Chapter 10 Gene and Chromosome Structure英文电子书.pdfVIP

10 MOLECULAR STRUCTURE OF GENES AND CHROMOSOMES These brightly colored RxFISH-painted chromosomes are both beautiful and useful in revealing chromosome anomalies and in comparing karyotypes of different species. [© Department of Clinical Cytogenetics, Addenbrookes Hospital/Photo Researchers, Inc.] y the beginning of the twenty-first century, molecular rare in bacteria. Sequencing of the same protein-coding gene biologists had completed sequencing the entire genomes in a variety of eukaryotic species has shown that evolution- of hundreds of viruses, scores of bacteria, and the bud- ary pressure selects for maintenance of relatively similar se- B ding yeast S. cerevisiae, a unicellular eukaryote. In addition, quences in the coding regions, or exons. In contrast, wide the vast majority of the genome sequence is now known sequence variation, even including total loss, occurs among for several multicellular eukaryotes including the roundworm introns, suggesting that most intron sequences have little C. elegans, the fruit fly D. melanogaster, and humans (see functional significance. Figure 9-34). Detailed analysis of these sequencing data has The sheer length of cellular DNA is a significant prob- revealed that a large portion of the genomes of higher eukary- lem with which cells must contend. The DNA in a single otes does not encode mRNAs or any other RNAs required by human cell, which measures about 2 meters in total length, the organism. Remarkably, such noncoding DNA constitutes must be contained within cells with diameters of less than more than 95 percent of human chromosoma