The central dogma explains the flow of genetic information from DNA nucleotides to the amino acid sequence of proteins.
RNA is the Missing Link Between DNA and Proteins
In the early 1900s, scientists discovered that DNA stores all the information needed for cellular functions and that proteins perform most of these functions. However, the mechanisms of converting genetic information into functional proteins remained unknown for many years. Initially, it was believed that a single gene is directly converted into its encoded protein. Two crucial discoveries in eukaryotic cells challenged this theory. First, protein production does not occur in the nucleus. Second, DNA is not present outside the nucleus. These findings sparked the search for an intermediary molecule that connects DNA with protein production. This intermediary molecule was found to be RNA.
RNA is synthesized in the nucleus using DNA as a template. The newly-synthesized RNA is similar in sequence to the DNA strand, except thymine is replaced by uracil. In eukaryotes, this primary transcript is further processed, removing the protein non-coding regions, capping the 5' end, and adding a 3' poly-A tail, to create mature mRNA, which is then exported to the cytoplasm.
The Genetic Code is Redundant
Proteins are created from 20 amino acids in eukaryotes. Combining four nucleotides in sets of three provides 64 (43) possible codons.This means that more than one codon can encode an individual amino acid. Thus, the genetic code is said to be redundant or degenerate. Often, but not always, codons that specify the same amino acids differ only in the third nucleotide of the triplet. For example, the codons GUU, GUC, GUA, and GUG all represent the amino acid valine. However, AUG is the only codon that represents the amino acid methionine. The codon AUG is also the codon where protein synthesis starts and is, therefore, called the start codon. Redundancy in the system minimizes the harmful effects of mutations. For instance, an alteration at the third position of the codon might not necessarily change the amino acid encoded, thereby, causing no change in the protein functionality.
The Genetic Code is Universal
With a few exceptions, most prokaryotic and eukaryotic organisms use the same genetic code for protein synthesis. This universality of the genetic code has enabled advances in scientific research in the field of agriculture and medicine. For instance, human insulin can now be manufactured on a large scale in bacteria using recombinant DNA technology. Recombinant DNA technology involves the use of genetic material from different species. Genes encoding human insulin are joined with bacterial DNA and inserted into a bacterial cell. The bacterial cell then performs transcription and translation to produce the human insulin protein, which can treat diabetes in patients.
