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THE GENETIC CODE:
What songs do the "notes of DNA" dictate?
So, now we've made mRNA in the nucleus. So where does this newly synthesized molecule go from here? As you recall from the Central Dogma of Molecular Genetics, the next step after transcription is translation, the process of making proteins. Now that the mRNA has the DNA's instructions, the molecule must travel OUT of the nucleus to the CYTOPLASM where protein synthesis takes place.
Before we continue with the process of translation, let's examine the "players" in this process. The terms important for this process are:
 the Ribosome
tRNA (transfer RNA)
the A site
the P site
Codons
Anticodons
Amino Acids
Now let's review what amino acids are. Amino acids are the building blocks of proteins. Everything your DNA codes for is protein, so your DNA codes for amino acids. There are only 20 amino acids total, but each one has a generalized structure.
Each of the 20 different amino acids shares the amino group, the carboxyl group, the Hydrogen atom, and the central Carbon atom. The only group which differentiates them is the "R" group. R is simply a symbol for the side group.
There is the specialized apparatus for making proteins called the ribosome. There are many ribosomes in the cytoplasm of a cell, and all the ribosomes are made of a small subunit and a large subunit. These two subunits open up like a "pac-man" allowing the mRNA message to slide through. Once the mRNA message is in place and protein synthesis is ready to begin, the two subunits close again so that the mRNA is now in between the two subunits.
The next player on the list is the tRNA (transfer RNA molecule). This molecule (first discussed on the "Structure and Function of RNA" page) is responsible for bringing in the proper amino acids. Remember, the mRNA is now held within the two subunits of the ribosome and is relatively immobile. The amino acids (which, you remember, are the building blocks of proteins) are floating free in the cytoplasm.
So how can we bring the amino acids down to the mRNA?
This problem is solved by the action of tRNA. The tRNA molecule acts as a "taxi" whose job is to read the code from the mRNA and bring the corresponding amino acid into place. What do I mean by "corresponding" amino acid? Every tRNA molecule has its own set of three bases which is called an anticodon. This anticodon is complementary to mRNA codons. The other "end" of the tRNA molecule has an "acceptor" site where the tRNA's specific amino acid will bind.
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