When cataloguing a collection of genetic strings, we should have an established system by which to organize them. The standard method is to organize strings as they would appear in a dictionary, so that “APPLE” precedes “APRON”, which in turn comes before “ARMOR”.
In “Transcribing DNA into RNA”, we mentioned that a strand of DNA is copied into a strand of RNA during transcription, but we neglected to mention how transcription is achieved.
In the nucleus, an enzyme (i.e., a molecule that accelerates a chemical reaction) called RNA polymerase (RNAP) initiates transcription by breaking the bonds joining complementary bases of DNA. It then creates a molecule called precursor mRNA, or pre-mRNA, by using one of the two strands of DNA as a template strand: moving down the template strand, when RNAP encounters the next nucleotide, it adds the complementary base to the growing RNA strand, with the provision that uracil must be used in place of thymine.
The war between viruses and bacteria has been waged for over a billion years. Viruses called bacteriophages (or simply phages) require a bacterial host to propagate, and so they must somehow infiltrate the bacterium; such deception can only be achieved if the phage understands the genetic framework underlying the bacterium’s cellular functions. The phage’s goal is to insert DNA that will be replicated within the bacterium and lead to the reproduction of as many copies of the phage as possible, which sometimes also involves the bacterium’s demise.
In “Translating RNA into Protein”, we examined the translation of RNA into an amino acid chain for the construction of a protein. When two amino acids link together, they form a peptide bond, which releases a molecule of water. Thus, after a series of amino acids have been linked together into a polypeptide, every pair of adjacent amino acids has lost one molecule of water, meaning that a polypeptide containing n amino acids has had n − 1 water molecules removed.
Point mutations can create changes in populations of organisms from the same species, but they lack the power to create and differentiate entire species. This more arduous work is left to larger mutations called genome rearrangements, which move around huge blocks of DNA. Rearrangements cause major genomic change, and most rearrangements are fatal or seriously damaging to the mutated cell and its descendants (many cancers derive from rearrangements). For this reason, rearrangements that come to influence the genome of an entire species are very rare.