Protein synthesis in cells

AP BIO U2

Step 1

• Transcription: The cell's DNA is stored in the nucleus, where it is transcribed into messenger RNA (mRNA). This mRNA molecule carries the genetic instructions for building a specific protein. The mRNA then exits the nucleus through nuclear pores. 

Step 2

Free ribosomes: After leaving the nucleus, some mRNA molecules travel to ribosomes floating freely in the cytoplasm. These ribosomes produce proteins that will function within the cytoplasm itself, or within organelles like the nucleus, mitochondria, or peroxisomes. 

Step 3

Ribosome-studded membranes: Other mRNA molecules carry a signal sequence that directs the ribosome to the RER. As the ribosome and mRNA attach to the RER, the newly forming protein chain is threaded directly into the ER's internal compartment (the lumen).

Step 4

• Protein modification and packaging: Transport vesicles containing the newly made proteins bud off from the RER and fuse with the Golgi apparatus. Here, the proteins are further modified, sorted, and packaged into new vesicles for delivery to their final destination. This destination could be other organelles, the cell membrane, or outside the cell. 

Map of locations throughout the cell

nucleus, cytoplasm, ribosomes, rough ER, golgi apparatus, vesicles, and to their final destination (organelles, cell membrane, or outside of the cell)