mRNA Processing: 5' Cap and Poly (A) Tail

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Questions and Answers

Draw out the pathway for gene expression in eukaryotic cells. What does this large pathway indicate?

Multiple ways and levels from beginning to end to influence gene expression.

What are the two types of RNA?

Ribosomal RNA

Polycistronic mRNA (correct)

Transfer RNA

Monocistronic mRNA (correct)

What is monocistronic mRNA?

It encodes a single polypeptide chain and has a single start site for protein synthesis.

What is polycistronic mRNA?

It encodes multiple polypeptide chains and has multiple start and stop signals. Signup and view all the answers

Why are prokaryotic RNA processing unique in relation to the generalization that RNA requires processing before it can be used?

They are synthesized in a form ready for translation immediately. Signup and view all the answers

What are the biggest differences in the processes for synthesis of functional mRNA in prokaryotes and eukaryotes?

Separation between processes, 5' cap addition, polyA tail addition, and intron removal. Signup and view all the answers

What does transcription yield in eukaryotic RNA processing?

A precursor mRNA (pre-mRNA) called a primary transcript. Signup and view all the answers

What does pre-mRNA undergo before it can become functional in the cell?

Chemical changes. Signup and view all the answers

What is an important distinction between eukaryotic and prokaryotic RNA transport?

Transcription and translation are kept separate by the nuclear membrane. Signup and view all the answers

What is the definition of RNA processing and its phases?

It consists of all chemical modifications necessary to make a mature RNA product, including 5' cap addition, 3' poly (A) tail addition, and splicing. Signup and view all the answers

What is splicing?

The hydrolytic removal of pre-mRNA portions (introns). Signup and view all the answers

What is the 5' cap?

Methylated guanosine at position 7 on the purine ring. Signup and view all the answers

When does 5' capping occur?

Shortly after initiation of RNA synthesis. Signup and view all the answers

What does 5' capping protect?

It protects mRNA from degradation by nucleases that attack RNA at the 5' end. Signup and view all the answers

What is the role of 5' capping in translation?

It helps in positioning mRNA on the ribosome for initiation of translation. Signup and view all the answers

What does the 5' capping consist of?

A terminal nucleotide, 7-methylguanylate (m7G) in a 5' - 5' linkage with the initial nucleotide of the mRNA chain. Signup and view all the answers

What are the roles for 5' caps?

Signaling proper splicing, regulation of ribosome binding for translation, and docking with nuclear pore during export. Signup and view all the answers

What is eIF4E?

Eukaryotic initiation factor 4E, a translation initiation factor that binds the 5' cap in the cytoplasm. Signup and view all the answers

When would there not be a polyA tail?

During the maturation of mRNAs for major histones, tRNA, and rRNA, which lack tails. Signup and view all the answers

What is the typical length of a polyA tail?

200-250 nucleotides long. Signup and view all the answers

When is the polyA tail added?

It is added after transcription at the 3' end. Signup and view all the answers

What is the process and enzyme involved in polyA tail addition?

The process is called 3' polyadenylation, and it involves poly (A) polymerase or polyadenylate polymerase. Signup and view all the answers

What sequence must pre-mRNA have to get a polyA tail?

AAUAAA. Signup and view all the answers

What prompts the addition of the polyA tail?

Cleavage of pre-mRNA 10-35 bp downstream. Signup and view all the answers

Outline the polyadenylation process.

CPSF binds AAUAAA, CSF binds GU/U, CFs bind near the cleavage site, CPSF and CstF form an RNA loop, cleavage occurs, and poly (A) polymerase adds a tail. Signup and view all the answers

What are the functions of 3' polyadenylation?

Protects mRNA from nuclease attack, aids ribosome recognition of mRNA, induces protein export from the nucleus, and isolates mRNAs in the lab. Signup and view all the answers

Study Notes

Gene Expression Pathway

Indicates multiple ways and levels to influence gene expression from start to finish in eukaryotic cells.

Types of RNA

Monocistronic mRNA: Encodes a single polypeptide chain and has a single start site for protein synthesis; predominant in eukaryotes.
Polycistronic mRNA: Encodes multiple polypeptide chains, with multiple start and stop signals; found primarily in prokaryotes.

Prokaryotic RNA Processing

RNA synthesized in a form ready for immediate translation.
Transcription and translation occur concurrently without membrane separation, influencing one another.

Comparison of mRNA Synthesis in Prokaryotes and Eukaryotes

Key differences include separation of transcription and translation processes, presence of a 5' cap, polyA tail, and intron removal in eukaryotic mRNA.

Eukaryotic RNA Processing

Transcription yields a precursor mRNA (pre-mRNA), referred to as a primary transcript.
Pre-mRNA undergoes chemical changes before becoming functional.

RNA Transport Distinction

In eukaryotes, transcription and translation are separated by the nuclear membrane.
RNA processing in the nucleus is required to convert pre-mRNAs to mature mRNAs.

RNA Processing Defined

Comprises all chemical modifications necessary to mature RNA.
Phases include:
- 5' cap addition (present in all)
- 3' polyA tail addition (most, at both ends)
- Intron splicing (removal)

Splicing

Involves hydrolytic removal of non-coding regions (introns) from pre-mRNA.

5' Cap

Methylated guanosine added at the 7th position on the purine ring.
Applied shortly after RNA synthesis begins and protects mRNA from degradation by nucleases.

Functions of 5' Cap

Serves in the initiation of translation by positioning mRNA on ribosomes.
Involved in signaling proper splicing, regulating ribosome binding, and docking with the nuclear pore during export.

5' Cap and Nuclear Pore Docking

mRNP (messenger ribonuclear protein complex) packages mRNA for export.
RNA-binding proteins interact with mRNA, shuttling between nucleus and cytoplasm.

eIF4E

Eukaryotic initiation factor 4E that binds the 5' cap in the cytoplasm, essential for translation initiation.

PolyA Tail

Length ranges from 200-250 nucleotides, added to the 3' end after transcription.
Not encoded by the gene itself, signified by the absence in DNA.

PolyA Tail Addition Process

Known as 3' polyadenylation, performed by the enzyme poly(A) polymerase, which adds adenines using ATP.

Required Sequence for PolyA Tail

Pre-mRNA must contain the AAUAAA sequence to initiate tailing.

Polyadenylation Process Steps

CPSF binds the AAUAAA sequence, forming a stable loop.
Cleavage occurs 10-35 base pairs downstream, followed by polyadenylation and binding of poly(A) binding proteins for translation stimulation.

Functions of 3' Polyadenylation

Protects mRNA from nucleases, aids ribosome recognition for translation, facilitates nuclear export of mRNA, and assists in isolating mRNAs in laboratory settings.

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Description

Explore the essential processes of mRNA processing in eukaryotic cells, focusing on the 5' cap and poly (A) tail. This set of flashcards covers critical concepts such as gene expression pathways and types of RNA, enhancing your understanding of molecular biology.