Prokaryotic Ribosomes: Study with Video Lessons, Practice Problems & Examples

Prokaryotic ribosomes, essential for protein synthesis, consist of a large (50S) and a small (30S) subunit, forming a complete 70S ribosome. The sedimentation coefficient, measured in Svedberg units, indicates the rate of sedimentation in centrifugation. Prokaryotic ribosomes contain ribosomal RNA (rRNA) components: 23S and 5S in the large subunit, and 16S in the small subunit. Notably, archaeal ribosomes differ from bacterial ones in rRNA sequence and protein composition, making them resistant to certain antibiotics targeting bacterial ribosomes.

Prokaryotic Ribosomes

In this video, we're going to begin our lesson on prokaryotic ribosomes. A ribosome is a cell component found inside all cells, regardless of their type. These ribosomes consist of a large and a small ribosomal subunit. The large and small ribosomal subunits will associate with each other to form the complete ribosome. This complete ribosome will be able to perform protein synthesis or build proteins. Ribosomes are also referred to as the site of protein synthesis. Although all cells have ribosomes, they are not always identical in different cells. Prokaryotic ribosomes will differ from eukaryotic ribosomes. We'll be able to talk more about that as we move forward in our course. The large and small ribosomal subunits that come together to make up the full ribosome consist of multiple proteins, along with ribosomal RNA, or rRNA for short. The ribosome itself is really just a big mixture of proteins and ribosomal RNA. We will talk more about this idea as we move forward. In this set of videos, we're mainly going to be focusing on prokaryotic ribosomes. We'll discuss the prokaryotic ribosome structure in our next video. So, I'll see you all there.

Structure of Prokaryotic Ribosomes

In this video, we're going to talk more about the structure of prokaryotic ribosomes. In order to better understand the structure of prokaryotic ribosomes, it's first helpful to address what the sedimentation coefficient is. This sedimentation coefficient is measured in units called Svedberg units, named after the scientist who helped discover them. Svedberg units are abbreviated with the capital letter s. What you're going to see is that the ribosomal subunits and the ribosomes themselves are basically named using the Svedberg units; thus, you'll see this 's' throughout this lesson. What is this sedimentation coefficient, anyway? It's a value that characterizes the rate of sedimentation and the particles' behavior in an instrument known as a centrifuge, which spins really fast and is commonly used to help separate materials using centrifugal forces. What you should know is that the greater the s value, the faster the molecule will centrifuge. By labeling these ribosomal components with the s value, you can get a sense of the size of the subunit.

It's important to note that prokaryotic organisms have 70S ribosomes. The "70S" refers to the sedimentation coefficient for that ribosome. These 70S ribosomes have two ribosomal subunits: a large 50S ribosomal subunit and a small 30S ribosomal subunit. The large 50S ribosomal subunit and the small 30S ribosomal subunit come together to create the 70S ribosome, and this is only the case in prokaryotic organisms. In eukaryotic organisms, the ribosomes are different. We'll talk more about the eukaryotic ribosomes later in a different video in our course. For now, one thing to note is that 50 + 30 does not equal 70. Students often have the tendency to try to simply add the Svedberg units, but that is not how they work. You cannot just simply add them. Instead, the 50S large ribosomal subunit, when it complexes with the small 30S ribosomal subunit, forms the 70S ribosome, as we'll be able to see down below in our image.

The large 50S ribosomal subunit contains two ribosomal RNAs, a 23S ribosomal RNA, and a 5S ribosomal RNA. The 30S small ribosomal subunit has a 16S ribosomal RNA. We'll be able to see this below in our image, focusing on the prokaryotic ribosome structure. Notice that the 70S ribosome consists of a large subunit and a small subunit. The large subunit contains two ribosomal RNAs highlighted in yellow: the larger one being the 23S ribosomal RNA, and the smaller one being the 5S ribosomal RNA. In the small ribosomal subunit, there is also a ribosomal RNA highlighted in yellow, the 16S ribosomal RNA. Note that there are three compartments in the middle of the ribosome. These three compartments are the active site of the ribosome.

One thing to also note is that archaea, although they are prokaryotes along with bacteria, the ribosomes are not identical. Archaeal and bacterial ribosomes are the same in size, so they're both 70S ribosomes. However, they differ in two important ways. The first way is that the sequences of the ribosomal RNAs are different. The 23S rRNA, the 5S rRNA, and the 16S rRNA all have different sequences in archaea compared to bacteria. The second way archaeal ribosomes differ from bacterial ribosomes is that archaeal ribosomes have a greater number of proteins in each subunit, reflecting their differences. Because archaeal ribosomes are not identical to bacterial ones, this is why archaea are unaffected by antibiotics that target protein synthesis in bacteria. These ribosomes are crucial for protein synthesis, and antibiotics that specifically target bacterial ribosomes may not work on those in archaea due to these differences. This concludes our lesson on the structure of prokaryotic ribosomes, and we'll be able to apply these concepts as we move forward in our course. I'll see you all in our next video.

Which of the following statements about ribosomes is FALSE?

• A. Ribosomes synthesize new lipids.
• B. Ribosomes synthesize new proteins.
• C. Ribosomes are made of protein.
• D. Ribosomes are made of RNA.

Prokaryotic 70S ribosomes are composed of which of the following subunits?

• A. 40S and 40S
• B. 40S and 50S
• C. 30S and 50S
• D. 30S and 40S
• E. 50S and 50S

Svedbergs are a measurement of the size and sedimentation rate of cellular structures such as ribosomes.

• A. True
• B. False

How are prokaryotic and archaeal ribosomes different from one another?

• A. Large and small subunits are different in size.
• B. The sequences of rRNAs are different.
• C. Archaeal ribosomes have less proteins per subunit than prokaryotes.
• D. Archaeal ribosomes have more proteins per subunit than prokaryotes.
• E. A and C
• F. B and D
• G. B and C

Here’s what students ask on this topic:

What are the main differences between prokaryotic and eukaryotic ribosomes?

Prokaryotic ribosomes are 70S, composed of a 50S large subunit and a 30S small subunit. Eukaryotic ribosomes are 80S, consisting of a 60S large subunit and a 40S small subunit. The Svedberg unit (S) measures the rate of sedimentation during centrifugation, not a direct sum of the subunits. Additionally, prokaryotic ribosomes contain different rRNA components: 23S and 5S in the large subunit, and 16S in the small subunit. Eukaryotic ribosomes have 28S, 5.8S, and 5S rRNA in the large subunit, and 18S rRNA in the small subunit. These structural differences are crucial for the function and antibiotic targeting of ribosomes.

What is the sedimentation coefficient and how is it measured?

The sedimentation coefficient, measured in Svedberg units (S), characterizes the rate at which a particle sediments in a centrifuge. It reflects the particle's size, shape, and density. The greater the S value, the faster the molecule sediments. For example, prokaryotic ribosomes are 70S, with a 50S large subunit and a 30S small subunit. The Svedberg unit is named after Theodor Svedberg, who developed the ultracentrifuge. This coefficient is crucial for distinguishing between different ribosomal subunits and understanding their behavior during centrifugation.

Why can't the Svedberg units of ribosomal subunits be simply added together?

The Svedberg units (S) of ribosomal subunits cannot be simply added because they are not linear measures of mass or size. Instead, they reflect the rate of sedimentation, which depends on the shape, density, and interaction of the particles. For example, the prokaryotic ribosome is 70S, composed of a 50S large subunit and a 30S small subunit. Adding 50S and 30S directly would suggest 80S, but the actual sedimentation rate of the combined ribosome is 70S due to the complex interactions between the subunits.

How do archaeal ribosomes differ from bacterial ribosomes?

Archaeal ribosomes, like bacterial ribosomes, are 70S, but they differ in two key ways. First, the sequences of their ribosomal RNAs (rRNAs) are different. Archaeal ribosomes have unique sequences for the 23S, 5S, and 16S rRNAs compared to bacterial ribosomes. Second, archaeal ribosomes contain more proteins in each subunit than bacterial ribosomes. These differences make archaeal ribosomes resistant to certain antibiotics that target bacterial ribosomes, highlighting the evolutionary divergence between these two prokaryotic domains.

What are the components of the large and small subunits in prokaryotic ribosomes?

The large subunit (50S) of prokaryotic ribosomes contains two ribosomal RNAs: 23S rRNA and 5S rRNA. The small subunit (30S) contains one ribosomal RNA: 16S rRNA. These rRNAs, along with multiple proteins, form the complete 70S ribosome. The 50S and 30S subunits come together to facilitate protein synthesis, with the rRNAs playing crucial roles in the ribosome's structural integrity and function during translation.