Final answer:
The absence of a peak at S phase in a cell count versus DNA content graph is because during DNA replication, the DNA content of a cell increases continuously, resulting in various amounts of DNA being present at different times.
Step-by-step explanation:
The reason there is no peak at S phase on a plot of number of cells versus amount of DNA per cell could be due to the fact that during the S phase, DNA is being replicated, which means that various amounts of DNA are present at different times during this phase. Option 5 - "The DNA is replicating, so various amounts of DNA are present" - most accurately explains the lack of a distinct peak in the S phase.
The S phase, which typically lasts between 8-10 hours, involves the process of DNA replication and the synthesis of new histone and other proteins required for assembling new chromatin. Since the DNA content of a cell increases continuously throughout the S phase, no discrete peak is observed.
The other proposed answers could be misleading. The S phase is bounded by G1 and G2 phases, where cells most significantly vary in the duration of G1. The G1 phase is the span of time between mitosis and the beginning of DNA replication, while the G2 phase is characterized by cell growth and preparation for mitosis. Neither the longer duration of S phase, nor the inability to determine fluorescence level, nor single-stranded DNA during replication are significant factors in the absence of a peak in S phase on the graph.