C’est la Vie
The genetic code of a cell is housed in the nucleus as DNA. However, before this code can be read, interpreted, and used to create proteins, a copy must be made and shipped into the cytoplasm where protein synthesis occurs. The process of making this copy of the genetic code is called transcription and is very similar to DNA replication, with only a few differences.
Transcription
During transcription, nucleotides are assembled into an RNA molecule by the enzyme RNA polymerase. This enzyme, which is much larger than DNA polymerase, is capable of binding to specific sequences of DNA, unwinding the DNA, reading a single complementary strand of DNA as a template, and generating a single strand of RNA that will contain the genetic code for making protein.
Once the RNA molecule is produced, it detaches from the RNA polymerase and the DNA strands bind back to one another to reform the double-stranded molecule.
RNA nucleotides
While the nucleotides in DNA are A:T and G:C pairs, the nucleotide uracil (U) replaces thymine (T) in RNA. Thus, the nucleotide pairs in RNA are A:U and G:C.
The idea of a transcript in written language is to produce a word-for-word copy that uses the exact same language as is spoken. In the case of the genetic code and nucleotide alphabet, a strand of RNA is the transcript, consisting of the same code as found in the coding strand of DNA.
Types of RNA
Several types of RNA will be produced by transcription. Messenger RNA (mRNA) is the genetic code in RNA form. Ribosomal RNA (rRNA) will be combined with proteins to form ribosomes, which synthesize protein. Lastly, transfer RNA (tRNA) is molecules that transport amino acids in the correct order to the ribosome for assembly into protein.
Translation
When a translation of a spoken language occurs, the communication happens in a completely different language, often using a different alphabet. The same is true in cellular translation. Here, the machinery of the cell reads the genetic language of nucleotides and assembles proteins using an alphabet of amino acids. The essential components of translation are the 3 RNA molecules: mRNA, rRNA, and tRNA. Cellular translation can be considered the carrying out of the instructions in a genetic code. The code describes what to do and the translation does it.
RNA
Much like words are composed of letters, the genetic alphabet of nucleotides is arranged on mRNA into 3-letter words called codons, which are simply small units of genetic code—like syllables. With 4 different nucleotides arranged into 3-letter codons, there are 64 distinct codons that can be used by the translation machinery of the cell. While mRNA represents the code, the location where this code is read and interpreted and proteins are assembled is the ribosome.
Some RNA molecules can bind to specific amino acids on one end while recognizing codons of the mRNA on the opposite end. Just as the coding strand of DNA binds to its complementary strand via nucleotide pairs (A:T, C:G), tRNA has 3 nucleotides (the anticodon) arranged that are complementary to the mRNA codons. This means that as the ribosome slides along the mRNA molecule, tRNA molecules bind to their respective codons and, in doing so, bring amino acids into the interior of the ribosome in the order directed by the mRNA code, and therefore into the correct sequence for the protein.
The space inside the ribosome can only hold 2 tRNA molecules at a time. As the ribosome slides along the mRNA, the amino acids link together via peptide bonds, thus freeing 1 amino acid from its respective tRNA molecule and forming a growing chain of amino acids attached to the other tRNA. At this point, the empty tRNA molecule is ejected, the other tRNA and its growing chain slide into the space vacated by the ejected tRNA, and a new tRNA molecule with an amino acid enters. This assembly continues until the entire protein is finished.
Codons and anticodons
Given an mRNA codon with the nucleotides A-U-G, what would be the complementary anticodon of the tRNA molecule that would bind here? The anticodon that would recognize A-U-G would be U-A-C. Remember that there are no T nucleotides in RNA.
Codons
Although there are 64 possible codons, only 20 amino acids can occur in nature and be assembled into proteins. Does this mean many of the codons are irrelevant? The answer is no. Multiple codons can be recognized by the same tRNA. In addition, multiple codons encode for the same amino acid. For instance, the codons GGU, GGC, GGA, and GGG are all recognized by the tRNA for the amino acid glycine. In fact, many tRNA molecules can recognize 4 different codons.
Specific codons called start codons and stop codons signal the beginning and end of protein synthesis. The codon AUG is the one and only start codon, and signals the assembly of the first amino acid in all proteins: methionine. The codons UAA, UAG, and UGA stop translation and signal for the ribosome complex to free the newly generated protein.