9.11: Proteins: From Genes to Degradation
Within a biological system, the DNA encodes the RNA, and the nucleotide sequence in the RNA further defines the amino acid sequence in the protein. This is referred to as “The Central Dogma of Molecular Biology” - a term coined by Francis Crick. Central dogma is a firm principle in biology that defines the flow of genetic information within any life form. The two fundamental steps in central dogma are - transcription and translation.
Transcription is the synthesis of RNA molecules by RNA polymerase and other accessory proteins using DNA as the template. It happens in three phases - initiation, elongation, and termination and leads to premature RNA molecules that need to be further processed. While in eukaryotes, the whole transcription process happens inside the defined cell nucleus; transcription in prokaryotes occurs in the cytoplasm itself. Besides, while prokaryotes have only one type of RNA polymerase, eukaryotic cells have three types of RNA polymerase - I, II, and III to transcribe different RNA classes.
In eukaryotes, the processing of the pre-mRNA molecules into mature mRNA usually happens alongside transcription. It is also necessary for its transport into the cytoplasm, where it can be translated into the protein. The ribosomes in the cytoplasm decode the mRNA molecules the help of tRNA molecules, and synthesize a chain of amino acids. It is within the ribosomes that peptide bonds are formed between the amino acids leading to a polypeptide. This polypeptide is later folded into an active protein that can perform its functions inside the cell.
Any defects in this entire process can turn out to be detrimental to the cell. Therefore, cells enforce quality checks at various stages to ensure the synthesis of the right protein. Any defective RNA or protein molecule synthesized is hence degraded through pre-defined mechanisms.