Outline the processes of protein synthesis in eukaryotic cells highlighting the role of ribosomes, the endoplasmic reticulum and the golgi apparatus

Let's outline the process of protein synthesis in eukaryotic cells.

Protein synthesis involves two main stages: transcription (DNA to mRNA) and translation (mRNA to protein), followed by post-translational modification and targeting.

1. Transcription:

   • In the nucleus, the DNA sequence of a gene is transcribed into a messenger RNA (mRNA) molecule. This mRNA then leaves the nucleus and enters the cytoplasm.

2. Translation (Role of Ribosomes):

   • Ribosomes are the cellular machinery responsible for translation. They read the mRNA sequence in codons (sets of three nucleotides).
   • Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome according to the mRNA codons.
   • The ribosome catalyzes the formation of peptide bonds between amino acids, building a polypeptide chain.
   • Proteins destined for the cytoplasm, mitochondria, or peroxisomes are synthesized on free ribosomes in the cytoplasm.

3. Role of the Endoplasmic Reticulum (ER):

   • Proteins destined for secretion, insertion into membranes (plasma membrane, ER, Golgi, lysosomes), or delivery to lysosomes begin their synthesis on free ribosomes.
   • As the polypeptide chain emerges, a signal peptide sequence directs the ribosome-mRNA complex to the rough endoplasmic reticulum (RER).
   • The ribosome attaches to the RER membrane, and the growing polypeptide chain enters the ER lumen (for secreted proteins) or becomes embedded in the ER membrane (for membrane proteins).
   • Inside the ER, proteins undergo initial folding, disulfide bond formation, and glycosylation (addition of carbohydrate chains). Chaperone proteins assist in proper folding. Misfolded proteins are retained or degraded.

4. Role of the Golgi Apparatus:

   • After processing in the ER, proteins are transported in vesicles to the Golgi apparatus.
   • The Golgi apparatus consists of flattened sacs called cisternae (cis, medial, trans).
   • Here, proteins undergo further modification, such as additional glycosylation, trimming of carbohydrate chains, and proteolytic cleavage.
   • The Golgi sorts and packages proteins into new vesicles based on their final destination.
   • These vesicles then bud off from the trans-Golgi network and transport proteins to their final destinations, such as the plasma membrane (for secretion or insertion), lysosomes, or back to the ER.