Science Lessons: Sixth Lesson: Translation continued

Lesson 6: Translation continued.

In the previous lesson we began to discuss the concept of Translation. Remember, Transcription involves taking the nucleotide gene sequence found in the DNA and converting it into an mRNA molecule. Translation is performed next and changes the mRNA that was originally defined by the DNA into a particular protein sequence. Through the combination of these two processes, DNA is converted into proteins. The main mechanistic workhouse of Translation that performs the vast majority of the work is called the ribosome.

Before going into more detail about Translation, I need to discuss some basic protein chemistry. The topic of the next lesson is going to cover proteins in much more detail, but for now let us discuss some simple pointers. Like DNA, which is made up of small building blocks called nucleotides, proteins are also made up of small building blocks called amino acids. In either case, these building blocks are like Lincoln Logs, in which they are simple pieces that come together to build a larger structure. There are 20 different types of amino acids found in nature, and amino acids bond together with a special type of covalent bond called a peptide bond. (See Lesson 2: Chemical Bonding if you are confused about the term covalent bonding. )

Translation occurs in the ribosome (just like the building of a car occurs in a factory) but requires another special class of RNA molecule called a tRNA. tRNA’s serve as important players during Translation. (Remember that RNA and DNA are both made up of nucleotides.) Together the ribosome and tRNA’s function to take the mRNA molecule and convert it into a protein sequence.

Basically, take the mRNA molecule and count every three nucleotides, (1, 2, 3…1, 2, 3….1, 2, 3…). For every triplet you counted, this is called a codon. Remember tRNA’s? Well, there are many different types of tRNA molecules and for each molecule there is a very important region found on them comprised of three nucleotides also. This is called the anticodon. The anticodon of a given tRNA matches a particular codon of an mRNA through complementary base pairing rules. For example, if a given codon on the mRNA is GAU then the complementary anticodon of the tRNA is CUA.

G -> C

A -> U

U -> A

Every possible combination of three nucleotides (codon) on a mRNA has a matching anticodon sequence on a tRNA. In other words, there is a given tRNA molecule that contains a matching anticodon for every possible codon combination (there are 64 mathematical combinations).

The importance of the anticodon-codon base-pairing combination is due to the fact that this is the means the cell uses to distinguish what is supposed to be the correct amino acid building blocks to use. Amino acids are attached to tRNA’s. A particular amino acid will be found on only the right tRNA with the right corresponding anticodon.

Yet, why are there 64 codon-anticodon combinations but only 20 amino acids? The simple answer to this question is that there are more than one codon-anticodon combinations that encode for the same amino acid. For example, the amino acid glycine is encoded by either 4 different codons: GGU, GGC, GGA, and GGG. The term molecular biologist use is called "redundant" or "redundancy" to explain this concept. You can use this table to visual this for yourself: