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Chapter 17:
From Gene to Protein
• Gene:
A segment of DNA that specifies the
amino acid sequence of a
polypeptide
• DNA does not directly control protein
synthesis, instead its information is
transcribed into RNA
1
Overview: The Flow of Genetic Information
• The information content of DNA
– Is in the form of specific sequences of
nucleotides along the DNA strands
– http://www.dnai.org/text/mediashowcase/index2.html?id=588
2
Genes specify proteins via transcription and translation
• 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
called gene expression
– Includes two stages, called transcription and translation
• The Central Dogma of Molecular Genetics:
– There are 3 major classes of genetic biopolymers: DNA and RNA
(both nucleic acids), and protein.
3
One Gene, One Enzyme hypothesis
• Synthesis of all substances in living things is dictated by enzymes
– Remember that enzymes are proteins whose 1' structure (sequence of
linked amino acids) are coded for by DNA base triplets.
• Beadle and Tatum experiments (1941)
• Purpose: "to determine if and how genes control known biochemical
reaction"
– Work with red bread mold Neurospora crassa to find “nutritional
mutants”
– Used radiation to create “auxotrophs”, organisms, such as a strain
of bacteria, that have lost the ability to synthesize certain
substances required for its growth and metabolism as the result of
mutational changes.
4
Beadle and Tatum’s experiment
X-ray  mutations  loss of enzyme
 lack of an AA (ex. Arg.)  mold
could only grow on argininesupplemented media
Beadle and Tatum proposed that a
single gene (thru a single mutation)
codes for a single specific enzyme =
Nobel Prize (1958)
5
The Products of Gene Expression: A Developing
Story
• Beadle and Tatum developed
the :"One Gene - One
Enzyme" correlation:
– Which states that the
function of a gene is to
dictate the production of a
specific enzyme
– Later found out not
necessarily true: only some
proteins are enzymes. It is
also true of structural
proteins, chains of
polypeptides, or hormones.
6
Ribonucleic Acid
• Why would the cell want to have an intermediate
between DNA and the proteins it encodes?
– The DNA can then stay pristine and protected,
away from the caustic chemistry of the
cytoplasm.
– Gene information can be amplified by having
many copies of an RNA made from one copy of
DNA.
– Regulation of gene expression can be effected
by having specific controls at each element of
the pathway between DNA and proteins.
– The more elements there are in the pathway,
the more opportunities there are to control it in
different circumstances.
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24.2 Gene Expression
• RNA (ribonucleic acid)
8
24.2 Gene Expression
• Three Classes of RNA
– Messenger RNA (mRNA)
• Takes a message from DNA to the ribosomes
• strand
– Ribosomal RNA (rRNA)
• Makes up ribosomes (along with proteins)
• globular
– Transfer RNA (tRNA)
• Transfers amino acids to ribosomes
• Hairpin shape
9
24.2 Gene Expression
• Gene Expression Requires Two Steps:
• Transcription
– Is the synthesis of RNA under the direction of DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which occurs
under the direction of mRNA
– Occurs on ribosomes
http://highered.mcgrawhill.com/sites/dl/free/0072835125
/126997/animation1.html
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24.2 Gene Expression
• Transcription
– During transcription, a segment of the DNA serves as a template
for the production of an RNA molecule
– Messenger RNA (mRNA)
• RNA polymerase (enzyme) binds to a promoter (“start”
sequence)
• DNA helix is opened so complementary base pairing can
occur
• RNA polymerase joins new RNA nucleotides in a sequence
complementary to that on the DNA, in a 5’ to 3’ direction
11
Transcription of DNA to form mRNA
12
Messenger RNA
• mRNA - of the 64 possible 3-base combinations:
– 61 code for the twenty different amino acids
– 3 code for "stop"; i.e. chain termination
• Specific nucleotide sequences call for “start” of
transcription (usually AUG = methionine) = PROMOTOR
sequence
• “stop” of mRNA synthesis = TERMINATION sequence
(UAA, UGA, UAG)
• Finished mRNA strands are ~500-10,000 nucleotides
long
13
Cracking the Code
• A codon in messenger RNA
Figure 17.5
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
lle
Met or
start
GCU
GCC
Val
GCA
GCG
Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
– Is either translated into an amino acid or serves as
a translational stop signal
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• During transcription
– The gene determines the sequence of bases along
the length of an mRNA molecule
The Process of
Transcription
http://highered.mcgrawhill.com/sites/dl/free/007283
5125/126997/animation20.h
tml
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
(template)
A C C A A A C C G A G T
5
TRANSCRIPTION
mRNA
U G G U U U G G C U C A
5
3
Codon
TRANSLATION
Protein
Figure 17.4
Trp
Amino acid
Phe
Gly
Ser
15
transfer RNA
Small, ~80 nucleotides long. tRNA exists as a
single-stranded molecule. However,
regions of double helix can form where
there is some base pair complementation
(U and A , G and C), resulting in hairpin
loops. The RNA molecule with its hairpin
loops is said to have a secondary
structure.
It can bind an amino acid at one end, and
mRNA (anticodon) at the other end. It
acts as an adaptor to carry the amino acid
elements of a protein to the appropriate
place as coded for by the mRNA codon
(complementary).
The "Wobble Phenomenon": There are only
40 different types of t-RNA and 64 codons.
This means that some of the t-RNA can
pair up with several different codons. This
can occur because there is some third
base “flexibility”.
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Transfer RNA: Amino Acid Carrier
17
rRNA
– Ribosomal RNA is the most abundant type of RNA in
cells
– Ribosomes: comprised of subunits 2/3 RNA, 1/3
protein
• Two populations of
ribosomes are evident
in cells, Free and
bound
• Free ribosomes in
the cytosol initiate the
synthesis of all
proteins
18
• The ribosome has three binding sites for
tRNA
– The P site
– The A site
– The E site
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
Figure 17.16b
P
A
Small
subunit
(b) Schematic model showing binding sites. A ribosome has an mRNA
binding site and three tRNA binding sites, known as the A, P, and E sites.
This schematic ribosome will appear in later diagrams.
19
Translation (Building a polypeptide)
requires Three Steps:
– Initiation (requires energy)
– Elongation (requires energy)
– Termination
Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
3
mRNA
5
Animation: How
Translation
Works.
http://highered.mcg
rawhill.com/sites/00725
07470/student_vie
w0/chapter3/animat
ion__how_translati
on_works.html
Codons
(c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its
anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to
the growing polypeptide. The A site holds the tRNA carrying the next amino acid to
be added to the polypeptide chain. Discharged tRNA leaves via the E site.
20
Proteins: A review of structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
H3N+
COO–
a. primary structure
O
C
CH
C N
O
R
CH
H
C N
R
CH
O
H
R
C
O
N
CH
C N
H
R
CH
O
H
N R
C
O
H
CH
C
O
N R
CH
C N
R
CH
C
O C
C
N H
N
O
R C
C
R C
C
C H
H N
N
C
C R
O C
C
N H
N
O C
R C
R
C
C O
H
H N
N
C
(alpha) helix
b. secondary structure
c. tertiary structure
C
C
N
R C H
b (beta) sheet =
C
O
pleated sheet
O
H N
R
C R
H O
C
H O
N H
R C
C C
O
O H
N
• Concept 17.4: Translation is the RNAdirected synthesis of a polypeptide: a closer
look
Narrated animation: Protein Synthesis (with quiz)
http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter3/animation__protein_synth
esis__quiz_3_.html
Interactive practice: Transcribe & Translate a Gene
http://learn.genetics.utah.edu/content/begin/dna/transcribe/
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Polyribosomes
– Several ribosomes may move along the same mRNA
• Multiple copies of a polypeptide may be made
• The entire complex is called a polyribosome
Completed
polypeptide
Growing
polypeptides
Incoming
ribosomal
subunits
Start of
mRNA
(5 end)
End of
mRNA
(3 end)
(a) An mRNA molecule is generally translated simultaneously
by several ribosomes in clusters called polyribosomes.
Ribosomes
mRNA
0.1 µm
Figure 17.20a, b
(b) This micrograph shows a large polyribosome in a prokaryotic
cell (TEM).
23
Overview of Gene Expression
Simple Gene
Expression animation
http://glencoe.mcgrawhill.com/sites/98340923
39/student_view0/chapt
er15/simple_gene_expr
ession.html
Detailed Protein
Synthesis animation
http://highered.mcgrawhill.com/olcweb/cgi/plugi
npop.cgi?it=swf::535::53
5::/sites/dl/free/0072437
316/120077/micro06.sw
f::Protein%20Synthesis
24
Summary of Gene Expression
25
Regulation of gene expression
• Genes are activated in some cells, but not others
• Genes can be active some of the time, but not others
• The mechanics of the “on/off” switch for genes was first identified in
bacteria.
• 1965 Nobel Prize in Medicine François Jacob, Jacques Monod and
André Lwoff.
• This operon enables the metabolism of lactose in Escherichia coli
Animation of the lac operon http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter15/the_lac_operon.html
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Regulation of Gene Expression
in Eukaryotes
Animation: http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120080/bio31.swf::Control%20of%20Gene%2
0Expression%20in%20Eukaryotes
4 levels:
1. Transcriptional control (nucleus):
•
e.g. chromatin density and transcription factors
2. Posttranscriptional control (nucleus)
•
e.g. mRNA processing
3. Translational control (cytoplasm)
•
e.g. differential ability of mRNA to bind ribosomes
4. Posttranslational control (cytoplasm)
•
e.g. changes to the protein to make it functional
Regulation of gene expression
Transcriptional control (nucleus):
–
e.g. chromatin density and transcription factors
–
–
Euchromatin: Loosely packed form of DNA; genes are transcibed
Heterochromatin: tightly packed form of DNA; genes are “silenced”
–
A transcription factor (sometimes called a sequence-specific DNAbinding factor) is a protein that binds to specific DNA sequences,
thereby controlling the flow (or transcription) of genetic information from
DNA to mRNA. Transcription factors perform this function alone or with
other proteins in a complex, by promoting (as an activator), or blocking
(as a repressor) the recruitment of RNA polymerase (the enzyme that
performs the transcription of genetic information from DNA to RNA) to
specific genes.
Transcription factors in Prokaryotes
Induction
Animation: The lac operon induction
http://highered.mcgrawhill.com/sites/0073377988/student_view0/chapter21/the_lac_operon__inducti
on_.html
Repression
Animation: The trp operon
http://highered.mcgrawhill.com/sites/0072995246/student_view0/chapter7/the_trp_operon.html
Transcription factors in Eukaryotes, animation/quiz
http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/chapter15/transcription_factors.htm
l
29
Posttranscriptional control (nucleus)
Processing of mRNA
• After Transcription
• Primary “Pre-”mRNA must be modified into mature
mRNA
– Introns are intragene segments (often, junk)
– Exons are the portion of a gene that is expressed
• Intron sequences are removed, and a poly-A tail is
added
– Ribozyme splices exon segments together
Eukaryotic mRNA modification: RNA splicing animation
http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animation22.html
30
mRNA Processing
pre-RNA must be modified before translation
31
The Functional and Evolutionary Importance of
Introns
• The presence of introns
– Allows for alternative RNA splicing
– Additional animations of RNA processing:
• Processing of Gene Information: Prokaryotes –vs- Eukaryotes:
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/12
0077/bio25.swf::Processing%20of%20Gene%20Information%20%20Prokaryotes%20versus%20Eukaryotes
• How Spliceosomes Process RNA: http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/12
0077/bio30.swf::How%20Spliceosomes%20Process%20RNA
32
Transposons
• “jumping genes”
• Sections of DNA that can move to new locations and
disrupt gene sequences
• Animation http://highered.mcgrawhill.com/sites/0073377988/student_view0/chapter21/transposons__shifting_segments_of
_the_genome.html
See Barbara McClintock
Chromosome 11 flyover
33