Cytoskeleton and Cell Motility: Microtubules, Actin, and Intermediate Filaments

What is the general function of the cytoskeleton in eukaryotic cells?

It provides structure, mechanical support, intracellular transport, and cell motility.

What are the three basic types of cytoskeletal elements?

Microtubules, microfilaments, and intermediate filaments.

What are microtubules composed of?

Tubulin, with a diameter of 25 nm.

What are microfilaments made of?

Actin, with a diameter of 7 nm.

What is the diameter range of intermediate filaments?

8-12 nm.

How are microtubules structured?

They are straight hollow cylinders built from 13 protofilaments.

What is a protofilament in microtubules?

A linear chain of alpha-tubulin and beta-tubulin heterodimers.

What is the significance of the GTP molecules in microtubules?

Each dimer binds 2 GTP molecules, with one being nonexchangeable (alpha-tubulin) and the other exchangeable (beta-tubulin).

What is the difference between the plus and minus ends of microtubules?

The plus end has rapid growth/shrinkage, while the minus end is slower and anchored by the microtubule organizing center (MTOC).

What is dynamic instability in microtubules?

It refers to the alternation between growth and shrinkage of microtubules.

What role do MTOCs play in microtubule dynamics?

They are sites where microtubules nucleate and determine polarity.

What is the major MTOC in animal cells?

The centrosome.

What are antimitotic drugs like colchicine and nocodazole used for?

They bind beta-tubulin, inhibit microtubule assembly, and prevent spindle formation.

What is the role of taxol in relation to microtubules?

It stabilizes microtubules, prevents disassembly, and blocks mitosis.

What are microfilaments primarily composed of?

G-actin monomers that polymerize into filamentous actin (F-actin).

What is the polarity of actin filaments?

Barbed (+) end for faster growth and pointed (-) end for slower growth.

What is the function of actin in muscle contraction?

Actin interacts with myosin to facilitate muscle contraction.

What do cytochalasins do to actin filaments?

They cap the plus ends, stopping addition.

What are intermediate filaments known for?

They are the most stable and least soluble cytoskeletal elements, providing mechanical strength.

What is the structure of intermediate filaments?

They are composed of long alpha-helical rods that form coiled coils.

What classes of intermediate filaments exist?

Class 1-2: keratins, Class 3: desmin and vimentin, Class 4: neurofilaments, Class 5: lamins, Class 6: nestin.

How do spectraplankins contribute to cytoskeletal integrity?

They crosslink microtubules, microfilaments, and intermediate filaments.

What are the three broad functions of motility?

1. Movement of the entire cell or organism through the environment. 2. Movement of the environment past or through a cell. 3. Movement of components within the cell.

What role does the cytoskeleton play in motility?

It serves as a scaffold with microtubules and microfilaments acting as tracks for motor proteins that convert chemical energy (ATP) into mechanical work.

What is the structure of motor proteins?

Motor proteins have a filament-binding head domain with ATPase activity, a stalk connecting the head and tail, and a cargo-binding tail for attaching vesicles or organelles.

What is the function of kinesins?

Kinesins move toward the plus end of microtubules, facilitating anterograde transport and resemble 'walking' by alternating head steps.

What distinguishes dyneins from kinesins?

Dyneins move toward the minus end of microtubules, functioning in retrograde transport, and work with the dynactin complex to link to cargo.

What is the axoneme structure of motile cilia and flagella?

It has a 9 + 2 arrangement: 9 outer doublets and 2 central microtubules.

How do cilia and flagella differ in their movement?

Cilia are short and numerous, moving with oar-like strokes, while flagella are long and few, generating a wave-like motion.

What is intraflagellar transport (IFT)?

Kinesin moves material to the flagellum tip (plus end), while dynein moves material back to the base (minus end).

What are the main types of myosin and their functions?

Myosin I links actin to membranes, while Myosin II forms thick filaments for contraction.

What is the sliding-filament model of muscle contraction?

Thin filaments slide past thick filaments without changing length, with myosin heads forming cross-bridges with actin.

What initiates the cross-bridge cycle in muscle contraction?

Myosin head (ADP + Pi) binds actin; release of Pi tightens the bond, followed by a power stroke that causes filament sliding.

How is skeletal muscle contraction regulated?

At rest, tropomyosin blocks myosin-binding sites on actin. Calcium binding to troponin C shifts tropomyosin, exposing binding sites for contraction.

What is the role of calcium in cardiac muscle contraction?

Calcium influx triggers large release via ryanodine receptors, controlling rhythmic contractions.

How does smooth muscle contraction differ from skeletal muscle contraction?

Smooth muscle contraction is involuntary and regulated through calmodulin and myosin light-chain kinase, leading to slower, sustained contractions.

What is the role of actin and myosin in nonmuscle cell motility?

They drive cell crawling through protrusion, attachment, tension, and forward movement, as well as cytoplasmic streaming.

What is the structure of skeletal muscle fibers?

Skeletal muscle fibers are long, multinucleate cells containing many myofibrils divided into repeating sarcomeres.

What are the components of a sarcomere?

Thin filaments (actin, tropomyosin, troponin) and thick filaments (myosin II), with A bands (dark) for thick filaments and I bands (light) for thin filaments.

What is the function of titin in muscle structure?

Titin connects thick filaments to Z lines and helps keep them aligned.

What happens to tropomyosin when calcium levels increase?

Calcium binds to troponin C, causing tropomyosin to shift and expose myosin-binding sites on actin.

What is the significance of the pacemaker region in cardiac muscle?

It controls depolarization waves, initiating rhythmic contractions of the heart.

What is the role of myomesin in muscle structure?

Myomesin bundles myosin at the H zone of the sarcomere.

What is the function of nexin in cilia and flagella?

Nexin links adjacent doublets in the axoneme, converting sliding into bending during movement.

What is cytoplasmic streaming and its significance?

Cytoplasmic streaming (cyclosis) is the actomyosin-driven flow of cytoplasm within cells, especially in plants, aiding in nutrient distribution.

What are the two main types of tissue in animals?

Epithelial tissue and connective tissue.

What is the function of adhesive junctions?

To anchor the cytoskeleton of one cell to that of another, providing mechanical strength.

What are the two main types of adhesive junctions?

Adherens junctions and desmosomes.

What proteins mediate adherens junctions?

Cadherins, specifically E-cadherin in epithelial cells.

What is the role of desmosomes?

To provide strong adhesion between adjacent cells, contributing to tissue mechanical strength.

What are cell adhesion molecules (CAMs)?

Members of the immunoglobulin superfamily that facilitate cell-cell adhesion.

What is the function of selectins?

To mediate transient cell-cell adhesion in the bloodstream, especially during inflammation.

What is the primary function of tight junctions?

To seal cells together, preventing the passage of molecules and ions between cells.

What is the function of gap junctions?

To provide direct communication between cells by allowing small molecules and ions to pass.

What is the extracellular matrix (ECM)?

A network that surrounds and supports cells in animal tissues.

What are the three major classes of molecules in the ECM?

Structural proteins, protein-polysaccharide complexes, and adhesive glycoproteins.

What is the main structural protein in the ECM?

Collagen, which provides strength and structural integrity.

What is the structure of collagen?

Collagen molecules consist of three α chains twisted into a triple helix.

What is the role of elastins in the ECM?

To provide elasticity and flexibility to tissues.

What are proteoglycans?

Core proteins with long chains of carbohydrates (glycosaminoglycans) that provide a gel-like matrix.

What is the function of fibronectins?

To link cells to ECM components and play a role in cell migration and wound healing.

Where are laminins primarily found?

In the basal lamina beneath epithelial cells.

What are integrins?

Transmembrane receptors that bind ECM molecules and integrate the ECM with the cytoskeleton.

What are focal adhesions?

Specialized adhesion sites in motile or non-epithelial cells that connect to the cytoskeleton.

What is the main function of the ECM?

To provide structural and biochemical support to surrounding cells.

How do tight junctions maintain cell polarity?

By preventing membrane protein diffusion between apical and basolateral surfaces.

What is the significance of connexons in gap junctions?

They span the gap between adjacent plasma membranes, forming an open channel for communication.

What types of cells are desmosomes particularly important for?

Cells in tissues that require mechanical strength, such as skin, heart, and uterus.

What are glycosaminoglycans (GAGs)?

Large, negatively charged polysaccharides that are components of proteoglycans.

What is the role of β-catenin in adherens junctions?

It binds to the cytosolic domain of cadherins and connects to actin filaments.

What are nucleic acids and their primary functions?

Nucleic acids (DNA and RNA) store, transmit, and express genetic information.

What is a gene?

A sequence of DNA that encodes proteins or functional RNA molecules.

What is the genome?

The total genetic content of an organism.

What was the conclusion of Griffith's Experiment (1928)?

A 'transforming principle' from dead S-strain cells converted R-strain cells to S-type.

What did Avery, MacLeod, and McCarty (1944) conclude about DNA?

DNA is the molecule responsible for heredity, as only the DNA-containing fraction could transform R to S.

What did the Hershey-Chase Experiment (1952) demonstrate?

DNA, not protein, carries genetic information, as labeled DNA entered bacterial cells while protein did not.

What significant contribution did Rosalind Franklin make to DNA research?

She provided evidence of helical DNA using X-ray diffraction (Photo 51).

What model of DNA structure did Watson and Crick propose?

They built the double-helix model of DNA.

What are Chargaff's rules?

A = T and G = C, indicating base pairing in DNA.

What are the main differences between eukaryotic and prokaryotic DNA?

Eukaryotic DNA is linear and located in the nucleus, while prokaryotic DNA is circular and located in the nucleoid region.

What is the structure of DNA?

DNA is a double helix made of two antiparallel polynucleotide strands with complementary base pairing (A-T and G-C).

What is semi-conservative replication?

Each daughter DNA contains one old (template) strand and one newly synthesized strand.

What is supercoiling in DNA?

Supercoiling is the twisting of DNA to condense its structure, with positive and negative supercoils affecting tension during replication.

What is the role of topoisomerases?

Topoisomerases regulate supercoiling by creating transient single-strand or double-strand breaks.

What is DNA denaturation?

Denaturation is the separation of DNA strands by heat or pH increase, monitored by increased light absorption at 260 nm.

What is the melting temperature (Tm) of DNA?

The temperature at which 50% of DNA is denatured; higher G-C content results in a higher Tm.

What is the basic unit of eukaryotic DNA packaging?

The nucleosome, which has a 'beads-on-a-string' structure with DNA wrapped around histone octamers.

What are the levels of eukaryotic DNA packaging?

1. Nucleosome, 2. 30-nm chromatin fiber, 3. Looped domains, 4. Higher-order folding into chromosomes.

What is the function of the nuclear envelope?

It separates the nucleus from the cytoplasm and consists of inner and outer membranes with nuclear pores for transport.

What is the nuclear lamina?

A dense fibrous mesh lining the inner membrane of the nuclear envelope, providing structural support and shape.

What is the nuclear pore complex (NPC)?

A massive structure composed of ~30 proteins (nucleoporins) that allows transport between the nucleus and cytoplasm.

How are proteins imported into the nucleus?

Proteins with a nuclear localization signal (NLS) are recognized by importin receptors and transported through the NPC.

What is the role of Ran-GTP in nuclear transport?

Ran-GTP binds importin inside the nucleus to release cargo and is involved in the export of RNA and proteins with nuclear export signals (NES).

What maintains the Ran-GTP gradient?

Maintained by GEF (guanine exchange factor) in the nucleus and GAP (GTPase activating protein) in the cytoplasm.

What is the purpose of DNA replication?

To accurately copy genetic material before cell division.

What is the key concept of DNA replication?

Replication is semiconservative; each new DNA molecule has one old strand and one new strand.

What must happen to the double helix during DNA replication?

The double helix must be unwound by breaking hydrogen bonds between bases.

What enzyme adds nucleotides to the growing DNA strand?

DNA polymerase.

In which direction does DNA synthesis proceed?

5′ to 3′ direction.

How does DNA polymerase correct errors during replication?

Through its 3′ to 5′ exonuclease activity.