Interphase is the portion of the cell life cycle where the cell is preparing for division. This portion of the life cycle includes growth, as well as reproducing organelles and proteins. The genome is also replicated in interphase. Interphase is composed of the G1, S, and G2 phases. G1 and G2 correspond to periods of growth, protein synthesis, and organelle replication. The S phase is the period of DNA replication.

The M phase is mitosis, and is the portion of the cell cycle during which the cell is undergoing division. As a result, it is not part of interphase.

When the cell is not preparing to divide any time soon, it will spend its time in the G₀ phase. This phase is distinct from the actual cell cycle, and can prevent the cell from actively growing and preparing for division. Cells arrested in the G₀ phase are considered quiescent.

Interphase includes S phase plus the two gap phases . The M phase is the remainder of the cell cycle, made up of mitosis (and meiosis in germ cells) and cytokinesis. Cytokinesis (physical cell division) occurs at the end of M phase, but before interphase.

Cells progress through the cell cycle by controlled expression and degradation of cyclin proteins. Cyclin-dependent kinases (CDKs) are always present in the cell and are not degraded after progression to a new stage of the cell cycle. Cyclins bind their respective CDKs to activate them. This activation causes a chain of events that allow the cell to progress to the next phase of the cell cycle. Afterwards, cyclins are ubiquinated and degraded until they are needed again.

During anaphase, sister chromatids are pulled apart towards opposite poles. The cell becomes longer and the cleavage furrow (contractile ring) forms. This marks the beginning of cytokinesis. The process completes during telophase, producing two new daughter cells. Cytokinesis must be preceeded by karyokinesis (physical movement of the chromosomes).

If the cell cannot progress past metaphase, the cell is most likely having trouble separating the sister chromatids. This process is mediated by attaching the kinetochore microtubules to the kinetochore on the sister chromatids. Our most likely explanation for a problem proceeding past metaphase is that the mutation is affecting the formation of microtubules. The progression from metaphase to anaphase is regulated by the metaphase checkpoint in the cell cycle, which is used to ensure proper attachment of spindle fibers to the centromeres. Problems with spindle fiber formation and binding would cause the cell to be arrested in metaphase.

Actin and myosin are not directly involved in this portion of mitosis, but are very important during cytokinesis. Chromosome condensation has already occurred (during prophase), so our mutation cannot be affecting those proteins.

Cellular division usually takes place in four steps before undergoing cytokinesis. In prophase, the chromosomes condense and become visible, and the spindle apparatus begins to form. In metaphase, the chromosomes line up on the equator of the cell. In anaphase, the chromatids are pulled apart and separated to opposite sides of the cell. Finally, telophase involves the nuclear membrane reforming around the chromosomes, which begin to decondense.

Once the cell has split and transported its sister chromatids to opposite ends of the cell, the nuclear envelopes can begin to regenerate around the genetic material at each pole. This event occurs during the end of mitosis, commonly known as telophase.

Combrestatin interferes with the formation of microtubules, which make up the cytoskeletal architecture of a cell; therefore, the correct answer choice is involved with some microtubule-based process. DNA and protein synthesis do not involve microtubules, and would not be affected by the lack thereof. Muscle contraction depends on myosin, actin, troponin, etc., and not on microtubules. Membrane depolarization involves sodium/potassium channels, neurotransmitters, etc., and is not directly affected by microtubule inhibition.

The only answer that remains is mitosis, which involves microtubules in chromosomal segregation. The mitotic spindle in this separation is primarily composed of microtubules. The polymerization and depolymerization of microtubules is crucial for mitotic division. Combrestatin therefore prevents proper mitosis.

In the process of meiosis, crossing over (the exchange of genetic material between homologous chromosomes resulting in recombination) only occurs during prophase I, when pairs of homologous chromosomes recombine during synapsis. This contributes to the increase in diversity due to sexual reproduction.