Mutations

Recall that I wrote a while back that modern biology has come to depend upon the two-part mechanism of random mutation and natural selection--that in order for evolution to proceed there must be an accumulation of positive mutations upon which natural selection acts to "guide" the subsequent diversity and complexity of living things. The structure of nucleic acids, the DNA replication process, and the process by which information is taken from DNA and made into proteins suggest a number of mechanisms for these necessary mutations.

A missense mutation occurs when a mistake is made in base-pairing such that a substitute nucleotide is placed in the sequence. This may have no effect (there is duplication of codons for most amino acids and a substitute may simply change the codon to another within the same amino acid family) or may change the amino acid specified (again, possibly to no effect if the substituted amino acid doesn't significantly change the protein; on the other hand, the biochemical properties of the substituted amino acid could radically alter the shape and function of hte protein to a greater or lesser extent).

A nonsense mutation occurs whenever a reading or transcriptional mistake produces a "stop" codon--there are three sequences that signal a halt to the protein-producing enzymes, sort of like a period in a sentence. The stunted protein that results may not work at all or may do its job poorly.

A frameshift mutation occurs when the "reading frame" of rRNA gets off track; this may occur if either a base pair gets deleted or an extraneous base pair is added to the sequence. As an example, read the following sentence taking the letters three at a time: theonereddogsatforthetwomen. What do you get if a random letter is placed between the two "d's"? "The one red xdo gsa tfo rth etw ome n." You start off just fine and then in the middle of the sentence it all goes haywire. So the sequence of amino acids is completely thrown off. Furthermore, most frameshift mutations result in a premature "stop" codon, truncating the malformed protein.

The vast majority of genetic mutations are deleterious to the organism, especially in simpler life forms where there is a lot riding on the functionality of a single cell or group of cells. This is particularly true in early development; a disasterous mutation typically results in such disordered functionality that premature death occurs. These genes lose the natural selection game--their bodies didn't live long enough even to compete on the reproduction level.