RNA and the Genetic Code

Ribonucleic acid, RNA, which makes up most of the nucleic acid found in the cell, is involved with transmitting the genetic information needed to operate the cell. Similar to DNA, RNA molecules are polymers of nucleotides. However, RNA differs from DNA in several important ways  

The sugar in RNA is ribose rather than the deoxyribose found in DNA. 

Messenger RNA (mRNA) carries genetic information from the DNA, located in the nucleus of the cell, to the ribosomes located in the cytoplasm. A gene segment of DNA will produce a separate mRNA for a particular protein that is needed in the cell. The size of the mRNA depends on the number of nucleotides in that gene. 

Transfer RNA (tRNA), the smallest of the RNA molecules, interprets the genetic information in mRNA and brings specific amino acids to the ribosome for protein synthesis. Only tRNA can translate the genetic information into amino acids for proteins. There 

FIGURE 17.9 Atypical ribosome consists of a small subunit and a large subunit. 

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RNA World hypothesis The recognition that RNA can self-replicate leads to the idea that information propagation and the genetic code first started with RNA. 

In a nutshell, RNA transcribes the genetic code of the DNA, and transfers it to the cell s protein factories. This process also requires a shearing and splicing of the RNA molecules, because DNA strands contain regions which are not essential for... 

Genetic information is accessed by a process known as transcription, in which the double-stranded DNA splits and the genetic code is transcribed onto a single-strand messenger RNA(mRNA). The mRNA is comprised of the same bases as the DNA, arranged in the same sequence, but in a complementary fashion. The mRNA migrates out of the nucleus and into the cytoplasm, where it attaches to ribosomes. The ribosomes assemble amino acids to form protein molecules through a process known as translation. 

RNA has a number of functions. For the purposes of protein synthesis, there are three main forms, messenger RNA, transfer RNA and ribosomal RNA. In addition, some RNAs are thought to be involved in catalytic processes, while in certain viruses RNA carries the genetic code. In the AIDs virus, for example, there are two (single) strands of RNA carrying the code for a new virus. Here we will confine our discussion to those RNA molecules that are involved in expressing genetic information. 

Messenger RNA carries the genetic code in the form cf purine and pyrimidine base triplets, called codons. Each... 

Wolf YI, Koonin EV On the origin of the translation system and the genetic code in the RNA world by means of natural selection, exaptation, and subfunctionalization. Biol Direct 2007, 2 14. 

Protein synthesis occurs in vivo in two steps. In the first step the polymerizing enzyme RNA-nucleotidyl transferase (RNA polymerase) catalyzes the formation of messenger RNA (m-RNA) from ribonucleotides at the site of the stored information (in the cell nucleus). DNA is the template. Messenger RNA contains the genetic code (amino acid code). Thus, m-RNA is complementary to the strand of DNA used as a template, i.e., cytosine corresponds to guanine, and vice versa uridine corresponds to adenine, and adenine corresponds to thymine (see also Chapter 29),... 

Nucleic acids Polymers made of phosphate-linked sugars bearing the heterocyclic bases adenine (A), guanine (G), cytosine (C), and thymine (T) in DNA or uracil (U) in RNA contain the genetic code for protein biosynthesis... 

Section 28 11 Three RNAs are involved m gene expression In the transcription phase a strand of messenger RNA (mRNA) is synthesized from a DNA tern plate The four bases A G C and U taken three at a time generate 64 possible combinations called codons These 64 codons comprise the genetic code and code for the 20 ammo acids found m proteins plus start and stop signals The mRNA sequence is translated into a prescribed protein sequence at the ribosomes There small polynucleotides called... 

RNA structures, compared to the helical motifs that dominate DNA, are quite diverse, assuming various loop conformations in addition to helical structures. This diversity allows RNA molecules to assume a wide variety of tertiary structures with many biological functions beyond the storage and propagation of the genetic code. Examples include transfer RNA, which is involved in the translation of mRNA into proteins, the RNA components of ribosomes, the translation machinery, and catalytic RNA molecules. In addition, it is now known that secondary and tertiary elements of mRNA can act to regulate the translation of its own primary sequence. Such diversity makes RNA a prime area for the study of structure-function relationships to which computational approaches can make a significant contribution. 

Fig. 1.5 Schematic representation of the evolution of life from its precursors, on the basis of the definition of life given by the authors. If bioenergetic mechanisms have developed via autonomous systems, the thermodynamic basis for the beginning of the archiving of information, and thus for a one-polymer world such as the RNA world , has been set up. Several models for this transition have been discussed. This phase of development is possibly the starting point for the process of Darwinian evolution (with reproduction, variation and heredity), but still without any separation between genotype and phenotype. According to the authors definition, life begins in exactly that moment when the genetic code comes into play, i.e., in the transition from a one-polymer world to a two-polymer world . The last phase, open-ended evolution, then follows. After Ruiz-Mirazo et al. (2004)...

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

Transfer RNA (tRNA) RNA with a triplet nucleotide sequence that is complementary to the triplet nucleotide coding sequences of mRNA. tRNAs in protein synthesis bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA... 

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