cell cycle homework questions
1. during which stage of the cell cycle does replication of the dna occur?
s stage
2. why is it necessary that the cell copies its dna?
3. why are chromosomes visible during mitosis but not at other times?
4. under a microscope, some cells can appear to be between metaphase and anaphase. explain this observation.
5. which stage or phase of the cell cycle corresponds to each of the descriptions below?
a) a new cell wall begins to form.
b) the membrane of the nucleus dissolves.
c) daughter chromosomes begin to separate.
d) the cell begins to pinch together along its centre.
e) thick chromosome threads are visible in two distinct regions of the cell.
f) the cell grows and copies its dna.
6. create a table to summarize what happens during the three stages of the cell cycle. use the back of this page.
7. biology books used to describe interphase as the ‘resting phase’. based on what we know now, what was wrong with this term?

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### Question 2: Why is it necessary that the cell copies its DNA? # Brief Explanations: Cell division (mitosis/meiosis) produces daughter cells. To ensure each daughter cell gets a complete set of genetic information (genome) identical to the parent cell, DNA must be copied. This maintains genetic continuity, proper cell function, and inheritance of traits. # Answer: It is necessary for cell division (e.g., mitosis/meiosis) to ensure each daughter cell receives a complete, identical set of genetic information (DNA) as the parent cell, maintaining genetic continuity, proper cell function, and inheritance of traits. ### Question 3: Why are chromosomes visible during mitosis but not at other times? # Brief Explanations: In interphase (non - mitotic stages), DNA is in a loose, uncoiled form called chromatin (to allow gene expression and DNA replication). During mitosis, DNA condenses into tightly coiled chromosomes (short, thick structures) to facilitate accurate segregation of genetic material to daughter cells. # Answer: In interphase, DNA is uncoiled as chromatin (for functions like replication/expression). During mitosis, DNA condenses into tightly coiled chromosomes (short, thick) to enable accurate separation of genetic material to daughter cells, making them visible. ### Question 4: Under a microscope, some cells can appear to be between metaphase and anaphase. Explain this observation. # Brief Explanations: The cell cycle is a continuous process, and the transition between mitotic phases (like metaphase to anaphase) is not instantaneous. Cells are captured at different moments in this continuous transition during microscopic observation, so some seem to be “between” phases. # Answer: The cell cycle (including mitotic phases) is continuous, not discrete. Cells are observed at different moments of the continuous transition between metaphase (chromosomes aligned) and anaphase (chromosomes separating), creating the appearance of being “between” these phases. ### Question 5: #### a) A new cell wall begins to form. # Brief Explanations: In plant cells, during cytokinesis (after mitosis), a new cell wall starts to form in the region of the cell plate. This occurs in the telophase - cytokinesis stage. # Answer: Telophase (cytokinesis in plant cells) - a new cell wall (cell plate development) begins to form. #### b) The membrane of the nucleus dissolves. # Brief Explanations: During prophase of mitosis, the nuclear membrane (envelope) breaks down (dissolves) to allow spindle fibers to access chromosomes and for chromosome movement. # Answer: Prophase - the nuclear membrane dissolves. #### c) Daughter chromosomes begin to separate. # Brief Explanations: Anaphase is the stage where sister chromatids (now daughter chromosomes) are pulled apart by spindle fibers and begin to separate towards opposite poles of the cell. # Answer: Anaphase - daughter chromosomes begin to separate. #### d) The cell begins to pinch together along its centre. # Brief Explanations: In animal cells, during cytokinesis (after mitosis), the cell membrane pinches in (cleavage furrow formation) along the cell's center. This occurs in the telophase - cytokinesis stage for animal cells. # Answer: Telophase (cytokinesis in animal cells) - the cell begins to pinch together (cleavage furrow) along its centre. #### e) Thick chromosome threads are visible in two distinct regions of the cell. # Brief Explanations: During telophase, chromosomes (thick threads) have reached opposite poles of the cell, so they are visible in two distinct regions (near the spindle poles). # Answer: Telophase - thick chromosome threads are visible in two distinct regions (at opposite poles) of the cell. #### f) The cell grows and copies its DNA. # Brief Explanations: Interphase (specifically the G₁ (growth) and S (DNA synthesis) phases) is when the cell grows, performs its normal functions, and copies its DNA (in S phase) in preparation for cell division. # Answer: Interphase (G₁ and S phases) - the cell grows and copies its DNA. ### Question 6: Create a table to summarize what happens during the three stages of the cell cycle. | Stage of Cell Cycle | Key Events | | ---- | ---- | | Interphase | - G₁: Cell grows, performs normal metabolic functions, and prepares for DNA replication.
- S: DNA replication (synthesis) occurs, resulting in duplicated chromosomes (sister chromatids).
- G₂: Cell continues to grow, synthesizes proteins/enzymes needed for mitosis, and prepares for cell division. | | Mitosis | - Prophase: Chromosomes condense, nuclear membrane dissolves, spindle apparatus forms.
- Metaphase: Chromosomes align at the cell's equator (metaphase plate).
- Anaphase: Sister chromatids separate and move to opposite poles.
- Telophase: Chromosomes de - condense, nuclear membranes reform around each set of chromosomes, spindle apparatus disassembles. | | Cytokinesis | - Animal cells: Cell membrane pinches in (cleavage furrow) to divide the cytoplasm into two daughter cells.
- Plant cells: A cell plate forms (which becomes a new cell wall) to separate the two daughter cells. | ### Question 7: Biology books used to describe interphase as the'resting phase'. Based on what we know now, what was wrong with this term? # Brief Explanations: Interphase is not a “resting” phase. During interphase, the cell is metabolically active: it grows (G₁), replicates its DNA (S), and prepares for mitosis (G₂). It performs essential functions like protein synthesis, organelle duplication, and is preparing for cell division, so it is highly active, not resting. # Answer: Interphase is misnamed as “resting” because the cell is metabolically active: it grows (G₁), replicates DNA (S), prepares for mitosis (G₂), synthesizes proteins/organelles, and performs normal functions, rather than being inactive.

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Question 2: Why is it necessary that the cell copies its DNA?

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Question 3: Why are chromosomes visible during mitosis but not at other times?