Although double stranded RNA is not uncommon, mainly RNA is found as single stranded molecules. Chemical analysis shows the following two main differences in the polynucleotide chains of DNA and RNA :
While deoxyribose sugar is found in DNA, in RNA ribose sugar is found. This difference between DNA and RNA is the most important and would distinguish the two under all conditions.
While in DNA, adenine, guanine, thymine and cytosine are the four common bases, in RNA the four bases are adenine, guanine, uracil and cytosine. Since there is a large number of other unusual bases found in RNA as well as in DNA and also since thymine may rarely be found in RNA also, this distinction is not as important as that of sugar molecule.
There was yet a third distinction described, i.e., double stranded nature of DNA and single stranded nature of RNA. This distinction is no longer tenable in so far as single stranded DNA (e.g. phage Φ x 174, parvovirus) and double stranded RNA (see Table 25.4) are now known in a number of cases.
While DNA is perhaps always genetic in nature, RNA is only rarely genetic in nature. This would mean that although the primary function of storing and carrying the genetic information is rarely also associated with RNA in some organisms, but mainly RNA, found in all organisms, would perform different important functions during protein synthesis. Therefore, two types of RNA are really known, namely genetic RNA and non-genetic RNA.
Genetic RNA. In
most plant viruses, genetic material consists of RNA, although now some plant viruses are known which have DNA. In many bacteriophages also, genetic material is RNA rather than DNA. Viroids
are other two classes of small RNAs (~ 350 bases) found in plants. Viroids are subviral pathogens and function independently without encapsidation by a protein coat (e.g. potato spindle tuber viroid or PSTV). Virusoids (also called satellite RNAs)
are encapsidated by plant viruses, packaged together with a viral genome. Virusoids can not replicate independently, and do so with the help of an associated virus. Such genetic RNA contains information which is normally found in DNA in higher organisms. In other words, RNA has replaced DNA in such cases. The RNA found in these viruses could be single stranded or double stranded (Table 25.4).
When RNA is double stranded, it generally follows the same rules of base pairing as in case of DNA. Different viruses having single stranded or double stranded RNA are listed in Table 25.4.
Non-genetic RNA. In
organisms, where genetic information is contained in, and transmitted through DNA, RNA though present in good quantity, but does not serve as a genetic material. This non-genetic RNA is synthesized on DNA template and is of following three different types.
(a) Messenger RNA (mRNA)
carries genetic information contained in DNA and makes a small fraction (5%-10%) of total RNA present in the cell. This species of RNA is rather short-lived and therefore has a rather rapid turnover. The molecular weight of this RNA varies, usually from 500,000 to 2,000,000.
(b) Transfer RNA (tRNA),
also known as soluble RNA (sRNA), makes another small fraction (10%-15%) of RNA. These are the smallest molecules of RNA and work as adapter molecules for carrying amino acid molecules to the site of protein synthesis. Being smallest in size, these molecules have been most thoroughly investigated. More detailed information on these molecules is given in Expression of Gene : Protein Synthesis 1. Proteins and Protein Synthesis Apparatus (tRNA and Ribosomes)
(c) Ribosomal RNA (rRNA)
is the most stable kind of RNA and is associated with the ribosomes, of which this RNA species makes 40%-60% by weight. This species of RNA also makes about 80% of the total RNA in the cell.
Besides the above three major classes of RNA, there is a variety of other non-genetic RNA species found in cells for specific functions. These include the following : (i) Anti-sense RNA
also called mic RNA
(messenger RNA inhibiting complementary RNA) is synthesized sometimes on the strand complementary to the one used for mRNA synthesis. This is used for regulation of DNA synthesis and gene expression, both in vivo
and in vitro
(consult Regulation of Gene Expression 1. Operon Circuits in Bacteria and other Prokaryotes
). (ii) HnRNA
is synthesized from split genes in eukaryotes, and particular class (group 1) of intron sequences are self-spliced due to ribozyme activity of RNA (consult Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)
). (iii) SnRNA
(small nuclear RNA) and ScRNA
(small cytoplasmic RNA) are found in the nucleus and cytoplasm respectively. SnRNAs (Ul, U2, U4, U5, U6) are used for the formation of spiiceosome
leading to splicing of another category (group 2) of intron sequences, (iv) Guide RNA (gRNA),
synthesized from minicircle mitochondrial DNA (mtDNA), is used for RNA editing leading to modification of RNA transcribed from DNA template (consult Expression of Gene : Protein Synthesis 3. RNA Processing (RNA Splicing, RNA Editing and Ribozymes)