Studied by 4 peoplewhere does processing occur
eukaryotes and bacteria
what RNA polymerase transcribes non protein coding RNAs
RNA polymerase I
cleavage of non protein coding RNAs
occurs in the nucleolus, RNA transcript is cut into smaller pieces
what performs tRNA processing in eukaryotes
endo and exonucleases
exonucleases
break phosphodiester bonds between nucleotides at the end of the strand
endonucleases
break the phosphodiester bond between nucleotides within a strand
what are endo and exonucleases composed of
RNA and protein
ribozyme
RNA molecule with catalytic activity
which organism has colinear transcriptional units
bacteria
what organism has introns
eukaryotes
introns
intervening sequences within genes that don’t code for protein, must be removed
exons
coding sequences within genes that do code for protein
are introns and exons transcribed in the pre-mRNA
yes
what removes introns
the spliceosome, recognizes intron boundaries and removes them, common in protein coding genes of complex eukaryotes, holds the pre-mRNA in position for splicing and rejoining the exons
what is the spliceosome composed of
5 subunits called snRNPs (U1, U2, U4, U5, U6)
snRNPs
composed of small nuclear RNA and a set of proteins
where do the snRNP subunits bind into
an intron sequence to recognize intron-exon boundaries
what are the ends of the intron called
the 5’ and 3’ splice sites
where is the branch site located
somewhere in the middle of the sequence
where does the U1 bind to
the 5’ splice site
where does the U2 bind to
the branch site
what does the U4, U5, U6 subunits do
U4/ U6 dimerize and U5 associates, U6 catalyzes the reaction; intron loops out and exons brought closer together, 5’ splice site is cut and U1 and U4 are released, 3’ splice site is cut and the exons are covalently linked
alternative splicing
a pre-mRNA can be spliced in more than one way, produces two or more polypeptides from the same gene
constitutive exons
always found in mature mRNA
alternative exons
vary from one cell to another
splicing factors
proteins that regulate alternative splicing
when does 5’ capping occur
while pre-mRNA is being synthesized
5’ capping process
at the 5’ end of the mRNA 3 phosphate groups are present, RNA 5’ triphosphatase takes one of the phosphates, guanyltransferase hydrolyzes GTP (gets rid of two of the phosphate groups) and attaches GMP to the 5’ end. Methyltransferase attaches a CH3 group to the N at position 7 in the guanine, resulting in a 7-methylguanosine attached to the 5’ end of mRNA
what does the 5’ cap do
cap binding proteins assist in moving mRNAs out of the nucleus, cap is recognized by translation initiation factors, cap may be important for splicing as well
is the poly A tail encoded in the DNA
no
how is the poly A tail added
by polyApolymerase
what are the functions of 3’ polyadenylation
facilitates mRNA export from nucleus, stabilizes mRNA in cytosol, translation of the mRNA
mRNA editing
making a change to the base sequence of an RNA molecule, can occur through addition, deletion or conversion of a base, not common
post transcriptional tRNA base modification
carried out by a variety of enzymes that can add or remove functional groups, lack of modifications increases errors during translation, common in eukaryotes and bacteria
Note
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