|MadSci Network: Evolution|
One does not find grossly different levels of genetic activity across different kingdoms of life. Fundamentally, DNA serves as the blueprint for all of the molecules necessary for a functional cell, tissue, organ, and organism. These necessary molecules are mostly proteins, which are strings of amino acids. To get from DNA to proteins life uses an intermediate class of molecules: RNA (specifically, messenger RNA, or mRNA). A cluster of proteins called a transcription complex will bind to a specific site on the DNA and use the sequence of nucleotides to build a mRNA molecule (which is itself a string of nucleotides). Complexes of RNA and protein called ribosomes will then attach to the mRNA molecule and build the protein. This basic process is found in all living organisms, from the most simple single-celled bacterium to the most complex multicellular plant, animal, or fungi. Whether the organism’s genome consists of a single circular chromosome or tens of linear chromosomes they will all exhibit the same genetic activity. The level of activity will vary mostly due to environmental cues, such as a lack of food or an excess of heat. While it might be expected that having more genes will lead to more “genetic activity”, one finds that similar genes under unstressed conditions are transcribed and translated at similar rates. Also, the number of genes will vary from organism to organism, but generally speaking complex multicellular animals have been found to have the number of genes being of the same order of magnitude. Estimates for the recently sequenced rice genome range from 32,000 to 55,000 genes, while the only other plant to have it’s genome sequenced (Arabidopsis thaliana, a small weed-like plant) is thought to have around 26,000 genes. For humans the estimated number of genes falls within this range, with the latest number being around 30,000 genes. Most plants do have more DNA in their genomes than animals. This is mostly due to two factors: the ability of plants to duplicate their genomes in order to reproduce (a process known as polyploidization) and the susceptibility of plants to mobile genetic elements. Polyploidization allows plants to more easily form hybrids when pollen and ova from different species com together. The result of such hybridization events are plants with genomes that are the sum of the two parent genome sizes (as opposed to half of one parent’s genome and half of the other parent’s genome as in normal sexual reproduction). The susceptibility of plants to mobile genetic elements (sometimes called “jumping genes”) is one of the reasons why Barbara McClintock was able to discover and characterize them in corn. Further investigations have shown that plant genomes are riddled with short stretches of DNA that are active and inactive mobile genetic elements. While animal genomes also contain active and inactive mobile genetic elements, for some reason they have much, much fewer than plants, either due to being better able to eliminate the sequences from the genome or not to get so many in the first place.
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