Lecture 13 Flashcards
Early experiments indicated that RNA was synthesized as a longer –
precursor (pre- mRNA)
Observation: The mRNA found associated with ribosomes was on average much – than pre-mRNA.
smaller
T/F: Pre-RNA exist as free RNA molecules in the nuclei of eukaryotic cell
false
RNA molecules are associated with an abundant set of nuclear proteins called –
heterogeneous ribonucleoprotein or hnRNP
– : a term refering to pre-mRNA and other nuclear RNAs of various sizes.
heterogeneous nuclear RNAs (hnRNAs)
Obvious difference: hnRNAs are – than cytoplasmic mRNAs (mature mRNA)
longer
cytoplasmic mRNA is derived from hnRNA by
splicing
The size difference between hnRNA and mature mRNA is primarily due to removal of –.
non-coding sequences called introns
Eukaryotic mRNAs have a 5’ cap formed by a –
5’-5’ triphosphate linkage of a 7-methylguanylate (GTP) residue
5’ cap functions
transport out of the nucleus, initiation of translation, protect from exonucleases
– coding sequence of a gene that contributes to the mature mRNA; is translated into protein
Exons:
Introns: non-coding sequence of a gene that is transcribed but is not part of the mature mRNA; is not –
translated
Specific sequences are required to remove introns during splicing, one of which is the
branching point
RNA splicing is performed by
spliceosome
Spliceosome: large assembly of –
snRNAs (small nuclear RNAs) and snRNPs (small nuclear ribonucleoproteins)
A pre-mRNA of a gene can be spliced in –
different ways
– of the spliceosome recognize of the 5’ splice junction, the branch-point site, and the 3’ splice junction through base-pairing.
snRNAs
Introns are removed and formed a branded structure a –
lariat
Poly(A) polymerase adds A residues to the –
3’ end
-- can be added to make the complete Poly(A) tail
200-250 Adenosine residues
Poly(A) tail functions to protect the mRNA from – by 3’ –> 5’ exonuclease
premature degradation
– bind to AAUAAA to signal poly A tail
CPSF
– cbinds to GU-rich element to signal polyA tail
CstF
