Biology Lecture 4 (2024-2025) PDF

Biology By: Dr. Shaimaa Zaher  Objectives List non-membranous organelles Describe histological structure of proteasome and recognize its function Recognize parts, structure and function of each part of cytoskeleton  Non-membranous organelles Ribosomes Pr...

Biology By: Dr. Shaimaa Zaher  Objectives List non-membranous organelles Describe histological structure of proteasome and recognize its function Recognize parts, structure and function of each part of cytoskeleton  Non-membranous organelles Ribosomes Proteasomes Cytoskeleton Microtubules Microfilaments Intermediate Filaments  Proteasomes ▪ Proteasomes are non membranous cell organelles. ▪ They are protein complexes including proteases that digest proteins. ▪ The protein targeted for destruction is marked by its attachment to ubiquitin which is a cytosolic protein found in all cells. ▪ Proteasomes deal primarily with free proteins as individual molecules. Whereas lysosomes deal with bulk materials in the form of vesicles and organelles.  The proteasome consists of a core particle that is cylindrical in shape. At each end of the core particle is a regulatory particle that contain ATPase and recognizes the protein attached to ubiquitin. ❑Function of proteasomes: Protein degradation which is essential to remove: ✓ Misfolded and denatured proteins. ✓Proteins that become unnecessary after performing their normal functions as cyclins that regulate cell cycle.  Cytoskeleton Definition: It is a network of microtubules, microfilaments, and intermediate filaments. Each is formed of structural proteins. LM: difficult to be seen. Function: 1- Maintains the shapes of cells. 2- Provides structural support to the cells. 2- Allows movement of organelles and cytoplasmic vesicles. 3- Allows movement of the entire cell.  1- Microtubules EM: electron dense hollow cylinders About 25 nm in diameter Formation and structure of microtubules: The structural protein subunits are α and β tubulin dimers that polymerize longitudinally to form protofilaments. Each 13 protofilaments are arranged to form a microtubule. Tubulin Dimers→ Protofilament→ 13 Protofilaments → Microtubules Site of formation: - Microtubule organizing centers (MTOCs) act as nucleating sites for polymerization & formation of microtubule. - The MTOC or centrosome is an area near the nucleus containing the 2 centrioles and pericentriolar matrix including tubulin protein.  Microtubules show dynamic instability i.e continuous tubulin polymerization (adding tubulin) & depolymerization (removing tubulin) Types of cellular microtubules (according to stability): 1) Unstable form: (that form mitotic spindle). 2) Stable form: (that form centrioles, cilia & flagella) Function of microtubules: Formation of mitotic spindle during cell division. Formation of centrioles, cilia and flagella. Intracellular transport of organelles and vesicles that occurs under control of motor proteins (kinesin & dynein) which use ATP for movement along microtubule.  Kinesin carries organelles or vesicles away from the center of the cell toward cell membrane; while cytoplasmic dynein carries vesicles in the opposite direction. Medical application Cancer chemotherapy depends on using antimitotic drugs (Vinblastin) because they arrest mitosis as they prevent formation of microtubules and mitotic spindles. STRUCTURES RELATED TO MICROTUBULES CENTRIOLES❑ CILIA❑ FLAGELLA❑ MITOTIC SPINDLE❑  Centrioles Definition: They are non-membranous organelles. There are two centrioles arranged perpendicular to each other, located near the nucleus in a region called centrosome or MTOC By E.M: ▪ Each centriole is cylinder in shape. ▪ In cross section, the wall is formed of 9 triplets of microtubules (i.e. 9 bundles of microtubules and each bundle is formed of 3 microtubules). Centrioles  E.M of a centriole 1  Cilia Cilia Definition: They are motile hair-like apical processes. Their core is formed of microtubules. Structure: LM: acidophilic hair-like striations EM: each cilium is formed of basal body and shaft (axoneme) Basal body: has the same structure of centriole?? Shaft or axoneme consists of peripheral 9 doublets and central 2 singlets of microtubules. Each doublet has a pair of arms formed of protein dynein (axonemal dynein) which has an ATPase activity that allow movement of cilia. EM of cilia Function of cilia : Motility of cilia is important in respiratory and female genital systems. Flagella: ❑Have the same structure of cilia but ✓ longer ✓ single (i.e. each cell has one flagellum) ✓ only in the tail of sperms ✓ Function: movement of sperms  2- Microfilaments (Thin or actin filaments) EM: electron dense fine filaments About 6 nm in diameter. Structural protein subunit is G-actin (globular G-actin polymerize to form double helix of filamentous F-actin)  Functions of actin filaments: Formation of the core of microvilli. In muscle tissue, actin filaments interact with myosin filaments for muscle contraction. Form network beneath cell membrane involved in endocytosis and exocytosis. Formation of contractile ring during cell division.  3- Intermediate filaments Structure: formed by polymerization of different rod like protein subunits that form coiled tetramers. About 10 nm in diameter They are more stable (not dynamic) & provide mechanical stability to the cell.  Different types e.g. ▪ Keratins: found in epithelial cells. ▪ Neurofilaments: found in neurons to support their long processes. ▪ Desmin: found in muscle cells. ▪ Lamins: associated with nuclear envelope. The main function is structural support of cells.