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Structure of Ribonucleic Acid (RNA)


Do you know what is RNA? The below article will explain the structure of RNA, the types of RNA and their functions. You can also find the structure and functions of mRNA, tRNA and rRNA in the below article.

Introduction

Nucleic acid with ribose sugar is called ribonucleic acid (RNA). RNA is a single chain (Single-stranded) polynucleotide chain functions as a carrier of coded genetic information from DNA to cytoplasm take part in protein synthesis. RNA contains about 70-12,000 ribonucleotides, which are arranged in a linear sequence and are joined together by 3'-5' phosphodiester bonds. The axis of the RNA molecule is composed of the alternate residue of the sugar-phosphate backbone. Carbon combines with carbon 3' end of next sugar and phosphate with carbon 5' end of its sugar, just like that of DNA strand. It contains four nitrogen bases, Adenine (A), Cytosine (C), Guanine (G) and Uracil (U). The nitrogenous bases are complementary to a sequence on the DNA template.

Types of RNA

RNA is of two types:

  1. Genetic RNA : RNA which is present as genetic material is called genetic RNA. E.g., Plant viruses like TMV, tumour viruses, riboviruses. These RNAs act as genetic material. RNA may be single-stranded (TMV) or double-stranded (Riboviruses). These RNAs forms DNA in the cell and this DNA produces RNA of its own kinds.

  2. Non-genetic RNA : In higher organisms, RNA does not serve as genetic material; therefore it is called non-genetic RNA. Non-genetic RNA is synthesized on a DNA template. In Eukaryotes, three types of non-genetic RNAs are found, which plays an important role in protein synthesis:

    1. Messenger RNA (mRNA)
    2. Transfer RNA (tRNA)
    3. Ribosomal RNA (rRNA)


Messenger RNA

mRNA carries genetic information from DNA to ribosomes during protein synthesis. For this Jacob and Monod named it as messenger RNA. mRNA is either found free in the cytoplasm of the cell or associated with ribosomes. mRNA is synthesized from the coding strand of DNA within the nucleus and its base sequence is complementary to the coding strand. Therefore mRNA contains the same genetic information as coded in that part of DNA. The base sequence of mRNA in which information for the synthesis of protein is coded is called genetic code . The genetic code is formed of several codons. Each codon is a sequence of three nitrogenous bases which codes for one amino acid. As each codon is formed of three nitrogenous bases, it is called a triplet code. mRNA has a molecular weight of about 5,00,000 with the sedimentation of the coefficient of 8S. It is always single-stranded and there is no base pairing. mRNA is short-lived as it withers away after a few translations. In bacterial cells, its life span is about two minutes, but in eukaryotes, it is metabolically more stable and can function for a number of hours and even for days.

Structure of mRNA


mRNA
(Image courtesy: www.wikipedia.org)

Structurally eukaryotic mRNA is composed of following structures :

  • Cap: Cap is found at 5' end of mRNA, which is formed of methylated guanine. Cap helps the mRNA to bind with ribosome.

  • Non coding region I: This follows the cap and consists of 10-100 nucleotides in which adenine and guanine is found in higher amount. There are no codes for proteins in this region.

  • Initiation codon: Initiation codon is situated after non-coding region I. It encodes methionine (AUG) in all types of cells.

  • Coding region: Coding region lies just after initiation codon and is consists of 1500 nucleotides. Coding region is responsible for translation of protein.

  • Termination codon : The coding region is followed by a termination codon (UAA, UAG or UGA) which stops translation.

  • Non-coding region II: This part of mRNA is composed of 50-150 nucleotides. It also does not have any codes for protein synthesis.

  • Poly-A sequence : it is the last part of mRNA at the 3' terminus. It is adenine rich therefore is also called to be s poly-adenylate sequence. Initially it has 20-250 nucleotides but the number of nucleotides decreases with the increasing age.



  • Functions of mRNA
    1. mRNA carries genetic information for the synthesis of polypeptide chain.
    2. mRNA acts as a template for the union of different amino acids for protein synthesis.


    Transfer RNA


    t RNA
    (Image courtesy: www.wikipedia.org)

    The transfer RNA is also called as soluble RNA since it is soluble in 1 M NaCl and is the second most common RNA in the cell. Transfer RNA constitutes about 15% of the total weight of RNA of the cell. The tRNA is a ribonucleic acid which transfers the activated amino acid to the ribosomes to synthesize proteins. tRNA is so small that it remains in the supernatant during centrifugation. tRNA has a sedimentation coefficient of 3.8S. It has a molecular weight of 25000 to 30000 and is made up of 73 to 93 nucleotides. tRNA is synthesized in the nucleus on a DNA template. tRNA consists of a single polynucleotide chain or strand which is bent in the middle and looped about itself.

    Functions of tRNA
    Transfer RNA plays a key role in protein synthesis. tRNA picks up a specifically activated amino acid from the cytoplasm and transfer it to the ribosome in the cytoplasm where proteins are synthesized. tRNA attaches itself to ribosome with the sequence specified by mRNA and finally, it transmits its amino acid to the new polypeptide chain.

    Ribosomal RNA

    rRNA is a ribonucleic acid present in the ribosomes and hence it is called ribosomal RNA. rRNA is insoluble and it constitutes about 80% of the cellular RNA. The ribosomal RNA is formed of a single strand. Each strand of rRNA is formed of many nucleotide units. Each nucleotide of rRNA is composed of a nitrogenous base, a ribose sugar and a phosphate molecule. The nitrogenous bases are adenine (A), Guanine (G), Cytosine (C) and Uracil (U). In some regions, the single strand is twisted upon itself to form a double helix. The helical region is joined by intervening single-stranded regions. In helical regions, most of the base pairs are complementary and are joined by hydrogen bonds. In the unfolded single-strand regions, the base pairs are not complementary. Because of this nature in the rRNA, the purine and pyrimidines have no equality. rRNA is also synthesized in the nucleus. DNA associated with the nucleolus is responsible for coding of rRNA. This part of DNA is known as nucleolar organizer region.

    Functions of rRNA
    1. rRNA brings about correct orientation of mRNA and tRNA in ribosomes.
    2. rRNA is involved in the formation of ribosomes.

    So, we can say that RNA is the genetic material of plant viruses and it plays an important role during protein synthesis in eukaryotic cells.


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