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Chapter 12: From DNA to Protein: Genotype to Phenotype
CHAPTER 12
From DNA to Protein:
Genotype to Phenotype
Chapter 12: From DNA to Protein: Genotype to Phenotype
Genotype to Phenotype
Genes are made up of DNA (genotype).
 Genes cannot directly produce a
phenotype.
 Genes must be expressed (phenotype)
as polypeptides.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
DNA, RNA, and the Flow of
Information
RNA differs from DNA in three ways:
 It is single-stranded,
 Its sugar molecule is ribose rather than
deoxyribose, and
 Its fourth base is uracil rather than
thymine.
 Adenine pairs with uracil
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Chapter 12: From DNA to Protein: Genotype to Phenotype
DNA, RNA, and the Flow of
Information
The central dogma of molecular biology
is DNA  RNA  protein.
 Review Figure 12.2
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Chapter 12: From DNA to Protein: Genotype to Phenotype
DNA, RNA, and the Flow of
Information - Summary
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A gene is transcribed to produce messenger
RNA (mRNA).
mRNA is complementary to one of the DNA
strands
Transfer RNA ((tRNA) translates sequence of
bases in mRNA into appropriate sequence of
amino acids.
Amino acids join together (peptide bonds) to
form proteins.
Review Figure 12.3
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.3
Figure 12.3
figure 12-03.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Transcription – synthesis of
RNA from DNA
Transciption requires the enzyme RNA
polymerase, RNA nucleotides, and a
DNA template.
 Transcription occurs in the nucleus.
 The product, a RNA transcript, is sent to
the cytoplasm where translation occurs.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Transcription
Transcription is divided into three
processes:
 Initiation,
 Elongation, and
 termination
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Initiation
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Transcription begins at a promoter – a
special sequence of DNA .
The promoter determines the direction, which
strand to read, and direction to take
RNA polymerase binds to the promoter.
Once the polymerase is attached to the
promoter DNA, the DNA strands unwind and
transcription begins.
Chapter 12: From DNA to Protein: Genotype to Phenotype
Elongation
As RNA polymerase moves along the
DNA, it continues to unwind DNA about
20 bases pairs at a time.
 One side of the unwound DNA acts as
template for RNA synthesis
 RNA transcript is formed by
complementary base pairings.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Termination
Specific DNA base sequences terminate
transcription.
 Pre-mRNA is released.
 Review Figure 12.4
 The resulting RNA transcript may be
mRNA, tRNA, or rRNA.
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Chapter 12: From DNA to Protein: Genotype to Phenotype

Messenger RNA (mRNA) carries a
genetic message from DNA to the
protein synthesizing machinery of the
cell (ribosomes)
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.4
– Part 1
Figure 12.4 – Part 1
figure 12-04a.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.4
– Part 2
Figure 12.4 – Part 2
figure 12-04b.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
The Genetic Code
The genetic code consists of triplets of
nucleotides (codons).
 Since there are four bases, there are 64
possible codons (43)
 There are more codons than different
amino acids.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
The Genetic Code
AUG codes for methionine and is the
start codon.
 UAA, UAG, and UGA are stop codons.
 Stop codons indicate the end of
translation.
 The other 60 codons code only for
particular amino acids.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
The Genetic Code
Since there are only 20 different amino
acids, the genetic code is redundant;
that is, there is more than one codon
for certain amino acids.
 However, a single codon does not
specify more than one amino acid.
 Review Figure 12.5
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Chapter 12: From DNA to Protein: Genotype to Phenotype
The Universal Genetic Code
The genetic code appears to be nearly
universal.
 Provides a common language for
evolution.
 Implications for genetic engineering.
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.5
Figure 12.5
figure 12-05.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Preparation for Translation:
Linking RNA’s, Amino Acids,
and Ribosomes
Translation occurs at the ribosomes.
 Translate the message from sequence
of nucleotides to sequence of amino
acids
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Components of Translation
Ribosomes – small and large subunits
 mRNA (messenger RNA)
 tRNA (transfer RNA)
 tRNA transfer an amino acid.
 tRNA has a sequence of 3 bases
known as the anticodon that is
complementary to mRNA codon
 amino acids are linked in codonspecified order per mRNA.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.7
Figure 12.7
figure 12-07.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Example of Process
The DNA coding region for proline is
GGG which is transcribed to
 The mRNA codon CCC which binds to
 The tRNA with the anticodon GGG
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Activating Enzymes link tRNA
and amino acids
A family of activating enzymes –
aminoacyl-tRNA synthetases_ attach
specific amino acids to their
appropriate tRNA’s to from charged
tRNA
 The amino acid is attached to the 3’
end of tRNA with a high energy bond
 Review Figure 12.8
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Ribosomes
The ribosome consist of a large and a
small subunit.
 When not active in translation, the
ribosomes exist as separate units.
 They can come together and separate
as needed.
 Review Figure 12.9
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.9
Figure 12.9
figure 12-09.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Three Phases of Translation
Initiation
 Elongation
 Termination
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Translation: Initiation
A sequence of mRNA (initiation factors)
binds to the small subunit of a
ribosome.
 Aminoacyl tRNA bearing UAC binds to
the start codon.
 Large subunit of ribosome joins the
complex Review Figure 12.10
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.10
Figure
12.10
figure 12-10.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Elongation – A Four Step
Process
A charged tRN moves into the ribosome
and occupies the A site. Its anticodon
matches the mRNA codon
 The polypeptide chain is transferred.
 Ribosome moves along the mRNA.
Empty tRNA is ejected via the E site.
 tRNA with peptide chains moves to P
site. A is empty. Repeat
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.11
– Part 1
Figure 12.11 – Part 1
figure 12-11a.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.11 –
Part 2
Figure 12.11 – Part 2
figure 12-11b.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Translation: Termination
The presence of a stop codon (UAA,
UAG, or UGA) in the A site of the
ribosome causes translation to
terminate.
 Both tRNA and the polypeptide are
released from the P site.
 The ribosomes separate
 Review Figure 12.12
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.12
Figure 12.12
figure 12-12.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
4 Sites for tRNA Binding
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T (transfer) site is where tRNA + amino acids
first attaches to the ribosome.
The A (amino acid) site is there the tRNA
anticodon binds to mRNA codon
The P (polypetide) site is where the amino
acids are bonded together.
The E (exit) site is where the tRNA will leave
the ribosome to pick up additional amino
acids.
Chapter 12: From DNA to Protein: Genotype to Phenotype
Regulation of Translation
Antibiotics can interfere with translation
 Erythromycin plugs the exit channel so
the polypeptide chain cannot leave the
ribosome.
 Review Table 12.2
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Regulation of Translation
In a polysome, more than one ribosome
moves along the mRNA at one time.
 Multiple copies of the same protein is
made for a single mRNA.
 Review Figure 12.13
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.13
Figure 12.13
figure 12-13.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Posttranslational Events
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The functional protein may vary from the
polypeptide chain that is originally released.
Signals contained in the amino acid
sequences of proteins direct them to cellular
destinations.
And polypeptides may be altered by the
addition of chemical groups that affect
function of the protein.
Review Figure 12.14
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.14
Figure 12.14
figure 12-14.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Posttranslational Events
Protein synthesis begins on free
ribosomes in the cytoplasm.
 Those proteins destined for the nucleus,
mitochondria, and plastids are
completed in the cytoplasm and have
signals that allow them to bind to and
enter destined organelles.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Posttranslational Events
Proteins destined for the ER, Golgi
apparatus, lysosomes, and outside the
cell complete their synthesis on the ER
surface.
 They enter the ER by the interaction of
a hydrophobic signal sequence with a
channel in the membrane.
 Review Figure 12.15
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ure
12.
15
–
Part
1
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.15 – Part 1
figure 12-15a.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.15
– Part 2
Figure 12.15 – Part 2
figure 12-15b.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Posttranslational Events
Covalent modifications of proteins after
translation include:
 proteolysis – polypeptide chain is cut
 Glycosylation – additions of sugars to
proteins
 Phosphorylation – add phosphate
groups to protiens.
 Review Figure 12.16
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.16
Figure 12.16
figure 12-16.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Mutations: Heritable Changes
in Genes
Mutations in DNA are often expressed
as abnormal proteins.
 However, the result may not be easily
observable phenotypic changes.
 Some mutations appear only under
certain conditions, such as exposure to
a certain environmental agent or
condition.
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Mutations: Heritable Changes
in Genes
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Point mutations (silent, missense,
nonsense, or frame-shift) result from
alterations in single base pairs of DNA.
Chapter 12: From DNA to Protein: Genotype to Phenotype
Mutations: Heritable Changes
in Genes
Chromosomal mutations (deletions,
duplications, inversions, or
translocations) involve large regions of
a chromosome.
 Review Figure 12.18
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure
12.18
Figure 12.18
figure 12-18.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Mutations: Heritable Changes
in Genes
Mutations can be spontaneous or
induced.
 Spontaneous mutations occur because
of instabilities in DNA or chromosomes.
 Induced mutations occur when an
outside agent damages DNA.
 Review Figure 12.19
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Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.19
– Part 1
Figure 12.19 – Part 1
figure 12-19a.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
Figure 12.19
– Part 2
Figure 12.19 – Part 2
figure 12-19b.jpg
Chapter 12: From DNA to Protein: Genotype to Phenotype
One Gene, One Polypeptide
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Certain hereditary diseases in humans have
been found to be caused by the absence of
certain enzymes.
Table 3-4
Phenylketonuria (PKU) is a recessive disease
caused by a defective allele for phenylalanine
hydroxylase.
In the absence of the enzyme, phenylalanine
in food is not broken down and accumulates.
Chapter 12: From DNA to Protein: Genotype to Phenotype
PKU
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At high concentrations phenylalanine is
converted to phenylpyruvic acid.
Phenylpyyruvic acid interferes with the
development of the nervous system.
Solution: An infant is put on a low
phenylalanine diet so phenylalanine does not
accumulate, no phenylpyruvic acid is made
and the child develops normally.
Figure 3-28
These observations supported the one-gene,
one-polypeptide hypothesis.