What is the Difference Between 16s rRNA and 18s rRNA

The main difference between 16S rRNA and 18S rRNA is that 16S rRNA occurs in the small subunit of the prokaryotic ribosome, whereas 18S rRNA occurs in the small subunit of the eukaryotic ribosome.

16S rRNA and 18S rRNA are two types of rRNA that occur in the small subunit of the ribosomes. The small subunit of ribosomes has a key role in decoding genetic information and initiating protein synthesis. 

Key Areas Covered

1. What is 16s rRNA
– Facts, Features, Behaviour
2. What is 18s rRNA
– Facts, Features, Behaviour
3. Similarities Between 16s rRNA and 18s rRNA
– Outline of Common Features
4. Difference Between 16s rRNA and 18s rRNA
– Comparison of Key Differences

Key Terms  

16S rRNA, 18S rRNA

What is 16S rRNA

16S rRNA is a type of rRNA responsible for making up the small subunit of the prokaryotic ribosome. Therefore, it is the RNA component of the 30S ribosome. 70S ribosomes is another name for prokaryotic ribosomes, which also occur in plastids of eukaryotic cells. Also, the size of the 16S rRNA is 1542 nt. Generally, it has a structural role similar to the rRNA in the large subunit to scaffold ribosomal proteins in defined positions. Moreover, it facilitates the binding of the small subunit to the large subunit by interacting with the 23S rRNA in the large subunit.

Figure 1: 16S rRNA

Furthermore, the three-dimensional structure of the 16S rRNA consists of four domains. Also, the 3′ end of the 16S rRNA contains the anti-Shine-Dalgarno sequence, which can bind to the Shine-Dalgarno sequence of the mRNA to be translated by the ribosome. Generally, this sequence occurs on the mRNA around the eight bases upstream of the start codon, AUG. On the other hand, 16S rRNA stabilizes the pairing of codon and anticodon in the A-site of the ribosome.

What is 18S rRNA

18S rRNA is a part of the small subunit of eukaryotic ribosome. It is a structural RNA homologous to the 16S ribosomal RNA in prokaryotes. 18S rRNA is also homologous to the 12S rRNA in mitochondria. 40S is the small subunit of the eukaryotic ribosome. Eukaryotic ribosomes occur in the cytosol of the eukaryotic cells. 18S rRNA genes are the most widely used genes in phylogenetic analysis to reconstruct the evolutionary history of organisms. They contain flanking regions that are highly conserved. Therefore, universal primers can be used in the amplification. 

Figure 2: 40S Subunit

Similarities Between 16S rRNA and 18S rRNA

• 16S rRNA and 18S rRNA are two types of ribosomal proteins that occur in the small subunit of the ribosomes.
• They have a function in decoding genetic information and initiation of protein synthesis. 

Difference Between 16S rRNA and 18S rRNA

Definition

16S rRNA refers to the RNA component of the 30S subunit of a prokaryotic ribosome. In contrast, 18S rRNA refers to the RNA component of the eukaryotic ribosomal small subunit.  

Occurrence 

16S rRNA occurs in the small subunit of the prokaryotic ribosome, while 18S rRNA occurs in the small subunit of the eukaryotic ribosome.

Size of the Small Subunit

16S rRNA forms 30S subunit while 18S rRNA subunit forms 40S rRNA subunit. 

Function

16S rRNA binds to the Shine-Dalgarno sequence, while 18S rRNA is important in phylogenetic analysis. 

Conclusion

In brief, 16S rRNA and 18SrRNA are two types of rRNA that occur in the small subunit of the ribosomes. 16S rRNA occurs in the small subunit of the prokaryotic ribosomes. It also occurs in the small subunit of ribosomes of plastids. The small subunit of these ribosomes is the 30S subunit. 16S rRNA binds to the Shine-Dalgarno sequence in prokaryotes. In comparison, 18S rRNA occurs in the small subunit of eukaryotic ribosomes. The small subunit of eukaryotic ribosomes is the 40S subunit. 18S rRNA is important in reconstructing the phylogenetic history of organisms. Therefore, the main difference between 16S rRNA and 18S rRNA is where they occur.   

References:

1. 16S ribosomal RNA. 16S Ribosomal RNA – an overview | ScienceDirect Topics. (n.d.). 
2. 18S ribosomal RNA. 18S Ribosomal RNA – an overview | ScienceDirect Topics. (n.d.). 

Image Courtesy:

1. “OSC Microbio 03 03 Ribosome” By CNX OpenStax – Own Work (CC-BY 4.0) via Commons Wikimedia
2. “40S solvent 2XZM” By Fvoigtsh – Own work (CC BY-SA 3.0) via Commons Wikimedia