26 Questions
What is the primary function of the spliceosome?
To remove introns and join exons in pre-mRNA
What is the function of the U1 snRNP in the spliceosome?
To bind to the GU motif at the 5' splice site
What is the purpose of capping in mRNA maturation?
To protect the 5' end of pre-mRNA from degradation
What is the outcome of alternative splicing?
The production of multiple isoforms of mRNA from a single gene
What defines the exon-intron boundary?
The GU-AG motif
What is the outcome of intron removal and exon joining?
The production of a mature mRNA molecule
What is a common characteristic shared by tRNA introns?
Similar size and tertiary structure
What is a key difference between introns in mRNA and tRNA?
mRNA introns have variable lengths, unlike tRNA introns
What is the function of the conserved sequences at the 5' and 3' ends of introns?
To recognize the intron for processing
What is a characteristic of the consensus sequence in introns?
It is highly conserved, but not identical
What is the typical sequence found at the 5' end of introns?
GU
What is the typical sequence found at the 3' end of introns?
AG
What is the significance of the branching point in introns?
It is the site of a highly conserved adenine
What is the significance of the G nucleotide at the exon-intron boundary?
It is a conserved sequence at the exon-intron boundary
What is the position of the branching point in the intron?
Very close to the 3' end of the intron
What is the significance of the GU and AG consensus sequences in introns?
They are highly conserved and mark the intron for processing
What is the significance of the variable length of the central part of the RNA?
It allows the RNA to fold in a specific way to bring distant parts together.
What is the role of the 2'OH group in the reaction of intron excision?
It performs a nucleophilic attack on the phosphodiester bond between two guanines.
Why does the reaction of intron excision occur only in RNA and not in DNA?
Because RNA has a 2'OH group that can perform a nucleophilic attack.
What is the consequence of the second nucleophilic attack in the intron excision reaction?
The release of the intron in a lariat form.
What is the role of the U-snRNAs in the intron excision reaction?
They form a complex with the intron and position the adenine correctly.
What is the significance of the consensus sequence in the RNA?
It is recognized by RNAs and proteins that play a role in the splicing reaction.
What is the structure formed by the adenine and the intron during the reaction?
A lariat structure.
What is the difference between the first and second nucleophilic attacks in the intron excision reaction?
The first attack is performed by the 2'OH group, while the second is performed by a 3'OH group.
What is the significance of the autocatalytic intron in mitochondria?
It does not require any external factors for function.
What is the outcome of the intron excision reaction?
The release of the intron in a lariat form.
Study Notes
Spliceosome Function
- The spliceosome is a large ribonucleoprotein complex responsible for removing introns and joining exons in pre-mRNA
- Composed of 5 snRNPs (small nuclear ribonucleoproteins) and over 100 proteins
- Catalyzes two transesterification reactions to form a lariat structure and release the intron
- Leaves a continuous mature mRNA molecule
GU-AG Recognition
- The spliceosome recognizes the GU-AG motif at the intron-exon boundaries
- GU at the 5' splice site and AG at the 3' splice site
- The U1 snRNP binds to the GU motif, and the U2 snRNP binds to the AG motif
- This recognition is crucial for the spliceosome to accurately remove introns and join exons
mRNA Maturation
- After transcription, the pre-mRNA undergoes several processing steps to mature into functional mRNA
- These steps include:
- Splicing: removal of introns and joining of exons
- Capping: addition of a 7-methylguanosine cap to the 5' end
- Polyadenylation: addition of a poly(A) tail to the 3' end
- Editing: modification of nucleotides to change the amino acid sequence
Alternative Splicing
- The process by which different exons are included or excluded to produce multiple isoforms of mRNA from a single gene
- Allows for the generation of different protein isoforms with varying functions
- Regulated by various factors, including:
- Splicing factors
- Hormones
- Cellular signals
- Tissue-specific expression
Intron-Exon Definition
- Introns: non-coding regions of DNA that are removed during splicing
- Exons: coding regions of DNA that are joined together to form the mature mRNA
- The exon-intron boundary is defined by the GU-AG motif
- The spliceosome recognizes this motif to accurately remove introns and join exons
This quiz covers the function of the spliceosome in removing introns and joining exons in pre-mRNA, including GU-AG recognition, mRNA maturation, alternative splicing, and exon-intron definition.
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