9.E: Cell Reproduction (Exercises)
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9.1: Cell Division
The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new daughter cells. The mechanisms involved in the cell cycle are highly regulated.
Review Questions
A diploid cell has_______ the number of chromosomes as a haploid cell.
- one-fourth
- half
- twice
- four times
- Answer
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C
An organism’s traits are determined by the specific combination of inherited _____.
- cells.
- genes.
- proteins.
- chromatids.
- Answer
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B
The first level of DNA organization in a eukaryotic cell is maintained by which molecule?
- cohesin
- condensin
- chromatin
- histone
- Answer
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D
Identical copies of chromatin held together by cohesin at the centromere are called _____.
- histones.
- nucleosomes.
- chromatin.
- sister chromatids.
- Answer
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D
Free Response
Compare and contrast a human somatic cell to a human gamete.
- Answer
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Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a pair. This is the 2 n or diploid condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes, one of which is a sex chromosome. This is the n or haploid condition.
What is the relationship between a genome, chromosomes, and genes?
- Answer
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The genome consists of the sum total of an organism’s chromosomes. Each chromosome contains hundreds and sometimes thousands of genes, segments of DNA that code for a polypeptide or RNA, and a large amount of DNA with no known function.
Eukaryotic chromosomes are thousands of times longer than a typical cell. Explain how chromosomes can fit inside a eukaryotic nucleus.
- Answer
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The DNA double helix is wrapped around histone proteins to form structures called nucleosomes. Nucleosomes and the linker DNA in between them are coiled into a 30-nm fiber. During cell division, chromatin is further condensed by packing proteins.
9.2: The Cell Cycle
The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase.
Review Questions
Chromosomes are duplicated during what stage of the cell cycle?
- G 1 phase
- S phase
- prophase
- prometaphase
- Answer
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B
Which of the following events does not occur during some stages of interphase?
- DNA duplication
- organelle duplication
- increase in cell size
- separation of sister chromatids
- Answer
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D
The mitotic spindles arise from which cell structure?
- centromere
- centrosome
- kinetochore
- cleavage furrow
- Answer
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B
Attachment of the mitotic spindle fibers to the kinetochores is a characteristic of which stage of mitosis?
- prophase
- prometaphase
- metaphase
- anaphase
- Answer
-
B
Unpacking of chromosomes and the formation of a new nuclear envelope is a characteristic of which stage of mitosis?
- prometaphase
- metaphase
- anaphase
- telophase
- Answer
-
D
Separation of the sister chromatids is a characteristic of which stage of mitosis?
- prometaphase
- metaphase
- anaphase
- telophase
- Answer
-
C
The chromosomes become visible under a light microscope during which stage of mitosis?
- prophase
- prometaphase
- metaphase
- anaphase
- Answer
-
A
The fusing of Golgi vesicles at the metaphase plate of dividing plant cells forms what structure?
- cell plate
- actin ring
- cleavage furrow
- mitotic spindle
- Answer
-
A
Free Response
Briefly describe the events that occur in each phase of interphase.
- Answer
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During G 1 , the cell increases in size, the genomic DNA is assessed for damage, and the cell stockpiles energy reserves and the components to synthesize DNA. During the S phase, the chromosomes, the centrosomes, and the centrioles (animal cells) duplicate. During the G 2 phase, the cell recovers from the S phase, continues to grow, duplicates some organelles, and dismantles other organelles.
Chemotherapy drugs such as vincristine and colchicine disrupt mitosis by binding to tubulin (the subunit of microtubules) and interfering with microtubule assembly and disassembly. Exactly what mitotic structure is targeted by these drugs and what effect would that have on cell division?
- Answer
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The mitotic spindle is formed of microtubules. Microtubules are polymers of the protein tubulin; therefore, it is the mitotic spindle that is disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during mitosis. The cell will arrest in mitosis and die.
Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.
- Answer
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There are very few similarities between animal cell and plant cell cytokinesis. In animal cells, a ring of actin fibers is formed around the periphery of the cell at the former metaphase plate (cleavage furrow). The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two. In plant cells, a new cell wall must be formed between the daughter cells. Due to the rigid cell walls of the parent cell, contraction of the middle of the cell is not possible. Instead, a phragmoplast first forms. Subsequently, a cell plate is formed in the center of the cell at the former metaphase plate. The cell plate is formed from Golgi vesicles that contain enzymes, proteins, and glucose. The vesicles fuse and the enzymes build a new cell wall from the proteins and glucose. The cell plate grows toward and eventually fuses with the cell wall of the parent cell.
List some reasons why a cell that has just completed cytokinesis might enter the G 0 phase instead of the G 1 phase.
- Answer
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Many cells temporarily enter G 0 until they reach maturity. Some cells are only triggered to enter G 1 when the organism needs to increase that particular cell type. Some cells only reproduce following an injury to the tissue. Some cells never divide once they reach maturity.
What cell cycle events will be affected in a cell that produces mutated (non-functional) cohesin protein?
- Answer
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If cohesin is not functional, chromosomes are not packaged after DNA replication in the S phase of interphase. It is likely that the proteins of the centromeric region, such as the kinetochore, would not form. Even if the mitotic spindle fibers could attach to the chromatids without packing, the chromosomes would not be sorted or separated during mitosis.
9.3: Control of the Cell Cycle
The length of the cell cycle is highly variable, even within the cells of a single organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for epithelial cells, and to an entire human lifetime spent in G0 by specialized cells, such as cortical neurons or cardiac muscle cells. There is also variation in the time that a cell spends in each phase of the cell cycle.
Review Questions
At which of the cell cycle checkpoints do external forces have the greatest influence?
- G 1 checkpoint
- G 2 checkpoint
- M checkpoint
- G 0 checkpoint
- Answer
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A
What is the main prerequisite for clearance at the G 2 checkpoint?
- cell has reached a sufficient size
- an adequate stockpile of nucleotides
- accurate and complete DNA replication
- proper attachment of mitotic spindle fibers to kinetochores
- Answer
-
C
If the M checkpoint is not cleared, what stage of mitosis will be blocked?
- prophase
- prometaphase
- metaphase
- anaphase
- Answer
-
D
Which protein is a positive regulator that phosphorylates other proteins when activated?
- p53
- retinoblastoma protein (Rb)
- cyclin
- cyclin-dependent kinase (Cdk)
- Answer
-
D
Many of the negative regulator proteins of the cell cycle were discovered in what type of cells?
- gametes
- cells in G 0
- cancer cells
- stem cells
- Answer
-
C
Which negative regulatory molecule can trigger cell suicide (apoptosis) if vital cell cycle events do not occur?
- p53
- p21
- retinoblastoma protein (Rb)
- cyclin-dependent kinase (Cdk)
- Answer
-
A
Free Response
Describe the general conditions that must be met at each of the three main cell cycle checkpoints.
- Answer
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The G 1 checkpoint monitors adequate cell growth, the state of the genomic DNA, adequate stores of energy, and materials for S phase. At the G 2 checkpoint, DNA is checked to ensure that all chromosomes were duplicated and that there are no mistakes in newly synthesized DNA. Additionally, cell size and energy reserves are evaluated. The M checkpoint confirms the correct attachment of the mitotic spindle fibers to the kinetochores.
Explain the roles of the positive cell cycle regulators compared to the negative regulators.
- Answer
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Positive cell regulators such as cyclin and Cdk perform tasks that advance the cell cycle to the next stage. Negative regulators such as Rb, p53, and p21 block the progression of the cell cycle until certain events have occurred.
What steps are necessary for Cdk to become fully active?
- Answer
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Cdk must bind to a cyclin, and it must be phosphorylated in the correct position to become fully active.
Rb is a negative regulator that blocks the cell cycle at the G 1 checkpoint until the cell achieves a requisite size. What molecular mechanism does Rb employ to halt the cell cycle?
- Answer
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Rb is active when it is dephosphorylated. In this state, Rb binds to E2F, which is a transcription factor required for the transcription and eventual translation of molecules required for the G 1 /S transition. E2F cannot transcribe certain genes when it is bound to Rb. As the cell increases in size, Rb becomes phosphorylated, inactivated, and releases E2F. E2F can then promote the transcription of the genes it controls, and the transition proteins will be produced.
9.4: Cancer and the Cell Cycle
Cancer is the result of unchecked cell division caused by a breakdown of the mechanisms that regulate the cell cycle. The loss of control begins with a change in the DNA sequence of a gene that codes for one of the regulatory molecules. Faulty instructions lead to a protein that does not function as it should. Any disruption of the monitoring system can allow other mistakes to be passed on to the daughter cells. Each successive cell division will give rise to daughter cells with even more damage
Review Questions
___________ are changes to the order of nucleotides in a segment of DNA that codes for a protein.
- Proto-oncogenes
- Tumor suppressor genes
- Gene mutations
- Negative regulators
- Answer
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C
A gene that codes for a positive cell cycle regulator is called a(n) _____.
- kinase inhibitor.
- tumor suppressor gene.
- proto-oncogene.
- oncogene.
- Answer
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C
A mutated gene that codes for an altered version of Cdk that is active in the absence of cyclin is a(n) _____.
- kinase inhibitor.
- tumor suppressor gene.
- proto-oncogene.
- oncogene.
- Answer
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D
Which molecule is a Cdk inhibitor that is controlled by p53?
- cyclin
- anti-kinase
- Rb
- p21
- Answer
-
D
Free Response
Outline the steps that lead to a cell becoming cancerous.
- Answer
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If one of the genes that produces regulator proteins becomes mutated, it produces a malformed, possibly non-functional, cell cycle regulator, increasing the chance that more mutations will be left unrepaired in the cell. Each subsequent generation of cells sustains more damage. The cell cycle can speed up as a result of the loss of functional checkpoint proteins. The cells can lose the ability to self-destruct and eventually become “immortalized.”
Explain the difference between a proto-oncogene and a tumor suppressor gene.
- Answer
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A proto-oncogene is a segment of DNA that codes for one of the positive cell cycle regulators. If that gene becomes mutated so that it produces a hyperactivated protein product, it is considered an oncogene. A tumor suppressor gene is a segment of DNA that codes for one of the negative cell cycle regulators. If that gene becomes mutated so that the protein product becomes less active, the cell cycle will run unchecked. A single oncogene can initiate abnormal cell divisions; however, tumor suppressors lose their effectiveness only when both copies of the gene are damaged.
List the regulatory mechanisms that might be lost in a cell producing faulty p53.
- Answer
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Regulatory mechanisms that might be lost include monitoring of the quality of the genomic DNA, recruiting of repair enzymes, and the triggering of apoptosis.
p53 can trigger apoptosis if certain cell cycle events fail. How does this regulatory outcome benefit a multicellular organism?
- Answer
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If a cell has damaged DNA, the likelihood of producing faulty proteins is higher. The daughter cells of such a damaged parent cell would also produce faulty proteins that might eventually become cancerous. If p53 recognizes this damage and triggers the cell to self-destruct, the damaged DNA is degraded and recycled. No further harm comes to the organism. Another healthy cell is triggered to divide instead.
9.5: Prokaryotic Cell Division
In both prokaryotic and eukaryotic cell division, the genomic DNA is replicated and then each copy is allocated into a daughter cell. In addition, the cytoplasmic contents are divided evenly and distributed to the new cells. However, there are many differences between prokaryotic and eukaryotic cell division. Bacteria have a single, circular DNA chromosome but no nucleus. Therefore, mitosis is not necessary in bacterial cell division.
Review Questions
Which eukaryotic cell cycle event is missing in binary fission?
- cell growth
- DNA duplication
- karyokinesis
- cytokinesis
- Answer
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C
FtsZ proteins direct the formation of a _______ that will eventually form the new cell walls of the daughter cells.
- contractile ring
- cell plate
- cytoskeleton
- septum
- Answer
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B
Free Response
Name the common components of eukaryotic cell division and binary fission.
- Answer
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The common components of eukaryotic cell division and binary fission are DNA duplication, segregation of duplicated chromosomes, and division of the cytoplasmic contents.
Describe how the duplicated bacterial chromosomes are distributed into new daughter cells without the direction of the mitotic spindle.
- Answer
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As the chromosome is being duplicated, each origin moves away from the starting point of replication. The chromosomes are attached to the cell membrane via proteins; the growth of the membrane as the cell elongates aids in their movement.