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6.7Problems

 

Outline the reasons why eukaryotes - and animal cells in particular - have such large and complex cytoskeleton system

Actin filaments that make up the cytoskeleton perform a treadmilling movement.

1)

Describe the phenomenon of treadmilling observed in cytoskeleton.

2)

Explain the mechanism of treadmilling.

 

Actin filaments are bundled together, forming various tertiary structures. List the names of the molecules involved in this structural formation. Also, describe the structures formed by actin filament bundles.

 

Explain the roles that motor proteins play when chromosomes move during mitosis.

 

Explain how an increase in the Ca2+ concentration in a cell affects the regulation of skeletal muscle contraction.

 

After animals die, there is a strong muscular contraction known as rigor mortis. Explain the reasons for this phenomenon.

 

Outline the reasons why eukaryotes - and animal cells in particular - have such large and complex cytoskeleton system

[1]Such cytoskeletons are necessary for animal cells, which do not have cell walls, to support themselves.
[2]Actins and myosins are necessary for the movement of eukaryotic cells.
[3]In eukaryotic cell division, chromosomes must be distributed accurately to daughter cells, and microtubules - which form the spindle - are necessary for this process.
[4]The locations of organelles in a cell are regulated by motor proteins that move along microtubules.

Actin filaments that make up the cytoskeleton perform a treadmilling movement.

1)

Describe the phenomenon of treadmilling observed in cytoskeleton.

Treadmilling is a phenomenon in which, in the case of actin, an actin filament (a polymer of G-actins) repeatedly undergoes polymerization and depolymerization, seemingly maintaining a constant state as a whole.

2)

Explain the mechanism of treadmilling.

Treadmilling is a result of the difference in the polymer formation rate between the G-actin-ATP complex and the G-actin-ADP complex. Although both repeatedly perform polymerization and depolymerization, the reaction equilibrium of the G-actin-ATP complex leans toward polymerization, while that of the G-actin-ADP complex leans toward depolymerization. Consequently, when the polymerization rate at the plus end and the depolymerization rate at the minus end are balanced, a constant state is maintained as a whole (see the Column in 6.1).

 

Actin filaments are bundled together, forming various tertiary structures. List the names of the molecules involved in this structural formation. Also, describe the structures formed by actin filament bundles.

The molecules that bundle actin filaments together are classified as actin-binding proteins. Well known examples are α-actinins and fimbrins as well as filamins that cross and bundle actin filaments.
Examples that show the importance of the tertiary structure made by actin filament bundles include the contractile apparatus in muscle cells, the processes of moving cells (i.e., pseudopodia) and the special skeleton structure formed in the core of microvilli (see the text in 6.1 and Figs. 6-7 and 6-10).
[Commentary]
In muscle cells, special contractile apparatuses called sarcomeres are formed, in which actin and myosin filaments are bundled together. Myosin filaments thrust into the spaces between actin filaments, thus causing contractile movement.
Branches of bundled actin filaments exist in pseudopodia formed in the moving cells, playing an important role in the formation and extension of processes.
Densely bundled actin filaments are found in the core of the microvilli formed on the surface of epithelial cells in the intestines and other organs, contributing to microvillus formation and material transport.

 

Explain the roles that motor proteins play when chromosomes move during mitosis.

In mitosis, replicated sets of chromosomes are pulled and equally separated by spindle fibers (consisting of microtubules) to both poles. As a result, two cells are created, each containing one set of chromosomes. Microtubules extending from the centrosomes located in both poles and the movement of motor proteins play central roles in this chromosome movement.
[Commentary]
The chromosome-side end of microtubules is the plus end, and the centrosome-side end is the minus end. The motor proteins that move on microtubules are kinesins and dyneins, which move in opposite directions.

 

Explain how an increase in the Ca2+ concentration in a cell affects the regulation of skeletal muscle contraction.

The trigger for muscle contraction is a transient increase in the Ca2+ concentration in a cell caused by the release of Ca2+ from sarcoplasmic reticula. The binding of Ca2+ to contraction regulation proteins that bind to actin filaments and inhibit their binding to myosins changes the tertiary structure of the regulation proteins, which allows interaction between actin filaments and myosin heads, thereby causing muscle contraction.
[Commentary]
Through the establishment of the direct linkage between the Ca2+ channel in sarcoplasmic reticula and that in transverse tubules, signals (excitations) are transferred from the plasma membrane to sarcoplasmic reticula, causing the release of Ca2+. As a result of the increased Ca2+ level, Ca2+ binds to a calcium-binding contraction regulation protein called troponin C.

 

After animals die, there is a strong muscular contraction known as rigor mortis. Explain the reasons for this phenomenon.

As a result of the leakage of Ca2+ from sarcoplasmic reticula, all muscles remain contracted until every ATP molecule is used.

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