Final answer:
Proteins destined for the nucleus, mitochondria, chloroplasts, or remaining in the cytosol are all initially synthesized in the cytoplasm. They contain specific oligopeptide signals that direct them to their final destinations. Mitochondrial and chloroplast proteins encoded by nuclear genes are imported post-translationally, while proteins for the ER and secretion are translocated co-translationally.
Step-by-step explanation:
The translation of proteins occurs in the cytoplasm of the cell where ribosomes are responsible for synthesizing proteins. However, proteins that are destined for the nucleus, chloroplasts, and mitochondria, or those that remain in the cytosol, have different pathways and signals that guide their final destination after translation. Proteins that will be located in the nucleus, mitochondria, or chloroplasts are synthesized in the cytoplasm and contain oligopeptide traffic signals which direct them to their appropriate destinations. Some proteins synthesized on free ribosomes in the cytoplasm do not move through the Golgi apparatus; instead, they may go directly to their final destination within the cell.
Mitochondria and chloroplasts contain their own genomes and translate some of their own proteins independently, but many proteins found in these organelles are encoded by nuclear genes and are imported post-translationally. They typically expose an N-terminal signal peptide that is recognized by receptor proteins on the organelle's membrane, facilitating their import into the organelle. In contrast, proteins for the endoplasmic reticulum (ER) and those that will be secreted or embedded in the plasma membrane begin their synthesis on ribosomes bound to the ER (co-translational translocation) and are later sorted within the endomembrane system.