during translation movement of ribosome from end of mrna to other end bt the distance of one triplet codon is called???
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Answer:We have followed individual ribosomes as they translate single messenger RNA hairpins tethered by the ends to optical tweezers. Here we reveal that translation occurs through successive translocation-and-pause cycles. The distribution of pause lengths, with a median of 2.8 s, indicates that at least two rate-determining processes control each pause. Each translocation step measures three bases—one codon—and occurs in less than 0.1 s. Analysis of the times required for translocation reveals, surprisingly, that there are three substeps in each step. Pause lengths, and thus the overall rate of translation, depend on the secondary structure of the mRNA; the applied force destabilizes secondary structure and decreases pause durations, but does not affect translocation times. Translocation and RNA unwinding are strictly coupled ribosomal functions.
Current understanding of the ribosome and the mechanism of translation has been significantly strengthened and expanded by recent advances in crystallography1-6 and cryo-electron microscopy7-10. The ribosome undergoes several dynamical structural changes as it moves relative to the mRNA and transfer RNAs during translation8,11. Kinetic experiments have given a quantitative description of some of these dynamics during the main steps of the elongation cycle of protein synthesis12. During elongation, the secondary structures present in all mRNAs are disrupted to allow movement of the mRNA through the 30S subunit, and the reading of each codon. This task is aided by the mRNA helicase activity of the ribosome that has been localized to the downstream tunnel of the 30S subunit13. Moreover, interactions of mRNA pseudoknots or hairpins with the helicase region of the ribosome can shift the reading frame of the mRNA to the -1 frame, and play an important role in regulating gene expression in retroviruses14-16.
It is extremely difficult to follow the steps of ribosomes during translational elongation using ensemble methods, because the dynamics of individual ribosomes are stochastic17,18 and it is impossible to synchronize their activity. Here, we have used optical tweezers to follow the step-by-step translation of a single hairpin-forming mRNA molecule by a single ribosome. This approach has allowed us to characterize the dynamics of ribosome translation, measuring the time the ribosome spends at each codon, the number of mRNA nucleotides that move through the ribosome in each translocation step, and the time required per step. We have also determined the effects of mRNA structure on step size and rate, and have studied the effects of internal Shine-Dalgarno sequences19 on translation arrest. These experiments provide a dynamic picture of the movement of a messenger RNA through a ribosome.
In these experiments, we used a single mRNA hairpin with a ribosome stalled at the 5′ end by omission of a required aminoacyl-tRNA; the RNA was attached to two micrometre-sized beads by RNA-DNA handles. One of the beads was held on a micropipette and the other in an optical trap used to measure the changes in distance between the beads (in nanometres) and the forces applied to the hairpin (in piconewtons) (Fig. 1A)20-22. Translation is resumed at the single-molecule level by adding a mixture containing the required aminoacyl-tRNAs. During translation, the ribosome opens the hairpin as it moves through the RNA; thus, each base translocated requires the breaking of a base pair, which corresponds to an increase in the end-to-end distance of the mRNA by about 1 nm at the forces involved in these experiments (15-20 pN)23. Translation can thus be followed in real time by monitoring these changes in distance.
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