Cell Division.pdf

Cell Division Describe the phases of the eukaryotic cell cycle The cell theory states that all organisms are made of cells and all cells come from per-exisiting cells. Somatic cells, which make up most of your body’s tissues and organs (including skin, muscles, lungs, gut, and hair cells), divide pr...

Cell Division Describe the phases of the eukaryotic cell cycle The cell theory states that all organisms are made of cells and all cells come from per-exisiting cells. Somatic cells, which make up most of your body’s tissues and organs (including skin, muscles, lungs, gut, and hair cells), divide primarily through a process called mitosis. Let’s explore why cells divide and the differences between mitosis and meiosis: 1. Why Do Cells Divide? ○ Cells divide for various reasons: Tissue Renewal: When you accidentally bite your lip or skin your knee, cells divide to replace old, dead, or damaged cells. Growth: Organisms grow because cells continually divide to produce more and more cells. Healing: Cells divide to repair wounds and cuts. ○ Nearly two trillion cells divide in the human body every day! ○ Cancer Risk: It’s essential for cells to stop dividing at the right time. If a cell can’t stop dividing when it should, it can lead to cancer. 2. Mitosis: ○ Purpose: Mitosis is how somatic cells divide. ○ Result: It produces two genetically identical daughter cells with the same number of chromosomes as the parent cell (diploid, 2n). ○ Importance: Mitosis is crucial for growth, repair, and maintaining body tissues. 3. Meiosis: ○ Purpose: Meiosis occurs in reproductive cells (like eggs and sperm). ○ Result: It produces four haploid daughter cells with half the number of chromosomes (1n) as the parent cell. ○ Importance: Meiosis ensures genetic diversity during sexual reproduction. The eukaryotic cell cycle - Interphase 1. S Phase (Synthesis of DNA): ○ During this phase, DNA replication occurs. ○ The two strands of DNA are separated at the hydrogen bonds holding the nucleotides together. ○ A new strand of DNA is synthesised opposite each of the old strands. ○ This ensures that each daughter cell receives a complete set of genetic information. 2. G1 Phase (Gap Phase 1): ○ Most cellular activities occur during G1. ○ Also known as the growth phase. ○ The duration of G1 is variable and cell type-specific. ○ Cells grow, perform their functions, and prepare for the next phases. 3. G2 Phase (Gap Phase 2): ○ G2 serves several critical purposes: Checks for Correct DNA Synthesis: The cell ensures that DNA replication during S phase was accurate. Prepares for the Mitotic Phase: Proteins and enzymes required for cell division are synthesised. Replication of Centrosomes: Centrosomes, essential for organising microtubules during mitosis, complete their replication. Remember, the cell cycle is a tightly regulated process that ensures proper growth, repair, and maintenance of our bodies. Each phase plays a crucial role in maintaining cellular health and function. Describe the phases and purpose of mitosis The cycle phase What happens during the phase? A nuclear envelope encloses the nucleus. The nucleus contains one or more nucleoli (singular, nucleolus). Two centrosomes have formed by duplication of a single centrosome. Centrosomes are regions in animal cells that organise the microtubules of the spindle. Each centrosome contains two centrioles. Interphase Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. The chromatin fibres become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, often, all along their arms by cohesins, resulting in sister chromatid cohesion. The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). The centrosomes move away from each other, propelled partly by the lengthening micro- tubules between them The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes have become even more condensed. This is part of the prophase - no need for much detail then presented. The centrosomes are now at opposite poles of the cell. The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie at the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. Anaphase is the shortest stage of mitosis, often lasting only a few minutes. Anaphase begins when the cohesin proteins are cleaved. This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes an independent chromosome. The two new daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the centromeres are pulled ahead of the arms, moving at a rate of about 1 om/min. The cell elongates as the nonkinetochore microtubules lengthen. By the end of anaphase, the two ends of the cell have identical—and complete— collections of chromosomes. What is a sister chromatid? Interphase: ○ Interphase is like the cell’s preparation phase. It’s when the cell gets ready for division. ○ During interphase, DNA replicates. Imagine it copying itself, creating two identical copies of each chromosome. These copies are called sister chromatids. ○ Think of sister chromatids as twins—identical and inseparable. Mitosis Phases: ○ Prophase: This is like the cell’s warm-up. The DNA condenses, becoming visible as curvy lines. It’s like the chromosomes are putting on their workout gear. ○ Metaphase: Picture the sister chromatids lining up in the middle of the cell, like a team getting ready for a race. ○ Anaphase: Now, the sister chromatids split apart. It’s like they’re sprinting to opposite ends of the cell. ○ Telophase: The cell wraps things up. The nuclear envelope reforms, and the cell starts dividing. Daughter Cells: ○ The new cells formed after division are like clones of the parent cell. They have the same genetic information—just like mini-me versions! Cell Checkpoints The important ones of remember are M, G2 and G1 checkpoints Looking at G1 and M checkpoints 1. G1 Checkpoints: ○ Imagine the cell as a diligent worker. Before it proceeds with division, it asks itself three questions: Is the DNA undamaged? It checks if the genetic material is in good shape. Is cell size and nutrition OK? It ensures it’s well-fed and has enough space. Are appropriate signals present? It looks around for the green light from other cells. ○ If any of these conditions aren’t met, the cell might decide to take a break and exit to a resting phase called G0. 2. M Checkpoints: ○ Picture the cell in the middle of division (like a gymnast on a balance beam). It asks: Are all chromosomes attached to spindles? The cell wants to make sure everything is aligned properly. ○ Passing these checkpoints requires multiple signals—like getting a thumbs-up from different supervisors. Describe the phases and purpose of meiosis Overview: ○ Meiosis occurs in the gonads (ovaries and testes) of organisms. ○ Its primary purpose is to produce gametes (sperm and egg cells). ○ Gametes are haploid, meaning they have only one set of 23 chromosomes (half the usual number). ○ When fertilisation occurs (when sperm meets egg), the diploid number (2n) is restored in the resulting zygote. Genetic Diversity: ○ Unlike mitosis (which produces identical daughter cells), meiosis generates cells that are genetically different from the parent cell. ○ This diversity is crucial for evolution and adaptation. Two Stages of Meiosis: ○ Meiosis I: Prophase I: Imagine this as the “get-to-know-each-other” phase. Homologous chromosomes pair up (a process called synapsis), forming structures called tetrads. During this cosy pairing, they can even exchange genetic material in a process called crossing over. Metaphase I: Picture the tetrads lining up in the middle, like couples dancing at a ball. Each pair of homologous chromosomes takes its place. Anaphase I: The homologous chromosomes separate, moving toward opposite poles. But wait—sister chromatids remain attached at this point! Telophase I: The cell starts to divide, and the nuclear envelope reforms around the separated chromosomes. Meiosis II: ○ Prophase II: The cell gears up again. Chromosomes condense, and the nuclear envelope breaks down. ○ Metaphase II: Sister chromatids line up along the equator, ready for their encore performance. ○ Anaphase II: Now, the sister chromatids finally part ways and head toward opposite ends. ○ Telophase II: The cell wraps up its final act, forming two new cells, each with a unique genetic mix. Describe the differences between mitosis and meiosis Mitosis: ○ Occurs in: Both diploid (2n) and haploid (n) cells. ○ DNA replication: Takes place during interphase before mitosis begins. ○ Number of divisions: Only one division, including prophase, prometaphase, metaphase, anaphase, and telophase. ○ Synapsis of homologous chromosomes: Does not occur. ○ Daughter cells: Two, each genetically identical to the parent cell, with the same number of chromosomes. Meiosis: ○ Occurs in: Only diploid (2n) cells. ○ DNA replication: Occurs during interphase before meiosis I (not meiosis II). ○ Number of divisions: Two divisions, each including prophase, metaphase, anaphase, and telophase. ○ Synapsis of homologous chromosomes: Happens during prophase I, along with crossing over between nonsister chromatids. The resulting chiasmata hold pairs together due to sister chromatid cohesion. ○ Daughter cells: Four, each haploid (n); genetically different from the parent cell and from each other.