THE RNA (RIBONUCLEIC ACID) – ITS STRUCTURE, TYPES AND FUNCTION

RNA (ribonucleic acid) is a single-stranded nucleic acid present in cell of all living organisms and some viruses. It plays important role in protein synthesis and gene regulation. RNA is a polymer made of single chain of nucleotides. A nucleotide is composed of nitrogenous base (purine or pyrimidine), pentose sugar and phosphate. RNA is formed from DNA by a process called transcription in nucleus.

Unlike DNA, which has deoxyribose sugar, the sugar present in RNA is ribose. Another difference between DNA and RNA is that RNA has uracil in place of thymine. RNA typically exists as a single strand, whereas DNA is a double-stranded helical molecule. However, if complementary base sequence with opposite polarity is provided, RNA molecule can fold back on itself like a hairpin and form pairs with complementary bases thus acquiring double strand characteristics. (G pairing with C and A with U).

In eukaryotic cell, 50% of RNA is present in ribosome and rough endoplasmic reticulum. 25% of RNA is present in cytoplasm, 15% in mitochondria and rest 10% in nucleus and nucleolus.

TYPES OF RNA

• Messenger RNA
• Transfer RNA
• Ribosomal RNA
• Long non coding RNA (>/= 200 nucleotides)
• Small non coding RNA (< 200 nucleotides) – micro RNA, small nucleolar RNA, small interfering RNA, small nuclear RNA, PIWI interacting RNA.

MESSENGER RNA (m RNA)

Messenger RNA constitutes 2% – 5% of the total RNA in the cell. The m RNA in a eukaryotic cell is monocistronic as it codes for one protein. In prokaryotes, m RNA can code for more than one protein and is polycistronic.

They function as messengers conveying the information in a gene to the protein synthesizing machinery. m RNA serves as a template on which a specific sequence of amino acids is polymerized to form a specific protein molecule.

In mammalian cells, the pre-m RNA molecules undergo processing to generate mature m RNA molecules, which then enter the cytoplasm where they act as templates for protein synthesis (translation).

STRUCTURE OF M RNA

The 5′ terminal has 7-methyl guanosine triphosphate ‘cap’ . Cap performs function of mRNA recognition during protein synthesis and provides stability to mRNA by preventing degradation by 5′ exonucleases.

The 3′ hydroxyl terminal of mRNA has an attached polymer of adenylate residues 20 – 250 nucleotides in length. The poly (A) “tail” maintains the intracellular stability of mRNA by preventing its degradation by 3′-exonucleases and helps export mRNA from nucleus to cytosol.

Both the “cap” and “poly (A) tail are added post transcriptionally to precursor m RNA (pre – m RNA).

FUNCTION OF M RNA

Serves as template for protein synthesis in a process called translation.

TRANSFER RNA (t RNA)

These make roughly 20% of total cellular RNA. They vary in length from 74 to 95 nucleotides. They are also generated by processing of a precursor molecule.

There are at least 20 species of t RNA molecules in every cell, at least one (and often several) corresponding to each of the 20 amino acids required for protein synthesis.

STRUCTURE OF T RNA

The nucleotide sequence which makes the primary structure of all t RNA molecules, allows for extensive folding and complementary base pairing (guanine with cytosine and adenine with uracil) to generate a secondary structure that resembles a clover leaf.

All t RNA molecules contain four main arms :

• ACCEPTOR ARM – Composed of 7 – 9 base pairs terminating in nucleotide sequence CCA. The t RNA specific amino acid is attached or “charged” onto the 3′-OH group of the A moiety of the acceptor arm.
• ANTICODON ARM – It contains three nucleotide sequence that pair with complementary bases present on m RNA codon.
• D (DIHYDROURIDINE) ARM – Contains modified base, dihydrouridine and is involved in recognition by aminoacyl -t RNA synthetase.
• T ARM (Ribothymidine-Pseudouridine-Cytidine ARM) – Have nucleotide named ribothymidine. It helps t RNA to bind with ribosome.

A loop of variable length, called variable loop is situated between the anticodon and T loops.

FUNCTION OF T RNA

The t RNA molecules serve as adapters for the translation of nucleotide sequence of m RNA into specific amino acids.

It transports amino acids to the ribosome and pairs its anticodon with codon on m RNA to form protein.

RIBOSOMAL RNA (r RNA)

The r RNAs constitute roughly 70% of total cellular RNA. They are located on ribosome. It is a non coding RNA molecule that forms the structural and functional core of ribosome. r RNA forms ribosomal subunits during translation to convert m RNA into proteins.

STRUCTURE OF R RNA

The ribosome contains two major nucleoprotein subunits – a large and a small subunit. In eukaryotes, large subunit is 60 S (has 28 S, 5.8 S and 5 S) and small subunit is 40 S (has 18 S) . In prokaryotes, large subunit is a 50 S (containing 23 S and 5 S) and small subunit is a 30 S (has 16 S). [S stands for svedberg unit which corresponds to molecular size and shape].

FUNCTION OF R RNA

They form the catalytic site on which translation (protein synthesis) occurs. r RNA helps to bind m RNA and t RNA to facilitate the process of protein synthesis by catalyzing peptide fond formation.

LONG NON CODING RNA (Lnc RNA)

They contain more than 200 nucleotides.

FUNCTION

• Act as molecular scaffold to recruit and combine with multiple regulatory proteins.
• Their repeat units mediate histone modification.
• Lnc RNA can produce allosteric effect to help interaction with different ligand proteins.

MICRO RNA (mi RNA)

It is a small non coding RNA containing 19 to 22 nucleotides. It is synthesized in nucleus and has loops. There is incomplete complementary binding of the guide strand to target m RNA.

FUNCTION

• Perform RNA silencing.
• Involved in post transcriptional regulation of gene expression.

SMALL NUCLEOLAR RNA (Sno RNA)

They are 60 to 300 nucleotides long and are located in the nucleolus.

FUNCTION

Post-transcriptional modification and maturation of ribosomal RNAs, small nuclear RNAs and other cellular RNAs.

SMALL INTERFERING RNA (Si RNA)

They are 20 to 27 nucleotides in length. It is synthesized in cytoplasm. It has no loops. Complete complementary binding of guide strand to target m RNA occurs.

FUNCTION

• RNA silencing.
• Post-transcriptional regulation of gene expression.

SMALL NUCLEAR RNA (sn RNA)

It contains 150 nucleotides. It is located in nucleus.

FUNCTION

Post-transcriptional modification of m RNA.

PIWI INTERACTING RNA (Pi RNA)

They are 21 to 35 nucleotides long. PIWI is a type of protein that have RNA binding site on which Pi RNA attaches itself.

FUNCTION

• They regulate gene expression.
• They silence transposable elements (transposons/ jumping genes), thus reducing reversible mutations.