The Cell Cycle PDF

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. What is Cell Division and Reproduction? ======================================= 1. **Mitosis (Main Type of Cell Division)**...

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. 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. Importance of Mitosis ===================== 1. Growth: ========== o Multicellular organisms grow by increasing the number of cells through mitosis. ================================================================================= 2. Repair and Replacement: ========================== o Mitosis replaces worn-out or damaged cells, maintaining tissue health. ======================================================================== o Example: Skin cells and blood cells are continually replaced by mitosis. ========================================================================== 3. Asexual Reproduction: ======================== o Some organisms reproduce asexually through mitosis, producing clones of the parent organism. ============================================================================================== o Example: Bacteria and some plants reproduce using mitosis. ============================================================ 4. Genetic Stability: ===================== o Ensures the daughter cells have the same number and type of chromosomes as the parent cell. ============================================================================================= Advantages of Asexual Reproduction: ===================================== o No need for a mate. ===================== o Rapid reproduction allows quick population growth. ==================================================== Disadvantage: =============== o Lack of genetic diversity makes populations more vulnerable to environmental changes. ======================================================================================= 2. Sexual Reproduction (meiosis): ================================= - 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). - 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 long as a football field. There is a seal applied on the open end of DNA like a tape on a thread.  - 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. - 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. - Chromosome Count: ================= - Humans have **46 chromosomes** (23 pairs). One chromosome of each pair comes from the mother, and the other from the father. - 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). Phases of the Cell Cycle: ========================= 1. **Interphase (Preparation Phase):** - Before mitosis begins, the cell is in interphase, which is often referred to as the cell\'s resting stage. However, it is highly active internally. Key processes include: - DNA Replication: Chromosomes are copied, ensuring each daughter cell will have a complete set of genetic material. - Organelle Production: New organelles are synthesized to support the new cells. - ATP Production: Increased energy is produced to power the cell division process. - The cell prepares itself for division by growing and accumulating resources. 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 organelles (like mitochondria) and other materials. 2. Mitosis (Division of the Nucleus) (karyokinesis): ================================================= - The cell divides its nucleus, ensuring each new cell gets an identical set of chromosomes. - Mitosis is divided into four distinct stages: Prophase, Metaphase, Anaphase, and Telophase. A helpful mnemonic is PMAT. 1\. Prophase What Happens? o Chromosomes condense and become visible under a microscope. o Each chromosome consists of two identical sister chromatids joined at the centromere. o The nucleolus (where ribosomes are made) breaks down and disappears. o Centrioles (only in animal cells) migrate to opposite poles of the cell, initiating the formation of the spindle apparatus. o The spindle apparatus is crucial for organizing and separating chromosomes during mitosis. Key Features: o Chromosomes are tightly coiled and visible. o Spindle fibers start forming between centrioles. o The nuclear envelope begins to break down, allowing chromosomes to interact with spindle fibers. 2\. Metaphase What Happens? o The nuclear membrane has completely disintegrated. o Chromosomes align along the center of the cell, known as the metaphase plate or equator. o Spindle fibers attach to the centromeres of the chromosomes. Key Features: o Chromosomes are neatly lined up at the cell\'s center. o Spindle fibers connect centromeres to the poles of the cell. Importance: o This alignment ensures that each daughter cell will receive one copy of each chromosome during separation. 3\. Anaphase What Happens? o The centromeres split, separating the sister chromatids into individual chromosomes. o Spindle fibers pull the chromosomes to opposite poles of the cell. Key Features: o Chromatids are now individual chromosomes. o Chromosomes move quickly, powered by ATP. Importance: o This separation ensures each new cell gets an identical set of chromosomes. 4\. Telophase What Happens? o Chromosomes reach the poles and begin to uncoil, becoming less visible under a microscope. o A nuclear envelope reforms around each set of chromosomes. o The spindle fibers disassemble. Key Features: o Two distinct nuclei are visible within the cell. o The cell is nearly ready to split into two. 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 organelles. ======================================================================================================================= What Happens? =============== o The cytoplasm divides, creating two separate daughter cells. ============================================================== Animal Cells: =============== o A ring of contractile fibers forms around the cell\'s center. =============================================================== o These fibers tighten, pinching the cell into two, similar to ============================================================== pulling a drawstring. ===================== Plant Cells: ============== o A cell plate forms at the center of the cell. =============================================== o The cell plate develops into a new cell wall, dividing the two ================================================================ daughter cells. =============== End Result: ============= o Two genetically identical daughter cells enter interphase and =============================================================== begin preparing for their own divisions. ======================================== 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). **Key Terms to Remember** Chromatid: One of two identical halves of a replicated chromosome. Centromere: The region where sister chromatids are joined. Spindle Apparatus: The structure that separates chromosomes during mitosis. Cell Plate: A structure in plant cells that develops into a new cell wall. Asexual Reproduction: A method of reproduction that produces genetically identical offspring. **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. Mitotic Index ============= - **Definition**: The **mitotic index** measures how actively cells in a tissue are dividing. It is calculated as the ratio of the number of cells in mitosis (actively dividing) to the total number of cells in a sample. Key Concepts ============ 1. **Cell Division and Mitosis**: - Some cells, such as **nerve cells**, never divide once an organism matures. - Other cells, like **skin cells**, constantly divide to replace lost or damaged cells. - In certain cases, cells divide more than they should, such as in **cancer**, due to failures in control mechanisms. 2. Importance of the Mitotic Index: ================================ - It helps identify how actively cells in a tissue are dividing. - A high mitotic index may indicate cancerous tissue, as cancer cells divide uncontrollably. - It can also be used to monitor **treatment effectiveness for cancer**---a falling mitotic index suggests that treatment is working. How to Calculate the Mitotic Index ================================== 1. **Steps**: - Examine a tissue sample under a microscope. - Count the number of cells that are actively dividing (in mitosis). These cells are identified by their **visible chromosomes**, as shown in **Figure H**. - Count the total number of cells in the sample. 2. Formula: ======== Example Calculation: ==================== 𝑵𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒄𝒆𝒍𝒍𝒔 𝒊𝒏 𝒎𝒊𝒕𝒐𝒔𝒊𝒔 𝑻𝒐𝒕𝒂𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒄𝒆𝒍𝒍𝒔 - Cells with visible chromosomes (in mitosis): **12** - Total number of cells: **25**  How to Identify Cells in Mitosis ================================ - When cells leave the **interphase stage** and enter **mitosis**, their chromosomes become visible. - Visible chromosomes are a marker of actively dividing cells (see **Figure H** for an illustration of mitosis stages). Applications of the Mitotic Index ================================= 1. **Cancer Diagnosis**: - Tissues with a higher mitotic index are likely to contain cancerous cells. - Cancer cells divide more rapidly than normal cells. 2. Monitoring Cancer Treatments: ============================= - During effective cancer treatment, the mitotic index of a tumor decreases. - A high mitotic index persisting during treatment may indicate that the treatment needs adjustment. **Why is the Mitotic Index Important?** - It provides a way to measure the activity of cell division. - Helps in diagnosing cancer and assessing tissue health. - Offers insights into the progress of treatments for cancer and other conditions involving abnormal cell division. Example for Clarity =================== - Imagine observing 100 cells under a microscope: - If 20 of those cells show visible chromosomes (indicating mitosis), the mitotic index would be: - This indicates that 20% of the observed cells are actively dividing. Key Points to Remember ====================== - The **mitotic index** is always a fraction or percentage. - A **higher mitotic index** is usually associated with more active cell division, which may signal cancer or rapid tissue repair. - Observing **visible chromosomes** under a microscope helps identify cells in mitosis.