20. The 5′ cap of eukaryotic mRNAs contains
(A) a modified guanine nucleotide
(B) a modified adenine nucleotide
(C) a modified cytosine nucleotide
(D) a modified uracil nucleotide
5′ Cap of Eukaryotic mRNA: Modified Guanine Nucleotide and Its Functions Explained
Introduction
Eukaryotic messenger RNA does not usually function immediately after it is synthesized. The initial RNA transcript, commonly called pre-mRNA, undergoes several processing events before it becomes a mature mRNA capable of directing protein synthesis. The three major processing events are 5′ capping, RNA splicing, and 3′ polyadenylation.
One of the earliest and most important modifications is the addition of a specialized structure to the 5′ end of the growing RNA transcript. This structure is known as the 5′ cap. The cap contains a modified guanine nucleotide called 7-methylguanosine, commonly abbreviated as m7G.
The 7-methylguanosine cap is attached to the first nucleotide of the RNA through an unusual 5′-to-5′ triphosphate linkage. This special structure distinguishes the cap from the ordinary 3′-to-5′ phosphodiester bonds present throughout the RNA chain. The 5′ cap protects mRNA from degradation and contributes to several other essential steps in gene expression.
Therefore, among the given options, the correct answer is a modified guanine nucleotide.
Correct Answer
Correct Answer: (A) a modified guanine nucleotide
Detailed Explanation
The 5′ cap of most eukaryotic mRNAs contains a modified form of guanine known as 7-methylguanosine. In this nucleotide, a methyl group is added to the nitrogen at position 7 of the guanine base. The resulting cap is commonly represented as m7G.
The cap is connected to the first nucleotide of the mRNA through a highly unusual linkage:
m7G(5′)ppp(5′)N
Here, m7G represents 7-methylguanosine, ppp represents the triphosphate bridge, and N represents the first transcribed nucleotide of the RNA.
In a normal RNA chain, adjacent nucleotides are connected through 3′-to-5′ phosphodiester bonds. The 5′ cap is different because the modified guanosine is attached in a 5′-to-5′ orientation. This unusual structure is central to the protective and regulatory functions of the cap.
Explanation of Option (A): A Modified Guanine Nucleotide
This option is correct.
The characteristic nucleotide present in the 5′ cap of eukaryotic mRNA is 7-methylguanosine. It is derived from guanine and contains a methyl group at the N7 position of the purine ring.
The modified guanine nucleotide is added to the 5′ end of the nascent RNA transcript during transcription. The mature cap structure is recognized by specific cap-binding proteins that participate in mRNA processing, nuclear export, protection from degradation, and translation initiation.
Therefore:
5′ cap of eukaryotic mRNA = 7-methylguanosine = Modified guanine nucleotide
Hence, option (A) is correct.
Explanation of Option (B): A Modified Adenine Nucleotide
This option is incorrect.
Adenine is an important nitrogenous base present in RNA, and modified adenine nucleotides can occur in many cellular RNAs. However, adenine does not form the characteristic terminal nucleotide of the standard eukaryotic mRNA 5′ cap.
The confusion may arise because adenine is strongly associated with another major mRNA processing event: polyadenylation. During 3′ end processing, a long sequence of adenine nucleotides called the poly(A) tail is added to the 3′ end of many eukaryotic mRNAs.
Thus:
5′ end → 7-methylguanosine cap
3′ end → Poly(A) tail
Therefore, option (B) is incorrect.
Explanation of Option (C): A Modified Cytosine Nucleotide
This option is incorrect.
Cytosine and modified cytosine residues are found in nucleic acids, but a modified cytosine nucleotide does not constitute the standard 5′ cap of eukaryotic mRNA. The defining nucleotide of the conventional cap is 7-methylguanosine.
Therefore, although cytosine is a normal component of RNA sequences, it is not the characteristic modified nucleotide attached to the 5′ end during mRNA capping.
Explanation of Option (D): A Modified Uracil Nucleotide
This option is incorrect.
Uracil is one of the four major nitrogenous bases present in RNA and replaces thymine in most RNA molecules. However, the standard eukaryotic mRNA cap is not derived from uracil.
The 5′ cap specifically contains a methylated guanine nucleotide. Therefore, the presence of uracil in RNA should not be confused with the specialized modified nucleotide used for mRNA capping.
Summary of Each Option
| Option | Nucleotide | Correct/Incorrect | Reason |
|---|---|---|---|
| (A) | Modified guanine | Correct | The 5′ cap contains 7-methylguanosine. |
| (B) | Modified adenine | Incorrect | Adenine is associated with the 3′ poly(A) tail, not the standard 5′ cap. |
| (C) | Modified cytosine | Incorrect | Modified cytosine does not form the characteristic eukaryotic mRNA cap. |
| (D) | Modified uracil | Incorrect | The cap is derived from guanine, not uracil. |
What Is the 5′ Cap of Eukaryotic mRNA?
The 5′ cap is a specialized chemical modification added to the 5′ end of most eukaryotic mRNAs. It is formed shortly after transcription begins and is closely coordinated with transcription by RNA polymerase II.
The simplest cap structure contains 7-methylguanosine linked to the first nucleotide of the RNA through a 5′-to-5′ triphosphate bridge. This structure is often called the m7G cap or Cap 0.
The unusual orientation and chemical modification of the cap make it recognizable to specialized proteins and help distinguish properly processed cellular mRNAs from uncapped RNA molecules.
Structure of the 5′ Cap
The basic structure of the eukaryotic mRNA cap can be represented as:
m7G(5′)ppp(5′)N
This notation contains three important structural features. First, the cap nucleotide is guanosine. Second, the guanine is methylated at the N7 position. Third, the cap is connected to the first RNA nucleotide by an unusual 5′-to-5′ triphosphate linkage.
| Feature | Description |
|---|---|
| Modified Base | Guanine |
| Specific Modification | Methylation at N7 |
| Cap Nucleotide | 7-Methylguanosine |
| Abbreviation | m7G |
| Linkage | 5′-to-5′ triphosphate linkage |
How Is the 5′ Cap Added to mRNA?
Step 1: Removal of the Terminal Phosphate
The newly synthesized RNA initially has a 5′ triphosphate end. An RNA triphosphatase removes the terminal phosphate, leaving a diphosphate at the 5′ end of the nascent transcript.
Step 2: Addition of Guanosine
A guanylyltransferase adds a guanosine monophosphate residue to the 5′ end of the RNA. This reaction creates the unusual 5′-to-5′ triphosphate linkage that characterizes the cap structure.
Step 3: Methylation of Guanine
A methyltransferase transfers a methyl group to the N7 position of the newly added guanine. This produces the characteristic 7-methylguanosine cap.
The overall sequence can be summarized as:
Nascent RNA → Guanosine addition → N7 methylation → Mature m7G cap
Major Functions of the 5′ Cap
Protection of mRNA from Degradation
The 5′ end of an uncapped RNA molecule is vulnerable to degradation by exonucleases. The unusual 5′ cap structure protects the mRNA from rapid enzymatic breakdown and contributes significantly to RNA stability.
Role in Translation Initiation
The cap is recognized by cap-binding translation factors, particularly eIF4E. This recognition helps recruit the translation initiation machinery and ultimately assists the small ribosomal subunit in locating the start codon.
Role in Nuclear Export
Newly processed mRNAs must be transported from the nucleus to the cytoplasm. The cap is recognized by the nuclear cap-binding complex, which contributes to proper mRNA processing and export.
Contribution to Pre-mRNA Splicing
The 5′ cap can promote efficient processing of the transcript, particularly the removal of introns located near the 5′ end of the pre-mRNA.
Quality Control of mRNA
The cap helps identify properly processed transcripts and contributes to the coordination of mRNA maturation, export, translation, and degradation.
Summary of 5′ Cap Functions
| Function | Biological Importance |
|---|---|
| Protection from degradation | Increases mRNA stability |
| Translation initiation | Promotes recruitment of translation factors and ribosomes |
| Nuclear export | Supports transport of mature mRNA to the cytoplasm |
| RNA splicing | Contributes to efficient processing of pre-mRNA |
| mRNA quality control | Helps distinguish properly processed transcripts |
Different Types of Eukaryotic mRNA Caps
Additional methylation can produce different cap forms. The basic 7-methylguanosine structure is known as Cap 0. Methylation of the ribose sugar of the first transcribed nucleotide produces Cap 1, while additional methylation of the second nucleotide produces Cap 2.
| Cap Type | Major Modification |
|---|---|
| Cap 0 | 7-Methylguanosine cap |
| Cap 1 | Cap 0 plus 2′-O-methylation of the first transcribed nucleotide |
| Cap 2 | Additional 2′-O-methylation of the second transcribed nucleotide |
5′ Cap and 3′ Poly(A) Tail
The 5′ cap and 3′ poly(A) tail are two major terminal modifications of many eukaryotic mRNAs. Although they occur at opposite ends of the transcript, both contribute to mRNA stability and efficient translation.
| Feature | 5′ Cap | 3′ Poly(A) Tail |
|---|---|---|
| Location | 5′ end | 3′ end |
| Main Component | 7-Methylguanosine | Multiple adenine nucleotides |
| Key Protein Interaction | Cap-binding proteins and eIF4E | Poly(A)-binding proteins |
| Major Roles | Protection, export and translation initiation | Stability and translation efficiency |
Why Is the 5′-to-5′ Linkage Important?
Most nucleotides within an RNA chain are joined through conventional 3′-to-5′ phosphodiester bonds. The cap is unusual because its guanosine nucleotide is linked in the opposite orientation through a 5′-to-5′ triphosphate bridge.
This unusual arrangement makes the end structurally different from an ordinary RNA terminus. It protects the transcript from many enzymes that degrade RNA from a conventional exposed 5′ end and creates a specialized recognition site for cap-binding proteins.
Connection Between RNA Polymerase II and 5′ Capping
The 5′ capping of eukaryotic pre-mRNA is closely associated with transcription by RNA polymerase II. Capping enzymes are recruited to the transcription machinery through interactions with the phosphorylated C-terminal domain of the largest RNA polymerase II subunit.
As a result, the cap is added while the RNA transcript is still being synthesized. This coordination allows the nascent RNA to acquire protection and processing factors early during transcription.
Biological Significance
The 5′ cap is far more than a simple chemical decoration on mRNA. It is a central regulatory structure that influences the entire life cycle of a transcript. From the early stages of RNA processing in the nucleus to translation in the cytoplasm and eventual mRNA degradation, the cap interacts with different proteins that determine the fate of the RNA molecule.
The presence of 7-methylguanosine at the 5′ end therefore connects transcription, RNA processing, nuclear export, translation, and mRNA stability into a coordinated pathway of eukaryotic gene expression.
Final Answer
The 5′ cap of most eukaryotic mRNAs contains 7-methylguanosine, a modified guanine nucleotide attached to the RNA through a 5′-to-5′ triphosphate linkage.
5′ mRNA cap = 7-Methylguanosine = Modified guanine nucleotide
Correct Answer: (A) a modified guanine nucleotide
