1. Chapter 17.
Mutations 1

2. Universal code Code is redundant several codons for each amino acid
“wobble” in the tRNA
“wobble” in the aminoacyl-tRNA synthetase enzyme that loads the tRNA
Strong evidence for a single origin in evolutionary theory. 2

3. Mutations Point mutations single base change base-pair substitution
silent mutation
no amino acid change
redundancy in code
missense
change amino acid
nonsense
change to stop codon
When do mutations affect the next generation? 3

4. Point mutation leads to Sickle cell anemia
What kind of mutation? 4

5. Sickle cell anemia 5

6. Mutations Frameshift shift in the reading frame insertions deletions
changes everything “downstream”
insertions
adding base(s)
deletions
losing base(s) 6

7. What’s the
value of
mutations? 7

8. Chapter 17.
RNA
Processing 8

9. Transcription -- another look
The process of transcription includes many points of control
when to start reading DNA
where to start reading DNA
where to stop reading DNA
editing the mRNA
protecting mRNA as it travels through cell 9

10. Primary transcript Processing mRNA
protecting RNA from RNase in cytoplasm
add 5’ cap
add polyA tail
remove introns
AUG
UGA 10

11. UTR = Untranslated Region
Protecting RNA
5’ cap added
G trinucleoside (G-P-P-P)
protects mRNA
from RNase (hydrolytic enzymes)
3’ poly-A tail added
A’s
helps export of RNA from nucleus
UTR
UTR
UTR = Untranslated Region 11

12. Dicing & splicing mRNA Pre-mRNA  mRNA edit out introns
intervening sequences
splice together exons
expressed sequences
In higher eukaryotes
90% or more of gene can be intron
no one knows why…yet
there’s a Nobel prize waiting…
“AVERAGE”…
“gene” = 8000b
pre-mRNA = 8000b
mature mRNA = 1200b
protein = 400aa
lotsa “JUNK”!
average size gene (transcription unit) = bases
average size primary transcript = 8000 bases
average size mature RNA = 1200 bases
average size protein = 400 amino acids
lots of “junk DNA”

13. Discovery of Split genes
1977 | 1993
Discovery of Split genes
Richard Roberts
Philip Sharp
adenovirus
NE BioLabs
MIT
common cold 13

14. Splicing enzymes snRNPs Spliceosome RNA as ribozyme
Small Nuclear Ribinucleoproteins
small nuclear RNA + proteins
Spliceosome
several snRNPs
recognize splice site sequence
cut & paste
RNA as ribozyme
some mRNA can splice itself
RNA as enzyme 14

15. Ribozyme 1982 | 1989 RNA as enzyme Sidney Altman Thomas Cech Yale
U of Colorado 15

16. Splicing details No room for mistakes!
editing & splicing must be exactly accurate
a single base added or lost throws off the reading frame
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| 16

17. Alternative splicing Alternative mRNAs produced from same gene
when is an intron not an intron…
different segments treated as exons
Hard
to define a gene! 17

18. Domains Modular architecture of many proteins
separate functional & structural regions
coded by different exons in same “gene” 18

19. The Transcriptional unit (gene?)
enhancer
1000+b
translation
start
translation
stop
exons
20-30b
transcriptional unit
RNA
polymerase 3'
TAC
ACT 5'
TATA
DNA
transcription
start
UTR
introns
transcription
stop
UTR
promoter
DNA
pre-mRNA 5' 3'
mature mRNA 5' 3'
GTP
AAAAAAAA 19

20. Any Questions?? 20

21. The Transcriptional unit
enhancer
1000+b
exons
20-30b
transcriptional unit
RNA
polymerase 3'
TAC
ACT 5'
TATA
DNA
introns 5' 3' 5' 3' 21