Ribosomal ribonucleic acid (rRNA) forms part of the body's process for making proteins from genetic information. So, rRNA is one of the structural materials for ribosomes, which help build proteins. Ribosomes in eukaryotic, or animal, cells have two major subunits, and the 18s rRNA forms a section of the smaller part. The sequence of the 18s rRNA gene is also used to place eukaryotic organisms on the evolutionary tree.
Scientists divide cellular life into eukaryotes and prokaryotes. Eukaryotes are creatures like animals who have their genetic information packed into enclosed nuclei in the cells. Prokaryotes are forms of life like bacteria that do not enclose genetic information in a small space. Both forms use ribosomes to build proteins, but the 18s rRNA molecule is only found in eukaryotes.
Ribosomes are structures that stick to strands of information that code for a specific protein. There, they help bring together the appropriate smaller molecules to build that protein. Each ribosome is made up of a mixture of protein and ribosomal RNA molecules. Eukaryotic ribosomes have two subunits, which are the 40S subunit and the 60S subunit. The gap where the two subunits attach to each other allows the ribosome to latch onto the strand of information.
Each subunit is made up of various rRNA molecules and lots of proteins. The 60S subunit contains one each of the 5S, 5.8S, and 28S rRNA molecules and many different proteins. The 40S subunit only contains the 18S rRNA molecule and some proteins.
All rRNA is named using numbers and the letter "S." This reflects the rate at which each rRNA sediments, or falls out of solution, when centrifuged. The rate of sedimentation is measured in Svedberg units, which use the letter "S" for short.
As ribosomes are essential for life, the sequence of the gene that codes for 18s rRNA is quite similar across organisms. The 18S rRNA gene contains the information necessary for cells to produce 18S rRNA molecules. If the gene mutated easily, the function would be lost. Although the gene is conserved, or has remained quite unchanged over millennia, it still has enough tiny variations to allow scientists to compare sequences from different organisms.
By looking at the small differences in the sequence of the 18S gene, a geneticist can figure out how closely related species are. He or she can also estimate the point of time in evolutionary history when each species branched away from each others' ancestors. Therefore, the 18S gene is a useful tool in piecing together the evolution of eukaryotic organisms.