The enzyme telomerase prevents loss of telomeres after many mitotic cell cycles.
Which cells need to transcribe telomerase enzyme?
▶️ Answer/Explanation
Ans: B
Telomerase is needed by cells that undergo frequent division. Stem cells (1) constantly divide for tissue renewal, and memory B-lymphocytes (2) proliferate during immune responses. Helper T-lymphocytes (3) secrete cytokines but don’t require frequent division, so they don’t need telomerase. Thus, the correct answer is B (1 and 2 only).
A scientist stains the chromosomes of a plant cell with a fluorescent dye to observe the telomeres. This cell has 38 chromosomes.
How many telomeres will the scientist observe in one of the nuclei during telophase of mitosis?
▶️ Answer/Explanation
Ans: B
During telophase of mitosis, each chromosome consists of two sister chromatids, each with its own pair of telomeres. Since the cell has 38 chromosomes, the total number of telomeres observed is calculated as:
\[ 38 \text{ chromosomes} \times 2 \text{ chromatids/chromosome} \times 2 \text{ telomeres/chromatid} = 152 \text{ telomeres} \]
However, in one nucleus (after cytokinesis), only half of these telomeres are present (since the cell divides into two daughter cells). Thus:
\[ 152 \div 2 = 76 \text{ telomeres} \]
Therefore, the correct answer is B (76).
The graph shows the mean length of the spindle fibres during mitosis.
Which region of the graph shows when all the centromeres have detached from the spindle fibres?
▶️ Answer/Explanation
Ans: D
During mitosis, spindle fibres shorten during anaphase to separate sister chromatids. When all centromeres detach, spindle fibres disappear, leading to a sharp decline in their mean length. Region D represents this phase, where spindle fibre length drops to zero, indicating complete detachment of chromosomes.
The diagram shows a typical mitotic cell cycle and the point in the cell cycle that has been reached by each of four cells, V, W, X and Y.
Which row correctly identifies the cells that match the two descriptions?
▶️ Answer/Explanation
Ans: B
The first description refers to a cell with unreplicated chromosomes, which is found in the G1 phase (Cell V). The second description refers to a cell where chromosomes are aligned at the equator, which occurs during metaphase (Cell X). Therefore, the correct pairing is V and X, corresponding to option B.