Molecular Biology: Introns and Spliceosomes

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Questions and Answers

What is primarily responsible for the removal of introns from pre mRNA?

RNA polymerase

Ribosomes

Endoplasmic reticulum

Spliceosome (correct)

Which component of the spliceosome first recognizes the 5' splice site?

U2 snRNP

U4 snRNP

U1 snRNP (correct)

U6 snRNP

What is the significance of alternative splicing in eukaryotic cells?

It leads to the production of multiple protein products (correct)

It allows the degradation of mRNA

It permanently deletes certain exons

It prevents intron removal

Which codon serves as the start codon during protein synthesis?

AUG Signup and view all the answers

What type of RNA is involved in matching amino acids to codons in mRNA?

tRNA Signup and view all the answers

Which of the following statements about intracellular compartments is incorrect?

They are exclusive to prokaryotic cells. Signup and view all the answers

What is the role of specific enzymes in relation to tRNAs during protein synthesis?

They couple tRNAs to the correct amino acids. Signup and view all the answers

What happens to mature eukaryotic mRNAs after they are exported from the nucleus?

They are eventually degraded in the cytosol. Signup and view all the answers

What is the primary function of lysosomes within the cell?

Intracellular degradation of waste materials Signup and view all the answers

How do proteins enter the nucleus?

Via specialized channels known as nuclear pores Signup and view all the answers

Which component of the nuclear envelope provides structural support?

Nuclear lamina Signup and view all the answers

What structural component functions as the site of intron removal during mRNA splicing?

Spliceosome Signup and view all the answers

What is the primary role of the 5' methylated guanosine cap in eukaryotic mRNAs?

It provides stability and protects RNA from nucleases. Signup and view all the answers

What is the role of RAN in nuclear transport?

Facilitates the exchange of GDP for GTP during nuclear transport Signup and view all the answers

What is the purpose of splicing in the processing of eukaryotic mRNAs?

To remove introns and join exons. Signup and view all the answers

The sorting of endocytosed material is primarily performed by which organelle?

Endosomes Signup and view all the answers

During the transcription process, which transcription factor first binds to the TATA box?

TFIID Signup and view all the answers

Which organelle is primarily responsible for the synthesis of ATP and carbon fixation?

Chloroplast Signup and view all the answers

What is a characteristic feature of nuclear localization signals?

They often consist of proline and basic amino acids Signup and view all the answers

What is the primary function of the Golgi apparatus in relation to proteins?

Modification and sorting of proteins for secretion Signup and view all the answers

What is the result of incorrect splicing of mRNA?

The mRNA may translate into defective proteins. Signup and view all the answers

Which factor is crucial in the initiation phase of eukaryotic transcription?

Binding of the RNA polymerase to general transcription factors. Signup and view all the answers

Which enzyme is responsible for adding the poly A tail to the 3' end of eukaryotic mRNA?

Poly A polymerase Signup and view all the answers

What is a major function of the poly A tail in mRNA?

To protect mRNA from degradation. Signup and view all the answers

Study Notes

Introns

Introns are removed from pre-mRNAs by RNA splicing.
Intron 5' ends start with GV and terminate with AG at the 3' end.
Sequences adjacent to the 3' and 5' ends of the intron tend to be similar.
Branch point near the 3' end contains an A residue. L

Spliceosome

Intron removal is catalyzed by the spliceos ome.
The spliceosome is made up of 5 types of RNA and more than 200 proteins.
The spliceosome assembles from snRNPs (small nuclear ribonucleoprotein complexes).
Each snRNP contains 1 or 2 snRNAs (small nuclear RNAs).

Spliceosome Assembly

Spliceosome assembly involves the sequential binding of snRNPs to pre-mRNA.
U1 recognizes the 5' splice site.
U2 binds to the branch point sequence.
U4/U6 and U5 bring the intron ends together.
Pre-mRNA is cleaved at the 5' splice site and joined to the branch point A residue, forming a lariat.
The 3' splice site is cleaved, and the two ends of the exon are joined together.

Alternative Splicing

Introns allow pre-mRNA to be spliced in multiple ways.
This leads to the production of multiple protein products.
Different mechanisms can activate or skip certain splice sites.

From RNA To Protein

mRNA synthesis takes place in the nucleus.
Mature eukaryotic mRNAs are exported from the nucleus.
mRNA is single-stranded RNA involved in protein synthesis.
mRNA molecules are eventually degraded in the cytosol (cytoplasm).
The mRNA sequence is decoded into sets of three nucleotides (codons).
tRNA molecules match amino acids to codons in mRNA.
Specific enzymes couple tRNAs to the correct amino acid.

From DNA to RNA

Eukaryotic mRNAs are processed in the nucleus.
5' capping involves adding a methylated guanosine cap.
Polyadenylation involves adding a 3' poly A tail.
Splicing removes introns.
RNA polymerase II phosphorylation is coupled to pre-mRNA processing.
The 5' cap protects RNA from nucleases and plays a role in positioning RNA on the ribosome for translation initiation.
The poly A tail protects mRNA from nucleases and is required for export of the transcript from the nucleus.

Protein Coding Genes

Protein coding genes are interrupted by non-coding introns.
Exons are coding sequences.
Introns are non-coding sequences.

Intracellular Compartments and Protein Transport

The main function of membrane-enclosed organelles of eukaryotic cells is compartmentalization.
The cytosol contains many metabolic pathways, protein synthesis, and the cytoskeleton.
The nucleus contains the main genome DNA and RNA synthesis.
The endoplasmic reticulum (ER) synthesizes mRNA for all protein coding genes, miRNA genes, and other non-coding RNAs.
RNA polymerase III transcribes non-coding RNAs in eukaryotic cells, including tRNA genes, 5s rRNA genes, and genes for many other small RNAs.

Transcription Initiation

Initiation begins when RNA polymerase binds to the promoter sequence.
RNA polymerase requires general transcription factors (TFs) such as TFIIB and TFIID.
The TATA box is a DNA sequence that signals where the genetic sequence can be read and decoded.
The TFIIB recognition element (BRE) is slightly upstream of the TATA box.
The downstream promoter element (DPE) is 30 nucleotides downstream from the start site.
TFIID binds to the TATA box via its TATA-binding protein subunit.
TFIIB forms a complex with TFIID.
The rest of the GTFs and RNA Pol II bind to the complex.
The TATA-binding protein binds to DNA.
TFIIH unwinds the DNA and phosphorylates RNA Pol II.
RNA Pol II is released and transcription begins.

Nuclear Envelope

The nuclear envelope separates the nucleus from the rest of the cell.
The inner and outer membranes are separated by the perinuclear space.
The outer membrane is continuous with the ER.
The nuclear lamina supports the nuclear envelope and is composed of lamins.
The nuclear envelope is perforated with nuclear pores, which are specialized channels where the inner and outer membranes are fused.
These channels allow the exchange of molecules between the nucleus and the cytoplasm.
Proteins are imported into the nucleus through nuclear pores.

Nuclear Import

Proteins responsible for copying DNA, synthesizing RNAs needed for assembling ribosomes, and other proteins are transported into the nucleus.
Over 5000 molecules are imported through the 2000-5000 nuclear pores every second.
Nuclear localization signals (NLSs) are required for importing proteins into the nucleus.
NLSs are usually 8-30 amino acids long.
They often contain proline and basic amino acids such as lysine and arginine.

Hydrolysis-Driven Nuclear Transport

Ran is a small GTPase that plays a role in nuclear transport.
Ran-GDP is found in the cytoplasm, and Ran-GTP is found in the nucleus.
GTP bound to Ran is hydrolyzed to GDP by Ran GAP (Ran GTPase activating protein) in the cytoplasm.
GDP bound to Ran is exchanged for GTP by Ran GEF (Ran guanine nucleotide exchange factor) in the nucleus.
This cycle of GTP hydrolysis and exchange drives the transport of proteins into and out of the nucleus.

ER

The ER is a network of membrane-bound sacs and tubules that is involved in lipid synthesis, protein synthesis, and distribution of proteins to other organelles and the plasma membrane.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

Lysosomes are responsible for intracellular degradation.

Endosomes

Endosomes are involved in the sorting of endocytosed material.

Mitochondria

Mitochondria carry out ATP synthesis by oxidative phosphorylation.

Chloroplasts

Chloroplasts are responsible for ATP synthesis and carbon fixation by photosynthesis.

Peroxisomes

Peroxisomes are involved in the oxidative breakdown of toxic molecules.

Protein Sorting

Proteins are transported into organelles by three mechanisms:
- Signal sequences direct proteins to the correct compartment.
- Proteins enter the nucleus through nuclear pores.
Signal sequences often reside at the N-terminus or C-terminus of a protein.

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Description

Explore the fascinating world of introns and spliceosomes in molecular biology. This quiz covers the processes of RNA splicing, the components of the spliceosome, and the significance of alternative splicing in gene expression. Test your knowledge on how introns are effectively removed from pre-mRNA and the role of snRNPs.