RNA: Types, Structure, Functions, mRNA, tRNA and rRNA

Botany Sir Himansu

RNA: Types, Structure, Functions, mRNA, tRNA and rRNA

RNA (Ribonucleic Acid) is an important biological macromolecule involved in genetic information transfer, protein synthesis and gene regulation. In some viruses, RNA also functions as the genetic material.

RNA World

The term RNA World was coined by Walter Gilbert. The RNA world hypothesis proposes that RNA may have played a central role in early life because it can function both as a carrier of genetic information and as a catalyst.

RNA as Genetic Material

RNA is considered to be an ancient form of genetic material. At present, RNA acts as genetic material in several viruses, whereas DNA is the primary genetic material in most cellular organisms.

RNA can store and transmit genetic information and can also possess catalytic activity. However, RNA is chemically less stable than DNA and is therefore less suitable for long-term storage of genetic information.

Why Is RNA Less Stable Than DNA?

RNA contains the sugar ribose, which has a 2′-OH group. This hydroxyl group makes RNA more reactive and susceptible to hydrolysis.

  • RNA contains ribose sugar.
  • DNA contains deoxyribose sugar.
  • RNA contains uracil instead of thymine.
  • RNA is generally more reactive and less chemically stable than DNA.
  • RNA generally undergoes mutation more rapidly than DNA in many RNA viruses.

RNA and Protein Synthesis

RNA plays a central role in the protein synthesis machinery. Different types of RNA perform different functions during gene expression.

The three major types of RNA involved in protein synthesis are:

  1. mRNA — Messenger RNA
  2. tRNA — Transfer RNA
  3. rRNA — Ribosomal RNA

Types of RNA

1. Genetic RNA

In some viruses, RNA itself serves as the genetic material.

2. Non-Genetic RNA

In cellular organisms, the major RNAs involved in protein synthesis are:

  • mRNA — carries genetic information from DNA to ribosomes.
  • tRNA — carries amino acids to the ribosome.
  • rRNA — forms an important structural and catalytic component of ribosomes.

mRNA — Messenger RNA

mRNA (messenger RNA) carries genetic information from DNA to the ribosome, where it serves as a template for protein synthesis.

Messenger RNA is generally a relatively unstable RNA molecule and may have a short half-life, particularly in bacteria such as Escherichia coli.

mRNA is transcribed from DNA and carries the information required for the synthesis of a specific protein.

Structure of Eukaryotic mRNA

A typical mature eukaryotic mRNA contains several important regions:

  1. 5′ Cap
  2. 5′ UTR
  3. Coding region
  4. 3′ UTR
  5. Poly-A tail

5′ Cap

The 5′ end of mature eukaryotic mRNA contains a 5′ cap. The cap helps protect mRNA and plays an important role in ribosome recognition and translation initiation.

5′ UTR

The 5′ untranslated region (5′ UTR) lies between the 5′ cap and the coding region. It is transcribed but does not encode the protein.

Coding Region

The coding region contains the information required for protein synthesis. It generally begins with a start codon, usually AUG, and ends with a termination codon.

3′ UTR

The 3′ untranslated region (3′ UTR) occurs after the coding region and plays important roles in the regulation, stability and processing of mRNA.

Poly-A Tail

A poly-A tail is added to the 3′ end of most mature eukaryotic mRNAs. It contributes to mRNA stability and helps regulate mRNA metabolism and translation.

Introns and Exons

Eukaryotic genes commonly contain introns and exons.

  • Introns: intervening sequences that are removed during RNA processing.
  • Exons: sequences retained in the mature RNA after processing.

During RNA processing, introns are removed by splicing and the exons are joined together to form mature mRNA.

tRNA — Transfer RNA

tRNA (transfer RNA) is involved in carrying specific amino acids to the ribosome during protein synthesis.

  • tRNA is one of the smallest major types of cellular RNA.
  • It is generally about 70–90 nucleotides long.
  • It functions as an adapter molecule during translation.
  • It recognizes the codon on mRNA through its complementary anticodon.
  • It carries a specific amino acid to the ribosome.

tRNA is also known as soluble RNA (sRNA) or adapter RNA.

Structure of tRNA

tRNA has a characteristic cloverleaf secondary structure. Its important regions include:

  • Acceptor stem
  • Anticodon loop
  • D-loop
  • TΨC loop
  • Variable loop

Anticodon

The anticodon is a sequence of three nucleotides present in the anticodon loop of tRNA. It recognizes and pairs with the complementary codon present on mRNA during translation.

Amino Acid Attachment Site

The amino acid is attached to the 3′ end of tRNA. The conserved terminal sequence is CCA.

rRNA — Ribosomal RNA

rRNA (ribosomal RNA) is an important structural and catalytic component of ribosomes. It plays a major role in protein synthesis.

rRNA is generally the most abundant type of RNA in a cell and is synthesized mainly in the nucleolus in eukaryotic cells.

Ribosomal RNA in Prokaryotes

Prokaryotic ribosomes are 70S ribosomes consisting of:

  • 30S small subunit
  • 50S large subunit

The major rRNA molecules are:

  • 16S rRNA — present in the 30S subunit.
  • 23S rRNA — present in the 50S subunit.
  • 5S rRNA — present in the 50S subunit.

Ribosomal RNA in Eukaryotes

Eukaryotic cytoplasmic ribosomes are 80S ribosomes consisting of:

  • 40S small subunit
  • 60S large subunit

The major rRNA molecules include:

  • 18S rRNA
  • 28S rRNA
  • 5.8S rRNA
  • 5S rRNA

Comparison of mRNA, tRNA and rRNA

RNA Full Form Main Function Important Feature
mRNA Messenger RNA Carries genetic information to ribosome Contains codons
tRNA Transfer RNA Carries amino acids Contains anticodon
rRNA Ribosomal RNA Forms structural and catalytic part of ribosome Associated with ribosomal subunits

RNA: NEET Biology Quick Revision

  • RNA = Ribonucleic acid.
  • RNA World term was coined by Walter Gilbert.
  • RNA contains ribose sugar.
  • RNA contains uracil instead of thymine.
  • The 2′-OH group makes RNA more chemically reactive than DNA.
  • mRNA carries genetic information for protein synthesis.
  • tRNA acts as an adapter and carries amino acids.
  • rRNA forms an important structural and catalytic component of ribosomes.
  • tRNA contains an anticodon.
  • The amino acid attaches to the 3′ CCA end of tRNA.
  • Prokaryotic ribosome = 70S = 50S + 30S.
  • Eukaryotic cytoplasmic ribosome = 80S = 60S + 40S.

Frequently Asked Questions

What are the three major types of RNA?

The three major types of RNA involved in protein synthesis are mRNA, tRNA and rRNA.

What is the function of mRNA?

mRNA carries genetic information from DNA to the ribosome and acts as a template during protein synthesis.

What is the function of tRNA?

tRNA carries specific amino acids to the ribosome and recognizes mRNA codons through its anticodon.

What is the function of rRNA?

rRNA forms a major structural and catalytic component of ribosomes and participates in protein synthesis.

Why is RNA less stable than DNA?

RNA contains a 2′-OH group in its ribose sugar, making it more chemically reactive and less stable than DNA.

What is the terminal sequence of tRNA?

The conserved terminal sequence at the 3′ end of tRNA is CCA, where the amino acid is attached.