Questions & Answers with three PhD biologists. Dr. Sy Garte, Dr. Sharon Hormer-Drummond and Dr. Mary B Moritz. Question 4: Posed by one of our readers by PM. How can new information arise in DNA. Can you give us examples? Mary: Let me outline 3 mechanisms: (1) Gene duplication leading to gene families with up to hundreds of similar genes (e.g. 390 different kinds of proteins produced in our noses, called olfactory receptors). Sometimes, though, genes change drastically and even lead to proteins with different tasks, e.g. hemoglobin belongs to a superfamily of proteins known for their ability to store and to transport oxygen, but additional globin functions such as sensing, signaling, and detoxification have been proposed (and of course where there are proteins, there is the corresponding DNA). (2) “de novo genes”, previously called “orphan genes”: The first step is for a tiny bit of DNA to mutate into a “start sequence.” All protein-coding genes have start sequences, which enable cells to recognize where genes begin. The new gene can then be copied and used as guide for building a protein. The new protein may turn out to be toxic, or it may serve no purpose. But once it emerges, new mutations to the new gene may make it more useful. It is known from fruit flies that they occur rather frequent in some specific regions, but most of them disappear rapidly within a few generations. (3) Horizontal gene transfer (HGT) – this is not per se new genetic information, but a way to transfer new information across species. The process is well known for microorganisms (e.g. microbial resistance) and has also been described between parasites and their hosts. Recently, examples in higher taxa, e.g. a moss-to-fern HGT, have been described. Sources: Carl Zimmer, The Continuing Evolution of Genes, NYT, 29 Apr 2014Miriam Blank and Thorsten Burmester, Widespread Occurrence of N-Terminal Acylation in Animal Globins and Possible Origin of Respiratory Globins from a Membrane-Bound Ancestor, Mol Biol Evol (2012) 29 (11):3553-3561.Fay-Wei Li et al, Horizontal transfer of an adaptive chimeric photoreceptor from bryophytes to ferns, PNAS vol. 111 no. 18, 6672–6677 Sy: To expand a bit on part of Marys answer, and also partially answer another question seen on the thread, some new genes arise from what used to be called junk DNA, long stretches of DNA inherited from viruses and non coding elements such as transposons that make up a large fraction of animal DNA. Much of those millions of base pairs in our DNA represent a potential source for new genes, much like a junk yard can be for an inventor, or a sculptor. The other thing I wanted to mention is that I have no idea where the idea came from, that there can not be new information in DNA, and the only possible change is loss of information. It seems to be a basic principle of creationists, but it has no basis in science. I would ask anyone who knows to explain the origin of this totally wrong concept. Even simple point mutations can create new information, so I really dont understand the idea at all. Sharon: Mechanistically, there are a number of ways that errors in copying or in meiosis can give rise to new genetic information. During Prophase I of Meiosis I, unequal crossover (where homologous chromosomes exchange matching pieces of the same gene as one step in making genetically unique gametes) is possible, allowing the chromosome arms to exchange unequal lengths within those genes. That creates unique gene alleles. In plants, polypoloidy frequently occurs, where there is no separation during Anaphase I of the homologs. One therefore ends up with gametes (and eventually zygotes) with multiple copies of the same gene (or whole chromosomes). The latter error is typically fatal in most animals, but can result in rapid speciation in plants. A third way are for viral or viral like actions to occur where genes are replicated, copied and move around (think of Barbara McClintocks jumping gene work in maize). We find the evidence for this in humans and throughout the animal kingdom where we might have one working copy of a gene, and several older versions that have either been repurposed or that are broken. These extra copies are essentially a tool kit - when new environmental challenges arise - members of populations that have extra gene copies may have selective pressures working on those copies to repurpose them. Some excellent work detailing this in the development of flagella has been done.
Posted on: Sat, 13 Sep 2014 15:02:38 +0000
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