Cell Biology Ribosomes

If you’ve taken a biology course, you’ve probably heard of these little guys. The name might be familiar, but how exactly do these tiny machines work? What are they for and where do they come from?

First let’s talk about what a ribosome is. A ribosome is, essentially, a microscopic machine. These machines are built from proteins and chains of molecules known as RNA (a molecule very similar to the infamous DNA). There are two major parts known as the “large subunit” and (you guessed it) the “small subunit”. These two pieces fit together somewhat like two halves of a hamburger bun. In between the two subunits (where the hamburger patty goes, for example) we can slide in a chain of Messenger RNA (or mRNA). How all these pieces come together is a little more complex than this, but essentially, this is how it all goes together. Once everything is in place, the ribosome “reads” the mRNA strand and pumps out a protein.

So now that we have the basics down, let’s look at how exactly the ribosome produces these proteins. First, an mRNA strand (which has been made in the nucleus) travels out into the cytoplasm of the cell. Here, a few molecules known as initiation factors stick to the mRNA strand and signal for the ribosome subunits. The two subunits come together and surround the mRNA strand. Now comes the fun part. Other tiny molecules floating around in the cytoplasm, called Transfer RNA (tRNA), find the ribosome and get fed into the machine through a spot called the “A-Site”. This site is named the A-Site because each tRNA is attached to an Amino Acid (the stuff that proteins are made from). Each tRNA has a short three-letter code called an “anti-codon”. This anti-codon matched up with a “codon” located on the mRNA strand sandwiched between the two ribosomal subunits. When the anti-codon and codon match up, they stick together.

Next step. Everything moves down one position. The tRNA moves down into a new site, called the “P-Site”, for PolyPeptide. Now that the A-Site is vacant, a new tRNA with a new Amino Acid move into this spot. This time, when the anti-codon and codon match up, the Amino Acid from this tRNA and the Amino Acid from the first tRNA bind together. Again, everything shifts down one spot. Now, the first tRNA is in the “E-Site”, which stands for “Exit”. As the name suggests, this is where the tRNA exists the ribosome. The Amino Acid, however, detaches from the tRNA and stays with the ribosome. With the second tRNA now in the P-Site, a third tRNA with a third Amino Acid can enter the A-Site and attach a third Amino Acid to the growing chain.

This process repeats over and over again, many times producing a chain which has hundreds of amino acids. This chain is known as a protein and can be used to do a number of functions within the cell, or can even be shipped out of the cell to do work in other places in the body.

Though this process is fairly similar in all cells, prokaryotic cells (bacteria) have slightly smaller ribosomes.