Download Transcription to RNA Figure 9.13 Telomeres and Telomerase Figure

9/20/16
Replication fork
Transcription to RNA
–  Telomeres
–  Mutation
•  The central dogma:
Leading strand
Lagging strand
Overall directions
of replication
Single-strand
binding protein
–  DNA->RNA->protein
–  One Gene, One Enzyme
Helicase
•  Transcription
Leading strand
5ʹ
DNA polymerase III
3ʹ
3ʹ
–  Initiation
–  Elongation
–  Termination
Parental DNA
Primer
5ʹ
Primase
3ʹ
5ʹ
DNA pol III
4
•  mRNA processing
–  Introns and exons
•  Other types of RNA
Overview
Origin of replication
Lagging strand
Leading strand
The leading strand
is synthesized
continuously
•  Finish DNA replication
Lagging strand
DNA pol I
3
DNA ligase
2
1
But the lagging strand is
synthesized in short
fragments. Why?
09/21/2016
In most cells, Chromosomes get shorter
with each
round of
replication
Figure
9.13
Telomeres
3ʹ 5ʹ
In most cells, Chromosomes get shorter
with each
round of
replication
Figure
9.13
Telomeres
and
Telomerase
3ʹ
5ʹ
and
Telomerase
telomerase
In germ line cells, telomerase
prevents chromosome
shortening. Why?
Chemical mutagens
Mutations have many causes:
• 
• 
• 
• 
Spontaneous errors
Chemical mutagens
Ionizing radiation
Slippage during DNA replication
After replication, new
strand will have an A
Deamination
changes C to U
--C----G--5
--U----G---
--U----A---
--T----A---
--G----C---
--G----C---
6
1
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DNA Repair
UV damage (e.g. thymine dimers)
DNA Proofreading
Mismatch repair
UV radiation can
cause thymine
dimers
Do Not
Memorize
7
MutS/L/H
Mismatch repair
Certain enzymes detect the
deformed helix that results from the
incorrect pairing
G
T
Here is a mis-paired base that must be
repaired:
The old (template) DNA
has methyl groups in
certain places
MutS/L/H
G
T
G
T
How is the mistake recognized?
Do Not
Memorize
How does the mismatch repair system
know which is correct and which
should be replaced?
CH3
GATC
CTAG
Cut the newly
synthesized strand
here
Do Not
Memorize
9
G
DNA pol I/III
DNA Ligase
Certain bacterial mutations cause
increased mutation rates
CH3
GATC
G
Re-synthesize DNA from the
template using the normal DNA
polymerases
CH3
Do Not
Memorize
G
C
10
GATC
CTAG
Defect in:
Rifr mutants per 108 cells
Wild-type (mut+ )
5-10
Pol III proofreading
(mutD)
4000-5000
Mis-match repair
(mutS)
760
Base excision repair
(mutY mutM)
8200
Corrected base pair
11
12
2
9/20/16
The consequences of a mutation depend on how it affects the protein
Figure 9.16 Mutation and
Phenotype
From Gene to Phenotype
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA strand
(template)
3ʹ
5ʹ
A
C
C
A
A
A
C
C
G
A
G
T
U G
G
U
U
U
G G
C
U
C
A
TRANSCRIPTION
mRNA
5ʹ
3ʹ
Codon
TRANSLATION
Protein
Trp
Phe
Gly
Ser
Amino acid
Chromosomes are one
long DNA double helix,
wound around proteins
DNA à RNA à Protein
One Gene à One Enzyme
Nucleosome
DNA
Histone
In Prokaryotes transcription and translation
occur simultaneously
TRANSCRIPTION
DNA
In Eukaryotes,
Transcription and
Translation occur
in separate
compartments
mRNA
Ribosome
TRANSLATION
Polypeptide
3
9/20/16
RNA Polymerase
Transcription
Non-template
strand of DNA
A
3ʹ
T
C
RNA nucleotides
C
C
A
A
T
U
G
A
A
T
U
A
C
G
C
A
G
T
G
U
C
5ʹ
T
3ʹ end
T
A
A
T
G
C
A
Template
strand of DNA
5ʹ
New RNA
DNA is copied to make messenger RNA
Synthesis of an RNA Transcript
Promoter
Transcription unit
5ʹ
3ʹ
–  Initiation
5ʹ
3ʹ
–  Elongation
3ʹ
5ʹ
Template strand of
Unwound RNA DNA
DNA
transcript
Rewound
5ʹ
3ʹ
RNA polymerase
binds to a promoter
sequence
3ʹ
5ʹ
Start point DNA
RNA polymerase
RNA
3ʹ
5ʹ
3ʹ
mRNA copy of gene is
synthesized
Nucleotides are added
at the 3’ end
5ʹ
RNA
–  Termination
transcript
5ʹ
3ʹ
3ʹ
5ʹ
5ʹ
Figure 17.7
Completed RNA
transcript
This is the non-template
strand. Note that it reads
almost the same as the
mRNA
5
3
5
This is the template
strand that is actually
copied
...ATGAATGTCAATGCA... 3
...TACTTACAGTTACGT... 5
...augaaugucà 3
Termination sequence
causes transcription to
stop
DNA template
RNA copy
Like DNA, RNA elongates
only at the 3 end
3ʹ
Both strands of DNA can serve as the template
Initiation in eukaryotes
Some genes are on one strand, other genes are on the other
1 Eukaryotic promoters
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
TATA box
TRANSLATION
Ribosome
Polypeptide
Promoter
5ʹ
3ʹ
3ʹ
5ʹ
T A T A A AA
ATAT T T T
TATA box
Start point
Template
DNA strand
2 Several transcription
factors
Several transcription
factors must bind to
promoter sequences
upstream of the gene
Transcription
factors
5ʹ
3ʹ
3ʹ
5ʹ
3 Additional transcription
factors
RNA polymerase II
Small segment of human genome
Then RNA
polymerase can bind
5ʹ
3ʹ
Transcription factors
3ʹ
5ʹ
5ʹ
RNA transcript
Transcription initiation complex
4
9/20/16
RNA processing in eukaryotes
2. Add poly A
tail to 3 end
1. Add 5 cap
A modified guanine nucleotide
added to the 5ʹ end
TRANSCRIPTION
50 to 250 adenine nucleotides
added to the 3ʹ end
DNA
mRNA
3. Splice out introns
TRANSCRIPTION
Protein-coding segment
5ʹ
Polyadenylation signal
G P P P
AAUAAA
Ribosome
TRANSLATION
5ʹ Cap
Polypeptide
5ʹ UTR
Start codon Stop codon
3ʹ UTR
A modified
GTP is added,
backwards, on
the 5 end
5ʹ
1
3ʹ
AAA…AAA
Intron
Exon
105
3ʹ
146
Introns cut out and
exons spliced together
Coding
segment
Ribosome
Exon
Poly-A tail
104
TRANSLATION
Polypeptide
Poly-A tail
mRNA
5ʹ Cap
Poly-A tail
1
5ʹ UTR
146
3ʹ UTR
RNA has many functions
RNA transcript (pre-mRNA)
Intron
Exon 1
•  4 main types:
Exon 2
Protein
Other proteins
snRNA
Special small
nuclear RNA
molecules do
the splicing
Spliceosome
5ʹ
Spliceosome
components
3
5ʹ Exon Intron
Pre-mRNA 5ʹ Cap
30
31
1
About 200 A s
added at 3
end
snRNPs
2
Pre-mRNA
mRNA
Spliceosomes
Pre- mRNA
Mature mRNA
DNA
RNA PROCESSING
Pre-mRNA
RNA PROCESSING
RNA processing in eukaryotes
5ʹ
mRNA
Exon 1
Cut-out
intron
–  Messenger RNA
–  Ribosomal RNA
–  Transfer RNA
–  Small nuclear RNA
•  Folds to many possible
shapes
–  Can act as an enzyme
( ribozyme )
SS RNA can base-pair
with itself to form loops.
Exon 2
DNA and RNA Compared
Four types of RNA
•  mRNA
–  Messenger RNA, encodes the amino acid sequence of a
polypeptide
•  rRNA
–  Ribosomal RNA, forms complexes with protein called ribosomes,
which translate mRNA to protein
•  tRNA
–  Transfer RNA, transports amino acids to ribosomes during protein
synthesis
1. 
2. 
3. 
4. 
RNA is single stranded.
Sugar is different.
RNA uses U instead of T
RNA leaves nucleus,
DNA doesn t.
•  snRNA
–  Small nuclear RNA, forms complexes with proteins used in
eukaryotic RNA processing
5