INTRODUCTION

The central dogma—that DNA makes RNA that makes protein—presented evolutionary biologists with a knotty puzzle: if nucleic acids are required to direct the synthesis of proteins, and proteins are required to synthesize nucleic acids, how could this system of interdependent components have arisen? One view is that an RNA world existed on Earth before cells containing DNA and proteins appeared.

According to this hypothesis, RNA—which today serves largely as an intermediate between genes and proteins—both stored genetic information and catalyzed chemical reactions in primitive cells. Only later in evolutionary time did DNA take over as the genetic material and proteins become the major catalysts and structural components of cells.

If this idea is correct, then the transition out of the RNA world was never completed; as we have seen, RNA still catalyzes several fundamental reactions in modern cells. These RNA catalysts—or ribozymes—including those that operate in the ribosome and in the RNA-splicing. Certainly, its chemical composition only differs from that of DNA in seemingly minor respects: a hydroxyl group in place of a hydrogen atom in its backbone and the absence of a methyl group on one of its four bases. Indeed, for many years, RNA was  seen as simply playing a supporting role to DNA  in information transfer.

Certain viruses have RNA genomes, but in the main, DNA is the repository of genetic information in Nature. Instead, RNA was largely seen as the shuttle that transferred genetic information from DNA to the ribosome, the adaptor that decoded that information, and as a structural component of the ribosome. We now recognize, however, that RNA is far richer and more intricate in structure than DNA and far more versatile in function than first appreciated.

RNA is principally found as a single-stranded molecule. Yet by means of intrastrand base pairing, RNA exhibits extensive double-helical character and is capable of folding into a wealth of diverse tertiary structures.

These structures are full of surprises, such as nonclassical base pairs, base–backbone interactions, and knot-like configurations. Most remarkable of all, some RNA molecules are enzymes, one of which performs a reaction that is at the core of information transfer from nucleic acid to protein and hence is of profound evolutionary significance.

STRUCTURE AND FUNCTION OF RNA

STRUCTURAL DIFFERENCE BETWEEN RNA AND DNA

RNA differs from DNA in three respects.

  • First, the backbone of RNA contains ribose rather than 2 -deoxyribose. That is, ribose has a hydroxyl group at the 20 position.
STRUCTURE AND FUNCTION OF RNA
FIGURE DEPICTING :- Structure of the backbone of the RNA
  • First, the backbone of RNA contains ribose rather than 2 -deoxyribose. That is, ribose has a hydroxyl group at the 20 position.
STRUCTURE AND FUNCTION OF RNA
FIGURE DEPICTING :- Uracil pairs with adenine
  • Third, RNA is usually found as a single polynucleotide chain. Except for certain viruses, such as those that cause influenza and acquired immune deficiency syndrome, RNA is not the genetic material and does not need to be capable of serving as a template for its own replication. Rather, RNA functions as the intermediate, the messenger RNA (mRNA), between the gene and the protein-synthesizing machinery
STRUCTURE AND FUNCTION OF RNA
FIGURE DEPICTING :- Uracil pairs with adenine

STRUCTURE OF RNA

(A) mRNA STRUCTURE

  • In bacteria and archaea, a single mRNA molecule may code for one or several polypeptide chains. If it carries the code for only one polypeptide, the mRNA is monocistronic; if it codes for two or more different polypeptides, the mRNA is polycistronic. In eukaryotes, most mRNAs are monocistronic. The minimum length of an mRNA is set by the length of the polypeptide chain for which it codes.

  • For example, a polypeptide chain of 100 amino acid residues requires an RNA coding sequence of at least 300 nucleotides, because each amino acid is coded by a nucleotide.  However, mRNAs transcribed from DNA are always somewhat longer than the length needed simply to code for a polypeptide sequence.  The additional, noncoding RNA includes sequences that regulate protein synthesis.

STRUCTURE AND FUNCTION OF RNA

(B) tRNA

  • tRNA  ( transfer RNA) is a small well characterized RNA molecule with key role in protein synthesis. It is also known as adaptor RNA. The name adaptor is given because it provides interface between nucleic acid language and protein language was introduce by crick in 1955.

     

  • tRNA  is single RNA chain of 73-93 nucleotide mostly 76, present in the cytosol. Primary  tRNA folds in clover leaf like secondary structure. Following are the parts of the secondary structure.

ACCEPTOR ARM

  • Always have seven base pair and 4 unpaired nucleotide sequence, including an absolutely conserved CCA sequence. CCA is universally conserved for all tRNA

D ARM

  • With 3 or 4 base pair stem and 5-7 nucleotide loop that frequently contains modified base dihydrouridine.

ANTICODON ARM

  • 5 base pair stem and loop that contains anticodon, a three nucleotide long sequence that is responsible for recognizing the codon by base pairing with mRNA.

T ARM

  • With 5 base pair and a loop that usually contains unusual base  pseudouridine.

VARIABLE ARM

  • Usually has 5 nucleotide but can contain upto 24 nucleotide.
FIGURE DEPICTING tRNA structure
FIGURE DEPICTING :- tRNA structure

RNA is looped out from the end of the doublehelical segment. Stretches of double-helical RNA may also exhibit internal loops (unpaired nucleotides on either side of the stem), bulges (an unpaired nucleotide on one side of the bulge), or junctions.

(C) rRNA

rRNA is a ribosomal RNA associated with set of protein to form ribosomes. The 80s ribosomes contains four different rRNA molecules 18s,28s,5.8s and 5s where as 70s ribosomes contains 16s,5s and 23s. this are the most abundant RNA most abundant molecule in the cell. They make up at least 80% of total RNA molecules found in a typical eukaryotic cells.

ROLE OF RNA

(A) IN GENE EXPRESSION

RNA plays a vital role in every step of gene expression.

  • The DNA molecule containing a gene is transcribed into RNA.
  • These instructions in the form of messenger RNA exit the nucleus into the cytoplasm.
  • RNA is translated into the protein by matching correct amino acid with a RNA codon sequence.

(B) RIBOZYME

One of the first ribozymes to be discovered was RNase P, an endoribonuclease that is involved in generating tRNA molecules from larger, precursor RNAs. Specifically, RNase P cleaves off a leader segment from the 5 end of the precursor RNA in helping to generate the mature and functional tRNA. RNase P is composed of both RNA and protein; however, the RNA moiety alone is the catalyst.

(C) OPERON

Metabolic operons in bacteria are sometimes under the control of regulatory RNA elements known as riboswitches that bind and respond to small molecule ligands in controlling gene transcription and translation. Examples of metabolites that are recognized by these riboswitches are the amino acid lysine, the nucleobase guanine.

(D) RNA AS PRIMER

If DNA polymerase cannot synthesize DNA de novo, where do primers come from? It turns out that they are not DNA, as might be expected, but rather RNA. DNA polymerase then extends this RNA primer, which is eventually excised and replaced by DNA. This extra complexity in DNA synthesis increases the fidelity of DNA replication.

CONCLUSION

RNA differs from DNA in the following ways: Its backbone contains ribose rather than 2 -deoxyribose; it contains the pyrimidine uracil in place of thymine; and it usually exists as a single polynucleotide chain, without a complementary chain. As a consequence of being a single strand, RNA can fold back on itself to form short stretches of double helix between regions that are complementary to each other.

RNA allows a greater range of base pairing than does DNA. Thus, as well as A:U and C:G pairing, non-Watson–Crick pairing is also seen, such as U pairing with G. This capacity to form noncanonical base pairs adds to the propensity of RNA to form double-helical segments.

Freed of the constraint of forming long-range regular helices,RNA can form complex tertiary structures, which are often based on unconventional interactions between bases and the sugar–phosphate backbone.

Some RNAs act as enzymes—they catalyze chemical reactions in the cell and in vitro. These RNA enzymes are known as ribozymes. Most ribozymes act on phosphorous centers, as in the case of the ribonuclease RNaseP. RNase P is composed of protein and RNA, but it is the RNA moiety that is the catalyst.

The hammerhead is a self-cleaving RNA, which cuts the RNA backbone via the formation of a 20 ,30 cyclic phosphate. Peptidyl transferase is an example of a ribozyme that acts on a carbon center. This ribozyme, which is responsible for the formation of the peptide bond, is one of the RNA components of the ribosome.

REFERENCES

  • Lehninger  principles of biochemistry seventh edition By  David L. Nelson and Michael M. Cox
  • Life sciences  fundamental and practices sixth edition, pathfinder publication By Pranav Kumar and Usha Mina
  • Molecular biology of the genes (seventh edition) by Watson, Baker, Bell, Gann, Levine, Losick

:- Article Written By Zahra Madraswala

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