Ribosomes and Translation – MCAT Biology | MedSchoolCoach


MCAT Biology

Sample MCAT Question: Translation

In terms of peptide chain extension during the process of translation, which of the following is true?

a) The A site is where empty tRNA exits from

b) The P site is where new tRNA with amino acids bind

c) The E site is closer to the 3’ end of mRNA

d) The P site contains the growing peptide chain

D is correct. The P site contains the growing peptide chain. The process of peptide chain extension during translation involves several components, including an A site, a P site, and an E site. The A site is where the new tRNA molecule with an amino acid binds, and the P site is where the tRNA with a growing peptide chain is bound. During the process of peptide chain extension, the existing peptide chain at the P site attaches to the amino acid at the A site. When this happens, the ribosome shifts and moves one position forward on the mRNA, moving the tRNA with the peptide chain back to the P site and the empty tRNA to the E site. The empty tRNA leaves from the E site, and another tRNA attaches to the newly empty A site. The process continues until translation is terminated. The E site is closer to the 5′ end of mRNA, as the mRNA strand is read by the ribosome from the 5′ direction to the 3′ direction.

Ribosome Structure

In order to understand the process of translation, it is first essential to understand the structure of ribosomes. Ribosomes are molecular complexes made up of both ribosomal RNA and ribosomal proteins. These RNA-protein complexes are responsible for translating mRNA into proteins. Within the cell, ribosomes can be found either in the cytosol as free ribosomes or bound to the rough endoplasmic reticulum, as membrane-bound ribosomes. The type of ribosome used to translate a protein depends on the type of protein that needs to be translated.

For example, cytosolic proteins are translated by free ribosomes, whereas proteins found in the membrane of the cell are translated by membrane-bound ribosomes on the rough ER. Figure 1 shows a ribosome. Notice how it is made up of two subunits. There is a small subunit, as well as a large subunit. As the mRNA is translated, it will be sandwiched between the subunits. For the MCAT exam, it is important to know the difference between prokaryotic and eukaryotic ribosomal subunits.

Translation - Ribosome
Figure 1. Ribosome Structure

Prokaryotic vs. Eukaryotic Ribosomes

Both prokaryotic and eukaryotic ribosomes have one large and one small subunit. The unit of each subunit is denoted by S, which measures the sedimentation rate of each subunit derived from centrifugation. For the MCAT exam, it is not important to understand the derivation process of the S-values. However, it is important to know which S-values correspond to which subunits on prokaryotic and eukaryotic ribosomes.


For prokaryotes, the small subunit is 30S, and the large subunit is 50S. The combined ribosome is 70S. Contrastingly, in eukaryotes, the small subunit is 40S, and the large subunit is 60S. The combined ribosome is 80S. It is interesting to note that several antibiotics work by targeting the various ribosomal subunits in prokaryotes and disrupting translation. The antibiotics will not harm eukaryotic ribosomal subunits because the structures are so different.


A useful tool to help remember the size of prokaryotic and eukaryotic ribosomes and their subunits is that Eukaryotic sizes are all Even numbers (40, 60, and 80S).

Translation - Initiation

The first step of translation is initiation. The process of initiation is defined by the ribosome assembling around the target mRNA, and the tRNA attaching to the start codon. This process is different for both prokaryotes and eukaryotes. In prokaryotes, ribosomes bind to what is called the Shine-Dalgarno sequence. The Shine-Dalgarno sequence is a specific sequence of nucleotides before the start codon. In essence, this sequence recruits the ribosomes to the mRNA molecule. Also, in prokaryotes, unlike in eukaryotes, both transcription and translation coincide, meaning that before the RNA polymerase is finished producing the mRNA, translation has already begun (Figure 2).

Transcription and Translation in Prokaryoties - MCAT Biology
Figure 2. Transcription and Translation in Prokaryoties

In eukaryotes, ribosomes are recruited by the 5’ cap on mRNA. Recall that the 5’ cap is one of the post-transcriptional modifications that convert pre-mRNA into the mature form of mRNA. Furthermore, in eukaryotes, translation starts at what is called the Kozak sequence. The Kozak sequence is a nucleic acid sequence that functions as the translation initiation site in eukaryotic mRNA and contains the start codon. For the MCAT exam, it is important to be able to distinguish the Shine-Dalgarno sequence in prokaryotes from the Kozak sequence in eukaryotes. Also, in eukaryotes, transcription occurs in the nucleus, while translation takes place in the cytosol or on the membrane of the rough endoplasmic reticulum (Figure 3).

Transcription and Translation in Eukaryotes - MCAT Biology
Figure 3. Transcription and Translation in Eukaryotes

Translation - Elongation/Extension

In eukaryotes, the start codon always codes for methionine with the nucleotide sequence AUG. When AUG is in the Kozak sequence, it will recruit a tRNA molecule with the amino acid methionine. After this occurs, the second step in translation, the extension or elongation of the peptide chain, is ready to take place.


Figure 4 shows three different sites on the ribosome that are essential for understanding the process of peptide chain elongation. There is the A site, the P site, and the E site. The A site is the site where the new tRNA with an amino acid binds. The P site is where the tRNA with the growing peptide chain binds. Lastly, the E site is where the empty tRNA molecule (without an amino acid) exits.

Translation - MCAT Biology
Figure 4. Translation Extension/Elongation

At the start of translation, the tRNA with the anticodon to AUG will bind to AUG. This tRNA is bound to methionine, and the site it occupies is the P site. A second tRNA with a different amino acid will bind to the next codon on the mRNA molecule at the A site. In step 2 of Figure 4, the peptide chain in the P site will attach to the amino acid in the A site. Once this attachment occurs, the ribosome will shift and move one position (3 bases, or 1 codon) forward along the mRNA transcript. Now, as step 3 shows, the peptide chain with two amino acids is occupying the P site, while the A site becomes empty. In step 4, the now-empty tRNA that got shifted down the mRNA will leave. In step 5, a new tRNA will bind to the A site, and the process will repeat itself.

Translation - Termination

At some point during translation, the ribosome will encounter one of three stop codons. It is important to note that the stop codons do not code for a tRNA. In other words, there is no tRNA with an anticodon that matches any of the three stop codons. Instead, a release factor protein binds at the stop codon (Figure 5). When this happens, the ribosome/mRNA complex will become disrupted and break apart. This action will release the polypeptide chain and signal the end of translation.

Termination of Translation - MCAT Biology
Figure 5. Termination of Translation
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