Cell Cycle and Cell Division

10.1 Cell Cycle

The cell cycle is a fundamental biological process that describes the sequence of events through which a cell duplicates its genome, synthesizes other cellular components, and divides into two daughter cells. This cycle is essential for growth, development, and reproduction in all living organisms. The process is tightly regulated to ensure that DNA replication and cell division occur accurately, maintaining genetic integrity across generations. Cell division is not an isolated event; it is accompanied by DNA replication and cell growth, which must be coordinated precisely. While cell growth in terms of cytoplasmic increase is continuous, DNA synthesis occurs only during a specific phase of the cell cycle. The replicated chromosomes are then distributed to daughter nuclei through a complex series of events during cell division, which are genetically controlled. The cell cycle thus encompasses all these phases, ensuring the proper duplication and segregation of genetic material and other cellular constituents.

• Cell cycle includes genome duplication, synthesis of cellular constituents, and cell division.
• DNA replication occurs only during a specific phase, not continuously.
• Cell growth (cytoplasmic increase) is continuous throughout the cycle.
• Cell division is genetically controlled to ensure accurate genome distribution.
• The cycle is essential for growth, reproduction, and maintenance of organisms.
• 📌 Cell cycle: Sequence of events by which a cell duplicates its genome and divides.
• 📌 DNA replication: Process of copying DNA during the cell cycle.
• 📌 Cell division: The process by which a parent cell divides into two daughter cells.

10.1.1 Phases of Cell Cycle

The eukaryotic cell cycle is broadly divided into two main phases: Interphase and M Phase (Mitosis phase). Interphase is the period between two successive mitotic divisions and is often referred to as the 'resting phase,' although it is a highly active period of cell growth and DNA replication. The M Phase is the period of actual cell division. In human cells cultured in vitro, the entire cell cycle takes approximately 24 hours, but this duration can vary widely among different organisms and cell types; for example, yeast cells complete the cycle in about 90 minutes. Interphase occupies more than 95% of the total cell cycle duration, while mitosis lasts about an hour. Interphase itself is subdivided into three phases: G₁ (Gap 1), S (Synthesis), and G₂ (Gap 2). During G₁, the cell grows metabolically and synthesizes proteins but does not replicate DNA. The S phase is dedicated to DNA replication, doubling the DNA content from 2C to 4C without changing chromosome number (diploid 2n remains 2n). In animal cells, centriole duplication also occurs during S phase. G₂ phase involves further protein synthesis and preparation for mitosis, with continued cell growth. Some cells exit the cycle into a quiescent stage called G₀, where they remain metabolically active but do not divide unless stimulated. In animals, mitosis typically occurs in diploid somatic cells, while in plants, mitotic divisions can occur in both haploid and diploid cells. Meristematic tissues in plants contain cells that divide throughout the plant's life, analogous to certain proliferative tissues in animals.

• Cell cycle divided into Interphase and M Phase (mitosis).
• Interphase includes G₁ (growth), S (DNA synthesis), and G₂ (preparation for mitosis).
• DNA content doubles during S phase from 2C to 4C, chromosome number remains constant.
• M Phase includes nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
• Some cells enter G₀ phase, a quiescent state without division.
• Mitosis occurs in diploid somatic cells in animals; plants show mitosis in haploid and diploid cells.
• 📌 Interphase: Phase of cell cycle involving cell growth and DNA replication.
• 📌 G₁ phase: Period of cell growth and metabolic activity before DNA synthesis.
• 📌 S phase: Phase of DNA replication.