Here's how ribosomes are made and get to the ER:
1. Ribosome Assembly in the Nucleolus: The rRNA and proteins needed for ribosome formation are synthesized and assembled in the nucleolus.
2. Ribosome Export from the Nucleus: Once assembled, ribosomes are exported from the nucleus through nuclear pores. They travel in two subunits: a large subunit and a small subunit.
3. Ribosome Binding to mRNA: In the cytoplasm, ribosomes bind to messenger RNA (mRNA) molecules. The mRNA carries the genetic code from DNA to the ribosome, providing instructions for protein synthesis.
4. Ribosome Binding to the ER: Some mRNAs contain a signal sequence that directs the ribosome to the ER. This signal sequence binds to a signal recognition particle (SRP), which then escorts the ribosome to the ER membrane.
5. Protein Synthesis at the ER: Once bound to the ER, the ribosome begins protein synthesis. As the protein is synthesized, it is threaded through a channel in the ER membrane, and it may fold into its final shape within the ER lumen.
Therefore, ribosomes are not transported from the nucleus to the ER. They are assembled in the nucleolus and then exported to the cytoplasm, where they can bind to mRNA and potentially be directed to the ER if the mRNA contains a signal sequence.
