the cell cycle until it is fixed. Apoptosis may occur if the cell is too far beyond repair, which is basically a cell’s suicide.

3. What is the difference between a cancerous tumor and metastasis?

There is a difference between a cancerous tumor and metastasis. Metastasis is the process in which a tumor grows, may interfere with surrounding tissue, and cells may break off and spread around the body. Checkpoint controls over cell division are damaged, so mutated proteins can progress through the cell cycle unchecked. In contrast, a cancerous tumor is the product of metastasis.

4. What are the functions of tumor-suppressor genes and proto-oncogenes in noncancerous cells?

The functions of tumor-suppressor genes in noncancerous cells are to inhibit cell division, especially when the DNA chain is damaged/mutated; they suppress the development of cells into tumors. Many cancer cells have mutations on these tumor-suppressor genes. The functions of proto-oncogenes are to help promote cell division by accelerating it directly and indirectly. Since many proto-oncogenes are involved in signal pathways, mutations can convert them into oncogenes, or cancer genes, which stimulate cells to leave the G0 phase and divide regardless of the presence of a signal, which then leads to uncontrollable cell division and growth.