Question

which of the following is true of kinetochores?
they are sites at which microtubules attach to chromosomes.
they attach to the ring of actin along the cytoplasmic surface of the plasma membrane, causing the actin to contract to form the cleavage furrow.
they are the primary centromere structures that maintain the attachment of the sister chromatids prior to mitosis.
they are located at the center of the centrosome; their function is to organize tubulin into elongated bundles called spindle fibers.
they interdigitate at the cells equator and then move apart, causing the cell to elongate.

Explanation:

Kinetochores are protein - DNA complexes. During cell division (mitosis or meiosis), microtubules of the spindle fiber attach to the kinetochores. This attachment is crucial for the proper segregation of chromosomes.

• The second option describes the role of the contractile ring (actin - myosin) in cytokinesis (formation of cleavage furrow in animal cells), not kinetochores.
• The third option: The centromere is the region of the chromosome, and cohesin proteins maintain the attachment of sister chromatids prior to mitosis, not kinetochores.
• The fourth option: The centrosome (with centrioles in animal cells) organizes tubulin into spindle fibers. Kinetochores are on chromosomes, not at the center of the centrosome.
• The fifth option: Microtubules (not kinetochores) interdigitate at the cell's equator (metaphase plate) and then move apart (anaphase) causing cell elongation.

Answer:

They are sites at which microtubules attach to chromosomes.