Ribonucleic Acid (RNA)

Ribonucleic Acid (RNA)

RNA is found in the nucleus and in the cytoplasm. It is a single-stranded, long chain macromolecules of ribonucleotides. Like DNA the ribonucleotide of RNA is also formed by cross-linking of three substances- phosphoric acid, a pentose sugar (ribose sugar) and nitrogen bases. The ribose sugar differs from the deoxyribose sugar of DNA in having four rather than three hydroxyl groups. The nitrogenous base of RNA is two types

Purine : adenine (A) and guanine (G)

Pyrimidine: Cytosine (C) and uracil(U)

Thus structurally DNA and RNA show two main differences:

DNA contains deoxyribose sugar and RNA contains ribose sugar.

DNA contains thymine and RNA contains uracil.

Being single stranded, the nitrogen bases of RNA remain unpaired and the complementary bases are found in the same strand. The four nucleotides in the RNA are A,G,C and U-nucleotides. Double-stranded RNA found in reovirus. In a double stranded RNA, the base pairing occurs between purines and pyrimidines. Guanine (G) and cytosine (C) are held together by two hydrogen bonds. The strand is made up of alternate bands of ribose sugar and phosphate (H₃PO₄) Molecule. A nitrogen base, a ribose sugar and a phosphate from a nucleotide and a nucleotide without the phosphate is called nucleoside. In most of the plant viruses and in some animal viruses the genetic material is RNA rather than DNA. The RNA is mainly classified into three types:

1. m-RNA (Messenger RNA)
2. r-RNA (Ribosomal RNA)
3. t-RNA (Transfer RNA)

• m-RNA (Messenger RNA): mRNA constitutes about 5-10% of the total RNA present in the cell. It carries the genetic information from DNA for protein synthesis. It is synthesized in the nucleus along the strand of DNA. The mRNA molecules leave the nucleus through the nuclear pores and reach the cytoplasm, where they get associated with the ribosome. Here, the mRNA code is translated into a sequence of amino-acids in a growing polypeptide chain.During protein synthesis, the ribosome is often found in clusters of linked together by a strand of mRNA. This complex is known as polysome.
• Size: The average molecular weight of the mRNA is about 500000 Dalton and the sedimentation coefficient is 8s. however, the molecular weight mRNA depends on upon the length or the molecular weight of gene.
• Characters of mRNA (prokaryotes)
• Polycistronic (single mRNA carry multiple genes and code multiple)
• Have the short half-life.
• No tail and cap region

• Characters

• Monocistronic
• 5’ end of eukaryotic mRNA consists of cap region which is composed of 7 methyl guanylate.
• 3’ end of the eukaryotic mRNA consists of poly A tail which consists of about 300 adenine residues which are catalyzed by the enzyme poly A polymerase.
• The translation of eukaryotic mRNA starts after completion of transcription whereas translation of prokaryotic mRNA starts before the completion of transcription.
• r-RNA (ribosomal RNA): r-RNA makes about 80% of the total RNA in the cell. It is the major components of ribosomes. It is synthesized by the genes present on the DNA of nucleolus called nucleolar organizer. It is found in the cytoplasm. It also provides proper binding sites for the mRNA on the ribosome.
• t-RNA (Transfer RNA): t-RNA forms about 10-15% of the total cell RNA. During protein synthesis, tRNA carries amino acid molecules from the amino acid pool to the ribosome. tRNA formed just like mRNA from the RNA template. Each amino acid has its own specific tRNA which transfers it from the cytoplasm to the ribosomes.
• The general structure of t-RNA: There are about 60 different t-RNA in the bacterial cell and 100-110 in mammalian cells.Transfer RNA molecules are the short single-stranded molecule that contains extensive secondary structures and have the length of about 73-93 nucleotides. Certain bases and secondary structures are constant for all t-RNAase whereas other parts are variable. Transfer RNA molecule also contain purine and pyrimidine bases that differ slightly from the normal bases found in RNA because they are chemically modified. The active t-RNA not only contains unusual bases but also extensive double-stranded region within the molecule. This secondary structure forms by internal base pairing when the single-stranded molecule folds back on itself. The structure of t-RNA can be drawn in conventional cloverleaf fashion. Some region of a t-RNA secondary structure are named after the base most often found there or after their specific functions (anticodon loop and acceptor stem).

• Each t-RNA has a specific anticodon and carries a specific amino acid. They show the following structural features:
• The 5’end of the t-RNA always ends in the base guanine. The 3’ end always ends in the base sequence of CCA. It is the site for binding to a specific amino acid. The amino acid becomes covalently attached to A of CCA sequence during polypeptide synthesis.
• An amino acid is transferred onto the t-RNA by a specific enzyme.
• The other side loop helps in the binding of t-RNA with the ribosome.
• The lowermost loop has 3 base sequences called anticodon. These are complementary to the triplet code of the mRNA. For example, if the triplet code at mRNA is AUG, the anticodon at t-RNA would be UAC.
• Thus,t-RNA molecule has two recognising sites, one for specific amino acid and the other for the codon of the mRNA.

Biological significance

RNA is very important for living cells. It plays an active role in cell metabolism.

• All the three types of RNA play a specific role in protein synthesis and hence in the expression of genes.
• In most plant viruses and in animal viruses, DNA does not occur. In them, RNA performs the genetic functions. This type of RNA is called genetic RNA or viral RNA.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.