Microtubules are always breaking down and building up. While the structure appears stable in the cell, you need to know that the process is instead quite dynamic. There are alternating alpha and beta tubulin molecules that wrap into a cylindrical shape. Microtubules, like actin, are polar molecules. The positively charged end grows fast, while the negatively charged end grows slowly. The beta end is always exposed on the positive end, while the alpha end is always exposed on the negative end. Microtubules are made at centrosomes in animal cells but not in plant cells or fungi. In plants and fungi, the microtubule organizing center is the nuclear envelope. They give structure to cilia and flagella. Cilia are seen in the female reproductive tract and in the respiratory lining. Flagella are seen in human sperm and certain bacteria. There are three types of microtubules that aid in the process of mitosis. These are astral microtubules, polar microtubules, and kinetochore microtubules. The astral microtubules radiate from the centrosomes to the cell membrane in order to keep the mitotic spindle in place. Polar microtubules help separate the chromosomes. Kinetochore microtubules actually connect to the chromosomes in order to pull them apart.
INTERMEDIATE FILAMENTS Intermittent filaments are just involved in structure and are not involved in cell movement. There are more than fifty different intermediate filament proteins. Some will form from hard keratin and will ultimately become hair, horns, and nails of animals. Soft keratins are also possible parts of intermediate filaments. Vimentin is found in fibroblasts, white blood cells, and smooth muscle cells. Desmin is mainly seen in muscle cells. Other cell types will have their own unique intermediate filaments. There are neurofilament proteins seen in neurons of all types. Lamins are seen in the nucleus. All intermediate filaments form an alpha-helical rod. These are the filaments that attach to desmosomes and hemidesmosomes.
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