Cell Cycle mindmap

Overview of the Cell Cycle

Definition

An ordered series of events in cells crucial for growth, development, and reproduction of organisms. The cell cycle is essential for the maintenance of healthy cells and encompasses the processes that lead to cell division and replication, ensuring genetic continuity and proper functioning of all living organisms.

Phases

• Interphase: This phase encompasses about 90% of the cell cycle and is crucial for preparing the cell for division. It is divided into three subphases:

  • G1 Phase (Gap 1): A period of cell growth where the cell produces proteins and synthesizes RNA, accumulating the necessary building blocks for chromosomal DNA and proteins. The cell also increases in size and prepares for DNA synthesis; importance is given to nutrient availability and overall cell health.

  • S Phase (Synthesis): The DNA within the nucleus is replicated, and centrosomes (the structures that anchor the microtubules) are duplicated. Energy stores are replenished, facilitating the energy-intensive process of DNA synthesis. This phase ensures that each sister chromatid is an exact copy of the original DNA.

  • G2 Phase (Gap 2): This phase further prepares the cell for mitosis. Proteins required for mitosis are synthesized, and the cell undergoes a final check on DNA replication and integrity. The cytoskeleton begins to dismantle in preparation for cell division.

• M Phase (Mitosis): This phase includes two key processes:

  • Mitosis: Characterized by the division of the nucleus and its contents, which is further divided into distinct stages: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. Each stage is vital for ensuring that daughter cells receive the correct number of chromosomes.

  • Cytokinesis: The division of the cytoplasm occurs simultaneously with telophase, resulting in the formation of two distinct daughter cells, each with identical genetic material.

Cell Cycle Regulation

• Importance: The cell cycle is strictly regulated by chemical signals, feedback loops, and checkpoints that monitor cell size, nutrient availability, and DNA integrity.

• Key Checkpoints: Checkpoints at G1, G2, and M phases ensure that each stage is completed correctly before the cell progresses to the next stage. These checkpoints can halt the cycle if conditions aren't favorable, allowing the cell to repair any damage or adapt to its environment.

  • G1 Checkpoint: Reviews cell size, nutrients, growth factors, and DNA damage. If conditions are not met, the cell may enter a non-dividing state called G0 phase.

  • G2 Checkpoint: Ensures that DNA has been fully and accurately replicated without damage. If issues are detected, the cycle is paused for repairs.

  • M-Spindle Checkpoint: Verifies that all chromosomes are attached to the spindle apparatus before the cell proceeds to anaphase, preventing errors in chromosome segregation.

Mitosis Phases

• Prophase: The chromatin condenses into visible chromosomes, and sister chromatids are joined at centromeres. The nuclear envelope begins to disintegrate, and spindle fibers start to form from centrosomes which have migrated to opposite poles of the cell.

• Prometaphase: The nuclear envelope is completely fragmented, allowing spindle fibers to connect to kinetochores - protein structures on the centromeres of chromosomes. Chromosomes begin moving due to the tension from spindle fibers.

• Metaphase: Chromosomes align at the cell's equatorial plane (metaphase plate). Each sister chromatid is connected to spindle fibers from opposite poles.

• Anaphase: The enzyme separase cleaves cohesin proteins, allowing sister chromatids to separate. The now individual chromosomes are pulled to opposite poles as the microtubules shorten, and the cell begins to elongate.

• Telophase: Chromosomes arrive at opposite poles and decondense back to chromatin. The nuclear envelope reassembles around each set of chromosomes, and nucleosomes reappear, marking the end of mitosis.

Cytokinesis

• Process: The cytoplasm divides, forming two identical daughter cells, each with a complete set of chromosomes. In animal cells, a cleavage furrow forms; in plant cells, a cell plate develops to separate the two daughter cells.

Regulatory Proteins in Cell Cycle

• Cyclins and Cyclin-dependent Kinases (Cdks): These proteins are crucial for advancing the cell through different phases of the cell cycle. Cyclins activate Cdks, which then phosphorylate target proteins to drive progression.

• MPF (Maturation-Promoting Factor): A cyclin-Cdk complex that triggers the transition from G2 to M phase. MPF induces events such as nuclear envelope breakdown and chromatin condensation, facilitating the onset of mitosis.

Checkpoints in the Cell Cycle

• G1 Checkpoint: Evaluates cell size, nutrients, growth factors, and DNA integrity before committing to DNA synthesis.

• G2 Checkpoint: Ensures DNA replication is complete and accurately checked for damage before mitosis begins.

• M-Spindle Checkpoint: Monitors the attachment of spindle fibers to kinetochores, ensuring proper chromosome segregation.

External and Internal Signals at Checkpoints

The integrity of the cell cycle is aided by both external signals (like growth factor availability and cell crowding) and internal checks within the cell’s machinery.

• Density-dependent inhibition: When cells are densely packed, they receive signals to halt division, preventing overcrowding and ensuring stable tissue structure.

Meiosis Overview

Meiosis produces haploid gametes through two successive divisions, creating genetic diversity through recombination and independent assortment.

Meiosis I

• Prophase I: Chromosomes condense, and homologous chromosomes undergo synapsis and recombination. The nuclear envelope disappears, allowing spindle fibers to form.

• Metaphase I: Paired homologous chromosomes align along the metaphase plate, increasing genetic variation.

• Anaphase I: Homologues are pulled apart (disjunction); chromatids remain together.

• Telophase I: The cell divides into two, each containing half the original chromosome number, but each chromosome still consists of two sister chromatids.

Cytokinesis**: The cytoplasm divides resulting in two haploid daughter cells, each with a unique combination of genes due to previous recombination events.

Meiosis II

• Prophase II: Chromosomes condense again; a new spindle apparatus forms in each haploid cell.

• Metaphase II: Chromosomes align at the metaphase plate, akin to mitosis but with half the chromosome numbers.

• Anaphase II: Sister chromatids separate, and individual chromosomes move toward opposite poles.

• Telophase II: The nucleolus reappears, and the nuclear envelope reforms around separated chromatids.

Cytokinesis**: Each of the two cells from Meiosis I divides again, resulting in four haploid cells, each genetically distinct from one another and from the parent cell due to the processes of crossing over and independent assortment that occurred in Meiosis I.