Question

Imagine protein X is a protein that is secreted out of the cell. How does the cell "know" that protein X needs to be synthesized at the endoplasmic reticulum (ER)?

A.The DNA that codes for protein X has a sequence of nucleotides that bind to a special protein/RNA complex

B. The ribosome that translates protein X has a sequence of nucleotides that bind to a special protein/RNA complex

C. Protein X has a sequence of amino acids that bind to a special protein/RNA complex

D. The mRNA that codes for protein X has a sequence of nucleotides that bind to a special protein/RNA complex

Answer 1

The correct option for this answer is option C.

Each protein has a special amino acid sequence called signal peptide. This signal peptide decides the location of protein to be transported. As soon as the protein emerges out of the ribosome, the signal peptide is recognized by a protein complex called Signal-recognition particle (SRP), which takes the ribosome to the ER. After that protein slides into the ER lumen where signal peptidase is removed by proteolytic cleavage and translation continues at the ribosome.

Option A is incorrect as DNA is first transcribed into mRNA which then translated into protein having a particular sequence of amino acids, not nucleotides which bind to special protein complex.

option B is incorrect as ribosomes translated into proteins it has a sequence of amino acids not of nucleotides as proteins are made up of amino acids, not nucleotides.

Option D is also wrong as the same case is applied in it like as option B. A protein is made up of amino acids, not nucleotides.