Question

Why is modification required for organisms

Answer 1

modification or variation is required for an organism to survive in changing environment
if environment changes then then all those organisms will survive who have variations ...

Answer 2

A, U, C, and G are boring. Truly, four nucleosides sometimes repeated in seemingly endless sequence has a numbing effect even in a computational and structural context. However, there is nothing boring about the chemistries and structures of post-transcriptional modifications of RNA as indicated by the rising interest and rapidly increasing numbers of publications. A plethora of papers demonstrate that modified nucleosides of RNA are functionally indispensable for RNA processing and translation. Over the last 20 years, efficiency, accuracy, enhanced activity and responsiveness are terms that accompany studies of modifications. Modifications are found in all types of RNAs, in every structural motif and in unstructured regions. The large numbers of modifications in tRNAs are quite familiar. The presence, chemistries and RNA sequence locations of modified nucleosides are only now beginning to be appreciated as both temporal—in response to environmental (internal and external) signals—and spatial, in their effect on RNA conformation, dynamics, and functions. Modifications can be recognition determinants for proteins as well as incredible determiners of RNA structure and function. Unfortunately, we tend to neglect the modified nucleosides. Using amplification and transcription in vitro that disregard the native modified nucleosides, we study the interesting low abundance, non-coding RNAs for which one molecule per cell is sometimes enough to affect gene expression. On the other hand, we lack technologies sensitive enough to explore modifications and their functions in these same low abundance RNAs. Our lack of understanding of RNA modification regrettably limits innovation in the use of modified nucleosides for synthetic biology and medicine. However, the past two decades have led to some of the most interesting of findings.

Modifications change everything
The presence of modified nucleosides in RNA has been recognized for more than half a century. Fifty years ago, inosine was identified by Holley's lab at the first position of the anticodon of yeast tRNAAla and quickly became the hallmark of Crick's Wobble Hypothesis. Wondrous as that seemed at the time, we now know that RNA editing, deaminations of A to I and C to U, occur in other RNAs, particularly mRNAs. The deamination of A to I at tRNAs’ anticodon's Wobble position allows tRNA to read three synonymous codons, NNU, NNC, and NNA. The non-canonical I•A base pair begs the question of how a purine•purine interaction is accommodated at the decoding site of the ribosome. With ribosome crystal structures, Ramakrishnan's and our group showed that the decoding site accommodates the distorted anticodon-codon double helix of I•A and extensively modified anticodon•codon base pairs. These biochemical and biophysical results certainly seem to be a simple enough explanation as to how the wobble position inosine and modified pyrimidines allow a number of tRNAs to read cognate and synonymous codons. However, nature did not leave well enough alone even when it comes to the inosine adaptation of tRNAs. Our lab demonstrated that 2-thiocytidine occurring in the anticodon loop of the isoaccepting tRNAArg1 species negates inosine's ability to read adenosine while allowing the recognition of U and C. Thus, sometimes multiple modifications that occur close in three dimensional space work together to affect RNA conformation, dynamics and function. Wobble at the third position of the anticodon together with modification chemistry and structure enable some 40 cytoplasmic tRNAs to translate the 61 Universal Genetic Codes, and sometimes the altered use of the three termination codes. In the human mitochondria, only 22 tRNAs with distinctive modified anticodons are used for decoding