mRNA processing Flashcards
How Eukaryotic mRNA is modified prior to leaving the nucleus
1) Addition of a cap to the front (5’) end
2) Addition of a poly-A tail to the back (3’) end
3) Cutting out of INTRONS from the mRNA
The of the growing mRNA is modified very soon after the start of transcription
5’ end
steps of Addition of a cap to the front (5’) end
a) A guanine is added to the absolute 5’ end
via a 5’-5’ linkage to the 1st nucleotide
- Different from the normal 5’-3’ phosphodiester linkages
b) That guanine and the 1st few nucleotides are then methylated
c) Cap-binding proteins then attach to the cap
- Cap-binding proteins function to:
1) Protect the mRNA from RNases in the cytoplasm
2) Indirectly allow mRNA to attach to the
small ribosomal subunit
A guanine is added to the absolute 5’ end
via a
5’-5’ linkage
attach to the cap
Cap-binding proteins
Cap-binding proteins function
1) Protect the mRNA from RNases in the cytoplasm
2) Indirectly allow mRNA to attach to the
small ribosomal subunit
steps of Addition of a poly-A tail to the back (3’) end
a) An enzyme (poly(A) polymerase) detects a consensus sequence
AAUAAA near the end of mRNA and cuts it ~25 nucleotides
downstream
b) The enzyme then adds 50-200 adenines to the cut end
Most genes are transcribed beyond the
coding sequence
The extra sequence will be cut off and a what tail will be added
poly A
detects a consensus sequence
AAUAAA near the end of mRNA and cuts it ~25 nucleotides
downstream
poly(A) polymerase
poly(A) polymerase adds this to the cut end
50-200 adenines
function of string of adenines
1) Protect the 3’ end of the mRNA from RNases
2) Allows cell to regulate mRNA stability
3) Help in mRNA-ribosome binding
the Longer the poly A tail the what the life span
longer
Most eukaryotic genes contain stretches of what sequence between what sequences
noncoding sequence (introns)
between
coding sequences (exons)
properties of introns
a) Common in eukaryotes, rare in prokaryotes
b) More complex the organism, more complex/abundant the introns
c) Intron abundance and size vary per gene within a species
- Some genes have no introns, others have as many as 60
do introns have to be removed from mRNA?
THEY MUST BE REMOVED FROM THE mRNA!!!
Introns are classified how
by how they are removed
classes of introns
Group I and II introns
Nuclear pre-mRNA introns
Found in rRNA genes and a few bacterial genes
(generally small introns)
Group I and II introns
Uses a molecule of guanosine to excise itself out and “glue” the remaining exons together
Group I and II introns
what molecule does Group I and II introns use
guanosine
Require help from an enzyme complex called the splicesome in order to remove its introns
- Larger and more complex
Nuclear pre-mRNA introns
what enzyme complex does Nuclear pre-mRNA introns use
splicesome
what attracts the splicesome
consensus sequences
at the borders
All introns are what
removed
exons are what
differentially cut out/retained
Different protein products
Different mRNAs
isoforms
ONE GENE ≠ONE PROTEIN
what will Processed eukaryotic RNAs exit the nucleus with
a cap, poly A tail, and no introns
where did Processed eukaryotic RNAs exit
through nuclear pore complexes
associate with processed mRNA molecules and direct them to and through the nuclear pore
mRNPs
mRNPs interact with a pore complexes called the
mRNA exporter
what happens to mRNAs that fail to be spliced (introns removed)
they will not exit the nucleus
