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Cytoskeleton

DR PALLEE SHREE

1. DETERMINE THE SHAPE OF THE CELLS AND PROVIDE STRENGTH shape & strength

• filaments are highly concentrated at the periphery of the cell where they form a 3D network beneath the plasma membrane

• This network of actin filaments and associated actin-binding and form cell cortex which determines cell shape and also help in cell surface activities Cont…. • The cortical actin is responsible for distinctive shape as biconcave discs

• As erythrocytes lack and intermediate filaments

• The principal advantage of red blood cells for these studies is that they don't contain internal , so their plasma membrane and associated proteins can be easily isolated Actin-binding of erythrocytes-

spectrin • The beta chain has a single actin- • Actin-binding protein- spectrin binding domain at its amino associate with short actin terminus. filaments • Link between the spectrin-actin network and the plasma • Result in the spectrin-actin membrane is provided by a network that forms the cortical protein called ankyrin cytoskeleton of RBC which binds both to spectrin and to a transmembrane protein • Spectrin is a member of the large called band 3. calponin family of actin-binding • An additional link between the spectrin-actin network and the • Spectrin is a tetramer consisting plasma membrane is providedby of two distinct polypeptide chains protein 4.1 called a and beta

Structure of spectrin (just for understanding) Association of the erythrocyte cortical cytoskeleton with the plasma membrane Cont…

• Member of the calponin family, filamin constitutes a major link between actin filaments and the plasma membrane of blood platelets.

• Additional member of the calponin family, dystrophin in muscles

• Absent or abnormal in patients cause muscular dystrophy

2. HELP IN ESTABLISHING CONTACTS WITH ADJACENT CELLS OR

Most cells have specialized regions of the plasma membrane that form contacts with adjacent cells, the extracellular matrix or with other substrata such as the surface of a culture dish. a. Establishing contacts with extracellular matrix

• Regions of attachment sites is contributed by for bundles of actin filaments that anchor the cytoskeleton of cell to areas of cell contact.

• The best Example: fibroblasts maintained in tissue culture.

• The fibroblasts attach to this extracellular matrix on the culture dish via the binding of transmembrane proteins called integrins.

• The sites of attachment are discrete regions called focal adhesions that also serve as attachment sites for large bundles of actin filaments called stress fibers. (Refer diagram in the next slide)

Attachment of stress fibers to the plasma membrane at focal adhesions b. Actin cytoskeleton is anchored to regions of cell-cell contact (adherens junctions)

• In sheets of epithelial cells, these junctions form a continuous belt-like structure called an adhesion belt around each cell

• Contact between cells at adherens junctions is mediated by transmembrane proteins called cadherins.

• The cadherins form a complex with cytoplasmic proteins called catenins, which associate with actin filaments. (Refer diagram)

Attachment of actin at adherence junctions 3. ACTIN HELP IN PROTRUSIONS OF THE CELL SURFACE

Most of these cell surface extensions are based on actin filaments, which are organized into either relatively permanent or rapidly rearranging bundles or networks. a. Permanent Protrusions of the Cell Surface • Best-characterized of these actin-based cell surface protrusions are microvilli on epithelial cells lining intestine they form brush border • Abundant on the surfaces of cells involved in absorption

• Another example of specilized microvilli is stereocilia of auditory hair cells, are responsible for hearing by detecting sound vibrations.

• Microvilli -parallel bundles of 20 to 30 actin filaments in these bundles are cross-linked in part by fimbrin and villin

• Along their length, the actin bundles of microvilli are attached to the plasma membrane by lateral arms consisting of the calcium-binding protein calmodulin in association with l b.Transient surface protrusions

• are extensions of moderate width, based on actin filaments cross-linked into a three- dimensional network

• Lamellipodia are broad, sheetlike extensions at the leading edge of fibroblasts, which similarly contain a network of actin filaments

• Many cells also extend microspikes or 4. ACTIN HELP IN Muscle Contraction • Skeletal muscles are bundles of muscle fibers

• Most of the consists of , which are cylindrical bundles of two types of filaments: thick filaments of myosin (about 15 run in diameter) and thin filaments of actin (about 7 nm in diameter).

• Each is organized as a chain of contractile units called sarcomeres, which are responsible for the striated appearance of skeletal and cardiac muscle.

Structure of muscle cells

Sarcomere • The ends of each sarcomere are defined by the Z disc.

• Within each sarcomere, dark bands (called A bands because they are anisotropic when viewed with polarized light) alternate with light bands (called I bands for isotropic).

• The I bands contain only thin (actin) filaments, whereas the A bands contain thick (myosin) filaments.

• The myosin and actin filaments overlap in peripheral regions of the A band, whereas a middle region (called the H zone) contains only myosin. Muscle contraction

• The basis for understanding muscle contraction is the sliding filament model, first proposed in 1954 both by Andrew Huxley and Ralph Niedergerke and by Hugh Huxley and Jean Hanson

• During muscle contraction each sarcomere shortens, bringing the Z discs closer together.

• There is no change in the width of the A band, but both the I bands and the H zone almost completely disappear.

• These changes are explained by the actin and myosin filaments sliding past one another so that the actin filaments move into the A band and H zone.

• Muscle contraction thus results from an interaction between the actin and myosin filaments that generates their movement relative to one another.

• The molecular basis for this interaction is the binding of myosin to actin filaments, allowing myosin ( convert chemical energy to mechanical) to function as a motor that drives filament sliding. Sliding filament model (sarcomere) Association of tropomyosin and troponins with actin filaments

Ca2+-binding protein calmodulin ***Refer to pdf shared for more detail. 5. NON MUSCULAR MYOSIN AND ACTIN LEADS TO 5. Non muscular myosin and actin leads to cytokinesis

Contractile assemblies in nonmuscle cells • The most dramatic example of actin-myosin contraction in nonmuscle cells, is provided by cytokinesis

• Toward the end of in and cells, a contractile ring consisting of actin filaments and myosin assembles just underneath the plasma membrane.

• Its contraction pulls the plasma membrane progressively inward, constricting the center of the cell and pinching it in two. Interestingly, the thickness of the contractile ring remains constant as it contracts, implying that actin filaments disassemble as contraction proceeds.

• The ring then disperses completely following

• In nonmuscle cells and in smooth muscle, however, contraction is regulated primarily by phosphorylation of one of the myosin light chains called the regulatory light chain 6.FORMATION OF PROTRUSIONS AND CELL MOVEMENT 6.Formation of Protrusions and Cell Movement • The movement of cells across a surface represents a basic form of cell locomotion employed by a wide variety of different kinds of cells. Examples includes:  The crawling of amoebas  The migration of embryonic cells during development  The invasion of tissues by white blood cells to fight  The migration of cells involved in wound healing  The spread of cancer

• All of these movements are based on local specializations and extensions of the plasma membrane driven by the dynamic properties of the actin cytoskeleton. Cell movement or extension involves a coordinated cycle of movements • First, cells must develop an initial polarity via specialization of the plasma membrane or the cell cortex. •

• Second, protrusions such as pseudopodia, lamellipodia, or filopodia must be extended to establish a leading edge of the cell. These extensions must then attach to the substrahtm across which the cell is moving.

• Finally, during the trailing edge of the cell must dissociate from the substratum and retract into the cell body.

Intermediate Filaments

• Intermed ate filaments have diameters between 8 and 11 nm

• Not involved in cell movements instead, play a structural role by providing mechanical strength to cells

• Intermediate filaments are apolar

Structure

• Intermediate filaments are composed of a variety of proteins that are expressed in different types of cells

• More than 65 different proteins have been identified

• These proteins are classified into six groups based on similarities between their amino acid sequences (refer table) Classes of intermediate filament and their functions

Functions Structure of Intermediate Filaments Assembly of intermediate filament

Step 1 Formation of dimers in which the central rod domains of two polypeptide chains are wound around each other in a coiled-coil structure

Step 2 The dimers of cytoskeletal intermediate filaments then associate in a staggered antiparallel fashion to form tetramers

Step 3 Tetramer assemble end-to-end to form protofilaments. A common step is the interaction of approximately eight protofilaments wound around each other in a ropelike structure. They are assembled from antiparallel tetramers, both ends of intermediate filaments are equivalent-apolar Assembly of intermediate filament Important points of assembly

• Filament assembly requires interactions between specific types of intermediate filament proteins. For example, keratin filaments are always assembled from heterodimers containing one type I and one type II polypeptide.

• In contrast, the type III proteins can assemble into filaments containing only a single polypeptide (e.g., vimentin) or consisting of two different type Ill proteins (e.g., vimentin plus desmin).

• The type III proteins do not, however, form copolymers with the keratins.

• Among the type IV proteins, a-internexin can assemble into filaments by itself, whereas the three proteins copolymerize to form heteropolymers. Characters

• More stable than actin or

• Donot exhibit dynamic behaviour

• Phosphorylation leads to disassembly and disappearance in a cell. One example is phosphorylation of the nuclear lamins which results in disassembly of the nuclear lamina and breakdown of the nuclear envelope during mitosis.

• Filament assembly requires interactions between specific types of intermediate filament proteins

• For example, keratin filaments are always assembled from heterodimers containing one type I and one type II polypeptide

Intermediate Filaments Functions 1. Intermediate filaments form elaborate network in cytoplasm of most cells, they extend from a ring surrounding the nucleus to plasma membrane and provide support

2. Keratin and vimentin filaments attach to the nuclear envelope, apparently serving to position and anchor the nucleus within the cell

3. Intermediate filaments thus provide a scaffold that integrates the other components of the cytoskeleton

4. The keratin filaments of epithelial cells are tightly anchored to the plasma membrane at two areas of specialized cell contacts: desmosomes and hemidesmosomes (refer fig in the next slide)

5. Desmin plays important role in nerve cell-it connects the individual actin-myosin assemblies of muscle cells both to one another and to the plasma membrane, thereby linking the actions of individual contractile elements and help in contraction

6. are the major intermediate filaments in most mature neurons. They are particularly abundant in the long of motor neurons where they appear to be anchored to actin filaments and microtubules by neuronal members of the plakin family For more function refer slide no 33 table

Attachment of intermediate filaments to desmosomes and hemidesmosomes (Refer PM unit)

Functions of Keratins and Neurofilaments: Diseases of the Skin and Nervous System

• Experimental evidence for such an in vivo role of intermediate filaments was first provided in 1991 by studies in the laboratory of Elaine Fuchs.

• These experiments also pointed to the molecular basis of a genetic disease, epidermolysis bullosa simplex (EBS).

• Motor neurons, particularly amyotrophic lateral sclerosis (ALS)., known as Lou Gehrig's disease (and the disease afflicting the renowned physicist Stephen Hawking Microtubules Fluorescence Imaging-Live cell study

An example of the role of microtubules in transporting organelles: The of this cell (shown in green and indicated by arrows) are closely associated with microtubules of the cytoskeleton Microtubules

• Microtubules are hollow, relatively rigid, tubular structures

• Components of mitotic spindle of dividing cells and the core of cilia and flagella

• An outer diameter of 24 nm and inner 14 nm

• wall of a microtubule is composed of globular proteins

• They are the dynamic structure

Structure • Building block of microtubule is tublin dimer of alpha and beta tublin

• A third type of tubulin y-tubulin is concentrated in the where it plays a critical role in initiating microtubule assembly

• Tublin dimer polymerize to form microtubule

• Tubulin protein are arranged in longitudinal rows, forms protofilaments, that are aligned parallel to the long axis of the tubule

• Microtubules are seen to consist of 13 protofilaments aligned side by side in a circular pattern within the wall

• Noncovalent interactions between adjacent protofilaments

• Tubulin polymerization can be studied in vitro

• Microtubules like actin filaments) are polar structures with two distinct ends: a fast-growing plus end and a slow growing minus end.

• This polarity is an important consideration in determining the direction of movement along microtubules Structure and assembly

• The tubulin dimers are organized in a linear array along the length of each protofilament

• The protofilament is asymmetric, with an a-tubulin at one end and β -tubulin at the other end

• All of the protofilaments of a microtubule have the same polarity Structure and assembly

• One end of a microtubule is known as the plus end and is terminated by a row of b-tubulin subunits

• The opposite end is the minus end and is terminated by a row of a- tubulin subunits.

• The structural polarity of microtubules is an important factor in the growth of these structures and their ability to participate in directed mechanical activities.

DYNAMIC ORGANIZATION OF MICROTUBULES Assembly of microtubules(three steps) Dynamic instability

• Microtubules like actin filaments are polar structures with two distinct ends: a fast-growing plus end and a slow growing minus end.

• This polarity is an important consideration in determining the direction of movement along microtubules, just as the polarity of actin filaments defines the direction of myosin movement.

• Microtubules can undergo rapid cycles of assembly and disassembly.

• Both a- and β -tubulin bind GTP to regulate polymerization.

• In particular, the GTP bound to β -tubulin (though not that bound to a- tubulin) is hydrolyzed to GDP during or shortly after polymerization. This GTP hydrolysis weakens the binding affinity of tubulin for adjacent molecules, thereby favoring depolymerization and result in dynamic behaviour Dynamic instability

• In microtubules, rapid GTP hydrolysis also results in the behavior known as dynamic instability in which individual microtubules alternate between cycles of growth and shrinkage

• Whether a microtubule grows or shrinks is determined in part by the rate of tubulin addition relative to the rate of GTP hydrolysis

• As long as new GTP-bound tubulin molecules are added more rapidly than GTP is hydrolyzed the microtubule retains a GTP cap at its plus end and microtubule growth continues.

• However, if the rate of polymerization slows, the GTP bound to tubulin at the plus end of the microtubule will be hydrolyzed to GDP. If this occurs, the GDP-bound tubulin will dissociate, resulting in rapid depolymerization and shrinkage of the microtubule.

• Dynamic instability, described by and Marc Kirschner in 1984 Dynamic instability of microtubule

Treadmilling and the role of GTP in microtubule polymerization Treadmilling of microtubule

• Like actin fi laments, microtubules undergo treadmilling a dynamic behavior in which tubulin molecules bound to GDP are continually lost from the minus end and replaced by the addition of tubulin molecules bound to GTP to the plus end of the same microtubule.

• The minus ends grow less rapidly than the plus ends of microtubules.

• This difference in growth rate is reflected in a difference in the critical concentration for addition of tubulin dimers to the two ends of the microtubule.

• Tubulin dimers with GTP bound to β -tubulin associate with the rapidly growing plus ends in a flat sheet, which then zips up into the mature microtubule just behind the region of growth.

• Shortly after polymerization the GTP bound to β tubulin is hydrolyzed to GDP, and since GDP-bound tubulin is less s table in the microtubule, the dimers at the minus end begin to peel off.

• Treadmilling takes place at tubulin dimer concentrations intermediate between the critical concentrations for the plus and minus ends.

• Under these conditions there is a net dissociation of dimers (bound to GOP) from the minus end, balanced by the addition of dimers (bound to GTP) to the plus end.

Microtubule-Associated Proteins • Microtubules prepared from living tissue typically contain additional proteins, called microtubule- associated proteins (or MAPs).

• The first MAPs to be identified are referred to as “classical MAPs” and typically have one domain that attaches to the side of a microtubule and another domain that projects outward as a filament from the microtubule’s surface.

• Some MAPs can be seen as cross-bridges connecting microtubules to each other, thus maintaining their parallel alignment.

• MAPs generally increase the stability of microtubules and promote their assembly.

• The microtubule-binding activity of the various MAPs is controlled by phosphorylation and deposphorylation. Microtubule-Associated Proteins

• An abnormally high level of phosphorylation of one particular MAP, called tau, has been implicated in the development of several fatal neurodegenerative disorders, including Alzheimer’s disease

• The brain cells of people with these diseases contain strange, tangled filaments (called neurofibrillary tangles) consisting of tau molecules that are excessively phosphorylated and unable to bind to microtubules.

• The neurofibrillary filaments are thought to contribute to the death of nerve cells.

Drugs

• Colchicine and colcemid commonly used as experimental drugs that bind tubulin and inhibit microtubule polymerization, which in turn blocks mitosis.

• Two related drugs vincristine and vinblastine are used in cancer chemotherapy because they selectively inhibit rapidly dividing cells.

• Another useful drug, taxol, stabilizes microtubules rather than inhibiting their assembly which block cell division and thus used as anticancer drug Intracellular organization of microtubules • During mitosis, microtubules similarly extend outward from duplicated to form the mitotic spindle, which is responsible for the separation and distribution of

• The centrosome is now known to function as microtubule-organizing center (MTOC) in which the minus ends of the microtubules are anchored.

• It serves as the initiation site for the assembly of microtubules, which then grow outward from the centrosome toward the periphery of the cell.

• The role of the centrosome is to initiate microtubule growth.

Intracellular organization of microtubules

• This dynamic behavior can, however, • Microtubules extend outward from the centrosome located adjacent to the be modified by the interactions of nucleus in microtubules with microtubule associated proteins (MAPs). • The key protein in the centrosome is y- tubulin, a minor species of tubulin • The centrosomes of most animal cells contain a pair of , oriented • y-tubulin is associated with eight or more perpendicular to each other and other proteins in a ring-shaped structure surrounded by amorphous called the r-tubulin ring complex

• This is thought to act as a seed for rapid • The centrioles are cylindrical microtubule growth structures based on nine triplets of

microtubules, similar to the basal • The centrosomes of most animal cells bodies of cilia and flagella contain a pair of centrioles, oriented perpendicular to each other and surrounded by amorphous pericentriolar • Centrioles are necessary to form material basal bodies, cilia, and flagella

Structure of

Centrioles are highly polar structure- Cartwheel at one end and several types of processes extending from the other end. These processes (called satellites and appendages) are thought to interact with specific proteins in the centrosome matrix.

Other centrosome matrix proteins such as gamma tubulin are associated with the lumen of the centriole. The triplet microtubules contain highly modified a- and β- and the unique γ-tubulin. fibers extend out from the triplet microtubules and connect to the other centriole. Cont…

• Microtubuleassociated proteins (MAPs) determine stability

• Many different MAPs -MAP-1, MAP-2, and tau (isolated from neuronal cells), and MAP-4, which is present in all non-neuronal vertebrate cell types.

• The tau protein has been extensively studied because it is the main component of the characteristic lesions found in the brains of Alzheimer's patients.

• The activity of many MAPs is regulated by phosphorylation, allowing the cell to control microtubule stability. • Maintaion polarity in neuron Tau+ and MAP2 in dendrites

FUNCTIONS OF MICROTUBULE 1. Microtubule in variety of cell movement a) Microtubules as Agents of Intracellular Motility b) Positioning of membrane vesicles and organelles Microtubule Motor Proteins which aids in movement • and , the prototypes of microtubule motor proteins

• They move along microtubules in opposite directions-most toward the plus end and toward the minus end

• Dynein, which was isolated by Ian Gibbons in 1965

• The development of in vitro assays for cytoplasmic motor proteins was based on the use of video-enhanced microscopy Motor Proteins that Traverse the Microtubular Cytoskeleton • The motor proteins of a cell convert chemical energy (stored in ATP) into mechanical energy, which is used to generate force or to move cellular cargo attached to the motor.

• Types of cellular cargo transported by these proteins include ribonucleoprotein particles, vesicles, mitochondria, , , and other cytoskeletal filaments.

• A single cell may contain a hundred different motor proteins Motor protein

• Collectively, motor proteins can be grouped into three broad superfamilies: kinesins, dyneins, and .

• Kinesins and dyneins move along microtubules, whereas myosins move along .

• Motor proteins move unidirectionally along their cytoskeletal track. As the protein moves along, it undergoes a series of conformational changes that constitute a mechanical cycle.

• The steps of the mechanical cycle are coupled to the steps of a chemical (or catalytic) cycle, which provide the energy necessary to fuel the motor’s activity • The steps in the chemical cycle include : – the binding of an ATP molecule to the motor, – hydrolysis of the ATP, – the release of the products (ADP and Pi) from the motor, – the binding of a new molecule of ATP.

• The binding and hydrolysis of a single ATP molecule is used to drive a power stroke that moves the motor a precise number of nanometers along its track.

Microtubule Motor Proteins

• Kinesin and dynein move in opposite directions along microtubules, toward the plus and minus ends, respectively.

• Kinesin consists of two heavy chains (wound around each other in a coiled-coil structure) and two light chains. The globular head domains of the heavy chains bind microtubules and are the motor domains of the molecule.

• Dvnein consists of two or three heavy chains (two are shown here) in association with multiple light and intermediate chains. The globular head domains of the heavy chains are the motor domains. Kinesin

• In 1985, and colleagues isolated a motor protein from the cytoplasm of squid giant axons

• They named the protein kinesin,

• Kinesin is a tetramer constructed from two identical heavy chains and two identical light chains. • es.”

• Each head (or motor domain) is connected to a neck, a rodlike stalk, and a fan-shaped tail that binds cargo Model of kinesin-catalyzed vesicle transport a. Microtubules as Agents of Intracellular Motility .

• Vesicles move in both directions within the axon.

• Vesicles containing transported materials are attached to the microtubules by crosslinking proteins, including motor proteins, such as kinesin and dynein.

b.Transport of vesicles along microtubules • Kinesin and other plus end- directed members of the kinesin family transport vesicles and organelles in the direction of microtubule plus ends, which extend toward the cell periphery.

• In contrast, dynein and minus end-directed carry their cargo in the direction of microtubule minus ends, which are anchored in the center of the cell. 2. IT HELPS IN POSITIONING MEMBRANE- ENCLOSED ORGANELLES WITHIN THE CELL.

Cont….

• Microtubules and associated motor proteins position membrane-enclosed organelles (such as the , , lysosomes, and mitochondria) within the cell.

• For example, positioning of ER to the periphery of the cell is in association with microtubules and kinesin I which pulls the ER along microtubules in the plus-end direction, toward the cell periphery.

• Similarly, kinesin appears to play a key role in the positioning of lysosomes away from the center of the cell

• Three different members of the kinesin family have been implicated in the movements of mitochondria.

• Conversely, cytoplasmic dynein is thought to play a role in positioning the Golgi apparatus.

• The Golgi apparatus is located in the center of the cell near the centrosome. If microtubules are disrupted, either by a drug or when the cell enters mitosis, the Golgi breaks up into small vesicles that disperse throughout the cytoplasm 3. BEATING OF CILIA AND FLAGELLA Cilia and flagella

• Cilia and flagella are microtubule-based projections of the plasma membrane that are responsible for movement of a variety of eukaryotic cells

• Eukaryotic cilia and flagella are very similar structures

• The minus ends of the microtubules of cilia and flagella are anchored in a , which is similar in structure to a centriole

• Basal bodies thus serve to initiate the growth of axonemal microtubules as well as anchoring cilia and flagella • to the surface of the cell. Structure of the of cilia and flagella • The fundamental structure of both cilia and flagella is the axoneme which is composed of microtubules and their associa ted proteins

• The microtubules are arranged in a characteristic "9 + 2" pattern in which a central pair of microtubules is surrounded by nine outer microtubule doublets.

• The two fused microtubules of each outer doublet are distinct: One (called the A tubule) is a complete microtubule consisting of 13 protofilaments; the other (the B tubule) is incomplete, containing only 10 or 11 protofilaments fused to the A tubule.

• The outer microtubule doublets are • connected to the central pair by radial spokes and to each other by links of a protein ca lled .

• In addition, two arms of dynein are attached to each A tubule, and it is the motor activity of these axonemal dyneins that drives the beating of cilia and flagella. Movement of microtubules in cilia and flagella (refer to structure of axoneme)

1. The bases of dynein arms are attached to A tubules, and dynein head groups interact with the B tubules of adjacent doublets.

2. Movement of the dynein head groups in the minus end direction (toward the base of the ) then causes the A tubule of one doublet to slide toward the base of the adjacent B tubule.

3. Because both microtubule doublets are connected by nexin links, this sliding movement forces them to bend and form the basis of beatin movement 4.CHROMOSOMAL MOVEMENT Reorganization of Microtubules during Mitosis • Helps in mitosis and distribution of chromosomes to daughter cells.

• Reassembled to form mitotic spindle, which is responsible for the separation of daughter chromosomes

• Mitotic spindle involves the selective stabilization of some of the microtubules radiating from the centrosomes. These microtubules are of four types, three of which make up the mitotic spindle. 1. microtubule 2. Chromosomal microtubules, 3. Polar microtubule 4. microtubule

Types of microtubules during mitotic spindle formation 1. Kinetochore microtubules attach to the condensed chromosomes of mitotic cells at their centromeres, which are associated with specific proteins to form the kinetochore Attachment to the kinetochore stabilizes these microtubules

2. Chromosomal microtubules, which connect to the ends of the chromosomes via chromokinesin.

3. Polar microtubule is not attached to chromosomes. Instead, the polar microtubules are stabilized by overlapping with each other in the center of the cell.

4. Astral microtubules extend outward from the centrosomes to the cell periphery and have freely exposed plus ends. Both the polar and astral microtubules contribute to chromosome movement by pushing the spindle poles apart. Mitotic spindle formation

• The centrioles and centrosomes duplicate during interphase.

• During of mitosis the duplicated centrosomes separate and move to opposite sides of the nucleus.

• Thenuclear envelope then disassembles, and microtubules reorganize to form the mitotic spindle.

• Kinetochore microtubules are attached to the of condensed chromosomes and chromosomal microtubules are attached to their ends.

• Polar microtubules overlap with each other in the center of the cell, and astral microtubules extend outward to the cell periphery.

• At metaphase, the condensed chromosomes are aligned at the center of the spindle. Movement of chromosome

• After alignment on the metaphase plate at Anaphase A and B chromosome movement and seperation occur

• Chromosome move towards spindle poles along the kinetochore microtubules.

• IT is driven by minus end-directed motor proteins associated with the kinetochore.

• The action of these motor proteins is coupled to disassembly and shortening of both the kinetochore and chromosomal microtubules. Spindle pole separation in anaphase B • The separation of spindle poles results from two types of movement • • First, overlapping polar microtubules slide past each other to push the spindle poles apart, probably as a result of the action of plus end-directed motor proteins.

• Second, the spindle poles are pulled apart by the astral microtubules. The driving force appears to be a minus end- directed motor anchored to the cell cortex. 5. Organization of microtubules in nerve cells determine cell polarity • kinesin l translocates along microtubules in only a single direction- toward the plus end.

• Because the plus ends of microtubules in axons are all oriented away from the cell body the movement of kinesin I in this direction transports vesicles and organelles away from the cell body toward the tip of the axon.

• Within intact axons, however, vesicles and organelles also had been observed to move back toward the cell body, implying that a different motor protein dynein responsible for movement along microtubules in the oppositedirection- toward the minus end.

References

• Cooper, G. M., & Hausman, R. E. (2004). The cell: Molecular approach. Medicinska naklada.

• Hardin, J., Bertoni, G. P., & Kleinsmith, L. J. (2017). Becker's World of the Cell. Pearson Higher Ed.

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