Cell Cycle Phases Explained PDF

Certainly! Here\'s a summary of the events in the **G1**, **S**, **G2**, and **M phases** of the **cell cycle** based on the contents of *The Molecular Biology of the Cell* by Alberts et al. **Overview of the Cell Cycle** The cell cycle is a tightly regulated process that leads to cell division. I...

Certainly! Here\'s a summary of the events in the **G1**, **S**, **G2**, and **M phases** of the **cell cycle** based on the contents of *The Molecular Biology of the Cell* by Alberts et al. **Overview of the Cell Cycle** The cell cycle is a tightly regulated process that leads to cell division. It is divided into two main phases: - **Interphase**: Where the cell grows and DNA is replicated. - **G1 phase** (Gap 1) - **S phase** (Synthesis) - **G2 phase** (Gap 2) - **M phase**: Where mitosis occurs, resulting in the division of the nucleus and cytoplasm. **G1 Phase (Gap 1)** - **Cell Growth and Preparation for DNA Replication**: - The G1 phase is the first phase of interphase, following cell division (M phase). - During G1, the cell grows in size, synthesizes RNA and proteins, and prepares for DNA replication. - The cell checks its internal and external environment to ensure conditions are favorable for DNA replication and division. - **Checkpoint Control**: The **G1 checkpoint** (also known as the **restriction point** in animal cells) ensures that the cell is ready to enter the S phase. If conditions are not right (e.g., DNA damage, lack of nutrients, etc.), the cell may enter a resting phase called **G0**, where it remains metabolically active but doesn\'t divide. - **Cyclin D** and **Cyclin E** are involved in the progression through G1, activating **CDKs** (cyclin-dependent kinases) that push the cell toward the S phase. - **Key Events**: - Protein synthesis, including cyclins, required for the progression through the cell cycle. - Growth in size and preparation for DNA replication. **S Phase (Synthesis)** - **DNA Replication**: - The S phase is where **DNA replication** occurs. Each chromosome is duplicated, resulting in two identical sister chromatids per chromosome. - DNA replication is tightly regulated by a series of enzymes, including **DNA polymerase** and **helicase**, which unwind the DNA and synthesize new strands. - The **centrosomes** (which help organize the microtubules for mitosis) are also duplicated during the S phase. - **Key Events**: - DNA synthesis begins at multiple **origins of replication** along each chromosome. - Each chromosome is duplicated, resulting in two identical sister chromatids. - Centrosome duplication begins. **G2 Phase (Gap 2)** - **Final Preparations for Mitosis**: - The G2 phase is a period of preparation for mitosis. It follows the completion of DNA replication in the S phase. - During G2, the cell continues to grow and produces more proteins necessary for mitosis. - The cell also checks to ensure that DNA replication has been completed without errors and that the cell is large enough for division. - The **G2 checkpoint** ensures that any DNA damage is repaired before the cell enters mitosis. If the DNA is damaged and cannot be repaired, the cell may undergo **apoptosis** (programmed cell death). - Cyclin B accumulates during G2, and once it binds to CDK1, it forms the **Maturation Promoting Factor (MPF)**, which helps the cell transition into mitosis. - **Key Events**: - Continued growth and preparation for mitosis. - Accumulation of cyclins (e.g., Cyclin B) and activation of CDKs. - Final checks for DNA integrity. **M Phase (Mitosis)** - **Mitosis and Cytokinesis**: - The M phase includes two major processes: **mitosis**, the division of the nucleus, and **cytokinesis**, the division of the cytoplasm. - Mitosis is broken down into several stages: - **Prophase**: - Chromosomes condense into visible structures. - The nuclear envelope begins to break down. - The **mitotic spindle** begins to form, which is made up of microtubules that will help separate the sister chromatids. - **Prometaphase**: - The nuclear envelope completely dissolves. - The spindle fibers attach to the **kinetochores** (protein complexes on the centromeres of the chromosomes). - **Metaphase**: - Chromosomes align along the **metaphase plate** (the center of the cell). - The spindle fibers ensure that each chromosome is properly attached to the microtubules from opposite spindle poles. - **Anaphase**: - The sister chromatids are pulled apart towards opposite poles of the cell. This separation is facilitated by the shortening of the microtubules. - **Telophase**: - The separated chromatids reach opposite poles. - The nuclear envelope re-forms around each set of chromosomes. - Chromosomes begin to de-condense back into chromatin. - **Cytokinesis**: - The cytoplasm is divided into two daughter cells. In animal cells, a **contractile ring** (actin filaments) forms and pinches the cell into two. - In plant cells, a **cell plate** forms, which eventually gives rise to the new cell wall. - **Key Events**: - The process of mitosis ensures that two genetically identical daughter cells are produced. - Chromosomes are evenly divided between the two daughter cells. - The cell undergoes **cytokinesis**, completing the division of the cell. **Cell Cycle Checkpoints** Throughout the cell cycle, there are **checkpoints** that ensure the cell is progressing correctly: - **G1 checkpoint**: Ensures the cell is ready to proceed to S phase. - **G2 checkpoint**: Verifies that DNA replication is complete and correct before entering mitosis. - **M checkpoint**: Ensures that the chromosomes are properly aligned and attached to the spindle fibers before proceeding to anaphase. **Summary of the Cell Cycle:** 1. **G1 Phase**: Cell grows, performs normal functions, and prepares for DNA replication. Cyclins and CDKs drive progression through the cycle. 2. **S Phase**: DNA replication occurs, and the chromosomes are duplicated. 3. **G2 Phase**: The cell prepares for mitosis, and checks for DNA damage or replication errors. 4. **M Phase**: Mitosis (nuclear division) and cytokinesis (cytoplasmic division) occur, resulting in two daughter cells. This overview is based on the descriptions in *The Molecular Biology of the Cell*, but is generalized. For in-depth information and to explore the specific molecular mechanisms in greater detail, I would highly recommend reading Chapter 17 Based on the references from the books and papers I mentioned earlier, here\'s a summary of organelle duplication and DNA replication in the context of the cell cycle: **Organelle Duplication and DNA Replication in the Cell Cycle** 1. **G1 Phase (First Gap Phase):** - **Organelle Duplication:** During this phase, cells grow and produce proteins and organelles, including mitochondria and chloroplasts. The organelles start to duplicate in preparation for cell division1. - **Preparation for DNA Replication:** The cell synthesizes enzymes and other proteins necessary for DNA replication. 2. **S Phase (Synthesis Phase):** - **Nuclear DNA Replication:** The primary focus of this phase is the replication of the cell\'s nuclear DNA. - **mtDNA and cpDNA Replication:** Mitochondrial and chloroplast DNA also replicate during this phase. This ensures that each daughter cell will inherit - directly. **Preparation for Mitosis:** The cell continues to grow and produce proteins, ensuring that all organelles, including mitochondria and chloroplasts, are properly duplicated and ready for segregation into daughter cells. - **Quality Control:** The cell checks that DNA replication has occurred correctly and repairs any errors.  **M Phase (Mitosis):** - **Distribution:** This phase involves the separation of the duplicated chromosomes into two daughter cells. - **Organelle Distribution:** Mitochondria and chloroplasts are distributed into the daughter cells, ensuring that both new cells have the necessary organelles to function properly yes ok tell me that where transcriptioon occur iin cell cycle in the regard of referances. also tells about protien synthesis **Transcription in the Cell Cycle:** Transcription, the process of converting DNA to mRNA, occurs throughout the cell cycle but is regulated differently at each stage. Here\'s a summary based on the references: 1. **G1 Phase:** - **Active Transcription:** Cells are highly transcriptionally active during this phase, preparing for DNA replication by synthesizing RNA and proteins necessary for cell growth. - **Gene Expression:** Genes involved in cell cycle regulation, metabolism, and growth are transcribed. 2. **S Phase:** - **Reduced Transcription:** While the primary focus of this phase is DNA replication, some transcription still occurs, particularly for genes required for DNA synthesis and repair. - **Specific Transcription Factors:** Certain transcription factors are active to ensure the replication machinery is properly synthesized. 3. **G2 Phase:** - **Increased Transcription:** Transcription activity increases again as the cell prepares for mitosis. Genes involved in mitosis and other necessary functions are transcribed.