The Cell Cycle PDF

The cell cycle ============== **Learning Objectives** - The cell cycle is a process that helps cells divide to produce new, genetically identical daughter cells. - These new cells are crucial for: - **Growth**: To increase the size of an organism. - **Repair**: To replace ol...

The cell cycle ============== **Learning Objectives** - The cell cycle is a process that helps cells divide to produce new, genetically identical daughter cells. - These new cells are crucial for: - **Growth**: To increase the size of an organism. - **Repair**: To replace old or damaged cells. - **Reproduction**: For some organisms, cell division creates entirely new organisms (asexual reproduction). What is Cell Division and Reproduction? ======================================= 1. **Mitosis (Main Type of Cell Division)** - **Purpose**: A process where a single cell divides into two cells with identical DNA. - Uses of Mitosis: ================ - **Asexual reproduction**: One parent creates offspring that are exact copies (e.g., Amoeba, bougainvillea plants). - **Growth**: To make more cells so the organism grows larger. - **Repair**: Replaces damaged tissues (e.g., healing a wound). 2. Sexual Reproduction =================== - Sexual reproduction depends on **meiosis**, which creates **gametes** (sex cells like eggs and sperm). - After fertilization (fusion of gametes), mitosis takes over to produce many new cells for the developing organism. **Key Point**: Mitosis ensures every new cell has the same set of instructions (DNA) as the original cell. What are Chromosomes? ===================== Chromosomes are like \"instruction manuals\" inside each cell that carry all the genetic information (DNA). 1. Structure and Importance: ========================= - Chromosomes are made of DNA (a molecule that carries genetic instructions) wrapped tightly around proteins called **histones**. - Histones: ========= - Act like spools, around which DNA wraps. - Help organize and condense DNA into neat bundles called nucleosomes. ============ - Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones. The distance between 2 nucleosome is almost 20 nucleotides. - This packaging is important because DNA is extremely long--- if stretched out, the DNA in one chromosome would be as  2. Visibility of Chromosomes: ========================== - **Inactive Cell**: When a cell is not dividing, chromosomes are thin, uncoiled, and hard to see. - **Dividing Cell**: Chromosomes coil tightly and become dense, making them easier to observe under a microscope. The word \"chromosome\" comes from \"colored body\" because they absorb stains easily. 3. How Chromosomes Work During Mitosis: ==================================== - Before a cell divides: - **Duplication**: Each chromosome is copied to make two identical copies (called **sister chromatids**). - During division: - The chromatids are split and shared equally between two new cells, ensuring that each daughter cell has the same DNA. 4. Chromosome Count: ================= - Humans have **46 chromosomes** (23 pairs). One chromosome of each pair comes from the mother, and the other from the father. 5. Karyotype: ========== - A **karyotype** is a photograph of all the chromosomes in a cell, arranged in pairs. - Example: Humans have 22 pairs of **autosomes** (non-sex chromosomes) and 1 pair of **sex chromosomes** (XX for females, XY for males). The Cell Cycle ============== The cell cycle is a repeating process where cells grow, divide, and form new cells. This cycle has specific steps to ensure proper division. Phases of the Cell Cycle: ========================= 1. **Interphase (Preparation Phase):** - A period when the cell is **not dividing** but is preparing for division by growing and duplicating its DNA. - Interphase has 3 stages: 1. G₁ (Gap 1): =========== - The cell grows in size and collects materials needed for the next steps. - This phase is variable---it can take hours in fast-dividing cells or years in cells that divide slowly. 2. S (Synthesis): ============== - The cell makes exact copies of its DNA, turning single chromosomes into double-stranded **chromatids**. 3. G₂ (Gap 2): =========== - The cell gets ready for division by creating more 2. Mitosis (Division of the Nucleus) (karyokinesis): ================================================= - The cell divides its nucleus, ensuring each new cell gets an identical set of chromosomes. 3. Cytokinesis (Division of the Cytoplasm): ======================================== - The cell\'s cytoplasm divides, forming two separate cells. Each cell gets its own nucleus and a full set of organelle. Control of the Cell Cycle ========================= The cell cycle is tightly regulated to ensure cells divide only when ready. 1. Cyclins and CDKs (Proteins That Control the Cycle): =================================================== - **Cyclins**: Proteins that control the timing of the cell cycle. - **CDKs (Cyclin-Dependent Kinases)**: Enzymes that work with cyclins to activate the next phase of the cycle. - Together, cyclins and CDKs ensure: - Chromosomes become dense for mitosis. - The nuclear membrane breaks down at the right time. 2. Checkpoints: ============ - The cell has built-in \"checkpoints\" to pause the cycle if something goes wrong (e.g. damaged DNA or incomplete replication). Did You Know? **Permanent Cells:** - Some cells do not divide once they are formed. These are called permanent cells. They are sometimes known to be in [**G**~**0**~]{.math.inline} which is the resting phase. {#permanent-cells.-they-are-sometimes-known-to-be-in-mathbfg_mathbf0-which-is-the-resting-phase.} ============================================================================================================ - They last a lifetime and cannot be replaced if damaged. - Examples: - **Nerve cells**: In your brain and spinal cord. - **Retina cells**: Detect light in your eyes. - **Lens cells**: Help your eyes focus. - **Heart muscle cells**: Make your heart beat continuously. Key Takeaways: ============== 1. **Mitosis is Essential**: - Ensures all new cells have the same DNA as the original. - Necessary for growth, repair, and some types of reproduction. 2. Chromosomes Must Be Duplicated: =============================== - Before dividing, chromosomes are copied to ensure each daughter cell has the full set. 3. The Cell Cycle is Controlled: ============================= - Proteins like cyclins and CDKs act as \"traffic lights,\" ensuring the cell only moves to the next step when ready. 4. Variability in Division Speed: ============================== - Some cells divide rapidly (e.g., skin cells), while others may take years or never divide (e.g., nerve cells). Learning Tip: ============= - After DNA replication, chromosomes consist of **two DNA helices** (chromatids). These separate during mitosis, becoming individual chromosomes in the daughter cells.