Evolucion

Notas del editor

• #42: Figure 6-21. Summary of the steps leading from gene to protein in eucaryotes and bacteria. The final level of a protein in the cell depends on the efficiency of each step and on the rates of degradation of the RNA and protein molecules. (A) In eucaryotic cells the RNA molecule produced by transcription alone (sometimes referred to as the primary transcript) would contain both coding (exon) and noncoding (intron) sequences. Before it can be translated into protein, the two ends of the RNA are modified, the introns are removed by an enzymatically catalyzed RNA splicing reaction, and the resulting mRNA is transported from the nucleus to the cytoplasm. Although these steps are depicted as occurring one at a time, in a sequence, in reality they are coupled and different steps can occur simultaneously. For example, the RNA cap is added and splicing typically begins before transcription has been completed. Because of this coupling, complete primary RNA transcripts do not typically exist in the cell. (B) In procaryotes the production of mRNA molecules is much simpler. The 5 end of an mRNA molecule is produced by the initiation of transcription by RNA polymerase, and the 3 end is produced by the termination of transcription. Since procaryotic cells lack a nucleus, transcription and translation take place in a common compartment. In fact, translation of a bacterial mRNA often begins before its synthesis has been completed . 真核生物やバクテリアの遺伝子から蛋白への過程の要約 細胞内の蛋白の最終的なレベルは各段階の効率と RNA や蛋白の分解割合に依存している。 (A) 真核細胞では転写のみで作られる RNA 分子は（時に、 primary RNA, （第一次 RNA ）とよばれる )coding （アミノ酸になるコード (exon 、エクソン ) と noncoding （アミノ酸のコードをもたない (intron, イントロン ) の配列を含んでいる。蛋白に翻訳（ translation) される前はこの RNA の両端は修飾され、イントロンは酵素的に触媒された RNA-splicing( 切断）反応により除去される。結果として出来た RNA は核から細胞質へ移動する。これらの段階は連なって一時に一つ起きるように描かれているが、実際はそれらは組み合わされ異なる段階が同時的に起きることができる。
• #45: Figure 8-42. The common origin of bacterial and eucaryotic RNA polymerases. Amino acid sequence similarities between the b' subunit of E. coli RNA polymerase and the largest subunit of eucaryotic RNA polymerase II are among the comparisons that reveal a common evolutionary origin for the bacterial and eucaryotic enzymes. The b' subunit is thought to bind to DNA. The regions of the sequence shown as green bars are more than 70% identical between yeast and Drosophila and more than 40% identical between Drosophila and E. coli . A uniquely eucaryotic sequence (of seven amino acid residues) is repeated 26 times at the carboxyl terminus of the yeast subunit and more than 40 times in the Drosophila subunit (indicated here by green circles ); these repeats are phosphorylated as part of the process that starts an RNA chain in eucaryotes (see Figure9-30 ). (After A.L. Greenleaf et al., in RNA Polymerase and the Regulation of Transcription [W.S. Reznikoff et al., eds.], pp. 459-464. New York: Elsevier, 1987.)
• #50: Figure 6-4. Start and stop signals for RNA synthesis by a bacterial RNA polymerase. Here, the lower strand of DNA is the template strand, whereas the upper strand corresponds in sequence to the RNA that is made (note the substitution of U in RNA for T in DNA). (A) The polymerase begins transcribing at the start site. Two short sequences ( shaded red ), about -35 and -10 nucleotides from the start, determine where the polymerase binds; close relatives of these two hexanucleotide sequences, properly spaced from each other, specify the promoter for most E. coli genes. (B) A stop (termination) signal. The E. coli RNA polymerase stops when it synthesizes a run of U residues ( shaded blue ) from a complementary run of A residues on the template strand, provided that it has just synthesized a self-complementary RNA nucleotide sequence ( shaded green ), which rapidly forms a hairpin helix that is crucial for stopping transcription. The sequence of nucleotides in the self-complementary region can vary widely. Pribnow box:AT-rich region, about 7 bp long, central to prokaryotic promoters at about position –10 from the initiation site. Palindrome sequence: nucleotide sequence that is identical to its complementary strand when each is read in ths same (5-3) chemical direction. aagctt ttcgaa
• #51: Figure 6-23. The "RNA factory" concept for eucaryotic RNA polymerase II. Not only does the polymerase transcribe DNA into RNA, but it also carries pre-mRNA-processing proteins on its tail, which are then transferred to the nascent RNA at the appropriate time. There are many RNA-processing enzymes, and not all travel with the polymerase. For RNA splicing, for example, only a few critical components are carried on the tail; once transferred to an RNA molecule, they serve as a nucleation site for the remaining components. The RNA-processing proteins first bind to the RNA polymerase tail when it is phosphorylated late in the process of transcription initiation (see Figure 6-16 ). Once RNA polymerase II finishes transcribing, it is released from DNA, the phosphates on its tail are removed by soluble phosphatases, and it can reinitiate transcription. Only this dephosphorylated form of RNA polymerase II is competent to start RNA synthesis at a promoter.
• #62: Figure 6-27. Alternative splicing of the a-tropomyosin gene from rat. a-tropomyosin is a coiled-coil protein (see Figure 3-11 ) that regulates contraction in muscle cells. The primary transcript can be spliced in different ways, as indicated in the figure, to produce distinct mRNAs, which then give rise to variant proteins. Some of the splicing patterns are specific for certain types of cells. For example, the a-tropomyosin made in striated muscle is different from that made from the same gene in smooth muscle. The arrowheads in the top part of the figure demark the sites where cleavage and poly-A addition can occur.
• #63: Figure 7-90. Negative and positive control of alternative RNA splicing. (A) Negative control, in which a repressor protein binds to the primary RNA transcript in tissue 2, thereby preventing the splicing machinery from removing an intron sequence. (B) Positive control, in which the splicing machinery is unable to efficiently remove a particular intron sequence without assistance from an activator protein.
• #73: Figure 11.14. The ribosome binding site for bacterial translation. In Escherichia coli , the ribosome binding site has the consensus sequence 5 -AGGAGGU-3 and is located between 3 and 10 nucleotides upstream of the initiation codon. Shine Dalgarno sequence The ribosome binding site upstream of an Escherichia coli gene
• #74: Subunidad pequenna ribosoma y ARNt iniciador para iniciación
• #76: Cuando el ribosoma alcanza un codón de terminación (en este ejemplo UGA), el polipéptido se escinde del último tRNA y el tRNA se desprende del sitio P. El sitio A es ocupado por un factor de liberación que produce la disociación de las dos subunidades del ribosoma.