RNA Structure and Synthesis - Eukaryotic Transcription

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Questions and Answers

What are the three classes of RNA polymerase in eukaryotic cells?

RNA polymerase I, II, and III

What does RNA polymerase I synthesize?

The precursor of the large ribosomal RNAs (28S, 18S, and 5.8S)

What is the role of promoters in eukaryotic gene regulation?

They serve as binding sites for RNA polymerase II and general transcription factors, initiating transcription.

Name three common elements found in eukaryotic promoters.

CAAT box, GC box, and TATA box Signup and view all the answers

What are cis-acting genetic elements?

DNA sequences that regulate genes located on the same molecule of DNA. Signup and view all the answers

What are general transcription factors?

Proteins that bind directly to the promoter and help regulate gene expression. Signup and view all the answers

What role do enhancers play in eukaryotic gene expression?

Enhancers increase the rate of transcription by RNA polymerase II. Signup and view all the answers

Enhancers must be located on the same chromosome as the gene they regulate.

True Signup and view all the answers

What are response elements?

DNA sequences within enhancers that bind specific transcription factors called activators. Signup and view all the answers

What is a-amanitin, and how does it affect eukaryotic gene expression?

A potent toxin produced by the poisonous mushroom Amanita phalloides that inhibits RNA polymerase II, preventing mRNA synthesis and ultimately protein synthesis. Signup and view all the answers

How does the mitochondrial RNA polymerase differ from the eukaryotic RNA polymerase?

It resembles bacterial RNA polymerase more closely. Signup and view all the answers

What are preribosomal RNAs?

Long precursor molecules that are processed into mature ribosomal RNAs. Signup and view all the answers

How do the rRNAs of prokaryotes and eukaryotes differ in their production?

In both prokaryotes and eukaryotes, rRNAs are produced from a single precursor molecule. Signup and view all the answers

What is the function of transfer RNA (tRNA)?

To carry specific amino acids to the ribosome, where they are used to build proteins. Signup and view all the answers

TRNA molecules are synthesized by RNA polymerase I.

False Signup and view all the answers

What are the major modifications that occur during tRNA processing?

Cleavage of specific sequences, addition of a CCA sequence at the 3' end, and base modifications. Signup and view all the answers

What are heterogeneous nuclear RNAs (hnRNAs)?

Precursor molecules for eukaryotic messenger RNA (mRNA). Signup and view all the answers

What are the three major modifications that occur during eukaryotic mRNA processing?

5' capping, addition of a poly-A tail, and removal of introns. Signup and view all the answers

What is 5' capping, and what is its significance?

The attachment of a 7-methylguanosine cap to the 5' end of mRNA, which is crucial for translation initiation and mRNA stability. Signup and view all the answers

What is polyadenylation, and what is its significance?

The addition of a poly-A tail (a string of adenines) to the 3' end of mRNA, which helps stabilize the mRNA and facilitate its export from the nucleus. Signup and view all the answers

What are introns, and how are they removed during mRNA processing?

Non-coding sequences within eukaryotic genes that are removed by splicing. Signup and view all the answers

What is alternative splicing, and what is its significance?

A process where different combinations of exons are spliced out of mRNA transcripts, creating multiple protein isoforms from a single gene. Signup and view all the answers

What are the key structural features of mature eukaryotic mRNA?

A 5' cap, a coding region, a 3' poly-A tail, and a monocistronic structure (containing a single coding region for a single protein). Signup and view all the answers

What is eukaryotic transcription?

The process by which a DNA sequence is used as a template to synthesize a complementary RNA molecule. Signup and view all the answers

Describe the steps involved in the initiation of eukaryotic transcription.

Initiation involves chromatin remodeling, the binding of general transcription factors and RNA polymerase to the promoter, and the unwinding of the DNA double helix. Signup and view all the answers

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

They bind to enhancer sequences and influence the efficiency of transcription. Signup and view all the answers

Describe the process of elongation in eukaryotic transcription.

Elongation involves the synthesis of the RNA transcript using the DNA template, with RNA polymerase moving along the DNA in the 3' to 5' direction. Signup and view all the answers

How does eukaryotic transcription terminate?

A termination signal sequence in the DNA is recognized by RNA polymerase, leading to the release of the RNA transcript and RNA polymerase from the DNA. Signup and view all the answers

What are the major post-transcriptional modifications that occur in eukaryotic mRNA?

Splicing, capping, polyadenylation, and cleavage, trimming, and base/sugar modifications. Signup and view all the answers

Explain the process of splicing in eukaryotic mRNA.

Non-coding introns are removed from the pre-mRNA, and the remaining coding exons are joined together to form mature mRNA. Signup and view all the answers

Explain the process of polyadenylation in eukaryotic mRNA.

A poly-A tail (a string of adenines) is added to the 3' end of the pre-mRNA, which helps stabilize the mRNA, facilitate its export from the nucleus, and protects it from degradation. Signup and view all the answers

What are the key differences between prokaryotic and eukaryotic transcription?

Prokaryotic transcription occurs in the cytoplasm, while eukaryotic transcription occurs in the nucleus. Prokaryotes have a single RNA polymerase, while eukaryotes have three. Prokaryotic mRNAs are polycistronic (can code for multiple proteins), while eukaryotic mRNAs are monocistronic (code for a single protein). Signup and view all the answers

Study Notes

RNA Structure and Synthesis (II) - Transcription of Eukaryotic Genes

Eukaryotic cells have three distinct RNA polymerase classes, each recognizing specific gene types.
RNA polymerase I synthesizes the precursor for large ribosomal RNAs (28S, 18S, and 5.8S) within the nucleolus.
RNA polymerase II synthesizes mRNA precursors and some small nuclear RNAs (snRNAs).
Promoters (class II genes) are specific DNA sequences within the eukaryotic promoter region.
These sequences are recognized by RNA polymerase II.
Sequences include the CAAT box, and the Hogness (TATA) box
Length ranges from approximately 45 to 55 bases.
Many promoters also contain a GC box (GGGCGG) sequence. These act as cis-acting genetic elements – meaning they're on the same DNA molecule as the transcribed gene.
These sequences serve as binding sites for general transcription factors.
Eukaryotic genes have enhancer sequences which are also cis-acting genetic elements.
These elements increase transcription initiation rates by RNA polymerase II.
Enhancers need to be on the same chromosome as the target gene.
They can be located upstream or downstream of the transcription start site. They can be relatively close to or thousands of bases distant from the promoter.
They can be located on either strand of DNA.
Enhancers stimulate RNA polymerase II by bending the DNA, allowing interaction with the transcription initiation complex.
Enhancers contain DNA sequences called response elements that bind specific transcription factors called activators.
RNA polymerase II is inhibited by α-amanitin, a potent toxin from the mushroom Amanita phalloides.
α-amanitin forms a tight complex with RNA polymerase II, therefore inhibiting mRNA synthesis and protein synthesis.
Mitochondria contain a single RNA polymerase that closely resembles bacterial RNA polymerase rather than a typical eukaryotic enzyme.
Ribosomal RNA (rRNA) of both prokaryotes and eukaryotes originate from long precursor molecules called preribosomal RNA.
In eukaryotes, the 28S, 18S and 5.8S rRNAs are produced from a single precursor molecule.
In prokaryotes, 23S, 16S, and 5S rRNAs are produced from a single precursor.
Eukaryotic 5S rRNA is synthesized by RNA polymerase III and subsequently modified.

Post-transcriptional Modification of RNA

Transfer RNA (tRNA)
Mature tRNA contains a 16-nucleotide sequence at the 5' end, and a 14-nucleotide intron contained in the anticodon loop area.
Uracil residues at the end are replaced by CCA sequences.
Messenger RNA (mRNA)
Synthesized by RNA polymerase II
Eukaryotic mRNAs have modifications:
5' capping
Addition of a poly-A tail
Removal of introns
Alternative splicing

5' Capping

The 5' cap is a 7-methylguanosine attached backward to the 5' terminal end.
This creates an unusual 5'→5' triphosphate linkage.
Adding the guanosine (guanylyltransferase) is a nuclear process.
Methylating the terminal guanine is a cytoplasmic process (guanine-7-methyltransferase) using S-adenosylmethionine as a methyl group donor.

Addition of a Poly-A Tail

A chain of 40-200 adenine nucleotides is added to the 3' end.
This poly-A tail is not part of the initial DNA sequence but is added after transcription by an enzyme (polyadenine polymerase).

Polyadenylation Signal Sequence

AAUAAA sequence found near the 3' end of the RNA molecule signals the addition of the poly-A tail.
Tails help stabilize mRNAs and promote their export from the nucleus after synthesis.
The tail is gradually degraded after mRNA exits the nucleus and enters the cytosol.

Removal of Introns

Eukaryotic mRNA maturation involves removing non-protein-coding RNA sequences called introns from the primary transcript.
Exons (coding sequences) are then spliced together to form the mature mRNA.
The molecular machine that performs this splicing is called a spliceosome.

Alternative Splicing of mRNA Molecules

Pre-mRNA from some genes can be spliced several ways
Gives rise to multiple mRNA forms (and therefore proteins).
A method for producing a wide range of proteins from a limited number of genes.

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Description

This quiz covers the intricacies of RNA structure and the transcription process of eukaryotic genes. It explores the roles of different RNA polymerases, the significance of specific promoter sequences, and the function of enhancer elements in gene expression. Test your knowledge on how these components contribute to understanding gene regulation in eukaryotic cells.