Ribose Nucleic Acid:

 

Introduction:

 

Ribose nucleic acids, when researchers’ realized that life originated on this planet as RNA world nearly 3.8 billion years ago. It is now considered as primeval molecule of life, and the life originated with RNA as the genetic material. Once DNA was deemed as the sole genetic materials of all living organisms, till the discovery of Tobacco Mosaic Virus (TMV) (Russian scientist), which contained RNA, now RNA can act as the genetic material.  Once RNA was considered as an accessory form of nucleic acids sub-serving, but soon people realized, that some RNAs have coded information and some were capable of performing catalytic activity, where proteins were once considered as the sole macromolecules capable performing enzymatic catalytic functions.  When life originated around 3.1-3.2 billion years ago, it was the RNA world. Everyone now trying to understand, how RNA world gave rise to DNA world. DNA world as genetic material is dominant, yet RNA has a hold on DNA’s function.  Ever since Feulgan demonstrated, by chromogenic reactive agents, that there are two types of nucleic acids, one Feulgan positive called DNA and the other Feulgan’s negative called RNA, understanding of the chemistry, structure and functions in detail has been the goal of a large number of molecular biologists, yet there are many aspects of RNA remain shrouded with mystery.

 

Quantity of RNA in a cell:

 

More than 90% of the total RNA of the cell is found in cytoplasm, the rest of it is in the nucleus especially in nucleolus region. 

• Total concentration of RNA changes depending upon the kind of cell and its state.

Active cells contain more RNA than inactive or resting cells. Perhaps highest concentration RNA is found in brain cells, as they are the most active cells in human body. 

• In comparison to DNA, certain types of RNAs are very unstable; their half-life is just few minutes, but some may remain longer than 120 days or more. 

Shorter half-life of some types of RNAs is of importance to cellular development and stability.

• Some RNA species are common to all cells; some are specific to cell types.

 

Chemistry:

 

Chemical analysis of RNA shows greater similarities with DNA with respect to the components it contains.  It is basically made-up of a ribose sugar (where as DNA contains deoxyribose sugar), a phosphate group (DNA also contain the similar phosphate groups), and nitrogenous bases such as adenine, guanine, cytosine (all the three are also present in DNA) and uracil (DNA contains Thymidine in the place of Uracil).

  

• These individual nucleotides are synthesized in a series of enzymatic reactions. They are named after their respective bases, such as rATP, rGTP, rCTP and rUTP.

 

• Among them rATP is the most important energy provider for a variety of cellular functions, whereas others like rGTP or rATP derived- cyclicAMP rGTP, cyclicGMP act as a secondary messengers.  rGTP also plays an important role in protein synthesis,  polymerization of Tubulins (~55kDa) into microtubules, protein synthesis, enzyme regulation, glycosylation  and  carbohydrate metabolism, to say that they are the few functions of rGTP.  Similarly rCTP, rUTPs are also involved in glycosylation and carbohydrate synthesis. rATPs are involved in polymerization of G-Actins to F-actins. Many of the nucleotides are substantially, in a specific manner, modified after RNA synthesis.

 

Composition of Nucleotides in RNA varies from one species to the other, and depends on what template on which it is produced and at what time.

 

• There is no equivalence of purines and pyrimidines in terms of quantity unlike DNA, where total pyrimidines is equal total purines, In most of the cases DNA is double stranded, in most cases RNA is single stranded with occasional secondary structures with helical or hair pin structures are found.

 

Structure:

 

All RNAs, whatever may be the type, structure and functions they perform exist as polynucleotide chains with 5’ to 3’ polarity.

 

• Each of the polynucleotide chains is synthesized on DNA or RNA templates.

 

• Pyro-phosphorylase called (Polynucleotide Phosphorylase) is the enzyme found in cells capable of producing polynucleotide chains by random polymerization and with out templates. 

 

Size and molecular weight of RNAs molecules vary from one species to the other, which range from 20-22 ntds (60x365= 21900 Daltons) to 10 000 or more (365 0000 Daltons).

 

• Basically RNAs are synthesized as polynucleotide chains (a primary structure), but they immediately or while they are in the process of synthesis, they are processed and fold, because of complementary base pairing into secondary structures and even possibly into tertiary structures. Binding of specific protein provides stability of 3-D structures of RNAs.

 

Even higher order of organization is possible with association of more than one RNA chains and a variety of proteins into a complex and compact 3-D structure ex. RNA+RNPs-Ribosomes.

 

Based on structural organization and functions, RNAs have been classified, into Ribosomal RNA (rRNA more than 90% of cellular RNA), Transfer RNA (tRNA) and Messenger RNA (mRNA). These are the major class of RNAs involved in decoding the information into polypeptides.  But there is another category of small Mol.wt RNAs called non coding RNAs (ncRNAs).  Recent estimation of them is approximately 10, 000 or more. They are now named as ncRNAs most of them are functional RNAs ex.  Sc RNA, Sn RNA, Antisense RNAs such as Si RNAs or Mi micro RNAs, Primer PriRNAs, GuideG  RNAs, Efference eRNAs,  Xist RNAs, piRNAs, pRNAs (p1RNase), Tm RNAs (transfer-mRNA like), SnoRNAs, , Telomeric RNA, 7sRNA, 7sK RNA, B2 RNA, Srp RNA,  and many more (Refer to the chapter ‘small molecular wt. RNAs’).  According to ENCODE information the HUGO people have given ~3000 specific names for the small ncRNAs. With the exception of mRNA, tRNA and Genetic RNAs all others can be clubbed into Non Coding RNAs (Nc small RNAs). Most of these ncRNA are transcribed by non-coding DNA and many of them derived from regular transcripts but most of them are regulatory in nature.