1. Which of the following is NOT involved in binary fission in prokaryotes? a. Replication of DNA b. Elongation of the cell c. Separation of daughter cells by septum formation d. Assembly of the nuclear envelope A. Answer a is incorrect. There must be two copies of the cell s genetic material in order for cell division to occur. Both daughter cells need their own copy of the parent cell s DNA. B. Answer b is incorrect. Elongation is a necessary step to increase cell volume and eventually divide into two new daughter cells. The correct answer is d C. Answer c is incorrect. Binary fission is not complete until the parent cell has divided into two daughter cells. This process occurs through the formation of a septum in prokaryotic cells. Assembly of the nuclear envelope D. Prokaryotic cells do not possess internal membranes. Their DNA exists within the cytoplasm and is not contained in a nuclear envelope. 2. Chromatin is composed of a. RNA and protein b. DNA and protein c. sister chromatids d. chromosomes A. Answer a is incorrect. Although RNA is associated with chromosomes, the term chromatin refers to the material that physically makes up the chromosome, which is DNA and protein. DNA and protein B. Answer b is correct. When DNA binds with histone proteins it forms chromatin. C. Answer c is incorrect. Sister chromatids are composed of chromatin, not the other way around. D. Answer d is incorrect. Chromosomes are composed of chromatin. 3. What is a nucleosome? a. It is a region in the cell s nucleus that contains euchromatin. b. It is a region of DNA wound around a collection of histone proteins. c. It is a region of a chromosome made up of multiple loops of chromatin. d. It is 30-nm fiber found in chromatin.
A. Answer a is incorrect. Euchromatin refers to regions of DNA that are actively expressed (transcribed into RNA) in the cell s nucleus. It is a region of DNA wound around a collection of histone proteins. B. Answer b is correct. The histones proteins function to coil the DNA and help package the DNA within the nucleus. C. Answer c is incorrect. The multiple radial loops of a chromosome involve the 30-nm fiber that already contains the nucleosome structures. They contribute to the larger overall structure of the chromosome. D. Answer d is incorrect. The 30-nm fiber represents the larger folding of the nucleosomes into solenoid coils. 4. How do sister chromatids differ from homologous chromosomes? a. Sister chromatids only represent the maternal genetic contribution. b. Homologous chromosomes are exact copies, but sister chromatids are just similar. c. Homologous chromosomes are similar, but sister chromatids are exact copies. d. Sister chromatids represent only half the genetic information stored in a chromosome. The correct answer c A. Answer a is incorrect. The term sister refers to the fact that these strands of DNA are derived from the same parent strand of DNA. The correct answer c B. Answer b is incorrect. The homologous chromosomes represent the maternal and paternal contributions to an organism s genome. They are not identical. The correct answer c Homologous chromosomes are similar, but sister chromatids are exact copies. C. Answer c is correct. The sister chromatids are formed when the DNA is replicated. They are exact copies. The correct answer c D. Answer d is incorrect. The sister chromatids are copies of the cell s DNA. The two chromatids are identical and contain all the information for any one chromosome. 5. What is the role of cohesin proteins in cell division? a. They organize the DNA of the chromosomes into highly condensed structures. b. They hold the DNA of the sister chromatids together. c. They help the cell divide into two daughter cells. d. They hold the microtubules onto the chromosome.
A. Answer a is incorrect. Condensin is the protein thought to be responsible for the condensation of chromosomes. They hold the DNA of the sister chromatids together. B. Answer b is correct. The cohesin protein links the two sister chromatids together. Cohesin is concentrated at the centromere in condensed chromosomes. C. Answer c is incorrect. The physical separation of the cell into two daughter cells is called cytokinesis. D. Answer d is incorrect. The kinetochore is the structure responsible for linking microtubules to sister chromatids. 6. Replication of the organelles of the eukaryotic cell occurs during a. interphase b. G 1 c. S d. G 2 A. Answer a is incorrect. The term interphase refers to all phases of the cell cycle other than mitosis. Organelle replication does occur during this time; however, there is a more precise answer possible. B. Answer b is incorrect. Many cellular processes occur during G 1 ; however, organelle replication is not associated with this phase. C. Answer c is incorrect. S phase is associated with DNA replication. G 2 D. Answer d is correct. Organelles replicate during G 2 in preparation for cell division. 7. Replicated copies of each chromosome are called and are connected at the. a. homologues; centromere b. sister chromatids; kinetochores c. sister chromatids; centromere d. homologues; kinetochore A. Answer a is incorrect. The DNA copies generated during S phase form the sister chromatids. They are, however, held together by the centromere.
B. Answer b is incorrect. The DNA copies generated during S phase do form the sister chromatids; however, the kinetochores function to link the chromatids to microtubules, not to each other. sister chromatids; centromere C. Answer c is correct. S phase replicates the DNA to form the sister chromatids. The chromatids are connected to each other by the centromere. D. Answer d is incorrect. The homologous chromosomes are separate strands of DNA and not copies. Kinetochores link the chromatids to microtubules and not to each other. 8. Kinetochores are associated with which of the following on a sister chromatid? a. Centromere b. Centriole c. Condensation d. Cohesion Centromere A. Answer a is correct. The centromere is the point along the chromosome where the two sister chromatids are connected and where the microtubules connect via the kinetochore. B. Answer b is incorrect. The centriole is a microtubule structure that organizes the mitotic spindle. It is not part of the chromatids. C. Answer c is incorrect. Condensation is the packaging of the cell s DNA into chromosomes. D. Answer d is incorrect. Cohesion links the two sister chromatids, but is independent of the presence of the kinetochores. 9. Separation of the sister chromatids and elongation of the cell occurs during a. prophase b. prometaphase c. anaphase d. telophase A. Answer a is incorrect. The nuclear envelope breaks down, and the mitotic spindles form during prophase. The sister chromatids are still connected. B. Answer b is incorrect. Prometaphase is a time when chromosomes are attaching to and moving along the mitotic spindle. The sister chromatids are still connected. anaphase
C. Answer c is correct. The cohesin proteins holding the two chromatids together are severed during anaphase. This allows the spindle microtubules to pull the two chromatids apart while also causing the cell to elongate. D. Answer d is incorrect. The nuclear envelope reforms during telophase, and the mitotic spindle disassembles. The separation of the sister chromatids and elongation of the cell has already occurred. 10. Why is cytokinesis an important part of cell division? a. It is responsible for the proper separation of genetic information. b. It is responsible for the proper separation of the cytoplasmic contents. c. It triggers the movement of a cell through the cell cycle. d. It is responsible for the elongation of the cell. A. Answer a is incorrect. The proper separation of the genetic information in a cell depends on the microtubule spindle. It is responsible for the proper separation of the cytoplasmic contents. B. Answer b is correct. Cytokinesis is the process that pinches the cell in two, distributing the cytoplasm between the two daughter cells. C. Answer c is incorrect. The regulation of the cell cycle is controlled by proteins called cyclins and the associated cyclin-dependent kinases. D. Answer d is incorrect. Elongation of the cell occurs independently of the process of cytokinesis. 11. At which stage in the cell cycle does a cell make a commitment to undergo cell division? a. At the G 1 /S checkpoint b. At the G 2 /M checkpoint c. At the spindle checkpoint d. At cytokinesis At the G 1 /S checkpoint A. Answer a is correct. Once a cell has passed the G 1 checkpoint and has replicated its DNA (in S phase), it must undergo cell division to separate those DNA copies into daughter cells. B. Answer b is not correct. The G 2 /M checkpoint is an important step that leads to mitosis; however, the cell can only reach this step if it has already replicated its DNA.
C. Answer c is incorrect. The spindle checkpoint is an important step that ensures that the sister chromatids are properly segregated into the daughter cells; however, the cell would not reach this stage if it had not replicated its DNA. D. Answer d is incorrect. Cytokinesis is the physical separation of the two daughter cells; however, a cell would not reach this stage without first committing to S phase. 12. How is the activity of cyclin-dependent kinases (Cdk s) regulated? a. Presence or absence of cyclins b. Phosphorylation c. Growth factors d. All of the above A. Answer a is incorrect. Cdk activity requires the presence of cyclins; however, other mechanisms regulate the activity. B. Answer b is incorrect. Phosphorylation is an important regulatory mechanism for Cdk, but it is not the only one. C. Answer c is incorrect. Growth factors can influence the activity of Cdk by altering the expression of cyclins; however, other mechanisms are involved in its regulation. All of the above D. Answer d is correct. All of the mechanisms listed above play a role in regulating Cdk. 13. Why is the function of the protein p53 important for preventing cancer? a. It ensures that the chromosomes are properly aligned for anaphase. b. It integrates signals in the cell leading to the production of cyclins. c. It checks for DNA damage before allowing replication during S phase. d. It functions as a phosphatase, removing phosphates to regulate Cdk. A. Answer a is incorrect. Alignment of the chromosomes at anaphase is important for the proper segregation of sister chromatids to the two daughter cells. This process involves the anaphase-promoting complex. B. Answer b is incorrect. Integration of cell signals is the job of proteins like Rb that interact both with transcription factors and Cdk. It checks for DNA damage before allowing replication during S phase. C. Answer c is correct. The role of p53 protein is specific to G 1. The activity of p53 prevents the replication of damaged DNA that might lead to cancer.
D. Answer d is incorrect. Phosphatases (enzymes that remove phosphate groups) are important in the regulation of the cell cycle; however, p53 is not a phosphatase. 14. What is a protooncogene? a. A mutant gene associated with the cell cycle that lead to cancer b. A normal gene that can result in cancer if it becomes mutated c. A gene introduced into a cell that can cause cancer d. A gene that suppresses unregulated cell divisions A. Answer a is incorrect. Mutant genes associated with cancer are called oncogenes. A normal gene that can result in cancer if it becomes mutated B. Answer b is correct. Protooncogenes are normal genes that have the potential to cause cancer if they become mutated. C. Answer c is incorrect. A protooncogene is one that is normally present in the cell. It is not introduced from an outside source. D. Answer d is incorrect. Genes that help to suppress uncontrolled cell division are called tumor-suppressor genes. Challenge Questions 1. Regulation of the cell cycle is very complex and involves multiple proteins, all interacting to control the process of cell division. MPF is the cyclin-cdk complex that is responsible for moving the cell past the G 2 /M checkpoint. The activity of the cyclin-dependent kinase of MPF is regulated by phosphorylation. Cdk is inhibited when it is phosphorylated by the kinase, Wee-1. Predict what would happen to a cell if Wee-1 were absent. Answer If Wee-1 were absent then there would be no way for the cell to phosphorylate Cdk. If Cdk is not phosphorylated, then it cannot be inhibited. If Cdk is not inhibited, then it will remain active. If Cdk remains active, then it will continue to signal the cell to move through the G 2 /M checkpoint, but now in an unregulated manner. The cells would undergo multiple rounds of cell division without the growth associated with G 2. As a consequence, the daughter cells will become smaller and smaller with each division hence the name of the protein! 2. Review your knowledge of signaling pathways (chapter 9). Create an outline illustrating how a growth factor (ligand) can lead to the production of a cyclin protein that would trigger S phase. Answer Growth factor = ligand 1. Ligand binds to receptor (the growth factor will bind to a growth factor receptor). 2. A signal is transduced (carried) into the cytoplasm.
3. A signal cascade is triggered. Multiple intermediate proteins or second messengers will be affected. 4. A transcription factor will be activated to bind to a specific site on the DNA. 5. Transcription occurs and the mrna enters the cytoplasm. 6. The mrna is translated and a protein is formed. 7. The protein functions within the cytoplasm possibly triggering S phase. If you study Figure 10.22 you will see a similar pathway for the formation of S phase proteins following receptor ligand binding by a growth factor. In this diagram various proteins in the signaling pathway become phosphorylated and then dephosphorylated. Ultimately, the Rb protein that regulated the transcription factor E2F becomes phosphorylated. This releases the E2F and allows it to bind to the gene for S phase proteins and cyclins.