Which of the following functions is not associated with the cytoskeleton in eukaryotic cells?

A. Determining the shape of animal cells.

B. The beating of cilia or flagella.

C. Maintaining the position of the nucleus in the cell.

D. The contraction of muscle cells in animals.

E. Movement of RNA molecules from the nucleus to the cytoplasm.

Diagram of an animal cell showing components of the cytoskeleton
Diagram of an animal cell showing some of the components of the cytoskeleton

The correct answer is E. Movement of RNA molecules from the nucleus to the cytoplasm.

The cytoskeleton of the cell in eukaryotic organisms is made of different types of protein fibers. These fibers help to provide shape and support to the cell, and in some cases enable the movement of organelles, chromosomes, and entire cells.

The three types of fibers from the smallest diameter to the largest diameter are actin microfilaments, intermediate filaments, and microtubules.

The intermediate filaments are most numerous in nerve cells and the epidermal cells of multicellular organisms where they help to provide support and structure. They are responsible for helping cells to closely organize and associate with the matrix and other cells in a tissue.

Microtubules are the largest of the proteins that are found in the cytoskeleton. They are important in making up the centrioles which form the spindle that allows chromosomes to separate during cell division. These microtubules are also the structures that form cilia and flagella.

Cilia and flagella are important structures produced in cells that allow for movement. Many single-celled Protista are ciliated or flagellated, which enable movement of the cell or the generation of feeding currents for the cell.

Actin filaments are the smallest structures of the cytoskeleton but are usually the most abundant in cells. They are important in connecting the plasma membrane to the cytoplasm and in enabling the contraction of muscle cells.

The cytoskeleton

The cytoplasm of the eukaryotic cell contains many protein fibers and filaments that form what is known as the cytoskeleton.

The cytoskeleton is a very important part of the cell which helps organize and support organelles and the cell. The fibers of the cytoskeleton also maintain the position of the nucleus within the cell.

It also plays a role in the movement of structures in the cell and the movement of unicellular organisms such as flagellates and ciliated protists.

Some parts of the cytoskeleton also help cells to link together and assemble to form the tissues of multicellular life forms. Some of the protein fibers also are important in forming cytoplasmic extensions such as the microvilli of cells.

There are three types of filaments that make up the elements of the cytoskeleton. These structures are intermediate filaments, microtubules, and actin microfilaments.

Intermediate filaments

These protein filaments are 10 nm in diameter and in between the size of actin microfilaments (7 nm) and microtubules (24 nm). The subunits of these structures consist of globular proteins. The subunits for intermediate filaments are alpha-helical proteins.

Several types of proteins are found making up these filaments. For instance, vimentin, glial fibrillary acidic protein (GFAP), desmin, keratins, and peripherin are some of the types of proteins that are present in these filaments.

The intermediate filaments are most commonly found in the cells of multicellular life forms. They are particularly abundant in the cells of the epidermis and in the axon region of nerve cells.

The role of these filaments is believed to be primarily to provide structure to the cell, they are not involved in the movement of cell organelles or for cell movement in the way that the other two protein fibers are.

Scientists have discovered that the intermediate filaments are very important in helping provide support where cells meet the matrix in tissues.  They appear to help support the plasma membrane and allow cells to group together and be arranged and organized into layers to form tissues.

Microtubules

Microtubules are made of the protein known as tubulin. These proteins enable the movement of vesicles through the cytoplasm of the cell and they help to give the cell support and shape.

The protein filaments of tubulin are arranged so as to form cilia and flagella, which are structures that allow for movement. In the case of the flagellum or cilium, protofilaments of alpha and beta tubulin are arranged to form a hollow cylinder.

The central part of the structure, cilium or flagellum, is known as an axoneme. The axoneme of a cilium consists of two microtubules surrounded by another nine such protein tubules.

The different subunits that form the microtubules have charges which confer polarity on the structure and determine how they become arranged during assembly. In fact, the alpha-tubulin has a negative charge while the beta-tubulin has a positive charge.

The flagellum has a similar arrangement and structure to a cilium, and both structures can bend to bring about movement. Microtubules present in the axoneme, move past each other in a sliding motion, which causes the structure to bend, and thus move.

Microtubule proteins also form the spindle fibers which are so important in cell division. These fibers attach to kinetochores of chromosomes and then contract to pull them apart during anaphase of mitosis and meiosis.

Actin microfilaments

Actin is a very abundant protein in eukaryotic cells, where it is an important component of the cytoskeletal structure of the cells.

These microfilaments are made of subunits of globular proteins known as F-actin and G-actin. The F-actin is actually formed from the polymerization of the G-actin protein.

Actin also has polarity in a similar way to microtubules, and they are very abundant just below the plasma membrane of the cell where they help to anchor it to the cytoplasm.

The actin is also an important part of the sliding-filament method of muscle cell contraction. Myosin and actin protein filaments slide past each other to enable muscle cells to shorten, contract.

This is how entire blocks of muscles are able to move and, thus, animals are able to locomote because of the presence of actin fibers.

These proteins of actin are also involved in the process of cytokinesis, the division of the cytoplasm, which occurs at the end of cell division. They also are responsible for forming the cytoplasmic projections, the microvilli of intestinal cells of animals.

References

  1. Editors of Encyclopedia Britannica (2019). Cytoskeleton. Retrieved from Encyclopedia Britannica.
  2. Lodish et al. (2000). Molecular Cell Biology, 4th edition. New York: USA, W.H. Freeman Publishers.
  3. Editors of Encyclopedia Britannica (2019). Cilium. Retrieved from Britannica.
  4. Editors of Encyclopedia Britannica (2019). Flagellum. Retrieved from Britannica.
  5. Nature Education (2014). Microtubules and Filaments. Retrieved from nature.com.

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