Download Chapter 17 A - HCC Learning Web

Chapter 17
From Gene to Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The information content of DNA
– Is in the form of specific sequences of
nucleotides
– located along the DNA strands
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The DNA inherited by an organism
– Leads to specific traits by:
• Dictating the synthesis of proteins
• The process by which DNA directs protein
synthesis is also called:
– Gene expression
• Gene expression includes two stages, called:
– Transcription
– Translation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The ribosome
– Is part of the cellular machinery for:
• Translation (polypeptide synthesis)
Figure 17.1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evidence from the Study of Metabolic Defects
• In 1909, British physician Archibald Garrod
– Was the first to suggest that:
• Genes dictate phenotypes through:
– Enzymes
– These enzymes catalyze specific chemical
reactions in the cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The “one gene–one enzyme hypothesis” states
that:
• The function of a gene is to:
– Dictate the production of a specific enzyme
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Products of Gene Expression: A Developing Story
• As researchers learned more about proteins
– They made minor revision to the one gene–
one enzyme hypothesis
• Genes code for:
– Polypeptide chains,
or
– RNA molecules
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Basic Principles of Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction of
DNA
– Produces messenger RNA (mRNA)
– Occurs in nucleus
• Translation
– Is the actual synthesis of a polypeptide
– Occurs under the direction of mRNA
– Occurs on ribosomes (cytoplasm)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In prokaryotes
– Transcription and translation occur together
–
mRNA is produced by transcription
– mRNA is immediately translated without
additional processing.
Figure 17.3a
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
(a)
Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
Figure 17.3a
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In eukaryotes
– RNA transcripts are modified before becoming
true mRNA
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Figure 17.3b
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(b) Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
• Cells are governed by a cellular chain of
command
– DNA RNA protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Genetic Code
• How many bases correspond to an amino
acid?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Codons: Triplets of Bases
• Genetic information
– Is encoded as a sequence of:
• Nonoverlapping base triplets
– These base triplets are called codons
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Transcription is:
– The DNA-directed synthesis of RNA
• During transcription
– The gene sequence determines:
• The sequence of bases along the length of an
mRNA molecule
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand
(template)
5
3
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
5
3
Codon
TRANSLATION
Protein
Trp
Amino acid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Phe
Gly
Ser
Cracking the Code
UUU
U
UUC
UUA
First mRNA base (5 end)
UUG
C
CUU
CUC
CUA
CUG
Phe
Leu
Leu
AUU
A
G
AUC
AUA
AUG
GUU
GUC
GUA
GUG
lle
Met or
start
Val
CCU
CCC
CCA
CCG
Pro
CAU
CAC
CAA
CAG
ACU
AAU
ACC
ACA
ACG
AAC
AAA
AAG
GCU
GCC
GCA
GCG
Thr
Ala
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
GAU
GAC
GAA
GAG
His
Gln
Asn
Lys
Asp
Glu
G
UGU
Cys
U
UGC
UGA
Stop
C
A
UGG
Trp
G
CGU
CGC
CGA
CGG
Arg
AGU
AGC
AGA
AGG
GGU
GGC
GGA
GGG
Ser
Arg
Gly
U
C
A
G
U
C
A
G
U
C
A
G
Third mRNA base (3 end)
U
Second mRNA base
C
A
UAU
UCU
Tyr
UAC
UCC
Ser
UCA
UAA Stop
UAG Stop
UCG
Cracking the Code
• A codon in messenger RNA:
– Is either:
• translated into an amino acid
– or
• Serves as a translational stop signal
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• For the specified polypeptide to be produced:
– Codons must be read
– Reading must be in the correct reading frame
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evolution of the Genetic Code
• The genetic code is nearly universal:
– Shared by organisms
– From the simplest bacteria to the most
complex animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In laboratory experiments:
– Genes can be:
• Transplanted from one species to another
• Transcribed into respective mRNAs
• Translated into respective proteins
Figure 17.6
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A tobacco plant expressing a firefly gene
Figure 17.6
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A jellyfish gene for fluorescent protein expressed in a pig
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Molecular Components of Transcription
• RNA synthesis
– Is catalyzed by RNA polymerase, which:
• Separates the DNA strands apart, and
• Hooks together the RNA nucleotides
– synthesis follows the same base-pairing rules
as DNA,except, in RNA:
• Uracil substitutes for thymine, and
• Ribose (not deoxyribose) is the sugar
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Synthesis of an RNA (stages of transcription)
• Initiation:
– RNA polymerase binds to the promoter
– DNA strands unwind, and
– Polymerase initiates RNA synthesis at the start point.
• Elongation:
–
–
–
The polymerase moves downstream unwinding the DNA &
Elongating RNA transcript in the 5 to 3 direction
The DNA strands re-form (rewound) a double helix
.
• Termination:
–
–
Eventually, the RNA transcript is released
The polymerase detaches from the DNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Synthesis of an RNA Transcript
Promoter
Transcription unit
5
3
3
5
Start point
RNA polymerase
DNA
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
1
5
3
Unwound
DNA
3
5
Template strand of
DNA
transcript
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
Rewound
RNA
RNA
5
3
3
5
3
5
RNA
transcript
3 Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Figure 17.7
Completed RNA
transcript
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
A
T
C
C
A
A
3
3 end
U
5
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
5
Newly made
RNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Template
strand of DNA
RNA Polymerase Binding and Initiation of Transcription
• Promoters signal the initiation of RNA synthesis
• Transcription factors
– Help eukaryotic RNA polymerase recognize
promoter sequences
1 Eukaryotic promoters
TRANSCRIPTION
DNA
RNA PROCESSING
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
Promoter
5
3
3
5
T A T A A AA
AT A T T T T
TATA box
Start point
Template
DNA strand
Several transcription
factors
2
Transcription
factors
5
3
3
5
3 Additional transcription
factors
RNA polymerase II
5
3
Transcription factors
3
5
5
RNA transcript
Figure 17.8
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transcription initiation complex
Elongation of the RNA Strand
• As RNA polymerase moves along the DNA, it:
– Continues to untwist the double helix
– Exposes about 10 to 20 DNA bases at a time
– Pairs these nucleotides with RNA nucleotides
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Following transcription, eukaryotic cells:
– Modify pre-mRNA
– Modification takes place in the nucleus
– Nuclear enzymes are used for modification.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Alteration of mRNA Ends
•
Each end of a pre-mRNA molecule is
modified in a particular way (3 modifications):
1. The 5 end receives a modified guanine
nucleotide cap
2. The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
RNA PROCESSING
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
5
mRNA
Protein-coding segment
Polyadenylation signal
3
G P P P
AAUAAA
AAA…AAA
Ribosome
TRANSLATION
5 Cap
5 UTR
Start codon Stop codon
Polypeptide
Figure 17.9
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3 UTR
Poly-A tail
Split Genes and RNA Splicing
3. RNA splicing
• Removes introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
5 Exon Intron
Pre-mRNA 5 Cap
30
31
1
Coding
segment
mRNA
Ribosome
Intron
Exon
Exon
3
Poly-A tail
104
105
146
Introns cut out and
exons spliced together
TRANSLATION
Polypeptide
mRNA 5 Cap
1
3 UTR
Figure 17.10
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Poly-A tail
146
3 UTR
• Proteins often have a modular architecture
– Consisting of discrete structural and functional
regions called domains
• In many cases
– Different exons code for the different domains
in a protein
Gene
DNA
Exon 1 Intron Exon 2
Transcription
RNA processing
Intron Exon 3
Translation
Domain 3
Domain 2
Domain 1
Figure 17.12
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Polypeptide