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#polymerization

7 public questions tagged with this topic.

Cytoskeleton

Which of the following conditions favors F-actin polymerization?

F actin formation prerequisites can be recapitulated in vitro using purified muscle actin from rabbit skeletal muscle. G actin in Ca ATP state low ionic strength is monomeric. Initiation requires exchange of Ca2+ for Mg2+ at high affinity site by adding 2 millimolar MgCl2 and 50 to 100 millimolar KCl to screen electrostatic repulsion, plus 1 millimolar ATP to maintain ATP actin. Crucially thermodynamic requirement total G actin concentration must exceed critical concentration about 0.1 micromolar at plus end. Below this no polymer forms, above this equilibrium free monomer remains at Cc and ex

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Conditions for F-Actin Polymerization and Cc.

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Cytoskeleton

Which actin-binding protein promotes actin polymerization by catalyzing the exchange of ADP for ATP on G-actin?

Actin nucleotide state dictates polymerization competence. ATP G actin incorporates into barbed end with high affinity Kd 0.1 micromolar, ADP G actin weak Kd 1 micromolar and depolymerizes. After filament turnover ADP actin accumulates needing recharge. Profilin, 15 kilodalton ubiquitous protein present at 50 to 100 micromolar, binds actin subdomain 1 and 3 cleft with 1 to 1 stoichiometry. Structural studies show binding opens nucleotide binding pocket between subdomains 2 and 4, reducing affinity for ADP 1000 fold and accelerating its dissociation rate from 0.02 to 1 per second, allowing abun

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Profilin and Nucleotide Exchange on G-Actin.

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Cytoskeleton

The rate of actin polymerization is highest at which end of the filament?

Polarity arises because actin protomers are asymmetric and assemble head to tail orienting ATP hydrolysis vector. Myosin S1 decoration electron microscopy historically defined barbed and pointed ends based on arrowhead appearance. Kinetic measurements show association rate constant for ATP G actin about 11.6 per micromolar per second at plus end versus 1.3 at minus end, dissociation rate 1.4 per second versus 0.8. Net result polymerization velocity ten times faster at barbed end at any monomer level above Cc. This asymmetry explained by structural differences: barbed end exposes W loop and hyd

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 18: Actin Polarity and Barbed End Growth.

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Cytoskeleton

The critical concentration (Cc) of actin filaments is defined as the concentration at which:

Assembly of G actin into F actin follows reversible nucleation condensation kinetics characterized by critical concentration. Defined as monomer concentration at which rate of subunit addition equals rate of loss at filament ends, Cc reflects equilibrium constant Kd equal to k off divided by k on. Experimental determination uses pyrene fluorescence plateau where increasing total actin beyond Cc goes into polymer, free monomer stays constant. Because barbed plus end and pointed minus end have distinct kinetic constants due to ATP versus ADP actin states, each end possesses its own Cc approximat

Ref: Pollard and Earnshaw, Cell Biology, Chapter 12: Actin Polymerization and Critical Concentration.

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Cytoskeleton

Which of the following proteins nucleates actin polymerization and remains bound to the (+) end?

Spontaneous formation of actin nuclei is thermodynamically unfavorable because dimers and trimers dissociate rapidly, creating lag phase in polymerization assays. Formins overcome this by stabilizing nucleation intermediate and remaining processively attached. Family members such as mDia1, mDia2, INF2, FMNL and yeast Bni1 and Bnr1 contain FH1 proline rich region and FH2 donut shaped dimer. Each FH2 hemidimer contacts actin, encircling barbed end and forming stable dimer nucleus that templates addition of subsequent ATP actin subunits. FH1 recruits profilin ATP actin via polyproline interaction

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 16: Actin Nucleation by Formins and FH Domains.

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Electrophoresis

Which of the following is a photochemical polymerization initiator?

Polyacrylamide polymerization can be initiated chemically via APS-TEMED or photochemically via riboflavin. Riboflavin (vitamin B2) undergoes photolysis under visible or UV light to generate free radicals that initiate acrylamide polymerization, particularly when combined with TEMED as co-catalyst. This light-controlled method allows slower, more uniform gelation, useful when rapid chemical polymerization is undesirable and for preparing low-percentage gels. Ethidium bromide is an intercalating fluorescent dye, TEMED alone is a chemical catalyst not photochemical, and TAE is an electrophoresis

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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PSS- Sec G- Sec metabolites

Five-carbon isoprene units polymerize to form:

The right choice is B: Terpenes. In PSS- Sec G- Sec metabolites, that matches how the process or concept actually works — the other choices mix up related ideas or use the wrong mechanism. Skip these: A) Phenolics; C) Alkaloids; D) Lignins. A quick check: if an option needs energy, pumps, or the opposite direction of movement, ask whether that really applies.

Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.

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