them to have lost a lot of their RNA. Another piece of evidence for thermoreduction comes from theProkaryotes genomes of prokaryotes:can survive inhigher • In eukaryotes, chromosomes are made of linear DNA.temperatures • In prokaryotes the genome is made of circular DNA.because, • Circular DNA is much less vulnerable to heat damage thanunlike linear DNA, which starts to get split ends at higheukaryotes, temperatures.their DNA is ina protectivecircular • Circular chromosomes are conspicuously absent fromconfiguration. eukaryotes and their widespread incidence in prokaryotes alone is best explained by the thermoreduction hypothesis. For eukaryotes to maintain linear DNA genomes, they require a special system for maintaining their ends: • An enzyme called telomerase, which has both a protein and an RNA component, does this job. • Telomerase is common to all eukaryotes, suggesting it is very ancient. • It seems unlikely that eukaryotes with their linear genomes, and many RNAs, including telomerase, could have emerged from the sauna of life. • It is more likely that the organisms which first braved high temperatures shed much of the evidence of their RNA world ancestry along the way, as well as linear DNA genomes and telomerase. • Modern prokaryotes appear to have a hot history, even though many now live at moderate or even cold temperatures. Clues to the origin of life in your own body Evolutionary biologists have traditionally studied the simplest organisms they can find in order to learn more about the origins of life. But simple doesnt necessarily mean ancient, so we should not restrict our search purely to simple organisms. All organisms have been evolving for 3.5 billion years or so, and the idea that there is some obscure bug that time forgot which resembles ancient life on Earth is outdated. As Forterres work shows, simplification has its merits, and it seems that bacteria have lost a lot of the molecular fossils of our ancient past. We know an enormous amount about theHuman cells, biochemistry of our own cells, and although theres layer uponwhich are layer of complexity, hidden underneath it all are clues to the originseukaryotic, of the earliest cells. How ironic it is that human cells harbor asmay harbor many if not more secrets of the origins of life than the simplesecrets to the bacteria! Its no wonder that evolutionary biologists are as excitedorigin of life. about the Human Genome Project as anyone else! It is important to keep in mind that eukaryotic cells have continued to evolve over time. While it is possible to uncover much about the
Were Bacteria the First Forms of Life on Earth?
learnmore links get involved glossary references back to topLife in ocean extremesDiscovery network provides pictures of some of the creatures that live in and aroundthe extreme environment of deep sea vents.http://dsc.discovery.com/convergence/blueplanet/photo/photo.htmlThe Origin of Life on EarthLearn more about the RNA world and the current and historical ideas on the origin oflife on Earth by reading this thought provoking article by Leslie Orgel of the SalkInstitute for Biological Studies.http://www.geocities.com/CapeCanaveral/Lab/2948/orgel.htmlThe Tree of Life pageA fun, interactive site that lets you surf up and down "The Tree of Life."http://phylogeny.arizona.edu/tree/phylogeny.htmlHuman genome project informationExplore this comprehensive site, sponsored by the U.S. Department of EnergysOffice of Science, for information about the U.S. and worldwide Human GenomeProject. Second link takes you to their Primer on Molecular Genetics to learn moreabout DNA and sequencing.http://www.ornl.gov/hgmis/http://www.ornl.gov/sci/techresources/Human_Genome/publicat/primer/toc.htmlEukaryotic OriginsAstrobiology Magazine article examines endosymbiosis (how bacteria is engulfed).http://www.astrobio.net/news/article243.htmlAstrobiologyThe astrobiology website for Australia and New Zealand.http://www.aao.gov.au/local/www/jab/astrobiology/Common ancestorNews story on Biomednet (requires free log-in) describing a presentation by AnthonyPoole about the common ancestor being more like an eukaryote, presented at theannual conference of the Society for Molecular Biology and Evolution, July 2001.http://news.bmn.com/news/story?day=010711&story=2Further reading (science journals)» Doolittle W.F. 2000. "The nature of the universal ancestor and the evolution of theproteome." Current Opinion in Structural Biology 10:355-358.» Forterre P., Philippe H. 1999. "Where is the root of the universal tree of life?"BioEssays 21:871-879.» Penny D., Poole A. 1999. "The nature of the last universal common ancestor."Current Opinion in Genetics and Development 9:672-677.» Poole A., Jeffares D., Penny D. 1999. "Early evolution: prokaryotes, the new kidson the block." BioEssays 21:880-889.» Ridley M. 2000. "The search for LUCA." Natural History 11:82-85. getinvolved links learn more glossary references back to top
The RNA SocietyBiology students and scientists can join this scientific society to share researchresults and information about emerging concepts in RNA. Membership application isavailable on-line. There is a sliding membership fee.http://www.rnasociety.orgFor teachers: origin of life lessonsInteractive lessons focus on origin of life studies. These lessons are intended for usein any high school biology course but many can be used in middle school, juniorcollege or lower division university levels.http://www.indiana.edu/~ensiweb/orig.fs.html
authorglossary learn more get involved references back to topChromosome - A single length of DNA which contains many genes. Eukaryotes haveseveral different linear chromosomes in each cell, and each contains some of thegenes cells need for life (humans have 23). Prokaryotes usually have one circularchromosome, which contains all their genes.DNA - Deoxyribose nucleic acid (DNA) is built up of four types of units, adenine (A),guanine (G), cytosine (C) and thymine (T), joined in a series. Genes are encoded bythe specific sequence of the DNA, and their products are usually proteins. Forexample, the sequence ATG indicates the start point of a gene, and tells the cell thatthe first amino acid in the protein is methionine.DNA polymerases - A class of enzyme that synthesises DNA.Enzyme - Enzymes speed up (catalyse) chemical reactions. Most enzymes areproteins but a few are made from RNA.Eukaryotes - Have cells with a nucleus that contains all the DNA. Eukaryotesinclude plants, animals, fungi, amoeba, algae, and many other organisms.Eukaryotes can be multicellular (many-celled) or single celled. Compare withprokaryotes.Genetic code - The language of genes. Three nucleotides together make a codonwhich codes for an amino acid. For example GAG codes for the amino acid glutamine.Ribosomes read the codons from an RNA copy of a gene, joining amino acidstogether to make the protein encoded in that gene. The genetic code is the same forall living things.Genome - The collection of genes that make up an organism. These genes may beon onechromosome, or many.Prokaryotes - Do not have a nucleus. There are two groups: bacteria and archaea,which appear quite different from each other. All prokaryotes are single celled.Protein - A chain of amino acids. The sequence of amino acids is specified by agene. Each protein is made by a ribosome, and it folds up into a specific shape thatis determined by the sequence of amino acids. The precise arrangement of aminoacids determines the properties of the protein. Some proteins are enzymes.Ribosomes - Ribosomes are the cells protein factories. Ribosomes read the geneticcode from the working RNA copies of genes, using these to synthesise the proteinencoded by the gene. Ribosomes have an RNA core, which is largely responsible forprotein production. The RNA core is stabilised by a protein scaffold.RNA - Ribose nucleic acid (RNA) is similar to DNA. For a protein to be made from agene a working copy of the gene is made from RNA. The RNA is read by theribosome, which "translates" it into protein. Other RNA molecules are enzymes,performing chemical reactions.RNA world - A proposed stage early in the evolution of life in which RNA acted asboth genetic material and enzyme.
articlereferences learn more get involved glossary back to topGeneral References:» Sidney Altman http://www.science.ca/scientists/scientistprofile.php?pID=3» Tom Cech http://petunia.colorado.edu/projects.html» Nobel Prize in Chemistry http://nobelprize.org/chemistry/laureates/1989/index.html» Patrick Forterre is a leading scientist in France who has been studying eukaryotes and their role in the origin oflife http://www-archbac.u-psud.fr/LabHome/PForterre/ePF.html» The University of Paris Sûd http://www.u-psud.fr/anglais.nsf/index.htm?OpenPage» Human Genome Project http://www.ornl.gov/hgmis/