Which of the following is involved in controlling gene expression? a. the occurrence of transcription b. activity of the polypeptide product c. life expectancy of the mRNA molecule in the cell d. All of these are involved.

Gene expression is the process by which genetic information stored in genes is used to produce functional products, such as proteins, necessary for the proper functioning of cells and organisms. This process is regulated at several levels, including transcription, translation, and post-translational events.

Now let's evaluate each option: a. the occurrence of transcription: Transcription is the first step of gene expression, where the genetic information in DNA is copied into mRNA. The occurrence of transcription determines which genes are expressed and at what levels, making it an essential factor in controlling gene expression. b. activity of the polypeptide product: After translation, proteins can undergo various modifications and regulatory processes that can impact their activity, stability, and localization in the cell. These modifications and processes can influence gene expression by altering the activity of the protein. c. life expectancy of the mRNA molecule in the cell: The stability and decay of mRNA molecules influence the levels of gene expression. An mRNA molecule with a longer life has a higher probability of being translated into protein, leading to a higher level of gene expression.

As we can see, all three options play a role in controlling gene expression, so the correct answer is: d. All of these are involved.