Chapter 14.1-2: mRNA Flashcards
(25 cards)
Colinear/colinearity
number of nucleotides in a gene should be proportional to the number of amino acids in the encoded protein (bacteria and many viruses)
Noncolinear
DNA much longer than mRNA - demonstrated through hybridization (eukaryotes)
Introns are removed by ___ _________. And are categorized by _____
RNA Splicing
How they are removed, splicing mechanism
Type of intron: Nuclear pre-mRNA
protein-encoding genes in the nucleus of eukaryotes (Spliceosomal)
Type of intron: tRNA
tRNA genes of eubacteria, archaea, and eukaryotes
Enzymatic
What is considered part of the gene?
DNA (exons & introns) RNA sequences NOT translated into protein, including... Transcription Unit Promoter RNA coding sequence Terminator
Function of mRNA
template for protein synthesis; it carries genetic information from DNA to a ribosome and helps to assemble amino acids in their correct order.
Structure of mRNA
- 5’ untranslated region (5’UTR or leader sequence), can be shine-Dalgarno sequence (ribosome binding site in bacteria)
- Protein coding region
- 3’ UTR- stability and translation
pre-mRNA
immature single strand of mRNA, that contains exons and introns
Addition of 5’ Cap
A nucleotide with 7-methylguanine; 5’-5’ bond is attached to the 5’-end of the RNA. NOT PHOSPHODIESTER. Cap-binding proteins recognize and attach the cap.
- Facilitates binding of ribosome to 5’ end to initiate translation
- Increases mRNA stability
- Enhances RNA splicing
- Enhances intron removal
Addition of poly(A) tail
~50-250 adenine nucleotides are added to the 3’-end of the mRNA.
Added by cleavage and polyadenylation at 3’ end
- Increase stability of mRNA
- facilitates binding of ribosome to mRNA
- Plays a role in export of mRNA into cytoplasm
RNA splicing
Main function, consensus sequences
Main function: Removes noncoding introns from pre-mRNA using consensus sequences
5’: GU A/G AGU: splice site
3’: CAGG
Branch point: Adenine “A”, ~18-40 nucleotides upstream of 3’ splice site
Other functions of RNA splicing
- Removes introns
- Facilitates export of mRNA to cytoplasm,
- Allows for multiple proteins to be produced through alternative splicing
RNA splicing: Spliceosome
Five RNA molecules + 300 proteins
Where splicing takes place
RNA Splicing Process (release Lariat)
- pre-mRNA cut at 5’ site
5’ end attaches to branch point forming lariat. G bonds with A through transesterification (ester exchange with alcohol) - Cut at 3’ splice site, and 3’ end of exon 1 becomes covalently attached to the 5’ end of exon 2. Lariat is released.
If a splice site were mutated so that splicing did not take place, what would the effect be on protein encoded by the mRNA?
a. it would be shorter than normal
b. it would be longer than normal
c. it would be the same length but would have different amino acids
a. it would be shorter
WHY? Because introns usually early stop codons.
Nuclear Organization of pre-mRNA processing: intron removal, mRNA processing, and transcription take place at
the same site in the nucleus
Minor Splicing
Use different types of consensus sequences & snRNAs
Self-splicing introns (groups 1&2)
in some rRNA genes in protists and mitochondria genes in fungi
Group 1- Nine Stemmed Loops, Large intron removed by splicing
Group 2- intron removed by lariat structure
Alternative processing pathways
- Multiple 3’ cleavage sites
- Alternative Splicing- one 3’ cleavage site, but alternative RNA splicing, either two introns removed, or counts it as one big intron and takes an exon with it.
Estimated that more than ___% of human genes undergo alternative splicing
90%
Alternative 3’ cleavage sites results in
a. multiple genes of different length.
b. multiple genes of pre-mRNA of different length.
c. multiple mRNAs of different length.
d. All of the above.
c. mRNAs of different length
RNA editing after transcription: Why?
So protein amino acid sequence differs from encoded gene.
Some nuclear genes in mammalian cells and in some mitochondrial genes in plant cells.
Can include insertion, deletion of nucleotides, and conversion of bases
gRNA plays role
Guide RNA (gRNA) is template for ____, ____ and _____ of bases
Addition, deletion, and alteration
