Final answer:
A chromosome consists of a single chromatid during the G1 phase of the cell cycle, before DNA replication occurs.
Step-by-step explanation:
The stage of the cell cycle where a chromosome would consist of only a single chromatid is G1 phase. This is before DNA replication occurs in the S phase. During the G1 phase, chromosomes are unduplicated and each chromosome is in the form of a single chromatid. In contrast, after the S phase and throughout the G2 phase and the stages of mitosis, the chromosomes consist of two sister chromatids joined at the centromere until they separate during anaphase.