The following statements refer to intermediate filaments. Choose the single INCO

Question

The following statements refer to intermediate filaments. Choose the single INCORRECT response. Intermediate filaments are highly polar structures with a 'plus and 'minus' end. Intermediate filaments can be comprised of different types of protein dependent on cell type. Intermediate filament subunits are comprised of molecules with a large alpha -helical domain that facilitates formation of coiled coil dimers, which can form tetramers. In the protofilaments the numerous interactions between staggered arrays of tetramers confers tensile strength to intermediate filaments. What is the role of the Thymosin complex in actin filament polymerisation? Choose the single CORRECT response. It increases the concentration of G-actin at the plus end of actin filaments. It effectively decreases the concentration of G-actin at both the plus and minus end of actin filaments. It competes with profilin for binding to G-actin and promotes actin growth at the plus end. It binds the minus ends of actin filaments and promotes directional growth and branching of actin filaments. Cell migration is a complex process involving coordination of several different processes? Which of the following statements about cell migration is INCORRECT? Extension of lamellipodium involves preferential actin polymerisation. The movement of a cell trailing edge involves the rapid depolymerisation of the actin filaments due to the release of energy by hydrolysis ATP-bound monomers. Arp complexes facilitate rapid actin ploymerisation at the plus ends and formation of branching actin filament networks in the extending lamellipodium. The activity of Rac and Rho can facilitate cell migration by regulating actin polymerisation and actin-myosin contractions, respectively. How does Ras become inactivated? Choose the single CORRECT response. Assisted by a guanine exchange factor, it releases GTP and binds GDP. By the action of a phosphatase, it dephosphorylates GTP to generate its GDP-bound state. Assisted by a GTPase activating protein, it hydrolyses GTP to generate its GDP-bound state. It becomes phosphorylated by the action of the kinase domains in receptor tyrosine kinases. Which of the following statements about receptor serine/threonine kinase signaling is CORRECT? TGF beta signaling is initiated when a ligand binds to a Type I receptor, which then dimerizes with and phosphorylates the Type II receptor. All TGF beta receptor signaling takes place at the plasma membrane. TGF beta signaling results in homodimers of Smad3 or Smad2 becoming phosphorylated and entering the nucleus to activate gene transcription. After its activation by the Type II receptor, the phosphorylated Type I TGF beta receptor recruits and phosphorylates Smad2 or Smad3.

Explanation / Answer

6. Ans: A

Intermediate filaments are thicker than actin filaments and thinner than microtubules. These subunits are elongated, non globular and do not have polarity.

7. Ans:C

Thymosin complex and profilin competes with each other for binding actin. As Thymosine complex and profilin act oppositively during actin barbed end elongation with Thymosine complex blocking and profilin allowing their bound G-actin to join the barbed end of the filament.Thus competition between the two proteoins creates an efficient way and extends actin polymerisation.

8. Ans: B

Lamellipodium is a kind of protrusion within which actin is polymerizing and often branched. The Arp2/3 complex is a heptamer that attaches itself to side of pre-existing actin filament and nucleates actin polymerization at a fixed angle.

9.Ans: C

Ras is a G-protein, guanosine nucleotide binding protein. With Guanosine nucleotide exchange factors and GTPase activating protein, bound nucleotide is exchanged. It functions as a binary signalling switch. In the deactivated state, is bound to the nucleotide and in activated state, Ras is bound to GTP.Ras will hydrolyze a bound GTP molecule into GDP with its intrinsic GTPase activity.

10. Ans: D

TGF-beta signals through two related transmembrane serine/threonine kinase receptors, the type I and type II receptors. Initiation of siganlling is done as ligand binds to type II receptor followed by recruitment of type I receptor into a heteromeric complex. Type II receptor transphosphorylates within the complex and activates type I receptor kinase that targets signalling components of the pathway. Smad 2 and Smad3 interact with type I receptor and are phosphorylated.

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