Pre-eukaryote

Glycolysis, a series of reactions that occurs in almost every living cell, is believed to be among the oldest of all the biochemical pathways. Both the enzymes and the number and mechanisms of the steps in the pathway are highly homologous in prokaryotes and eukaryotes. Also, glycolysis is an anaerobic process, which it would have had to be in the oxygen-poor atmosphere of pre-eukaryotic Earth. 

Eukaryotic mRNA synthesis results in a pre-mRNA precursor that contains extensive amounts of excess RNA (introns) that must be precisely removed by RNA splicing to generate functional, translatable mRNA composed of exonic coding and noncoding sequences. 

Eukaryotes have a specific signal for termination of transcription however, prokaryotes seem to have lost this mechanism. Once started, RNA polymerase keeps going, making a primary transcript [pre-mRNA or hnRNA (for heterogeneous nuclear)] until far past the end of the final mRNA message. 

Heterogeneous nuclear RNA (huRNA or pre-mRNA), which is found only in the nucleus of eukaryotic cells. It represents precursors of mRNA, formed during its post-transcriptional processing. 

The removal of introns from pre-messenger RNAs in eukaryotes is catalyzed by the spliceosome, which is a large ribonucleoprotein consisting of at least 70 proteins and five small nuclear RNAs (snRNA) [144]. This splicing pathway involves two phosphotransfer reactions. In the first step, the 5 splice site is attacked by a 2 hydroxy group of an adenosine nucleotide within the intron [indicated by A in Fig. 12] that corresponds to the branch point in the lariat intermediate (Fig. 12,middle). In the second step, the 3 -OH group of the free 5 exon attacks the phosphodiester bond between the intron and... 

C. Eukaryotic transcription is more complex than in prokaryotes, mainly in terms of the nature of the RNA polymerases, the assembly of the pre-initiation complex, and the need for processing eukaryotic RNAs. 

RNA polymerase I (Pol I) is responsible for the synthesis of only one type of RNA, a transcript called pre-ribosomal RNA (or pre-rRNA), which contains the precursor for the 18S, 5.8S, and 28S rRNAs (see Fig. 26-22). Pol I promoters vary greatly in sequence from one species to another. The principal function of RNA polymerase II (Pol II) is synthesis of mRNAs and some specialized RNAs. This enzyme can recognize thousands of promoters that vary greatly in sequence. Many Pol II promoters have a few sequence features in common, including a TATA box (eukaryotic consensus sequence TATAAA) near base pair —30 and an Inr sequence (initiator) near the RNA start site at +1 (Fig. 26-8). 

Posttranscriptional processing is not limited to mRNA. Ribosomal RNAs of both prokaryotic and eukaryotic cells are made from longer precursors called preribosomal RNAs, or pre-rRNAs, synthesized by Pol I. In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing (Fig. 26-21). 

In eukaryotes, a 45S pre-rRNA transcript is processed in the nucleolus to form the 18S, 28S, and 5.8S rRNAs characteristic of eukaryotic ribosomes (Fig. 26-22). The 5S rRNA of most eukaryotes is made as a completely separate transcript by a different polymerase (Pol III instead of Pol I). 

Removal of internal sequences in eukaryotes is not restricted to mRNA processing. It also occurs in the processing of rRNA and some tRNAs. In tRNAs the mechanism appears to be different in that the signal for splicing originates not from the primary sequence but from the secondary or tertiary structure of the pre-tRNA. 

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