Microcheck

Eukaryotic mRNA must be processed, which involves capping, polyadenylation, and splicing. In eukaryotic cells, the mRNA must be transported out of the nucleus before it can be translated in the cytoplasm. Eukaryotic mRNA is monocistronic.

■ Explain the mechanism of action of diphtheria toxin.

■ Would a deletion of two base pairs have a greater consequence if it occurred in an intron or in an exon?

Figure 7.16 Splicing of Eukaryotic RNA

7.5 Genomics

Increasingly rapid methods of determining the nucleotide sequence of DNA have led to exciting advancements in genomics. Fueled by the commitment to sequence the entire human genome, scientists honed the methodologies by first sequencing the genomes of select microorganisms. In 1995, the sequence of the chromosome of Haemophilus influenzae was published, marking the first complete genomic sequence ever determined. The genome sequences of more than 75 other organisms have now been determined (table 7.5). A draft of the human genome is also complete.

Although sequencing methodologies are becoming more rapid, analyzing the resulting data and extracting the pertinent information is far more complex than it might initially seem. One of the most difficult steps is to locate and characterize the potential protein-encoding regions. Imagine trying to determine the amino acid sequence of a protein encoded by a 1,000-base-pair (bp) stretch of DNA without

7.5 Genomics 181

PERSPECTIVE 7.2 RNA: The First Macromolecule?

The 1989 Nobel Prize in Chemistry was awarded to two Americans, Sidney Altman of Yale University and Thomas Cech of the University of Colorado, who independently made the surprising and completely unexpected observation that RNA molecules can act as enzymes. Before their studies, it was believed that only proteins had enzymatic activity.The key observation was made by Cech in 1982 when he was trying to understand how introns were removed from mRNA that coded for ribosomal RNA in the eukaryotic protozoan Tetrahymena. Since he was convinced that proteins were responsible for cutting out these introns, he added all of the protein in the cells' nuclei to the mRNA that still contained the introns.

As expected, the introns were cut out. As a control, Cech looked at the ribosomal RNA to which no nuclear proteins had been added, fully expecting that nothing would happen. Much to his surprise, the introns were also removed. It did not make any difference whether the protein was present—the introns were removed regardless. Thus, Cech could only conclude that the RNA acted on itself to cut out pieces of RNA.

The question remained of how widespread this phenomenon was. Did RNA have catalytic properties other than that of cutting out introns from rRNA? The studies of Altman and his colleagues, carried out simultaneously to and independently of Cech's, provided answers to these further questions. Altman's group found that RNA could convert a tRNA molecule from a precursor form to its final functional state. Additional studies have shown that enzymatic reactions in which catalytic RNAs, termed ribozymes, play a role are very widespread. Ribozymes have been shown to occur in the mitochondria of eukaryotic cells and to catalyze other reactions that resemble the polymerization of RNA. Whether catalytic RNA cuts out introns from mRNA in the nucleus is not known.

These observations have profound implications for evolution: Which came first, proteins or nucleic acids? The answer seems to be that nucleic acids came first, specifically RNA, which acted both as a carrier of genetic information as well as an enzyme. Billions of years ago, before the present universe in which DNA, RNA, and protein are found, probably the only macromolecule that existed was RNA. Once tRNA became available, these adapters could carry amino acids present in the environment to specific nucleotide sequences on a strand of RNA. In this scenario, the RNA functions as the genes as well as the mRNA.

knowing anything about the orientation of the promoter or the reading frame of the transcribed mRNA. Since either strand of the double-stranded DNA molecule could be the template strand, two entirely different mRNA molecules could potentially code for the protein. In turn, each of those two molecules has three reading frames, for a total of six reading frames. Yet only one of these actually codes for the protein. Understandably, computers are an invaluable aid and are used extensively in deciphering the meaning of the raw sequence data. In turn, this has resulted in the emergence of a new field,

Table 7.5 Representative Microorganisms Whose Genome Sequences Have Been Determined

Name of Organism

Genome Size (106 base pairs)

Important Characteristics

Agrobacterium tumefaciens

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