Gene Expression (Transcription) Overview

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Questions and Answers

What role does polyadenylation play in the RNA lifecycle?

It replaces exons with introns during RNA maturation.

It protects the 3' end from exonuclease degradation. (correct)

It enhances the binding of RNA polymerase to DNA.

It facilitates protein synthesis by enhancing ribosome binding.

Which small nuclear RNA is not involved in the formation of spliceosomes?

U3 (correct)

What modification is performed on tRNA during its processing?

Addition of a poly-A tail.

Incorporation of a 7-methylguanylate cap.

Capping with trimethylguanosine for ribosomal recognition.

Removal of the 5' extension and addition of CCA at the 3' end. (correct)

What is the outcome of alternative splicing in mRNA processing?

It generates distinct varieties of mRNA from the same gene. Signup and view all the answers

What is the primary function of small nuclear ribonucleoproteins (snRNPs) in RNA splicing?

To bind with snRNAs and splice introns from pre-mRNA. Signup and view all the answers

What characterizes the initiation phase of transcription in prokaryotes?

The sigma factor recognizes the promoter at the TATA box and binds it. Signup and view all the answers

How does termination of transcription differ between prokaryotes and eukaryotes?

Eukaryotic termination requires the dissociation of RNA polymerase by dephosphorylation. Signup and view all the answers

Which statement accurately describes eukaryotic transcription elongation?

RNA polymerase unwinds the DNA and then synthesizes RNA. Signup and view all the answers

What is the composition of the RNA polymerase in prokaryotic transcription?

It consists of one enzyme formed of 2α and 2β subunits. Signup and view all the answers

Which eukaryotic RNA polymerase is responsible for synthesizing mRNA?

RNA polymerase II Signup and view all the answers

Which factors are recognized during the initiation phase of transcription in eukaryotes?

TFIID binds to the TATA box and other specific boxes. Signup and view all the answers

What is the function of the TATA box in eukaryotic transcription?

It serves as the primary site for transcription factor binding. Signup and view all the answers

In the context of the preinitiation complex, which component directly binds to the TATA box?

Transcription factor IID (TFIID) Signup and view all the answers

Which of the following is NOT a feature of RNA polymerase II?

It directly binds to the promoter sequence. Signup and view all the answers

What is the primary role of transcription factors in eukaryotic transcription?

To regulate which genes are transcribed. Signup and view all the answers

What determines the frequency of transcription in eukaryotes?

The GC box located upstream of the transcription start site. Signup and view all the answers

What is the significance of the CAAT box in the transcription process?

It enhances transcription efficiency. Signup and view all the answers

Which mechanism is associated with the absence of a hairpin in histone mRNAs?

Processing by a specific histone mechanism. Signup and view all the answers

Which transcription factor is recruited after TFIIB during the formation of the transcription initiation complex?

TFIIF Signup and view all the answers

What initiates the transcription process by RNA polymerase II?

Phosphorylation by kinase Signup and view all the answers

What is the role of Polyadenylation Specificity Factor (CPSF) in transcription termination?

Binds to the AAUAAA sequence Signup and view all the answers

Which of the following statements about capping of mRNA is true?

Capping protects the RNA from ribonucleases Signup and view all the answers

What is the main consequence of enhancer sequences binding during transcription initiation?

Modification of initiation complex formation rate Signup and view all the answers

What role does Cleavage Stimulation Factor (CstF) play in transcription termination?

Binds the GU-rich sequence Signup and view all the answers

During transcription elongation, what process must occur to allow RNA synthesis to progress?

Unwinding of DNA Signup and view all the answers

What sequence in the emerging RNA is critical for transcription termination?

AAUAAA sequence followed by GU-rich sequence Signup and view all the answers

What is the role of the sigma factor in prokaryotic transcription initiation?

It recognizes and binds to the promoter at the TATA box. Signup and view all the answers

What is the significance of the TATA box in prokaryotic transcription?

It determines the frequency of transcription initiation. Signup and view all the answers

Which enzyme is responsible for relieving positive supercoiling during RNA transcription?

Gyrase Signup and view all the answers

What characterizes the Rho factor in prokaryotic transcription termination?

It binds at C-rich G-poor regions and moves towards RNA polymerase. Signup and view all the answers

What is the composition of prokaryotic RNA polymerase?

One sigma factor and two alpha and two beta subunits. Signup and view all the answers

Which statement about the palindromic GC-rich region at transcription termination is true?

It forms a hairpin structure in RNA due to base pairing. Signup and view all the answers

What is the function of topoisomerase during transcription in prokaryotes?

To relieve negative supercoiling behind RNA polymerase. Signup and view all the answers

What distinguishes Rho-independent termination of transcription from Rho-dependent termination?

Rho-independent termination does not require any protein. Signup and view all the answers

Study Notes

Gene Expression (Transcription) in Prokaryotes

Prokaryotes use only one RNA polymerase for synthesizing three types of RNA.
This polymerase is composed of a sigma factor and two alpha and two beta subunits.
The sigma factor recognizes and binds to the promoter region (TATA box).
RNA polymerase then binds to the start point (of transcription).
Rifampicin, an antibiotic inhibits RNA synthesis by binding to the beta subunit of the polymerase.
Two key sequences identify the start site for transcription:
- TATA box (10 bases upstream of start point)
- TTGACA box (35 bases upstream of start point). This sequence determines the frequency of transcription.

Gene Expression (Transcription) in Eukaryotes

Eukaryotes have three RNA polymerases (I, II, and III).
Each polymerase is involved in the synthesis of different types of RNA:
- RNA polymerase I synthesizes rRNA
- RNA polymerase II synthesizes mRNA
- RNA polymerase III synthesizes tRNA

Transcription Initiation in Eukaryotes

Preinitiation complex (PIC) formation is crucial for initiating transcription in eukaryotes.
The PIC comprises RNA polymerase II and general transcription factors.
TBP (TATA-box binding protein) of TFII D binds to the TATA box (-10 sequence)
The other transcription factors TFII A, TFII B, TFII F, TFII E, TFII H bind to the other factors to form the complete PIC.
These factors facilitate the unwinding of the DNA double helix.
The initiation complex forms in the promoter region, but RNA polymerase II does not bind directly to the promoter sequence itself, rather to the PIC.

Other Transcription Factors

CAAT box (CCAAT) is 40 bases from the initiation of transcription
GC box (GGGCGG) is 200 bases away from the initiation of transcription. Both affect transcription frequency.
TBP and TAFs (collectively known as TFIID) bind to the TATA box to initiate transcription.
Other transcription factors (TFII A, TFII B, etc.) then bind to the complex, leading to the unwinding of the DNA double helix.
Enhancer sequences can modify the initiation complex and affect the rate of transcription.

Transcription Elongation

RNA polymerase unwinds DNA and creates a new strand of RNA.
The process of making the new strand begins by phosphorylation of the RNA polymerase enzyme by a kinase.

Transcription Termination

The "end" of the RNA transcript is determined by a specific sequence (AAUAAA) followed by a GU-rich sequence. Proteins then bind to this sequence and a poly(A) polymerase adds a poly-A tail (100-200 adenine nucleotides) to the 3' end of the mRNA.
Polyadenylation is important to protect the 3' end of the mRNA (from enzyme degradation) and to initiate translation.
Capping and polyadenylation are post-transcriptional modifications to mRNA.

RNA Splicing

Splicing removes non-coding intron sequences and joins coding exon sequences from the primary RNA transcript.
Small nuclear ribonucleoproteins (snRNPs) are essential in splicing
Different splicing patterns result in multiple protein products from a single gene (alternative splicing). RNA polymerase III processes tRNA and RNA polymerase I processes rRNA.

Processing of other RNA molecules

tRNA is processed by removing extensions, adding CCA to the 3' end, removing introns, and modifying bases (methylation of uracil) for proper function.
rRNA is processed and cleaved (split into distinctive segments) in the nucleus to form functioning ribosomal subunits.

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Description

Explore the fundamental concepts of gene expression focusing on transcription processes in both prokaryotic and eukaryotic organisms. This quiz covers the role of RNA polymerase, key sequences like the TATA box, and differences between prokaryotic and eukaryotic transcription mechanisms. Test your understanding of these essential biological processes.