In which part of the cell does protein synthesis occur?

Protein synthesis occurs primarily at ribosomes, which are the cellular structures responsible for translating messenger RNA (mRNA) into polypeptide chains, ultimately forming proteins. Ribosomes can be found free-floating in the cytoplasm or attached to the endoplasmic reticulum, creating rough ER.

The process begins when mRNA, transcribed from DNA in the nucleus, exits into the cytoplasm where it encounters ribosomes. The ribosome reads the sequence of nucleotides in the mRNA and, with the help of transfer RNA (tRNA), assembles amino acids in the correct order to produce a functional protein. This interaction at ribosomes is crucial for all cellular functions that depend on proteins, including enzyme activity, structural support, and regulatory functions.

While the nucleus is involved in the first step of protein synthesis, specifically in the creation of mRNA, it does not carry out the synthesis of proteins directly. Mitochondria are primarily known for energy production through cellular respiration, and lysosomes are involved in breaking down waste materials and cellular debris. Thus, ribosomes are the key players in the actual synthesis of proteins, making this the correct answer.