The Molecular Basis of Protein Synthesis: RNA and the Genetic Code
The Three Major Types of RNA
Messenger RNA (mRNA)
The Messenger RNA (mRNA) molecule is a long, single strand that contains the codons which will be translated into the amino acid sequence of a protein.
Transfer RNA (tRNA)
Transfer RNA (tRNA) molecules bind to specific amino acids and deliver them to the ribosome, where they are incorporated into growing protein chains. There are many types of tRNA, with at least one type for each amino acid.
Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA) and a large amount of protein constitute ribosomes. Each ribosome consists of two subunits.
Understanding the Genetic Code
The genetic code is often compared to Morse code: a set of symbols (bases in nucleic acids, or dots and dashes in Morse code) can be translated into another set of symbols (amino acids in proteins, or letters of the alphabet).
Nucleic Acid Bases
There are four different bases in DNA:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Thymine (T)
There are four bases in RNA:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Uracil (U)
Codons and Redundancy
But there are 20 different amino acids in proteins, so the bases cannot serve as a code one by one. However, combining three bases gives us 64 (4 x 4 x 4) possible combinations, which are more than enough. These messenger RNA triplets are called codons.
The investigation showed that the AUG codon signals the start of a protein (the initiation codon). Three codons (UAA, UAG, and UGA) mark the end of a protein (termination codons).
Although only 20 amino acids must be encoded, there are 60 codons, in addition to the start and end signals. This code is therefore highly redundant, or degenerate. In other words, a single amino acid can be specified by several codons. For example, six different codons encode arginine. However, although the code is redundant, it is unambiguous: each codon specifies one amino acid and only one.
Initiation of Protein Synthesis (Translation)
The genetic code dictates where protein synthesis must be initiated, starting with the AUG initiation codon on the mRNA.
The process of translation initiation follows these steps:
- The first step involves the assembly of several protein initiation factors and a tRNA molecule containing the complementary UAC anticodon (sometimes referred to as the "initiator tRNA") within the small subunit of a ribosome.
- The small ribosomal subunit then binds to the mRNA molecule and moves along it until it locates the start codon.
- At this point, the UAC anticodon of the initiator tRNA pairs precisely with the AUG initiation codon.
- The large ribosomal subunit then joins the small subunit.
- As this occurs, the initiator tRNA simultaneously binds to the P site within the assembled ribosome.
- The ribosome is now fully assembled and ready to initiate the elongation phase of translation.