Protein Power: Translation
In Part 3 of this series on DNA, Ask Science explains how our cells take instructions from messenger RNA and use them to build the proteins essential for our bodies.
Lee Falin, PhD
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Protein Power: Translation
In Part 1 of my Protein Power series, I talked about how RNA works and how it differs from DNA. Then in Part 2 we looked at how our cells modify RNA in order to create messenger RNA or mRNA, a molecule which contains instructions for making protein. This week, we’ll finish up this series by looking at how our cells take those instructions and use them to build the protein..
Leaving the Nucleus
When we last left our RNA molecule, it had just received two tickets it needed to survive life outside of the nucleus: a 5’ cap and a poly-adenosine tail. Now our RNA can escape the confines of the cell’s nucleus through one of the many nuclear pores which dot the walls.
Once outside the nucleus, the mRNA is free to dock with a ribosome. A ribosome is a very large, very complicated mass of proteins and RNA that has the exciting job of translating the code embedded in mRNA molecules into proteins. This is such an exciting job that most mammals have millions of ribosomes floating around in every cell, just looking for mRNA molecules to attach to.
Ribosomes have two parts, cleverly called the “large sub-unit” and “small sub-unit.” They clamp onto the mRNA molecule like two halves of a clamshell. Once the ribosome is firmly attached, the actual process of translation can begin.
Building Proteins
As amazing as ribosomes are, just like any other complicated factory, they can’t make anything without regular shipments of raw materials. The raw materials needed for marking proteins are amino acids. Every protein in your body is made up of a long string of amino acids stuck together with a special chemical bond called a peptide bond. However, in order to string these amino acids together, the ribosome first needs to find them.
There are hundreds of different kinds of amino acids, but only 23 different ones are currently known to be used in protein building. You might have heard the phrase “essential amino acids” before, as in “Choco-coated sugar bombs provide your child with all essential amino acids!”
While your body is able to synthesize most of the types of amino acids it needs, there are 9 amino acids needed for protein synthesis which your body can’t synthesize. So the only way for your ribosomes to get access to those varieties are through the foods you eat.
See also: Breaking News on Protein and Aging: Episode 280
Crack the Code!
Now that we know what proteins are made of, let’s take a brief look at how ribosomes put them together. The neatest part of this process is how the genetic code works. Recall that we said that mRNA is made up of a sequence of nucleotides. Every set of of 3 nucleotides is called a codon, and different codons signal the ribosome to grab different amino acidsopens IMAGE file . As the ribosome moves along the mRNA strand it reads each codon and uses that information to decide which amino acid to stick onto the end of the protein sequence it is building.
Let’s say as the ribosome slides along the mRNA sequence that it encounters a guanine nucleotide, followed by a cytosine, followed by an adenine. This sequence (GCA) is the codon that corresponds to the amino acid alanine. So the ribosome would know that the next amino acid it needed to string onto the protein was an alanine.
Send in the tRNA!
Now you might be asking yourself: How does the ribosome match up the right amino acid with the corresponding codon? Well it doesn’t really. It relies on some helper molecules called tRNA, or “transfer” RNA. This is a special kind of RNA molecule which is shaped in just the right way to fit inside a ribosome.
There is a different tRNA for every type of codon. One side of the tRNA molecule has 3 nucleotides that are the base-pair complements of the codon sequence it binds to. (For more about base-pairing see Part 1 of the Protein Power series). The other side of the tRNA molecule carries around the corresponding amino acid.
Once it slips into the ribosome and binds to the mRNA sequence, the amino acid is stripped off the other side, attached to the protein sequence, and then the tRNA is kicked out the back door like an unwelcome houseguest.
Once the end of the mRNA sequence is reached, the ribosome ejects the long strand of amino acids, which folds up into a protein, ready to go out and face the world.
Conclusion
If you’ve followed this series from the start, you now know how your DNA is used to make proteins. If you haven’t, be sure to check out Part 1 and Part 2 of the Protein Power series for the whole scoop.
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Pyruvate kinase protein domains image, Thomas Splettstoesseropens IMAGE file at Wikpedia Creative Commons, CC BY- SA 3.0. Valine molecule image courtesy of Shutterstock.
