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RNA structure

             We know that, there are two major types of nucleic acids found in cells. RNA or Ribonucleic acid is a nucleic acid found in the nucleus as well as in the cytoplasm. So, in this article we will discuss about- What is RNA? Different types of RNA and their Functions, RNA structure diagram etc.

           There are two main types of RNA; 1. Genetic RNA  and 2. Non-genetic RNA. 
The genetic RNA acts as genetic material of some viruses. The non-genetic RNA functions for the protein synthesis in most of the organisms.

 RNA structure:

Structure of RNA

             Generally the RNA molecule is a single stranded structure. It may be simple and straight or may be variously folded or coiled upon itself. Chemically it is a polynucleotide molecule. Components of RNA are ribose sugar, phosphate as phoshoric acid and nitrogenous bases. 

                 The nitrogen bases present in RNA are as Purines: Adenine and Guanine. Pyrimidines: Uracil and Cytosine i.e. In RNA Thymine is replaced by Uracil). The strand of RNA is a long chain of nucleotides joined by phospho di-ester linkages. There are four different types of nucleotides present as A, G, U and C nucleotides. RNA strand has two ends, 3' end and 5'end.
                  In the RNA molecule if coiling or folding occurs then base pairing may occur between complementary bases as; A=U or U=A and G triple bond C or C triple bond G. 

             In the RNA molecule the base pairing is not fixed and definite. Therefore, purine : pyrimidine ratio may or may not be 1:1.

Types of RNA:

             There are three types of Non-genetic RNA. They play important role in protein synthesis in both prokaryotic and eukaryotic cells. These are m-RNA, r-RNA and t-RNA. All the three of RNA are produced on DNA template by the process called transcription. 

1. m-RNA or Messanger RNA:

a) The m-RNA molecule is always simple and straight without any fold. 

b) It carries message for protein synthesis from DNA to ribosomes.

c) About 3-5% of the total cellular RNA is of this type. 

d) It has molecular weight up to 5,00,000. It has two ends, 5' end and 3' end. 
   

m-RNA strand

            A sequence of three Nucleotides on m-RNA is called codon. Each 'triplet' codon on m-RNA codes for specific amino acid. This is called m-RNA language or genetic code or cryptogram. The codon present at 5' end of m-RNA is called start codon or initiation codon. It is usually AUG, in some case it may be GUG. The codon at 3' end is stop or non-sense codon. The codon 3' end is stop or sense codon. It is either UAA, UAG or UGA. 

               Each codon on m-RNA specifies a particular amino acid, e.g. the start codon AUG, specifies methionine or f-methionine, GGG specifies glycine, etc. Stop codons do not specify any amino acid hence are called nonsense codons. They are also called terminators as the synthesis of polypeptide chain stops and termination occurs. 

           Thus, m-RNA carries coded information about the sequence of amino acids in the polypeptide chain, from DNA to ribosomes, during protein synthesis. 

Codogen:

          It is the smallest possible sequence (triplet) of Nucleotides present on the DNA strand which can specify one particular amino acid. 
 
Codon:

          It is the smallest possible sequence (triplet) of Nucleotides present on m-RNA strand which can specify one particular amino acid. 

Anticodon: 

             It is a triplet of Nucleotides present on the anticodon loop of t-RNA which is complementary to the codon of m-RNA. 

2. r-RNA or Ribosomal RNA:

 1. It is a single stranded structure which is variously folded upon itself. 

2. In the folded regions it may show pairing between complementary bases.

 
3. It remains associated with ribosomes permanently. 

4. About 80% of the total cellular RNA is of this type. 

5. The molecular weight ranges between 40,000- 1,00,000.

r-RNA structure

           It provides proper binding site for m-RNA. It orients the m-RNA molecule in such a way that all the codons are properly read. 

3. t-RNA or Transfer RNA:

1. The t-RNA molecule is also single stranded. 

2. About 10-20% of the total cellular RNA is of this type. 

3. It is the smallest of all the RNA types containing 73-96 Nucleotides only with molecular weight ranging between 23,000-30,000. It shows two patterns of folding upon itself. The structure of t-RNA can be explained by two models. 

A) Hair pin model. 
B) Clover leaf model. 

             In hair pin model of t-RNA, by folding there is formation of one loop having a triplet of unpaired bases called anticodon. The 5' end has G-nucleotide while at 3' end there is a sequence of CCA Nucleotides. 

Hairpin model of t-RNA

             The clover leaf model of t-RNA shows presence of three arms namely DHU arm, middle arm and TYC arm. These arms have loops at their ends such as amino acyl binding loop, anticodon loop and ribosomal binding loop respectively. 

Clover- leaf model

       

     The anticodon loop has anticodon which is a triplet of unpaired Nucleotides. The anticodons present on t-RNA are complementary to codons present on the m-RNA (anticodons are also called as nodoc). In addition it also shows a small lump called variable lobe or extra arm. Like the hair-pin t-RNA it has G-nucleotide at 5' end and CCA- Nucleotides at 3' end. 

              t-RNA is also called s-RNA (soluble) because it cannot be easily separated even by ultra centrifugation technique. It carries specific type of amino acid at CCA end to the ribosomes during protein synthesis. It places the required amino acid properly in the sequence and translates the coded message of m-RNA in terms of amino acids. 

The genetic Code:

               DNA contains all the genetic information of an organism. The genes control, regulate and express the characters. The expression of a character take place through a specific protein. The gene is segment of DNA, m-RNA molecule is formed on DNA which later helps in construction of a polypeptide chain. Thus, the m-RNA act as an agent in conveying information from  DNA to cytoplasm. 

                A sequence of three Nucleotides on the m-RNA strand is called codon. Each codon codes for a specific amino acid. This was suggested by George Gamow in 1954 while the direct evidence for this was provided by Crick in 1961. Subsequently Marshall Nirenberg, Heinrich Matthaei and Har Gobind Khorana deciphered complete genetic code by using artificial m-RNA templates. 

Characteristics of genetic code:

1. Genetic code is triplet and commaless. 

2. It is non-ambiguous. 

3. Genetic code is degenerate, as 61 codons are available for 20 amino acids. 

4. The genetic code is universal. i.e it is similar in all the organisms, from simple bacteria to complex organisms.

 
5. The genetic code has start and stop signals. The codon AUG acts as start/ initiation codon which codes for methionine, while the codons UAG, UAA and UGA serve as stop codons or termination codons. 

6. Polarity is one of the important feature of the genetic code. It can be read only in 5' to 3' direction of m-RNA. 

Genetic code- Table

          So, this are the all about RNA, Types of RNA and their Functions, RNA structure etc.