If a mitotic cell is fused with a cell in the G1 phase of the life cycle, the second nucleus will also enter mitosis. What is the most likely cause of this behaviour?
(a) Because mitosis consumes the majority of the time in the cell cycle, it will dominate the newly formed fused cell.
(b) Molecules like Cycline are abundant in the cytoplasm of dividing cells and signal the fused cell nucleus to undergo mitosis.
(c) Spindle fibres from the first mitotic cell are unable to distinguish between the two nuclei's chromosomes.
(d) Following fusion, the cell detects that its surface area to volume ratio is too high and begins to divide.
VerifiedHint: Consider the role of cell cycle checkpoints and the factors that regulate cell division.
Complete step-by-step Answer: The most likely explanation for the second nucleus entering mitosis when a cell in metaphase of mitosis is fused with a cell in G1 phase of the cell cycle is (b) Molecules such as Cycline are in high concentration in the cytoplasm of a dividing cell and will signal the fused cell nucleus to undergo mitosis.
During mitosis, the cell cycle is tightly regulated by a series of checkpoints that ensure the accurate duplication and segregation of chromosomes. These checkpoints are controlled by various factors, including cyclin-dependent kinases (CDKs) and their associated cyclins. Cyclin B, in particular, is a key regulator of the G2/M checkpoint, which ensures that the cell has completed DNA replication and is ready to enter mitosis.
When a cell in metaphase of mitosis is fused with a cell in G1 phase, the high concentration of cyclin B in the cytoplasm of the mitotic cell diffuses into the G1 cell nucleus. This influx of cyclin B activates CDKs, which in turn trigger the entry of the G1 nucleus into mitosis. This phenomenon is known as nuclear fusion rescue.
Option (a) is incorrect because mitosis does not dominate the cell cycle. Interphase, the period between cell divisions, occupies most of the cell cycle.
Option (c) is incorrect because spindle fibers can distinguish between chromosomes from different origins. Spindle fibers attach to kinetochores, specialized proteins located on the centromeres of chromosomes. Kinetochores have unique sequences that allow spindle fibers to properly align and segregate chromosomes during mitosis.
Option (d) is incorrect because the surface area to volume ratio is not a direct signal for cell division. While changes in surface area to volume ratio can influence cell growth and division, they do not directly trigger the onset of mitosis.
Hence the correct answer is Option B
Note: This concept of nuclear fusion rescue is an example of how cell cycle checkpoints can be overridden under certain conditions. The presence of cyclin B from the mitotic cell provides a strong signal for the G1 nucleus to proceed with mitosis, even though it has not yet completed DNA replication.
