Question

What is necessary for a cell to pass the G2 checkpoint? a. cell has reached a sufficient size b. an adequate stockpile of nucleotides c. accurate and complete DNA replication d. proper attachment of mitotic spindle fibers to kinetochores

Short answer

The most critical prerequisite for a cell to pass the G2 checkpoint is accurate and complete DNA replication (option c).

Step-by-step solution

Understanding the G2 Checkpoint

The G2 checkpoint is a point in the cell cycle at which the cell ensures that DNA replication in the cell nucleus has been completed correctly. If DNA is damaged or replication is not complete, the cell cycle is halted to prevent the cell from dividing, potentially giving rise to genetically unstable daughter cells. This describes option c. If there are any errors, the cell can repair them before going into the M phase for mitosis.

Analyzing Other Options

Option a suggests the size of a cell is a condition for G2 checkpoint passage. Although it is important for cells to be of sufficient size before they divide, cell size is primarily assessed during the G1 checkpoint, rather than G2. Option b underscores the importance of having an adequate stockpile of nucleotides. While nucleotides are indeed necessary for DNA replication, the stockpiling of these components is also more relevant to the G1 and S checkpoints. Option d emphasizes the attachment of mitotic spindle fibers to kinetochores. While this is a critical event, it occurs during the M phase of the cell cycle, specifically at the M checkpoint, not the G2 checkpoint.

Key concepts

DNA Replication in the Cell Nucleus

DNA replication is a pivotal process that occurs within the cell nucleus. It's a highly regulated mechanism that copies the cell's entire genome in preparation for cell division. During the S phase of the cell cycle, enzymes like DNA polymerase start the replication process at specific regions of the DNA, known as origins of replication. As the helix unwinds, nucleotides are matched with their complementary bases on each strand, creating two identical molecules of DNA.

DNA replication must be accurate and complete for a cell to pass the G2 checkpoint. The cell has a series of surveillance mechanisms to ensure no errors have occurred. This is critical because any mistakes that are passed on during cell division can cause mutations, which might lead to diseases such as cancer. There are proofreading functions built into the DNA replication enzymes that correct any errors on-the-fly, promoting genetic fidelity. Should replication errors be detected at the G2 checkpoint, the cell cycle is put on hold, allowing for repair mechanisms to fix the issues before the cell proceeds to mitosis.

Cell Cycle Checkpoints

The cell cycle is comprised of a series of regulated steps that lead to cell division. Cell cycle checkpoints are like quality control stations, where the cell evaluates whether certain conditions have been met before progressing to the next stage. There are primary checkpoints: G1, S, G2, and the M checkpoint.

The G1 checkpoint ensures the cell is ready for DNA synthesis, checking for adequate resources and no DNA damage. During the S phase, replication is monitored for progress and precision. The G2 checkpoint, which is the focus of our exercise, confirms that DNA replication is complete and error-free. If any DNA damage is detected, the cell can activate repair processes or, in cases of severe damage, signal for apoptosis to prevent the faulty cell from proliferating. The M checkpoint is the final check, where the cell ensures that mitotic spindle fibers are properly attached to kinetochores before allowing sister chromatids to separate, leading to the formation of two new cells.

Mitosis and Kinetochore Attachment

Mitosis is a process where a single cell divides into two identical daughter cells, each with the same number of chromosomes as the parent cell. Mitosis is divided into stages: prophase, metaphase, anaphase, and telophase. A key event during mitosis is the attachment of mitotic spindle fibers to a structure on the chromosome known as the kinetochore.

During metaphase, chromosomes line up at the equatorial plate of the cell. Spindle fibers extend from the centrosomes at opposite poles of the cell and attach to kinetochores present on the centromeres of each chromosome. This alignment and attachment are crucial because they ensure that when the chromosomes are pulled apart during anaphase, each new cell will receive the correct number of chromosomes. Proper kinetochore attachment is assessed at the M checkpoint, not the G2 checkpoint as was erroneously suggested in the exercise. Without correct kinetochore-spindle fiber attachment, cells could end up with the wrong number of chromosomes, a condition known as aneuploidy, which can have severe consequences for cell function and overall health.