Ribosomal Structure - Online Tutor, Practice Problems & Exam Prep

The ribosome, an RNA-protein complex, translates mRNA into proteins. It consists of a large and small subunit, primarily made of ribosomal RNA (rRNA) with some protein for structural support. Translation occurs at three key sites: the aminoacyl (A) site, where charged tRNA pairs with mRNA codons; the peptidyl (P) site, where amino acids are added to the growing polypeptide chain; and the exit (E) site, where uncharged tRNA is released. The ribosome also features a decoding center for accurate tRNA-codon matching and a peptidyltransferase center for peptide bond formation.

Ribosome Structure

So the ribosome is composed of RNA and protein. It's an RNA-protein complex that's responsible for translating the mRNA into protein. Its structure consists of a large and a small subunit. They are initially separate in the cytoplasm, but upon translation, they come together and form the ribosome complex. It is primarily composed of RNA with just a small amount of protein. The RNA is the component responsible for catalyzing most of the reaction, while the protein mainly provides structural support. The ribosome is assembled in a special compartment called the nucleolus, which is inside the nucleus.

The ribosome contains various positions where translation occurs. The first position is called the A site (aminoacyl site), which means that the charged tRNA (tRNA with the amino acid attached) comes in, and the anticodon matches with the codon in the mRNA. Then you have the P site (peptidyl site), where the amino acid on the tRNA is added to the growing peptide chain. After donating its amino acid, it moves to the E site (exit site), where the deacylated tRNA is released to be recharged and repeat the process. These are the three main sites you need to know. There are other sites, such as the decoding center, which ensures that the proper tRNA is matched to the proper codon, primarily ensuring that the A site has been paired properly. Another essential site is the peptidyltransferase center, where the peptide bond is catalyzed between the new amino acid and the chain already formed.

Looking at the ribosome structure, the small subunit is depicted in dark green and the large subunit in light green. They are initially separate but come together for translation. You can see the A site, the P site, and the area for the E site, although it is not clearly drawn. Here, the tRNA, represented by these distinctive structures, enters and binds to the codon. Once bound at the A site, the P site facilitates the transfer of the amino acid to the growing polypeptide chain. Once released, it can start the cycle again with recharging. The visual illustrated here also highlights the protein forming at the ribosome. This overview provides a basic understanding of the ribosome and the different sites and how translation occurs, but we will explore more details in future videos.

With that, let's now move on.

Which of the following is true regarding the ribosome structure?

• A. Ribosomes are made up of only proteins
• B. Ribosomes are made up of only RNA
• C. Ribosomes are made up of both protein and RNA
• D. Ribosomes are made up of neither protein nor RNA

Which of the following is not a ribosomal position used to add amino acids to a growing polypeptide chain?

• A. A site
• B. C site
• C. P site
• D. E site

Which of the following ribosomal sites is responsible for adding the amino acid to a growing polypeptide chain?

• A. A site
• B. C site
• C. P site
• D. E site

Ribosomal assembly occurs where in the cell?

• A. Golgi
• B. Cytoplasm
• C. Endoplasmic Reticulum
• D. Nucleolus

Here’s what students ask on this topic:

What are the main components of a ribosome?

A ribosome is primarily composed of ribosomal RNA (rRNA) and proteins. It consists of two subunits: a large subunit and a small subunit. The rRNA makes up the majority of the ribosome's mass and is crucial for its catalytic activity, while the proteins provide structural support. The ribosome is assembled in the nucleolus, a specialized compartment within the nucleus. During translation, the two subunits come together to form a functional ribosome that translates mRNA into proteins.

What are the functions of the A, P, and E sites in the ribosome?

The A (aminoacyl) site, P (peptidyl) site, and E (exit) site are key functional regions of the ribosome during translation. The A site is where charged tRNA molecules pair with mRNA codons. The P site is where the amino acid from the tRNA is added to the growing polypeptide chain. Finally, the E site is where the uncharged tRNA is released from the ribosome after it has donated its amino acid. These sites work in a coordinated manner to ensure the accurate synthesis of proteins.

How does the ribosome ensure accurate translation of mRNA?

The ribosome ensures accurate translation of mRNA through its decoding center and peptidyltransferase center. The decoding center verifies that the correct tRNA anticodon pairs with the mRNA codon at the A site. This ensures that the correct amino acid is added to the growing polypeptide chain. The peptidyltransferase center catalyzes the formation of peptide bonds between amino acids, ensuring the proper assembly of the protein. These mechanisms work together to maintain the fidelity of protein synthesis.

Where is the ribosome assembled in the cell?

The ribosome is assembled in the nucleolus, a specialized compartment within the nucleus of the cell. The nucleolus is responsible for producing and assembling the ribosomal RNA (rRNA) and ribosomal proteins into the large and small subunits of the ribosome. Once assembled, these subunits are transported to the cytoplasm, where they come together during translation to form a functional ribosome that synthesizes proteins.

What role does rRNA play in the ribosome?

Ribosomal RNA (rRNA) plays a crucial role in the ribosome's function. It constitutes the majority of the ribosome's mass and is primarily responsible for its catalytic activity. The rRNA forms the core of the ribosome's structure and is involved in the binding of tRNA and mRNA. It also catalyzes the formation of peptide bonds between amino acids during protein synthesis. The proteins in the ribosome mainly provide structural support, while the rRNA carries out the essential catalytic functions.