Download AP Lesson #50 After transcription, do prokaryotes need to modify

Transcribe the following strand of DNA into mRNA
AP Lesson #50
EQ: After transcription has occurred,
does mRNA need to be modified?
RNA
polymerase
TGGTACAGCTACTCATCGTACCGA
3’
DNA Template Strand
5’
After transcription, do prokaryotes
need to modify their mRNA?
• NO!
– Protein coding gene is colinear with the mRNA
– mRNA is ready to be translated into a protein
Why would we need to modify mRNA?
• Eukaryotic genes are not continuous
– exons = the real gene
• expressed / coding DNA
– introns = the junk (noncoding)
• inbetween sequence
What about eukaryotes?
• Need to be modified
– Eukaryotic genes have junk DNA!
– Average gene contains 27,000 bases, of which about
1,200 bases actually code for proteins
How do we get rid of that junk?
• mRNA splicing
– Turn 27,000 bases into 1,200 bases
– Create a primary transcript = pre-mRNA
– mRNA splicing - edit out introns
– make mature mRNA transcript
intron = noncoding (inbetween) sequence
eukaryotic DNA
intron = noncoding (inbetween) sequence
~10,000 bases
eukaryotic DNA
exon = coding (expressed) sequence
exon = coding (expressed) sequence
primary mRNA
transcript
mature mRNA
transcript
~1,000 bases
spliced mRNA
What molecules help to cut out the junk?
• snRNPs
– small nuclear RNA proteins
– Recognize inton/exon and
attach
exon
• Spliceosome
snRNPs
snRNA
intron
exon
5'
3'
– several snRNPs join
– splice site sequence
• cut & paste gene
• Remove introns and paste
together exons
spliceosome
5'
mature mRNA
lariat
exon
5'
exon
excised
intron
3'
• Need to protect mRNA on its trip from nucleus to
cytoplasm
– Enzymes in cytoplasm attack mRNA (nucleases
nucleases)
• protect the ends of the molecule
• add 5′ GTP cap (ribosomal recognition site)
• add poly-A tail
–longer tail, mRNA lasts longer: produces more
protein because it won’t get broken down as
il
quickly
A ta
3'
oly3' p
5
5'
mRNA
– Alternative mRNA splicing
– Allows a single gene to possibly create more than one
protein
3'
What is left to do after splicing?
p
' ca
• A single gene can code for more than one protein
• It depends on which segment is treated as an exon
• Exon Shuffling
(creates variation)
3'
• Remaining exons form a
5'
continuous coding sequence
If we just cut them out anyways, why are
introns important?
A
A A
50-2
A
What does a cell do
with the mature
mRNA?
• Mature mRNA exits
the nucleus through
the nuclear pores
• mRNA enters the
cytoplasm where it
will go through
translation
Nuclear
envelope
DNA
TRANSCRIPTION
RNA PROCESSING
Pre-mRNA
mRNA
TRANSLATION
Ribosome
A
Polypeptide
’s
50 A
P P
G P
What happens if snRNPs screw up?
• A single base added or lost throws off the reading
frame
– Section of mRNA (codons) that will be translated
• One change in the mature mRNA could change the
protein
AUGCGGCTATGGGUCCGAUAAGGGCCAU
AUGCGGUCCGAUAAGGGCCAU
AUG|CGG|UCC|GAU|AAG|GGC|CAU
Met|Arg|Ser|Asp|Lys|Gly|His
AUGCGGCTATGGGUCCGAUAAGGGCCAU
AUGCGGGUCCGAUAAGGGCCAU
AUG|CGG|GUC|CGA|UAA|GGG|CCA|U
Met|Arg|Val|Arg|STOP|
Summarizing Strategy
• Quick Write:
– Explain the difference between prokaryotic and
eukaryotic mRNA modification and why it must occur.
Assessment
• HW: Chapter 17.1 thru 17.3 Wkst & Read Section
17.4 (Pgs. 337 – 344)