Final answer:
Before mitosis, DNA replication must occur during the cell cycle's S phase to ensure that each daughter cell receives an identical copy of DNA. This replication results in each chromosome having sister chromatids, which are essential for the maintenance of genetic consistency following cell division.
Step-by-step explanation:
Precursor to Mitosis: DNA Replication
Before a somatic cell can undergo mitosis, a critical process must take place to ensure that each resulting daughter cell receives an identical copy of DNA. This essential process is DNA replication, which occurs during the synthesis phase, or S phase, of the cell cycle. During DNA replication, each chromosome becomes composed of two identical copies, known as sister chromatids, which are later separated into daughter cells during mitosis. This mechanism guarantees that the resulting cells are genetically identical to the parent cell.
Homologous recombination and crossing over are processes associated with meiosis, not mitosis. These events contribute to genetic diversity in sexually reproducing organisms but do not participate in the preparatory steps for mitosis.
Thus, the correct answer to the question - Prior to mitosis, what must occur? - is c) DNA replication. None of the other options listed (synapsis, crossing over, homologous recombination) are involved in the lead-up to mitosis.