12.2: Monitoring cellular processes- mitosis

Once the DNA replication/repair checkpoint has been passed, the cell can divide. The first step of this process (in eukaryotes) is known as mitosis (→). Mitosis involves a molecular machine, the mitotic spindle, based on protein polymers, αβ-tubulin-based microtubules and actin-myosin based contractile ring. There is a molecular checkpoint that monitors the assembly of the mitotic spindle, and a checkpoint that monitors that each replicated chromosome has connected correctly to the mitotic spindle, made of organized microtubules (← see video link). Each replicated chromosome consists of two linear double stranded DNA molecules. The pair of replicated chromosomes interacts with the mitotic spindle through a specific protein structure known as the kinetocore. Kinetocores are assembled in association with specific chromosomal DNA regions known as centromeric sequences. Each replicated chromosome will have its own kinetocore and each interacts independently with the mitotic spindle (this is different from their behavior during meiosis, as we will see). The presence of the mitotic chromosome attachment checkpoint was recognized in experiments in which chromosomes were manipulated so that they could not connect correctly to the mitotic spindle; such a manipulation caused a delay or halt in mitosis until the "mistake" was corrected.^[425] The mitotic checkpoints serve to ensure that each sibling cell gets one and only one copy of each and every chromosome present in the parental cell.^[426] Once these checkpoints are passed, molecular links between replicated chromosomes are severed and the mitotic spindle moves chromosomes to opposites sides (poles) of the parental cell. The parental cell then divides using the contractile ring, a actin/myosin polymer based molecular machine. While the two cells inherit the same set of alleles as were present in the parental cell, they may behave differently due to differences in different environments they find themselves in - factors returned to when we consider developmental processes.

The cell cycle decision check points are composed of multicomponent interaction networks. While we consider check point mechanisms only briefly, they play important roles in developmental processes and many diseases. A typical check point is built around a protein kinase, an enzyme that phosphorylates various targets – such post-translational phosphorylation can lead to changes in protein structure, protein-protein interactions, protein activities and stability, and intracellular localization. Cell cycle checkpoints often involve a particular class of kinases, known as cyclin-dependent kinases (CDKs)(→). The activity of these CDKs can be regulated positively by the binding of a small regulatory protein, known as a cyclin, as well as negatively by other interacting proteins and post-translational modifications. Cyclin’s and other regulators are themselves the target of various forms of regulation, including proteolytic degradation, triggered by their post-translational modification. Typically cyclin-CDK complex formation and activity is inhibited by various factors (proteins). When the conditions involved in the checkpoint are met, inhibition is removed allowing for the cyclin-CDK complex to form and become active. The active kinase phosphorylates and regulates the activity and stability of its targets, allowing the cell to pass through the check point and proceed along the cell cycle. One effect of activating the CDK is often the rapid degradation (removal) of the cyclin; this makes the switch effectively irreversible until such time as cyclin protein levels increase again during the next cell cycle.

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