Osmosis from Elsevier - Cell cycle - microbiology, biology, physiology
The cell cycle is a series of events that somatic cells undergo from formation to division into two identical daughter cells. It consists of two main phases: interphase and mitosis. Interphase is the longest phase, where the cell grows, performs its functions, and replicates its DNA in preparation for division. Interphase is divided into three subphases: G1 (growth and preparation), S (DNA synthesis), and G2 (final preparations for mitosis). During G1, the cell grows and checks for DNA damage. If damage is detected, the cell may enter a non-dividing state (G0) or undergo apoptosis. If the cell passes the G1 checkpoint, it enters the S phase, where DNA is replicated. The G2 phase involves further growth and preparation for mitosis. Mitosis is the process of dividing the replicated DNA into two nuclei, followed by cytokinesis, which splits the cell into two daughter cells. Mitosis is divided into prophase, metaphase, anaphase, and telophase, followed by cytokinesis. The cell cycle ensures that each daughter cell receives identical genetic material, maintaining the integrity of the organism's genome.
Key Points:
- The cell cycle consists of interphase and mitosis, leading to the division of a parent cell into two identical daughter cells.
- Interphase includes G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis) phases.
- Mitosis involves prophase, metaphase, anaphase, and telophase, followed by cytokinesis to form two new cells.
- Cells may enter a non-dividing state (G0) if DNA damage is detected during the G1 checkpoint.
- The cell cycle ensures genetic consistency across cell generations, crucial for organismal development and repair.
Details:
1. 🔄 Overview of the Cell Cycle
- The cell cycle encompasses the stages somatic cells undergo from formation to division, resulting in two identical daughter cells.
- Cell cycle duration varies significantly among different cell types, influenced by their function and environmental conditions.
- Rapidly dividing cells, such as skin cells, complete the cycle in less than a day, demonstrating a high turnover rate essential for tissue maintenance and repair.
- Conversely, liver cells exhibit a much longer cycle duration, potentially extending to several years, reflecting their role in metabolic functions and stability.
- Factors influencing cycle length include cellular function, tissue requirements, and external stimuli such as growth signals and cellular damage.
- Understanding these variations is crucial for medical applications, such as cancer treatment, where controlling cell division is vital.
2. 📚 Interphase: Preparation for Division
- Interphase is the longest part of the cell cycle, crucial for preparation for division.
- During interphase, the cell performs its regular functions, grows, and replicates its DNA.
- After division, each daughter cell begins interphase, indicating its recurring role in cell cycles.
3. 🔍 G1 Phase: Growth and Checkpoints
- The G1 phase, or 'Gap 1', is the longest phase of the cell cycle, focusing on cell growth and regular cellular activities such as protein synthesis and energy production.
- During G1, chromosomes consist of a single chromatid, described as a thin spaghetti of DNA.
- A critical control point, the G1 checkpoint, exists to ensure DNA is undamaged and proteins are synthesized correctly in the right amounts.
- If the cell identifies any issues, it can either enter the G0 phase for DNA repair or trigger apoptosis if the damage is irreparable, ensuring that only healthy cells proceed to division.
4. 🧬 S Phase: DNA Replication
- During the S phase, DNA is replicated to ensure each daughter cell receives identical genetic material.
- DNA replication involves structural proteins, enzymes, and energy but does not increase the number of chromosomes.
- Human somatic cells maintain 46 chromosomes throughout the cell cycle, although the amount of DNA changes.
- Each chromosome starts the S phase with a single chromatid, which is duplicated, resulting in two identical chromatids (sister chromatids) connected at the centromere.
- Despite DNA doubling, the chromosome count remains at 46.
5. 🔧 G2 Phase: Final Preparations
- In the G2 phase, cells duplicate organelles to ensure there are enough for both daughter cells.
- By the end of the G2 phase, the cell resembles a large balloon filled with cytoplasm and organelles, ready to split.
- The cell must pass the final G2 checkpoint, confirming no DNA damage after replication, before proceeding.
6. 🔬 Mitosis: Division of the Nucleus
- Mitosis involves dividing a cell into two daughter cells by separating the replicated DNA into two distinct nuclei (Kinesis) and then separating the daughter cells (cytokinesis).
- Mitosis is divided into four subphases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis.
- During prophase, the nuclear membrane disintegrates, chromosomes condense and become visible under a microscope as X shapes, and centrosomes migrate to opposite poles.
- In metaphase, chromosomes align at the metaphase plate, and spindle fibers from centrosomes attach to chromosome centromeres.
- Anaphase involves the pulling apart of sister chromatids by spindle fibers, moving them to opposite poles of the cell.
- During telophase, a nuclear membrane forms around each new set of single chromatid chromosomes, leading to two nuclei.
- Cytokinesis completes the process by pinching the cell membrane to form two separate daughter cells.
7. ⏸ G0 Phase: Non-dividing State
- Cells in the G0 phase are outside the active cell cycle, meaning they are neither dividing nor preparing to divide.
- Hepatocytes are an example of cells that remain in G0 until liver damage occurs, prompting re-entry into the G1 phase to facilitate cell division and repair.
8. 🔁 Recap of the Cell Cycle Phases
- The cell cycle is divided into two main phases: interphase and mitosis.
- Interphase includes the G1 phase (cell growth and function), S phase (DNA replication), and G2 phase (further cell growth before mitosis).
- Mitosis consists of prophase, metaphase, anaphase, and telophase, during which replicated DNA is equally divided between two daughter cells.
- The process concludes with cytokinesis, where the cell membrane divides to form two new cells.