An onion root tip cell contains 16 chromosomes and a 2C amount of...

During the $G_1$ phase, the onion root tip cell has $16$ chromosomes and a $2\text{C}$ amount of DNA.

In the S phase (synthesis phase) of the cell cycle, DNA replication takes place. This causes the amount of DNA in the cell to double, increasing from $2\text{C}$ to $4\text{C}$.

However, the number of chromosomes remains unchanged because the newly synthesized sister chromatids remain attached at the same centromere. The chromosome number only changes when sister chromatids separate during anaphase.

Thus, after the completion of the S phase, the cell will still have $16$ chromosomes but the DNA content will be $4\text{C}$.

Answer: $16$ chromosomes and $4\text{C}$ DNA