Biology - Week 8a - Cell Division PDF

# Biology - Week 8a: ## Functions of Cell Division: - **Reproduction:** An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism. - **Growth and development:** This micrograph shows a sand dollar embryo shortly after the fertilized egg divided,...

# Biology - Week 8a: ## Functions of Cell Division: - **Reproduction:** An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism. - **Growth and development:** This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells. - **Tissue renewal:** These dividing bone marrow cells (arrow) will give rise to new blood cells. ## Cell Division - An integral part of the cell cycle - Results in genetically identical daughter cells - Cells duplicate their genetic material - Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA ## DNA - Genetic information - genome - Packaged into chromosomes ## Phases of the Cell Cycle - The cell cycle consists of - **Interphase** - normal cell activity - **The mitotic phase** - cell division - **In mitosis DNA is a consistent conformation** **Interphase** - G1 - primary growth - S-genome replicated - G2 - secondary growth - M-mitosis: cell divide - C-cytokinesis: cell begins to split into hour + to seperate. **Mitosis and Meiosis** - All normal cells have 46 chromosomes (diploid cells). - These cells duplicate and divide to form daughter cells, with 46 chromosomes - Process is called mitosis and can occur with most cells in the body ## Meiosis ### Sex Cells - Gametes - Unlike somatic cells, sperm and egg cells are haploid cells, containing only one set of chromosomes - At sexual maturity the ovaries and testes produce haploid gametes by meiosis ### What is Mitotic Cell Division? - Division of mitotic cells (non reproductive cells) in eukaryotic organisms. - A single cell divides into two identical daughter cells - Daughter cells have the same number of chromosomes as does the parent cell. ### Two Fundamental Cell Types - **Prokaryotic** - Prokaryotes cells/bacteria multiply by binary fission - **Eukaryotic** - Packing for the move… the chromosomes are in a much more relaxed conformation - Protein complexes called condensins, help to promote chromosome to condense and prepare for duplication ## Structure of a eukaryotic chromosome - **chromosome** - **arm** - **arm** - **centromere** - **sister chromatids** - **daughter chromosomes** - The centromere is a constricted region of the chromosome containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores. - Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division. - ***Kinetochore proteins:*** The microtubules interact here sister chromatids and they play a very important role in the movement of centromere during mitosis. ## Eukaryotic Cell Cycle - **2 major phases:** - **(3 stages)** - DNA uncondensed (= chromatin) - **(4 stages + cytokinesis)** - Nuclear division & division of cytoplasm - DNA condensed (= chromosomes) ## Mitosis - **1st - Prophase (chromosomes condense)** - **2nd - Metaphase (chromosomes attach and align)** - **3rd - Anaphase (chromosome separate)** - **4th - Telophase and Cytokinesis (cell divides and chromosomes relax)** ## Vitosis - **METAPHASE** - **Metaphase plate** - **Spindle** - **Centrosome at one spindle pole** - **Daughter chromosomes** - **ANAPHASE** - **TELOPHASE AND CYTOKINESIS** - **Cleavage furrow** - **Nucleolus forming** - **Nuclear envelope forming** ## G2 of Interphase - Two centrosomes have formed by replication of a single centrosome.  - In animal cells, each centrosome features two centrioles. - Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed ## Prophase - The chromatin fibers 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 together. - The mitotic spindle begins to format 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, apparently propelled by the lengthening microtubules between them. ## Metaphase - Metaphase is the longest stage of mitosis, lasting about 20 minutes. - The centrosomes are now at opposite ends of the cell. - The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles, The chromosomes’ centromeres lie on the metaphase plate. - For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. - The entire apparatus of microtubules is called the spindle because of its shape. ## Telophase - Two daughter nuclei begin to form in the cell. - Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. - The chromosomes become less condensed. - Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. ## Cells divide at different rates. - The rate of cell division varies with the need for those types of cells. - Some cells are unlikely to divide (GO).? - Are mitotic rells: - Neuron cells - Heart cells ## FIGURE 5.2 CELL DIVISION | CELL TYPE | APPROXIMATE LIFE SPAN | | :--------- | :----------------------- | | Skin cell | 2 weeks | | Red blood cell | 4 months | | Liver cell | 300-500 days | | Intestine-internal lining | 4-5 days | | Intestine-muscle and other tissues | 16 years | ## Abnormalities of Cell Division - May be due to chromosomal, genetic, or environmental factors, or combination of these - **Major chromosomal abnormalities** usually lead to spontaneous abortion of fetus eg. Down syndrome. - **Chromosomal disorders** are usually related to number or placement of chromosomes - Chromosomes may fail to separate properly during cell division causing daughter cell to have an extra chromosome while other daughter cell has no chromosomes. - **Abnormal number or structure of autosomal chromosomes** is usually incompatible with life because these chromosomes carry a large number of essential genes. - Also a cause cancer - uncontrol cell proliferation! ## Part 2- Regulation of the Cell Cycle: **The "cell cycle"** - The distinct events of the cell cycle are directed by a distinct cell cycle control system. - These molecules trigger and coordinate key events in the cell cycle. - The control cycle has a built-in clock, but it is also regulated by external adjustments and internal controls. **The "cell cycle"** - A checkpoint in the cell cycle is a critical control point where stop and go signals regulate the cycle. - Three major checkpoints are found in the G1, G2, and M phases. - Rhythmic fluctuations in the abundance and activity of control molecules pace the cell cycle. - Some molecules are protein kinases that activate or deactivate other proteins by phosphorylating them. - The levels of these kinases are present in constant amounts, but these kinases require a second protein, a cyclin, to become activated. - Levels of cyclin proteins fluctuate cyclically. - The complex of kinases and cyclin forms cyclin-dependent kinases (Cdks). - **Cyclin/CDK type control specific phases of the cell cycle** - **Cyclin A-CDK1** - **Cyclin B-CDK1** - **Cyclin A-CDK2** - **Cyclin D-CDK4** - **Cyclin D-CDK6** - **Cyclin E-CDK2** ## Three main checkpoints in the cell cycle: 1. **Is cell the correct size? Is DNA damaged?** 2. **Is DNA fully replicated? Is DNA damage repaired?** 3. **Have spindle fibers formed? Have they attached to chromosomes correctly?** ## Checkpoint Control of Cell Cycle * **Cyclin-dependent kinases** ## Cell Cycle Control (G1) - **Inactive kinase** - *p16* - *CycD* - *CDK4* - **Repressor of transcription of proteins required for DNA synthesis** - *Rb* - *E2F* - **Active kinase** - *CycD* - *CDK4* - *Rb* - *E2F* - **Activator of transcription of proteins needed for DNA synthesis** - **Role of p53 in Cell Cycle Control (G1)** - Uncontrolled cell division - cancerous cells - Many different types of cancers have mutations of the p53 regulatory protein - **p53** - repairs the DNA damage and allows mitosis to occur if damage is too severe. It activates the death pathway. - **Damaged DNA** - **Tumor suppressor genes** - Tumor suppressor genes are normal genes that slow down cell division, repair DNA mistakes, and tell cells when to die (a process known as apoptosis or programmed cell death). When tumor suppressor genes don't work properly, cells can grow out of control, which can lead to cancer. - **Cancer** - Cancer cells divide excessively and invade other tissues because they are free of the body’s control mechanisms.  - Cancer cells do not stop dividing when growth factors are depleted either because they manufacture their own, have an abnormality in the signaling pathway, or have a problem in the cell cycle control system. - If and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle. - **Why are cell cycle checkpoints important? Cancer** - Cancer Cells Evade One Major “Safety” Mechanisms Built into the Cell Cycle - p53 protein normally targets cells with severe DNA damage to undergo programmed cell death. - If the p53 gene is mutated, damaged cells will not be removed and may continue dividing in an uncontrolled manner.