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Classically - separated by long sequences that do not code...

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Classically, genes are identified by their function. That is the existence of the gene is recognized because of mutations in the gene that give an observable phenotypic change. Historically, many genes have been discovered because of their effects on phenotype. Now, in the era of genomic sequencing, many genes of no known function can be detected by looking for patterns in DNA sequences. The simplest method which works for bacterial and phage genes (but not for most eukaryotic genes as we will see later) is to look for stretches of sequence that lack stop codons. These are known as “open reading frames” or OOOOO s. This works because a random sequence should contain an average of one stop codon in every 21 codons. Thus, the probability of a random occurrence of even a short open reading frame of say 100 codons without a stop codon is very small (61/ 64) 100 = 8.2 x 10 –3 Identifying genes in DNA sequences from higher organisms is usally more difficult than in bacteria. This is because in humans, for example, gene coding sequences are
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Unformatted text preview: separated by long sequences that do not code for proteins. Moreover, genes of higher eukaryotes are interrupted by iiiii , which are sequences that are spliced out of the RNA before translation. The presence of introns breaks up the open reading frames into short segments making them much harder to distinguish from non-coding sequences. The maps below show 50 kbp segments of DNA from yeast, Drosophila, and humans. The dark grey boxes represent coding sequences and the light grey boxes represent introns. The boxes above the line are transcribed to the right ant the boxes below are transcribed to the left. Names have been assigned to each of the identified genes. Although the yeast genes are much like those of bacteria (few introns and packed closely together), the Drosophila and human genes are spread apart and interrupted by many introns. Sophisticated computer algorithms were used to identify these dispersed gene sequences....
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