Genetics Chapter on Gene Expression and Splicing

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Questions and Answers

What type of mutation is a change of a purine to a pyrimidine?

Silent mutation

Transition mutation

Missense mutation

Transversion mutation (correct)

What is the primary function of splicing in eukaryotic gene expression?

Translation of mRNA into a protein

Transportation of mRNA from the nucleus to the cytoplasm

Removal of introns and joining of exons (correct)

Addition of a 5' cap to the mRNA transcript

What is the significance of alternative splicing in human genetics?

It is responsible for the transcription of DNA into mRNA

It ensures that all genes are expressed in all cell types

It allows for the production of multiple protein isoforms from a single gene (correct)

It ensures that the correct sequence of amino acids is incorporated into a protein

Which of the following is an example of a protein produced through alternative splicing?

Tropomyosin Signup and view all the answers

What is the role of snRNPs in splicing?

They recognize and remove introns from the pre-mRNA Signup and view all the answers

Which autoimmune disease is associated with a defect in splicing?

Systemic lupus erythematosus Signup and view all the answers

What is the purpose of the poly(A) tail added to the 3' end of eukaryotic mRNA?

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

Where does the polyadenylation process take place?

In the nucleus Signup and view all the answers

What is the approximate length of the poly(A) tail in eukaryotic mRNA?

40-200 nucleotides Signup and view all the answers

What sequence is crucial for the addition of the poly-A tail to mRNA?

Polyadenylation signal sequence (AAUAAA) Signup and view all the answers

Which type of RNA is responsible for synthesizing proteins by decoding the genetic code?

mRNA Signup and view all the answers

How many codons code for the amino acid Leucine?

6 codons Signup and view all the answers

Which RNA polymerase is responsible for transcribing the primary rRNA transcript (45S RNA)?

RNA polymerase I Signup and view all the answers

What happens to the poly-A tail after the mRNA enters the cytosol?

It is gradually shortened Signup and view all the answers

What does the wobble hypothesis explain in codon-anticodon interactions?

The third base of the codon can pair nontraditionally Signup and view all the answers

What is the role of tRNA during protein synthesis?

Carries amino acids to the ribosome Signup and view all the answers

Which one of the following correctly indicates the number of stop codons in the genetic code?

Three stop codons Signup and view all the answers

Which of the following modifications is NOT a process that eukaryotes undergo to their mRNA after transcription?

Translation of mRNA into a protein Signup and view all the answers

What is the purpose of the 5' cap on eukaryotic mRNA?

All of the above Signup and view all the answers

What is the main difference between the post-transcriptional modification of mRNA in prokaryotes and eukaryotes?

Eukaryotic mRNA has a 5' cap and a poly(A) tail, while prokaryotic mRNA does not. Signup and view all the answers

What is the enzyme responsible for attaching the 5' cap to eukaryotic mRNA?

Guanyltransferase Signup and view all the answers

What is the source of the methyl group that is added to the 5' cap of eukaryotic mRNA?

S-adenosylmethionine (SAM) Signup and view all the answers

What is the role of snRNPs (snurps) in the splicing of eukaryotic mRNA?

All of the above Signup and view all the answers

Why is histone mRNA unique among eukaryotic mRNAs?

It does not contain introns and is not processed. Signup and view all the answers

What is alternative splicing, and what does it allow?

Alternative splicing is a process where multiple proteins are produced from a single gene. Signup and view all the answers

Study Notes

Course Information

Course: BMS 141
Lecture Number: 9
Title: Post-transcription modification, Genetic codon
Instructor: Dr. Lames Dawood
Program: Medicine and Surgery
Year: Fall 2024
University: Galala University, Powered by Arizona State University

Intended Learning Outcomes

Posttranscriptional modifications of mRNA
Posttranscriptional modifications of rRNA
Posttranscriptional modifications of tRNA
Define genetic codon
Enumerate different types of mutations
Explain the effect of different mutations on phenotypes

Post-transcriptional Processing of mRNA

In Prokaryotes

Little modification occurs in mRNA
The primary transcript is immediately available for translation
Can be mono-cistronic (one polypeptide chain) or poly-cistronic (many polypeptide chains)
tRNA and rRNA primary transcripts are broken into smaller units

In Eukaryotes

The primary transcript undergoes several modifications in the nucleus
Capping of 5' end by GTP (followed by methylation, e.g., 7-methyl GTP)
Removal of introns and splicing of exons
Addition of a poly(A) tail at the 3' end

Capping of 5' end

Enzyme: guanyltransferase
Source of guanine: GTP
Methylation follows in the cytosol (7-methyl GTP).
Enzyme responsible for methylation: guanine-7-methyltransferase
Source of methyl group: S-adenosylmethionine (SAM) (Active methionine)

Function of Cap

Protects mRNA against 5' exonucleases
Helps transport of mRNA to the cytoplasm
Helps recognition of mRNA by ribosomes and initiation of translation
Eukaryotic mRNA lacking the cap are not efficiently translated

Removal of Introns and Splicing of Exons

Coding regions of DNA are called exons
Exons are separated by non-coding regions called introns
Pre-mRNA contains exons and introns
Introns are removed, and exons are spliced together
Histone mRNA does not contain introns
snRNPs (snurps), which are part of spliceosomes (snRNA+protein), determine the splicing site

Alternative Splicing

Leads to different protein products from the same gene
Example: different muscle cell types produce tissue-specific tropomyosin proteins from same gene
Different mRNAs can be generated from one primary transcript during mRNA processing.
Example of alternative splicing: pre-mRNA for calcitonin produces different mRNAs in the thyroid gland and neurons (leading to different hormones)

Removal of Introns and Splicing of Exons - Clinical Significance

Produces proteins with different activities
Alternative splicing is involved in 60% of human genetic coding
Provides a mechanism for producing a diverse set of proteins from a limited set of genes

Addition of Poly(A) Tail at 3' end

Most eukaryotic mRNAs have a chain of 40-200 adenine nucleotides at the 3' end (poly(A) tail)
This tail is not transcribed from the DNA
Added after transcription by the enzyme, polyadenylate polymerase, using ATP.
The mRNA is cleaved downstream of the consensus sequence (AAUAAA) and the poly(A) tail is added to the new 3' end.

Significance of Poly A tail

Stabilizes mRNA
Facilitates the exit of mRNA from the nucleus

Eukaryotic rRNA Processing

Primary rRNA transcript (45S RNA) contains 18S, 5.8S and 28S rRNA
RNA polymerase I transcribes rRNA genes to produce a single 45S precursor pre-rRNA
Cleaving and processing of pre-rRNA in the nucleus yield 18S, 5.8S and 28S rRNA
5S rRNA gene is transcribed by RNA polymerase III

Synthesis of tRNAs

Eukaryotic tRNA genes transcribed by RNA polymerase III
pre-tRNA processing:
- Removal of the extra 5' sequence (leader sequence)
- Replacement of UU nucleotides with CCA at the 3' end
- Modification of some bases (e.g., methylation)
- Removal of a short intron in the anticodon loop

Genetic Code and Mutations

Genome: All DNA in a cell
Chromosome: Contains many genes
Gene: A DNA sequence that specifies a polypeptide
Genetic codon: A triplet of nucleotides that codes for an amino acid
Information is based on a triplet code (codons)
64 possible codons (4^3)
61 codons specify amino acids
3 codons are stop codons (UAA, UAG, UGA)
AUG is the initiator codon
The ordering and grouping of nucleotides is the reading frame

Characteristics of the Genetic Code

Specificity: Each codon codes for only one specific amino acid
Universality: All organisms use the same genetic code (except mitochondria)
Degeneracy: A given amino acid may have more than one codon (e.g., Leucine has 6 codons)
Reading frame: Codons are read from the initiation codon as a continuous sequence of 3 bases

The Wobble Hypothesis

Interaction between codon and anticodon needs to be exact at first two positions. Conventional base-pairing rules apply.
The third base is not in close apposition and can allow non-traditional base-pairing (wobble).
This allows codons for the same amino-acid to be recognized by multiple tRNAs/anticodons.

DNA Mutations

Mutation: Permanent change in a gene's nucleotide sequence
Mutagen: Physical or chemical agent that causes mutation
Mutagenesis: Process of producing a mutation (spontaneous or induced)

Single Base Mutations (Point Mutations)

Transition mutation: Changing a purine to another purine (A ↔ G) or a pyrimidine to another pyrimidine (C ↔ T).
Transversion mutation: Changing a purine to a pyrimidine (A ↔ T or G ↔ C) or vice versa.
Silent mutation: The resulting codon still codes for the same amino acid
Missense mutation: The resulting codon codes for a different amino acid
Nonsense mutation: The resulting codon codes for termination of the peptide chain
Sense mutation: The terminating codon is changed to one that codes for an amino acid

Addition or Deletion of Nucleotides

Addition or deletion of one or two bases results in a "frame-shift" mutation
Addition or deletion of three nucleotides results in addition or deletion of one amino acid.

In-Class Assessment Questions

Functions of poly(A) tail in mRNA (except)
Functions of 5' mRNA capping (choose one)
Systemic lupus erythromatosus results from (choose one option)
Processing of tRNA (except)

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Description

Test your knowledge on gene expression, alternative splicing, and the roles of various RNA types in eukaryotic cells. This quiz covers essential topics such as mutations, the significance of poly(A) tails, and the complexities of splicing mechanisms. Perfect for students studying genetics and molecular biology.