Cell Cycle

The Cell Cycle is an ordered series of events involving cell growth and division, which results in the formation of two daughter cells. For BISE board exams, it is crucial to understand that the cell cycle is broadly divided into two main phases: Interphase and the M-phase (Mitosis/Meiosis). Interphase is the non-dividing phase where the cell prepares for division and is subdivided into $G_1$ phase (cell growth and protein synthesis), S phase (DNA replication), and $G_2$ phase (organelle duplication and preparation for mitosis). Examiners frequently ask about the relative duration of these phases, noting that a cell spends most of its life in Interphase.

The M-phase is where actual cell division occurs, consisting of karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). Karyokinesis is further divided into four sequential stages: Prophase, Metaphase, Anaphase, and Telophase. Board questions heavily focus on identifying the key events of these stages, such as the condensation of chromatin into visible chromosomes during prophase, the alignment of chromosomes at the equator forming the metaphase plate, the splitting and movement of sister chromatids toward opposite poles during anaphase, and the re-formation of the nuclear envelope during telophase. Additionally, note the difference in cytokinesis between animal cells (via cleavage furrow) and plant cells (via cell plate formation).

Control of the cell cycle and cell death are also high-yield topics for both short and long questions. The cell cycle is strictly regulated by checkpoints ($G_1$, $G_2$, and M checkpoints) controlled by cyclins and cyclin-dependent kinases (Cdks) to prevent errors like mutations or damaged DNA from replicating. When these regulatory mechanisms fail, uncontrolled cell division leads to tumors, known as cancer. Conversely, programmed cell death, or apoptosis, is a vital normal process that eliminates damaged or redundant cells, preventing tissue damage and maintaining proper development.