Split GenesIn eukaryotes many genes are split into pieces, and the resulting RNA transcript must be processed before it can be translated...If you understood the above, this should be a simple review of what you already know or did know. If you did not understand most of it, you might want to go back and read the Basic Genetics page first. As I sit here writing this page, in front of me sits a cup from a summer class I taught many years ago on this topic to a number of high school biology teachers. It was a great class! On the cup is drawn a funny representation of split genes. I am using this cup and its drawing to write this web page. Back then, when I taught the course, the Internet and webpages did not even exist. Now back to our topic... What are eukaryotes? They are complex organism up above bacteria and bluegreen algae, called prokaryotes. Eukaryotes include mammals and, of course, by default, humans. Inside the cell they have a nucleus separated by a nuclear membrane, among many other differences. So in eukaryotes genes are often "split". The figure below depicts a single split gene:  Exons are the coding portion of the gene. They code for various parts of the protein that will be produced (translated). Here six exons are indicated, 1-6. Introns are non-coding DNA sequences. They do not code for any portions of the protein. Five introns, a-e, are shown in this example. ("f" is just some trailer DNA. Ignore it.) When RNA is produced (transcribed) processing involves snipping out the non-coding introns and rejoining the exons together into a single RNA strand. If the introns are left in, the resulting protein will be non-functional. Introns can be lost over time. Take the rat insulin genes. They arose by gene duplication. One of the genes has two introns while the other has only one. Studies in other mammals and birds indicates the one-intron gene is the older gene. Not all genes have introns. Using recombinant DNA technology, my laboratory once cloned an amylase gene from the fruitfly, Drosophila pseudoobscura, that lacked introns. Subsequent amylase genes cloned from the fruitfly had introns. Curious. There are many other examples of eukaryotic genes lacking introns, but most tend to have them. The processing out of the introns is another control step in gene regulation. |
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