They must occur in the DNA, which is in each cells nucleus. More specifically the nucleolus of the cell. A mutation occurs in an allele, where a base triplet (or a codon) gets transcribed incorrectly.
The answer is signal transduction. It is also known as cell signaling. It is the transmission of molecular signals beginning from a cell's exterior to its interior. Signals got by cells must be transferred efficiently into the cell to make certain a suitable response. This step is started by the cell-surface receptors.
Answer: This is an example of Homeostasis!! :D
Explanation:
Trees, plants, oxygen tank