Select the Ribosome closeup. How does the cell make proteins inside the ribosome?

Answer 1

The ribosome is a complex molecular machine found within all living cells that plays a crucial role in protein synthesis. It is made up of ribosomal RNA (rRNA) and proteins, and it functions as the site where translation occurs, converting messenger RNA (mRNA) into a polypeptide chain (protein).

Here's how proteins are synthesized inside the ribosome:

1. Initiation: The process begins when a ribosome assembles around the start codon of an mRNA strand. The small subunit of the ribosome binds to the mRNA, and the initiator transfer RNA (tRNA) carries the first amino acid (usually methionine) and binds to the start codon through complementary base pairing.

2. Elongation: Once the initiation complex is formed, the large subunit joins, forming a functional ribosome. The ribosome has three sites for tRNA: the A (aminoacyl), P (peptidyl), and E (exit) sites. The ribosome moves along the mRNA, reading its sequence in sets of three nucleotides (codons):

   • The tRNA corresponding to the next codon enters the A site, bringing an amino acid.
   • The amino acid in the P site, which is held by the tRNA, forms a peptide bond with the amino acid in the A site. This reaction is catalyzed by the rRNA within the ribosome, specifically the peptidyl transferase activity of the large subunit.
   • The ribosome then shifts (translocates) along the mRNA, moving the tRNA from the A site to the P site, while the empty tRNA in the E site exits the ribosome.

3. Termination: The process continues until the ribosome encounters a stop codon (UAA, UAG, or UGA) on the mRNA. When this happens:

   • A release factor (protein) binds to the stop codon, which stimulates the ribosome to release the completed polypeptide chain.
   • The ribosomal subunits then disassemble, and the mRNA is released, making it available for degradation or reuse.

This whole process of translation is highly regulated and involves various factors that ensure accuracy and efficiency. The ribosome is an essential part of this machinery, facilitating the assembly of amino acids into proteins based on the genetic code provided by the mRNA.