Ribosome Structure & Function – MCAT Biology | MedSchoolCoach

Ribosome Structure & Function

MCAT Biology - Chapter 2 - Section 1.5 - Gene Expression - The Central Dogma
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Sample MCAT Question - Ribosome Structure & Function

Which prokaryotic ribosomal subunit is correctly associated with its sedimentation coefficient (s)?

a) Large subunit: 60s

b) Combined subunits: 80s

c) Small subunit: 40s

d) Large subunit: 50s

D is correct. 50s is the sedimentation coefficient of the large subunit of prokaryotic ribosomes. Ribosomes are molecules that translate mRNA into proteins. They are composed of two subunits – a large subunit and a small subunit. These subunits are associated with numbers that measure the sedimentation rate of particles based on specific parameters such as mass, density, and shape. This number is the sedimentation coefficient denoted by the symbol, s. Prokaryotic and eukaryotic small, large, and combined (both large and small) subunits each have a unique sedimentation coefficient. Answer choice A describes eukaryotic large subunits. Answer choice B describes eukaryotic combined subunits. Answer choice C is the eukaryotic small subunit.

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Ribosome Structure and Function

Before you can 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.

Large and Small Ribosome Subunits - MCAT Biology
Figure 1. Large and Small Ribosome Subunits

Prokaryotic vs Eukaryotic Ribosomal Subunits

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).

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