RNA: World Genetic Material

Introduction

Ribonucleic Acid (RNA) is another genetic material that is found in living organisms. It is a single-stranded polymer of nucleotides. Each nucleotide contains a ribose sugar moiety, a nitrogen base, and a phosphate group. Nitrogen bases are Adenine, Guanine, Cytosine, and Uracil which are joined to the sugar by glycosidic bonds, and the backbone of RNA is made of a phosphodiester bond, similar to DNA. Though it is a single stranded structure it can fold upon itself, show complementary base pairing within, and form a 3D structure.

Types of RNA

There are three types of RNA’s found namely −

• mRNA − Known as messenger RNA, is a straight RNA molecule without any folds. It comprises 5-7% of total cellular RNA. It contains two 3' and 5’ ends. mRNA contains a triplet codon which is complementary to the DNA sequence. It is a coding RNA and codes for amino acids.

• tRNA − Known as transfer RNA or adaptor RNA. It is a non-coding RNA and is required in the translation process. tRNA decodes the information of the mRNA and attaches appropriate amino acids to it. tRNA comprises 10-20% of total cellular RNA. It is the smallest of all RNA found and is made up of 73-93 nucleotides. There are two tRNA models -

• Hairpin Model (by Hoagland)

• Clover- leaf Model (by Holley)

• rRNA − Known as ribosomal RNA. It is a non-coding RNA and helps in the attachment of the large and small ribosomal units. This RNA comprises 80% of total cellular RNA. These are transcribed in the nucleoli.

Structure

A mature mRNA consists of the following parts −

• Coding Sequence − It is a chain of nucleotides which are complementary to the DNA sequence, and is made of triplet codons which code for a specific amino acid. This is known as the genetic code. Codon at the 5’ end is the start codon and contains AUG which codes for methionine. Codon at the 3’ end is the stop codon i.e. UAA, UAG, UGA.

• 5’ cap − This is the 7 methylguanosine cap which is added by 5’-5’ triphosphate linkage. It is called 5’ methylation and is necessary to stabilize and protect mRNA from the activity of exonuclease enzymes. It helps in the detection of mRNA by ribosomes.

• 3’Poly A tail − A tail of 20-50 Adenine residues are attached at the 3’ end and is called Polyadenylation. This tail assists the movement of mRNA from the nucleus to the cytoplasm, prevents mRNA against exonuclease, and helps in process of translation.

tRNA

tRNAThe Clover-leaf model of tRNA consists of the following parts −

• Anticodon loop − It is complementary to the mRNA sequences and detects the codon and brings the correct amino acid.

• T loop − This arm contains modified bases called thymidine, pseudouridine and cytidine residues. This loop assists the interaction between tRNA and ribosomes.

• D loop − It contains dihydrouridine and is made of 4-6 bp. It helps maintain the stability of the tRNA structure and also helps in accurate translation.

• Acceptor End − Known as an amino acid attachment site and is responsible for the attachment of specific amino acids, corresponding to the anticodon present.

• Variable Arm − It is located between the T arm and the Anticodon arm and helps in aminoacyl tRNA synthetase detection.

rRNA

In Prokaryotes

• Large Subunit of Ribosome − 50 S subunit which is composed of 5 S and 23 S rRNA. 5 S rRNA is a short sequence of 120 nucleotides. 23 S rRNA is made of 3000 nucleotides.

• Small Subunit of Ribosome − 30 S subunit composed of 16 S rRNA, 1500 nucleotides in length.

In Eukaryotes

• Large Subunit of Ribosome − It is the 60 S subunit which consists of 5 S, 28 S and 5.8 S rRNA. The 5 S and 5.8 S rRNA are less than 200 nucleotides in length. 28 S rRNA is over 5 kilobases in length.

• Small Subunit of Ribosome − It is the 40 S subunit which consists of 18 S rRNA, close to 2 kilobases in length.

Functions of RNA

mRNA

• It encodes information from DNA for protein synthesis.

tRNA-

• It has the ability to read the mRNA instructions for protein synthesis.

• It brings the correct amino acid to the ribosomes for protein synthesis.

• Helps in the formation of the polypeptide bond between the amino acids.

rRNA-

• It helps in joining and maintaining the stability of the two ribosomal subunits.

• It attaches to mRNA and tRNA keeping them in the correct position for translation.

• Forms special helices and loops for Aminoacyl, Peptidyl, and Exit site.

• Required in initiation, elongation, and termination process of translation.

• It helps in the attachment of ribosomes to the mRNA.

The Difference between RNA and DNA

DNA	RNA
Deoxyribose Nucleic Acid.	Ribose Nucleic Acid.
Consists of 4 oxygen molecules.	Consists of 5 oxygen molecules.
It contains Thymine as a nitrogen base.	It contains Uracil as a nitrogen base.
Genetic material of Eukaryotes and Prokaryotes.	Genetic material of mostly viruses.
Double-stranded in structure.	Single-stranded in structure.
Present in the nucleus.	Present in nucleus and cytoplasm.
These are long-lived.	These are short-lived.
They are very stable and are not prone to mutations.	They are unstable and prone to many mutations.
It transfers genetic information.	It is required in the process of protein synthesis.
It is self-replicating.	It requires a template to replicate.

Conclusion

RNA is an important nucleic acid and is required to perform many important activities of the cell. These are short-lived and very specific in nature. They are of many types such as mRNA, tRNA, and rRNA. These are the most vital components to complete the Central Dogma of life i.e. DNA – transcription $\mathrm{\rightarrow}$ RNA – translation $\mathrm{\rightarrow}$ PROTEIN. In the absence of RNA, all organisms might cease to exist.

FAQs

Q1. What is transcription?

Ans. Transcription is the transfer of genetic information from DNA to RNA. During this process, mRNA is formed and is the messenger which carries information for protein formation to the Ribosomes.

Q2. What is the process of translation?

Ans. Translation is to translate the information available on mRNA to a chain of polypeptides. Ribosome is the site of protein synthesis. It requires Aminoacyl tRNA synthetase, Peptidyl transferase along with various other initiation, elongation, and termination factors.

Q3. What is splicing?

Ans. Pre-mRNA contains exons (coding regions) and introns (non-coding regions). The process of removal of introns is called splicing. Splicing can be catalyzed by spliceosomes which are small nuclear ribonucleoproteins called as SnRNPs or by selfsplicing introns.

Q4. Difference between prokaryotic and eukaryotic mRNA

Ans.

PROKARYOTIC mRNA	EUKARYOTIC mRNA
Formed in the cytoplasm.	Formed in the nucleus.
It is polycistronic and codes for many proteins.	It is monocistronic, and codes for one protein.
Less stable.	More stable.
Does not contain cap and tail.	Contains a 5’cap and 3’ tail.
Ribosomes detect mRNA by Shine- Dalgarno sequence.	Ribosomes detect mRNA by the 5’ capping.