Each DNA Molecule That Forms A Chromosome Must Contain A Centromere Sequence

In order for a DNA molecule to form a functional chromosome, it must be able to replicate, segregate its two copies at mitosis, and maintain itself between cell generations. In order to replicate, each DNA molecule must have at least one specific nucleotide sequence that acts as an origin of replication. This is the site where the enzymes involved in replication bind and initiate replication. Another specific nucleotide sequence, called a centromere, attaches any DNA molecule that contains it to the mitotic sequence during M phase. The centromeres are required for chromosome segregation.

At the start of M phase, each chromosome consists of two sister chromatids joined at their centromeres. Specific proteins bind to the centromere sequences; these are thought to initiate the formation of a multiprotein complex , called the kinetochore, which binds to one or more microtubules (the kinetochore microtubules). The kinetochore is firmly attached to the microtubules; it is from this point of attachment that the chromosomes are pulled through the cytoplasm.

Two alternative models of how the kinetochore might be able to generate a poleward force on its chromosome during anaphase. (A) Microtubule-walking proteins that resemble dynein or kinesin are part of the kinetochore, and they use the energy of ATP hydrolysis to pull the chromosome along its bound microtubules. (B) Chromosome movement is driven by microtubule disassembly: as tubulin subunits dissociate, the kinetochore tends to slide poleward in order to restore its binding to the walls of the microtubule. Similar mechanisms may be used at the spindle pole, which likewise seems to be able to hold onto microtubules while permitting their controlled depolymerization.

Download Printable (PDF) Version