Cytoskeleton Lecture Notes PDF

Non membranous cell organelles (Cytoskeleton) HDTD Module/Biology /First Grade 2024-2025 By Lecturer: Shoub yassin  Objectives -By the end of this lecture, you should be able to: List the components of cytoskeleton Define structure and function of microtubules Define the structure and fun...

Non membranous cell organelles (Cytoskeleton) HDTD Module/Biology /First Grade 2024-2025 By Lecturer: Shoub yassin  Objectives -By the end of this lecture, you should be able to: List the components of cytoskeleton Define structure and function of microtubules Define the structure and function of microfilaments Define the structure and function of intermediate filaments Cytoskeleton In 1903, Nikolai Koltsov proposed that the shape of cells is determined by a network of tubules that he termed the ‘cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells including bacteria and archaea Cytoskeleton It extends from the cell nucleus to the cell membrane and is composed of similar proteins in various organisms. Cytoskeleton’s primary function is to give the cell its shape and mechanical resistance to deformation. Cytoskeleton Helps in intracellular transport of vesicles and organelles within the cell. It can be a template for the construction of a cell wall. It can form specialized structures, such as cilia and flagella.  Cytoskeleton The structure, function and dynamic behaviour of the cytoskeleton can be very different, depending on organism and cell types. The cytoskeleton consists of three components: 1. Microtubules. 2. Microfilaments (Actin filament). 3. Intermediate filament  Examples of the cytoskeleton in epithelial cells In the epithelial cells of the intestine, all three types of fibers are present. Microfilaments project into the villi, giving shape to the cell surface. Microtubules grow out of the centrosome to the cell periphery. Intermediate filaments connect adjacent cells through desmosomes External cell movement Cellular movement is accomplished by cilia and flagella. Cilia are hair-like structures that found in the lining of trachea and tubes of female oviduct. Flagella are whip-like appendages ,They are longer than cilia, but have similar internal structures made of microtubules.  Both flagella and cilia have a 9 + 2 arrangement of microtubules. This arrangement refers to the 9 fused pairs of microtubules on the outside of a cylinder, and the 2 unfused microtubules in the center.  Dynein "arms" attached to the microtubules serve as the molecular motors. Defective dynein arms cause male infertility and also lead to respiratory tract and sinus problems. Below are two cross-sections of sperm tails (flagella). Internal cell movement The cytoskeleton acts as a "track" on which cells can move organelles, chromosomes and other things. Some examples are: 1-Vesicle movement between organelles and the cell surface, frequently studied in the squid axon. 2. Cytoplasmic streaming 3. Movement of pigment vesicles for protective coloration 4- Cell division—cytokinesis 5. Movement of chromosomes during mitosis and meiosis 1- Microtubules Microtubules are long, hollow cylindrical and filamentous or fibrilar structures found the cytoplasm of all eukaryotic cells. Absent in prokaryotes. Microtubules are found in the thrombocytes (blood platelets) of human and rat. They are about 25 nm in diameter and 200 nm to 25 micrometre in length.  1-Microtubules Microtubules are major components of the cytoskeleton. They are found in all eukaryotic cells, and they are involved in mitosis, cell motility, intracellular transport, and maintenance of cell shape. Microtubules are composed of alpha- and beta-tubulin subunits assembled into linear protofilaments. These two units are arranged alternately in the protofilament Microtubules 13 protofilament fold in a cylindrical shape to form microtubules. Microtubules-Function Microtubule are rigid structures which work as a supporting framework and give shape to the cell. They maintain shape of long processes such as cilia, flagella and axons of nerve cells. Microtubules-Function The motion of the cilia and flagella is created by the microtubules. The centrioles are morphologically identical to the basal body of cilia or flagella Microtubules changes the cell shape during cell differentiation. Help in the elongation of the cells in the lens of eye. 2-Microfilaments: structure Microfilaments also known as actin filaments are solid rods of protein. The diameter of filament is about 7 nm and they are smallest of the cytoskeletal filaments. -They are called as actin filaments because they are mostly composed of the protein actin. 2-Microfilaments Actin is the protein building blocks of microfilament. Actin is found abundantly in all eukaryotic cells. Their structure is two strands of actin wound in a spiral. Microfilaments range from 5 to 7nanometers in diameter  Microfilaments Actin which serves as a track for the movement of a motor protein called myosin. Actin and myosin are plentiful in muscle cells. When actin and myosin filaments slide to each other, the muscles will contract. Microfilaments also provide some rigidity and shape to the cell. Microfilaments  Microfilaments They can depolymerize (disassemble) and reform quickly, thus enabling a cell to change its shape and move. White blood cells make good use of this ability. They can move to the site of an infection and phagocytize the pathogen. 3-Intermediate filaments ntermediate filaments (IF) are referred to as intermediate because of their relative intermediate diameter of 10nm compared to actin at 5–7 nm and microtubules at 20–25 nm.  Intermediate filaments intermediate filaments are composed of a variety of proteins that are expressed in different types of cells. More than 50 different intermediate filament proteins have been identified and classified into six groups based on similarities between their amino acid sequences. Intermediate filaments  Intermediate filaments Types I and II consist of two groups of keratins,. Some type I and II keratins (called hard keratins) are used for production of structures such as hair, nails, and horns. The other type I and II keratins (soft keratins) are abundant in the cytoplasm of epithelial cells, with different keratins being expressed in various differentiated cell types.  Intermediate filaments The type III intermediate filament proteins include vimentin, which is found in the fibroblasts, smooth muscle cells, and white blood cells. The type IV intermediate filament proteins include the three neurofilament (NF) protein The type V intermediate filament proteins are the nuclear lamins,  Centrioles The centrosome is the area of the cytoplasm is a barrel-shaped organelle. It's next to the nucleus and within the centrosome there are two centrioles.  Centrioles Centrioles are very important for cell division. So when the cell is going to divide, those centrioles go to opposite ends of the nucleus. When the chromosomes are condensing to undergo mitosis, the centrioles form the areas that mitotic spindle forms from. And those mitotic spindles go and attach to each of the chromosomes and pull the chromosomes to opposite ends of the cell to allow cytokinesis. Centrioles Centrosomes are built by microtubule components. They are arranged in nine sets (in each centriole) where each set comprises three microtubules or tubulins which is also termed as 9+3 arrangement. The two centrioles are oriented in 90 degrees to one another.  Centrioles The centrosome is present in an animal cell. The centrosome is not present in plant cells Centrioles are present at the base of cilia and flagella so they are also called basal bodies.  Thank you