Protein synthesis broken transcripts

Before the hnRNA produced by RNA polymerase II (see p. 242) can leave the nucleus in order to serve as a template for protein synthesis in the cytoplasm, it has to undergo several modifications first. Even during transcription, the two ends of the transcript have additional nucleotides added (A). The sections that correspond to the intervening gene sequences in the DNA (introns) are then cut out (splicing see B). Other transcripts—e.g., the 45 S precursor of rRNA formed by polymerase I (see p. 242)—are broken down into smaller fragments by nucleases before export into the cytoplasm. 

Dealing with lost peptidyl-tRNAs and broken transcripts. Many problems arise during protein synthesis. For example, a peptidyl-tRNA may become detached from a ribosome. In E. coli this seems to happen most frequently with peptidyl-tRNA y5. A 193-residue peptidyl-tRNA hydrolase is essential for life 434-436 It releases the tRNA for reuse, recycling all peptidyl-tRNAs other than formylmethionyl-tRNA. Perhaps the enzyme is essential because detached peptidyl-tRNAs are toxic, but it is more likely to be to avoid a shortage of free tRNAl vs.434... 

There are two steps in protein synthesis where polarity of information is important. The first is the relationship between the 50 to 30 directionality of mRNA, and the NH3+ to COO- terminal direction of protein synthesis. The utilization of tRNA as the adaptor is the second step where polarity of information is crucial. The tRNA has a bipolar function, it needs to correctly link each amino acid to the corresponding position encoded by the mRNA. Figure 26.1 shows an overview of how mRNA synthesis and protein translation share the same polarity. Moreover, similar to transcription, translation can also be broken down into three discrete components initiation, elongation, and termination. 

For all forms of DNA, hydrogen bonding between the two spiral chains stabilizes the double helix. Replication of DNA occurs when the hydrogen bonds are broken, and the two strands are separated. These form the templates that are used to make identical copies, via enzymes called DNA polymerases. In fact, the second strand of the double helix is complementary to the first, it contains no extra information but is involved in replication. Ribonucleic acid (RNA) is also found in cells. It has a similar structure to DNA, but the sugar is instead D-ribose and uracil bases replace thymine bases. RNA is important in the synthesis of proteins. It is produced from DNA templates via the process of transcription. Further details of protein biochemistry can be found elsewhere (e.g. Voet and Voet, 1995). Here we simply emphasize that life itself is created from that special class of soft material called polymers. 

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