Q2. The DNA sequence is not enough to understand the functions of a genome, a functional unit of a genome is a gene, so how do you find one?
The are three different approaches to gene identification and localization. First, genes can be found by sequence inspection, either by eye, or by computer methods (bioinformatics). Second, genes can be found experimentally by identifying protein or RNA sequence found in tissues, and finding a corresponding DNA sequence on the chromosome. Third, genes can be identified by comparing sequences between genomes of different species (comparative genomics).
The are three different approaches to gene identification and localization. First, genes can be found by sequence inspection, either by eye, or by computer methods (bioinformatics). Second, genes can be found experimentally by identifying protein or RNA sequence found in tissues, and finding a corresponding DNA sequence on the chromosome. Third, genes can be identified by comparing sequences between genomes of different species (comparative genomics).
1. Gene location by sequence inspection Sequence inspection can be used to locate genes because genes are not random series of nucleotides (structural unit of DNA and RNA) but instead have distinctive features. For example, genes always start with a specific sequence, the initiation condon (ATG) (Figure 1). The issue is complicated, because genes are not continuous, and are interrupted by the non-coding sequences called introns. However, these elements can also be identified, because they have common features. Finally, gene sequences always end with a termination codon (for example: UGA). The sequence identified in this manner between initiation and termination codon is called an open reading frame (ORF
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| Figure 1. Start and stop codons |
2. Experimental techniques for gene location
Experimental methods for gene location use RNA molecules that are transcribed (copied) from genes. All genes are by definition transcribed into RNA. In these methods, you collect RNA that was transcribed from the DNA, and convert it back to DNA code using molecular methods. Since it is no longer the original DNA, but a copy, we call it a cDNA, and a library of fragments obtained by this methods, a cDNA library. Once gene sequences have been identified in the DNA code, all we need to do is to find them in the genome.
3. Comparative methods for gene location
Once a gene is identified in one species, it is often easy to find it in a related species, because their sequences are likely to be similar. This can be done by comparing sequences computationally (homology search), or by hybridizing (attaching sequences) genes between different species. One way of doing this is zoo blotting (Figure 2). The objective is to determine if a fragment of human DNA hybridizes to DNAs from related species.
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| Figure 2. Zoo Blotting |
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