Post Transcriptional Modifications and Processing of Eukaryotic RNA

Introduction

In eukaryotes, the primary RNA transcript synthesized from DNA is called heterogeneous nuclear RNA (hnRNA) or precursor mRNA (pre-mRNA). This RNA is not immediately functional. Before it can participate in protein synthesis, it undergoes several modifications and processing steps inside the nucleus. These modifications are collectively known as post-transcriptional modifications.

The processing of RNA ensures stability, protection from degradation, accurate transport to the cytoplasm, and proper translation into proteins.

Post-Transcriptional Modifications of Eukaryotic RNA

The newly synthesized pre-mRNA undergoes the following important modifications:

1. 5′ Capping

2. 3′ Polyadenylation (Poly-A tail addition)

3. RNA Splicing

These processes convert pre-mRNA into mature mRNA.

1. 5′ Capping

The first modification occurs at the 5′ end of the RNA transcript.

Process

• A modified guanine nucleotide called 7-methyl guanosine (m7G) is attached to the 5′ end.

• The cap is attached through an unusual 5′–5′ triphosphate linkage.

Functions of 5′ Cap

1. Protects mRNA from enzymatic degradation.

2. Helps in attachment of ribosome during translation.

3. Assists in transport of mRNA from nucleus to cytoplasm.

4. Increases stability of mRNA.

2. Polyadenylation (Addition of Poly-A Tail)

The second modification occurs at the 3′ end of the RNA.

Process

• A sequence of about 200 adenine nucleotides is added to the 3′ end.

• The enzyme poly-A polymerase catalyzes this process.

Functions of Poly-A Tail

1. Protects mRNA from degradation.

2. Enhances stability of mRNA.

3. Facilitates export of mRNA from nucleus.

4. Helps in efficient translation.

3. RNA Splicing

RNA splicing is the process by which non-coding regions are removed and coding regions are joined together.

Definition

• Introns are non-coding sequences removed during splicing.

• Exons are coding sequences retained in mature mRNA.

Split Genes

The genes containing both introns and exons are called split genes.

Discovery

Split genes were discovered independently by Richard J. Roberts and Phillip A. Sharp in 1977.

They received the Nobel Prize in 1993 for this discovery.

Concept of Introns and Exons

Introns

• Non-coding intervening sequences.

• Removed during RNA processing.

• Do not participate directly in protein synthesis.

Exons

• Expressed sequences.

• Remain in mature mRNA.

• Code for amino acids in proteins.

Importance

• Allows gene regulation.

• Increases protein diversity.

• Important in evolution of complex organisms.

Splicing Mechanism

Splicing occurs inside the nucleus with the help of a large RNA-protein complex called the spliceosome.

Components of Spliceosome

• Small nuclear ribonucleoproteins (snRNPs or “snurps”)

• Small nuclear RNAs (snRNAs)

• Associated proteins

Major snRNPs:

• U1

• U2

• U4

• U5

• U6

Steps in RNA Splicing

Step 1: Recognition of Splice Sites

The spliceosome recognizes:

• 5′ splice site

• Branch point

• 3′ splice site

Step 2: Formation of Lariat Structure

• The intron folds to form a loop called lariat.

• The 5′ end of intron joins the branch point adenine.

Step 3: Removal of Intron

• The intron is excised from pre-mRNA.

Step 4: Joining of Exons

• Adjacent exons are ligated together.

• Mature mRNA is formed.

Alternative Splicing

Alternative splicing is a process in which different combinations of exons are joined together to produce multiple mRNAs from a single gene.

Mechanism

• Certain exons may be included or excluded.

• Different mature mRNAs are produced.

Importance

1. Produces different proteins from the same gene.

2. Increases protein diversity.

3. Important in tissue-specific protein formation.

4. Helps in developmental regulation.

Examples of Alternative Splicing

1. Troponin gene produces different proteins in muscle tissues.

2. Calcitonin gene gives rise to calcitonin and CGRP by alternative splicing.

Significance of RNA Processing

1. Produces mature functional mRNA.

2. Protects RNA from degradation.

3. Regulates gene expression.

4. Enhances protein diversity.

5. Essential for proper cellular functioning.

Clinical Importance

Defects in RNA splicing can lead to diseases such as:

• β-thalassemia

• Certain cancers

• Spinal muscular atrophy

Thus, correct RNA processing is essential for normal cellular activities.

Conclusion

Post-transcriptional modification is an essential step in eukaryotic gene expression. Through capping, polyadenylation, and splicing, immature pre-mRNA is converted into mature mRNA capable of directing protein synthesis. The presence of split genes and alternative splicing greatly increases the complexity and diversity of proteins in eukaryotic organisms.

Exam-Oriented Questions

Short Questions

1. What is post-transcriptional modification?

2. Define 5′ capping.

3. What is polyadenylation?

4. Define introns and exons.

5. What are split genes?

6. What is spliceosome?

7. Define RNA splicing.

8. What is alternative splicing?

9. Name the snRNPs involved in splicing.

10. Mention one significance of alternative splicing.

Long Questions

1. Describe post-transcriptional modifications of eukaryotic RNA.

2. Explain the mechanism of RNA splicing.

3. Discuss split genes and the concept of introns and exons.

4. Write a detailed note on alternative splicing.

5. Explain the significance of RNA processing.

MCQs

1. The modified nucleotide present in 5′ cap is:

   • a) Adenine

   • b) Cytosine

   • c) 7-methyl guanosine

   • d) Uracil
     Answer: c) 7-methyl guanosine

2. Introns are:

   • a) Coding sequences

   • b) Non-coding sequences

   • c) Ribosomes

   • d) Amino acids
     Answer: b) Non-coding sequences

3. The complex involved in RNA splicing is:

   • a) Ribosome

   • b) Lysosome

   • c) Spliceosome

   • d) Centrosome
     Answer: c) Spliceosome

4. Poly-A tail is added at:

   • a) 5′ end

   • b) Middle region

   • c) 3′ end

   • d) Ribosome
     Answer: c) 3′ end

5. Alternative splicing results in:

   • a) DNA replication

   • b) Protein diversity

   • c) Mutation

   • d) Cell division
     Answer: b) Protein diversity

References

1. Alberts B. et al. Molecular Biology of the Cell.

2. Watson J.D. et al. Molecular Biology of the Gene.

3. Lodish H. et al. Molecular Cell Biology.

4. Lewin B. Genes.

5. Karp G. Cell and Molecular Biology.