What does mitosis do that the cell would do wrong if it just split down the middle in cell division?

Mitosis is when a DNA error, aka mutation, occurs in a body cell. When cells divide, it needs a copy of the DNA. During DNA replication, that error can cause the instructions for building proteins to differ. The change can not affect offspring. Meiosis is the same situation, but in sex cells. This can be transferred to offspring.

When a virus infects a host cell, it uses the cell's machinery to make copies of its own genetic material in a process called replication. finally, during the process of self-assembly, that protein shell and the copies of the virus's genetic material are assembled into new viruses.