The Human Genome Project, the undertaking begun in 1990 to determine the sequence of the human genome, has resulted in highly automated and efficient DNA sequencing techniques. This has enabled scientists to more readily determine the chromosomal sequence of other organisms, including both prokary-otes and eukaryotes, fueling the rapidly growing field of genomics. As discussed in chapter 7, the genomes of more than 75 organisms have been sequenced, and many more are in stages nearing completion. A draft of the entire human genome was completed in 2000. ■ genomics, p. 180
Genomic approaches are leading to many exciting advances, such as the discovery mentioned earlier of the bacterial rhodopsin gene in a marine bacterium that has never been cultivated. Cloning techniques contributed to the study, but DNA sequencing and subsequent analysis of predicted protein-encoding regions were also integral to the finding. Previous genomic studies of other organisms enabled the researchers to learn that the amino acid sequence of one predicted protein was similar to that of bacteriorhodopsin, a light-driven proton pump of a Halobacterium species, a member of the Archaea. ■ analyzing a prokaryotic DNA sequence, p. 182
Knowing the typical DNA sequence of a human gene helps scientists identify genetic alterations that result in certain diseases. For example, sickle cell anemia is due to a single base-pair change in the gene that encodes a part of the hemoglobin protein. This alteration results in a distorted protein that does not function properly. Likewise, the genetic disease cystic fibrosis is most often caused by a three-base-pair deletion in a protein involved with chloride transport in and out of cells. ■ protein structure, p. 27
DNA sequence analysis is also used to study evolutionary relatedness of organisms. Those with extensive sequence homology are likely to be closely related. Conversely, those with vastly different sequences are probably only distantly related. Comparing DNA sequences of ribosomal RNA genes has proven particularly valuable for determining relatedness. ■ comparing the sequences of 16S ribosomal nucleic acid, p. 262
9.4 Applications of the Polymerase Chain Reaction
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