ACT Science Question of the Day

The process by which cells divide and multiply is known as the cell cycle. This cycle consist of two main phases: interphase and mitosis. Each phase consists of a series of clearly defined and observable steps. At the conclusion of the cycle, each parent cell produces two genetically identical daughter cells that may undergo a cycle of replication.

Roughly 90 percent of the cell cycle is spent in interphase. Interphase is comprised of three main steps: the first gap phase, the synthesis phase, and the second gap phase. The initial gap phase is a period of cellular preparation in which the cell increases in size and readies itself for DNA synthesis. In the synthesis phase, DNA replication occurs. In the second gap phase the cell grows in size and prepares for cellular division in the mitotic phase. At the end of each gap phase the cell has to pass regulatory checkpoints to ensure proper cell growth and environmental conditions.

Mitosis is a form of nuclear division and is broken down into five distinct phases. During prophase, the genetic material contained in chromatin condenses into distinct chromosomes. Prometaphase is marked by the breakdown of the nuclear envelope and the formation of centrosomes at the poles of the cell. During metaphase, kinetochores attached to the microtubules migrate the chromosomes to the center of the cell. A checkpoint ensures that the chromosomes are aligned on the center and halts the cycle if an error occurs. Anaphase occurs when chromosomes break apart at their center or centromere and sister chromatids move to opposite ends of the cell. Last, telophase and cytokinesis occurs as nuclear membranes form in each new daughter cell and when chromosomes unwind into loose chromatin. Cytokinesis is defined as the division of the each cell’s cytoplasm and organelles. The conclusion of the cell cycle results in the production of two genetically identical daughter cells.

Ninety percent of the cell cycle is spent in which stage?