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.