Lecture 17 Flashcards
What are characteristics of mRNA?
Fully processed messenger RNA with 5’ cap, introns removed by RNA splicing, and a poly A tail
What are characteristics of pre-mRNA?
An mRNA precursor containing introns and not cleaved at the poly A site
What are characteristics of hnRNA?
Heterogeneous nuclear RNAs. These include pre-mRNAs and RNA processing intermediates containing one or more introns
What are characteristics of snRNA?
Five small nuclear RNAs that function in the removal of introns from pre-mRNAs by RNA splicing, plus two small nuclear RNAs that substitute for the first two at rare introns
What are characteristics of pre-tRNA?
A tRNA precursor containing additional transcribed bases at the 5’ and 3’ ends compared to the mature tRNA. Some pre-tRNAs also contain an intron in the anti-codon loop
What are characteristics of pre-rRNA?
The precursor to mature 18S, 5.8S, and 28S ribosomal RNAs. The mature rRNAs are processed from this long precursor RNA molecule by cleavage, removal of bases from the ends of the cleaved products, and modification of specific bases
What are characteristics of snoRNA?
Small nucleolar RNAs. These base-pair with complementary regions of the pre-RNA molecule, directing cleavage of the RNA chain and modification of bases during maturation of the rRNAs.
What are characteristics of siRNA?
Short interfering RNAs, approximately 22 bases long, that are each perfectly complementary to a sequence in an mRNA. Together with associated proteins, siRNAs cause cleavage of the “target” RNA, leading to its rapid degradation
What are characteristics of miRNA?
Micro RNAs, approximately 22 bases long, that base-pair extensively, but not completely, with mRNAs, especially over the six base pairs at the 5’ end of the miRNA. This inhibits translation of the “target” mRNA
Where does transcription of Pol 1 RNA occur specifically?
Nucleolus
Where does transcription of Pol 2 RNA occur specifically?
Nucleoplasm
Where does transcription of Pol 3 RNA occur specifically?
Nucleoplasm
What degrades abnormal RNAs that are formed from transcription in the nucleus before these RNAs can make their way into the cytoplasm of the cell?
Exosomes
What happens to RNA once it is used for translation in the cytoplasm?
Degradation by either deadenylation or removal of the 5’ cap
What type of RNA makes ribosomes?
mRNA
What does rRNA do?
rRNA acts as a translator between ribosomes and mRNA for translation
Describe the process of creating the 5’ cap on mRNA
1) Release of gamma phosphate group on pre-mRNA
2) Release of beta and gamma phosphate groups on GTP
3) Connect pre-mRNA without the gamma phosphate group to GTP with only alpha phosphate group
4) Use S-Adenosyl Methionine to methylate 5’ end of complex
5) Ribose ring also methylated
What kind of linkage is the 5’ cap in terms of 5’ and 3’ ends?
5’ to 5’ linkage. This inhibits ribonuclease activity
Describe the process of polyadenylation
1) Recognize Poly A recognition sequence
2) Recruit machinery
3) Poly A Polymerase (PAP) used to cleave excess nucleotides and creates Poly A tail
What is alternative splicing?
During splicing of pre-mRNA to make mature mRNA, exons can be rearranged or excluded from the mature mRNA, resulting in different arrangements of mRNA strands resulting from the same pre-mRNA strand
What is alternative polyadenylation?
There are multiple Poly A recognition sequences throughout a pre-mRNA strand that can be used to develop the Poly A tail. Rest of mRNA gets cut away
What is alternative capping?
There are multiple recognition sites for a 5’ cap that can alter the types of mature mRNA resulting from a single pre-mRNA strand
What is an example of two proteins resulting from alternative splicing of the calcitonin gene?
Thyroid - Calcitonin
Brain - Calcitonin Gene Related Protein (CGRP)
What is an example of two proteins resulting from alternative splicing of the Bcl-x gene?
Lymphocytes - Bcl-xS induces apoptosis of a cell
Brain - Bcl-xL inhibits apoptosis of a cell
Where exactly does splicing occur?
Splicing occurs adjacent to nucleotides that are part of a consensus sequence that is available 100% of the time
How exactly does splicing occur?
1) Transesterification (nucleophilic attack) on phosphate group at the end of an intron from a 2’ hydroxyl group at the other end of the intron (this produces a lariat structure and releases the first exon sequence)
2) The released exon sequence nucleophilic attacks (transesterification) the phosphate group on the opposite end of the intron, causing the two exon group to come together and the lariat structure containing the intron to be released
How is splicing mediated?
Through snRNAs and associated proteins
1) U1 snRNA perfectly pairs to the exon of mRNA next to the intro that is about to be spliced
2) U2 snRNA perfectly pairs within the intron to the branch point near the other exon, but does not pair with the branch point itself
What is odd about patients with the autoimmune disease systemic lupus erythematosus?
Antisera in these patients contains antibodies that bind to snRNA proteins (specifically to Sm sequences)
Can chain elongation of transcription occur at the same time as splicing mechanisms?
Yes. The long carboxyl terminal domain (CTD) of RNA Pol II can associate with multiple RNA-processing factors simultaneously. Chain elongation is coupled to RNA processing
What proteins allow for accurate splicing to occur?
Cross-exon recognition complexes
How does the HIV virus avoid splicing?
Through the use of the virus-specific protein Rev. Rev binds to pre-spliced mRNA and allows it to exit the nucleus without splicing
Post-transcriptionally, can RNA be edited?
Yes
What happens to mRNA if a deletion causes a premature stop codon?
It can go through a process called nonsense-mediated mRNA decay in which the mRNA is recognized to have a premature stop codon, its translation is repressed, and it is decayed
