Final answer:
The process that decreases the chromosome number by half in cells is meiosis, which is crucial for sexual reproduction in producing four genetically unique haploid gametes.
Step-by-step explanation:
The process that serves to decrease cellular chromosome number by half is meiosis. Meiosis is a type of cell division that produces haploid gametes with half the number of chromosomes compared to the original cell. This is crucial in sexual reproduction, allowing two haploid gametes to join and form a diploid cell with a complete set of chromosomes. Meiosis consists of two rounds of cell division, namely meiosis I and meiosis II, resulting in four genetically unique haploid cells. Unlike meiosis, mitosis results in the production of two diploid cells that are genetically identical to the original cell, and binary fission is a process of asexual reproduction in bacteria, producing two genetically identical cells.