| Talk in general about the position and the state of cytoskeleton. | They are found between different cell organelles in the cytosol, in eukaryotes especially animal cells, 3D network of complex proteins, highly dynamic, appears static in microscopy, consists of thin, intermediate and thick filaments and microtubules, that are closely attached, revealed by fluorescent microscopy. |
| Talk about the structure of microtubules. | Hollow tubular structures that occur in nearly all animal cells, components of the spindle of division of the basal body of cilia and flagella of centrioles and core of flagella and cilia, outer diameter is 24-25 nm and wall thickness is about 5nm, the length is variable depending on the structure in which it is found.
Highly dynamic since they undergo polymerization and depolymerization during various physiological processes. |
| Talk about the composition of microtubules. | Wall is composed of globular alpha and beta tubulin molecules, which are GTP binding proteins which assemble into dimers, in a cross section, each microtubule is found to consist of 13 protofilaments arranged in a circular pattern within the wall . |
| Talk about the protofilaments of microtubules. | Each is composed of a dimer of alpha and beta tubulin that are not covalently bonded, so it consists of a succession of alpha and beta monomers, so it has alpha on one end and beta on the other, All protofilaments that make a microtubule are oriented in the same direction, consequently, the entire microtubule is polarized having a plus extremity (beta) and a minus extremity (alpha), polarity is important for polymerization process which mainly occurs on the plus terminal, on the other hand, protofilaments in a bundle are helically aligned because there is a 1nm difference between two protofilament. |
| Talk about the associated proteins of the microtubules. | AKA MAP, whose activity is controlled by phosphorylation and dephosphorylation reactions (kinase/ phosphatase)
they may be involved in interaction between adjacent microtubules increasing their stability,controlling rigidity. |
| Talk about the stability of microtubules. | Microtubules have different stability, those of spindle are labile and disassemble quickly whereas those of cilia and flagella are highly stable lasting for almost the whole cell life, it is controlled by MAPs and other chemicals (acetyl) on particular aa of tubulin. depend on phosphorylation of MAPs. |
| How do microtubules interact with other cytoskeleton components? | They interact with intermediate filaments and motor proteins, which associate temporarily with microtubules that belong to two families, dyneins and kinesin, which mediate movement of vesicle and molecules along microtubules. |
| Talk about motor proteins. | Have many domains:
two bind to microtubules (feet) and one or more that bind to transported cargo (Hands) in addition they have a domain that binds ATP, hydrolyzes it and transforms its energy into mechanical one that serves for movement by change of protein conformation, they usually move in an unidirectional manner along the microtubules performing specific directed transport within the cell. |
| Talk about polymerization and depolymerization of microtubules. | Microtubules are sensitive to many drugs that inhibit polymerization and increase it which can be used as cancer treatments. |
| Talk about cytoplasmic filaments in general. | Totally different from microtubules, but not independent from them, elongate unbranched and result from the polymerization of globular or fibrous monomers. They were first described in muscle fibres but then found in all eukaryotic cells, three major classes which are all microfilaments (thin) intermediate filaments (IFs) and thick myosin filaments. |
| Talk about Microfilaments. | AKA F-actin, thin filaments, characterized by a diameter that is about 7-8 nm, and primarily composed of globular G-actin monomers, polymerization requires ATP and results in two chains which are helical, the thin filaments that are so abundant in muscle fibers. In all eukaryotes |
| What are the roles of microfilaments? | Amoeboid movements (by converting ATP into mechanical energy by ATPase at the plus side with nucleation and at the minus side for depolymerization causing movement maybe with the help of myosin in the cell cortex appear as networks, or as bundles with IFs)
pseudopods (lamellipodia), internal support of microvilli and cause their movement, intracellular transport, can be involved in motility of cell. |
| Talk about assembly of proteins to actin filaments and their roles. | Can associate with myosin for cell movement.
Associate with villin and fimbrin for internal structure of microvilli (connection between thin filaments present)
Attachement to Plasma membrane
Polymerization and depolymerization nucleaiton and inhibition.
Actin filaments can cooperate with myosin forming the contractile ring that separates the cells during cytokinesis. |
| Talk about intermediate filaments. | Have intermediate diameter about `10-11 nm, found in animal cells but not in plant cells, radiate through the cytoplasm and are connected to other cytoskeleton structures by thick bridges made up of proteins, which are filamentous proteins that have two binding sites each for cytoskeleton element. They are quite heterogenous since they are made up of more than 50 distinct types of proteins, grouped in six different classes depending on aa, some are keratins, desmins, and lamins. |
| Talk about the structure of IFs proteins. | All monomers consist of a helical rod-shaped domain with two globular heads at both ends, two identical or different monomers form a dimer, homodimers or heterodimers formation occurs by winding of the helical domain around each other, dimeric structure has a ropelike shape is polarized since C terminus of one monomer faces C terminus of the other, same for N, dimers then assemble together in an opposite orientation with their heads significantly shifted relative to one another. the resulting tetramer is not polarized since C faces N.
Tetramers assemble to form IFs that are not polarized but resistant to tensile (pulling) force. |
| Give an example of IFs. | Keratin network that makes up the outer layer if the skin, highly resistant to bacteria and mechanical pressure, and air and watertight, which is due to the compactness of molecules.
They radiate as a cage from the nucleus to the cytoplasm forming hemi-desmosomes plaques making necessary movement during cell cycle. |
| Talk about myosins | Filamentous proteins that cause movement by operation with actin
Many classes.
Found in Eukaryotes
Tail connected by a neck to a globular head
Heads are the same in all classes but tails differ
Myosin II causes muscle contraction (Has ATPase)
Myosin I binds to PM and causes microvillous movement |
| Talk about myosins forming thick filaments | Myosin II assembles to form a thick filament which are about 22 nm in diameter, consisting of 100s of molecules, its bipolar since its polarity is reversed at the center which is devoid of heads (M line)
Found in contractile fibrils
Maintain their positions in the sarcomere
Interact with thin filaments. |
| Talk about microtubule organizing centers. | Polymerization of microtubules is initiated by centers in the cell, they are named MTOCs which are the cetnrosomes and the basal bodies. Plant cells have no centriole but proteins of MTOCs are present in the nuclear envelope and in the cell cortex.
They are responsible for the nucleation of microtubules and number, length, polarity, and number of protofilaments that make up their walls, as weel as time and location. All of them share a specific form of tubulin named Y-tubuiln (gamma). |
| Talk about the centrosomes. | Consists of a pair of centrioles which are situated near the cell center at right angel with respect to each other, 0.2 micrometre in diameter.
No membrane around them, but enveloped with pericentriolar matter (protein) amorphous |
| What is a centriole. | Set of nine triplets of microtubules and many proteins, each set of triplets is referred to as A,B and C. A is central one is complete, and connected to center by radial spokes, whereas B and C are incomplete since they share protofilaments with the surrounding ones, (A and B respectively)
2 make centrosomes.
They are sites for nucleation
Polymerize MT for spindle (from centriole to the periphery since polymerization occurs in the + end)
They are duplicated by the nucleation of two other centrioles by the pericentriolar region of the parent cell. |
| Talk about the basal bodies. | located at the bases of the cilia and flagella have same cross-section as the cetrioles however some are extended to form cilium or flagellum core called axoneme.
Centrioles and basal bodies have same structure and components since they can regenerate each other, in the spermatid,centriole derived basal body is the origin of the flagellum formation. So basal bodies are involved in production of microtubules which are in flagellum and cilia. |
| Talk about the structure of cilia and flagellum. | membrane enclosed motile structures extending from the surface of eukaryotic cells, responsible for cell movement. Two versions, of the same organelle, cilia are usually shorter and occur in groups and beat rhythmically for cell movement, or move particles or fluid along the surface.
Flagella are not so numerous, prokaryotic (no membrane) differ from eukaryotic ones. |
| Talk about the axineme. | central core of cilia and flagella, covered by an extension of plasma membrane, its array is usually of nine peripheral doublets of microtubules and 2 central ones in a structure referred to as 9+2, all having same polarity minus at base, doublets are called A (complete) and B, but central ones are complete and run longititdually through the whole structure (covered by central sheath). |
| Talk about the peripheral microtubules of cilia and flagella axoneme connections. | A are connected by periodical bridges to adjacent B
They are also connected to neighboring doublets by dynein molecules located in and out for movement
Radial spooks connect A to central sheath
Cilia and flagella regenerate by basal bodies. |
| Talk about the mode of movement of cilia and flagella. | Cilia beat synchronously in two phases: one rigid opposite to the desired movement and one flexible desired.
Flagella have diverse beating patterns using ATP and dynein (doublets) |
| Talk about the cytoskeleton function | Structural support
Resistance to mechanical pressure
Cell shape
Plasma membrane structures
Muscle contraction
Myosin II and actin ring cytokinesis
Cell movement
oriented movement of vesicles, macromolecules and chromosomes according to polarity of MT and movement of motor proteins (+ to - or vice versa) |
| Talk about plant cell cytoskeleton. | differ compositionally from animal one, no IFs, centrosomes, proteins of centrioles one the cytosolic face of nuclear envelope (MTOC)
Functions relatively similar
actin filaments formation of pollen tube and nuclei movement for fertilization,
microtubules determine cell morphology controlling cell wall growth
MTs intercellular traffic
four stations through the cell cycle |
| Talk about the roles of centrosome. | It has y-tubulin in the core and pericentriolar matter which plays a role in nucleation of microtubules by forming a ring structure of 25 nm diameter serving as a template for alpha-beta dimers to polymerize |
