MadSci Network: Evolution

Re: Why do plants have more DNA but less genetic material than animals?

Date: Wed Aug 24 08:48:21 2005
Posted By: Kurt Wollenberg, Assistant Professor, Ophthalmology and Medicine
Area of science: Evolution
ID: 1123764679.Ev

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

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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