Francis Crick, who was credited with discovering the structure of DNA[note 1] (and also one of the earliest descriptions of coiled-coil proteins) described Crick's dogma, a rule about the flow of genetic information from DNA to protein. That flow of information is that DNA is transcribed to mRNA, and the mRNA is in turn translated into protein.
The rule is not a hard physical law that prevents any of the reverse taking place under any circumstances. Reverse transcriptase is able to convert RNA into DNA, which is how retroviruses like HIV can infect human cells. And reverse translate algorithms are used by scientists to derive suitable RNA and DNA sequences from protein sequences. Rather the dogma describes the flow of information from a gene to its functional gene product.
RNA Transcription
When DNA is transcribed into mRNA, an enzyme called RNA transcriptase binds to the DNA. The action of this enzyme is to produce the messenger, or mRNA from the DNA template. mRNA is written, as with DNA, in a 5' to 3' direction. In order for this to happen to produce an mRNA transcript of the DNA template on the sense strand, the RNA transcriptase polymerizes the mRNA using the antisense strand as a template, to which the growing mRNA binds to on the basis of reverse complementarity (Figure 1).
Figure 1: In mRNA transcription, the enzyme RNA transcriptase uses the antisense strand of the DNA of the gene to produce a strand of mRNA that is complementary to the antisense strand. This is because the enzyme can only add ribonucleotides in to the 3' end of the growing mRNA, and also that using the complementary strand as a template will produce an mRNA sequence equivalent to the sense strand of the gene DNA.
Protein Translation
In protein translation, the mRNA produced by RNA transcription is exported from the nucleus to the ribosomes, which are the cell's protein synthesis machinery. Inside the ribosomes there is transmission, or tRNA (Figure 2) which reads the mRNA in the ribosome. Each tRNA molecule has a sequence of three nucleotides that are complementary to the sequence of one of the sixty four possible codons found in mRNA. This allows for the mRNA and tRNA to bind on the basis of base complementarity. Onto this tRNA is also an amino acid, so it is this tRNA molecule that creates the physical link between the codon found in a DNA sequence and the amino acid it encodes for, thus being the basis for the genetic code.
Figure 2: Transmission RNA (tRNA) is a type of RNA that interacts with both the mRNA and the amino acid. By forming an interaction with the mRNA via a sequence of three nucleotides, and containing a specific amino acid, this means that a specific codon (three nucleotide sequence) encodes for a specific amino acid. The tRNA has loops and forms a secondary structure, and like other nucleotide structure, these are formed on the basis of charge complementarity. Because not all tRNA molecules occur at the same frequency, this gives rise to codon frequency, and rare codons may affect the efficiency of protein translation as a result.
During translation, two tRNA molecules will bind to adjacent codons (Figure 3). The two tRNA molecules with amino acids attached to them will bring the amino acids into close proximity with one another. The ribosomes will then attach the amino acids to one another via a peptide bond between the carboxyl group of the first amino acid and the amine group of the second amino acid. Once the peptide bond is made, the first tRNA detaches and the second tRNA attaches to the mRNA, and the process repeats, elongating the growing peptide each time.
Figure 3: During protein translation, tRNA molecules for adjacent codons are drawn into close proximity to one another and allow each amino acid to be added in turn to the growing peptide chain as it is secreted by the ribosome. Because tRNA molecules are brought into proximity during this process, this gives rise to a phenomenon of codon-pair frequency, and rare codon pairs may, like rare codons, affect the efficiency of protein translation.
Crick's dogma is a succinct explanation of the complex process of how a gene found in an organism becomes a functional protein.
- Francis Crick, James Watson and Maurice Wilkins won the Nobel Prize for the discovery of the structure of DNA. Many cite the importance of the work of Rosalind Franklin to this discovery, as it was Crick and Watson's access to her unpublished results that allowed their breakthrough. Franklin died before the Nobel Prize (which are not awarded posthumously) was awarded, so she was unrecognized in her lifetime, and as a woman scientist in the mid 20th century was a victim of the sexism of her era.
