Question

1. Discuss the process of protein synthesis.

2. Discuss in detail a particular system that is responsible for the export of proteins

Answer 1

The process of protein synthesis is also known as translation. It takes place in the cytoplasm of the cell at ribosomes. The information(in coded form)for the protein synthesis is stored in genetic material that is DNA. DNA is located in the nucleus of the cell and on the other hand the ribosomes, on which the translation occurs, are located in the cytoplasm. To overcome this situation DNA transferred the coded information to the mRNA(formed from preRNA). This process is known as transcription. mRNA carries coded information and goes to the ribosome. mRNA and ribosome complex forms and ribosome serve as a tool to break the coded information carried by mRNA.The released information goes to tRNA and tRNA picks the amino acid accordingly and bring them to the ribosomes. The ribosome has two sites A site(acyl site) and P site(peptide site). A site provides an attachment point for the new amino acid in the sequence and P site provide an attachment point for the elongated peptide chain.

The process of protein synthesis mainly has the following stages: Initiation, Elongation and Termination of the polypeptide chain.

The key components for protein synthesis are mRNA, Ribosome, tRNA and enzyme aminoacyl tRNA synthetase.

mRNA is a single-stranded molecule and has bases A, U, C, G. It has an initiator(start) codon at 5' end and has a terminator(stop)codon at the 3' end. During the course of protein synthesis, the four bases of mRNA are observed as three bases codon. Each codon base encoded with information for a specific amino acid.

On the other hand, tRNA has an anticodon complementary to the base codons of mRNA.

During the course of initiation, the small subunit of ribosomes binds with the 5' end of mRNA. The invitation of protein synthesis begins with the recognition of base codon (start codon)of mRNA at 5' end. mRNA has a start codon named AUG(in case of methionine amino acid in eukaryotes). This codon lies near to the P site of the ribosome. The tRNA has a complementary anticodon CAU. The complementary anticodon binds to the start codon present at mRNA by hydrogen bonds. There are some factors named IF 1, IF 2, IF 3 (initiator factors) and GTP promotes the initiation of the peptide chain. At this point, the large subunit of ribosome joins the small subunit. During this course, GTP is hydrolyzed to GDP. At this point, the ribosome has methionine attached with tRNA at the P site. This is the initiation process started with the binding of the start codon(at mRNA)and anticodon(at tRNA).

Now elongation of polypeptide chain takes place. There are three factors named EF-Tu, EF-Ts, EF-G facilitate the elongation process. As the coded information is released the charged tRNA picks new amino acid accordingly and enters the ribosome at A site. The anticodon of tRNA locates and binds to the complementary codon of mRNA. Now the methionine present at P site migrates on to the tRNA and newly arrived amino acid complex resulting in the formation of a peptide bond between methionine and newly arrived amino acid. Now methionine and newly arrived amino acid migrate to the P site. Now P site has a tRNA carries newly arrived amino acid and methionine. The elongation continues until the stop codon is not achieved.

At the 3' end of the mRNA chain, there is a codon present named stop codon. This codon is not located and joined by anticodon present at tRNA- amino acid complex. At this point termination and release of polypeptide chain take place.