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From DNA to Protein:
Genotype to Phenotype
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One Gene, One Polypeptide
• A gene is defined as a DNA sequence that encodes
information.
• In the 1940s, Beadle and Tatum showed that when
an altered gene resulted in an altered phenotype,
that altered phenotype always showed up as an
altered enzyme.
• Their results suggested that mutations cause a
defect in only one enzyme in a metabolic pathway.
• This lead to the one-gene, one-enzyme hypothesis.
Figure 12.1 One Gene, One Enzyme (Part 2)
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One Gene, One Polypeptide
• The gene–enzyme connection has undergone
several modifications. Some enzymes are
composed of different subunits coded for by
separate genes.
• This suggests, instead of the one-gene, one
enzyme hypothesis, a one-gene, onepolypeptide relationship.
 Today, we know some genes encode
functional RNA molecules, such as ribozymes.
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DNA, RNA, and the Flow of Information
• The expression of a gene takes place in two
steps:
 Transcription makes a single-stranded RNA
copy of a segment of the DNA.

For functional RNAs, this is the final step.
 Translation uses information encoded in the
RNA to make a polypeptide.
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DNA, RNA, and the Flow of Information
• RNA (ribonucleic acid) differs from DNA in three
ways:
 Single stranded.
 The sugar in RNA is ribose, not deoxyribose.
 RNA has uracil instead of thymine.
• RNA can base-pair with single-stranded DNA
(adenine pairs with uracil instead of thymine) and
also can fold over and base-pair with itself.
Figure 12.2 The Central Dogma
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DNA, RNA, and the Flow of Information
• Messenger RNA, or mRNA moves from the
nucleus of eukaryotic cells into the cytoplasm,
where it serves as a template for protein synthesis.
• Transfer RNA, or tRNA, is the link between the
code of the mRNA and the amino acids of the
polypeptide, specifying the correct amino acid
sequence in a protein.
Figure 12.3 From Gene to Protein
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Transcription: DNA-Directed RNA Synthesis
• In normal prokaryotic and eukaryotic cells,
transcription requires the following:
 A DNA template for complementary base pairing
 The appropriate ribonucleoside triphosphates
(rATP, rGTP, rCTP, and rUTP) to act as substrates
 The enzyme RNA polymerase
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Transcription: DNA-Directed RNA Synthesis
• The first step of transcription, initiation, begins at
a promoter, a special sequence of DNA.
• There is at least one promoter for each gene to be
transcribed.
• The RNA polymerase (synthesizes RNA during
transcription) binds to the promoter region when
that protein is needed by the cell.
Figure 12.4 (Part 1) DNA is Transcribed in RNA
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Transcription: DNA-Directed RNA Synthesis
• After binding, RNA polymerase unwinds the DNA
and reads the template in the 3-to-5 direction
(elongation).
• The new RNA elongates from its 5 end to its 3
end; thus the RNA transcript is antiparallel to the
DNA template strand.
• RNA polymerization is always 5’ to 3’ (needs free
3’ –OH to add nucleotide).
Figure 12.4 (Part 2) DNA is Transcribed in RNA
Figure 12.4 (Part 3) DNA is Transcribed in RNA
Particular base sequences in the DNA specify termination
– the signal that the end of the gene has been reached and
transcription can terminate.
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The Genetic Code
• A genetic code relates genes (DNA) to mRNA and mRNA
to the amino acids of proteins.
• mRNA is read in three-base segments called codons.
• The 64 possible codons code for only 20 amino acids and
the start and stop signals.
• Each codon is assigned only one amino acid.
Figure 12.5 The Universal Genetic Code
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• The codon in mRNA and the amino acid in a
protein are related by way of an adapter—a
specific tRNA molecule.
• tRNA has three functions:
 It carries an amino acid.
 It associates with mRNA molecules.
 It interacts with ribosomes.
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• A tRNA molecule has 75 to 80 nucleotides and a
three-dimensional shape.
• The shape is maintained by complementary base
pairing and hydrogen bonding.
• The three-dimensional shape of the tRNAs allows
them to combine with the binding sites of the
ribosome.
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• At the 3 end of every tRNA molecule is a site to which its
specific amino acid binds covalently.
• Midpoint in the sequence are three bases called the
anticodon.
• The anticodon is the contact point between the tRNA and
the mRNA.
• The anticodon is complementary (and antiparallel) to the
mRNA codon.
• The codon and anticodon unite by complementary base
pairing.
Figure 12.7 Transfer RNA
Anticodon
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• The ribosome is a complex protein assembly where protein
synthesis takes place.
• The ribosome binds to the mRNA, and then the correct
transfer RNA comes in and binds to bring in the correct
amino acid – thus building the protein chain.
• Each ribosome has two subunits: a large and a small one.
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Preparation for Translation:
Linking RNAs, Amino Acids, and Ribosomes
• The ribosome validates the three-base-pair match
between the mRNA and the tRNA.
• If hydrogen bonds have not formed between all
three base pairs, the tRNA is ejected from the
ribosome.
Figure 12.10 The Initiation of Translation
The ribosome
attaches to the
mRNA at a
special
ribosome
recognition
sequence
upstream of the
start sequence
AUG.
The start codon
(AUG)
designates the
first amino acid
in all proteins =
methionine.
Figure 12.11 Translation: The Elongation Stage
The ribosome helps form a peptide bond between
the last amino acid of the growing protein and the
amino acid attached to the incoming tRNA.
Figure 12.12 The Termination of Translation
When a stop codon—UAA, UAG, or
UGA—enters the ribosome, it
signals the ribosome to release the
formed protein.
Figure 12.13 A Polysome (Part 1)
Figure 12.13 A Polysome (Part 2)
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Posttranslational Events
• Two posttranslational events can occur after the
polypeptide has been synthesized:
 The polypeptide may be moved to another
location in the cell, or secreted.
 The polypeptide may be modified by the
addition of chemical groups, folding, or
trimming.
Figure 12.14 Destinations for Newly Translated Polypeptides in a Eukaryotic Cell
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Posttranslational Events
• As the polypeptide chain forms, it folds into its 3-D
shape.
• The amino acid sequence also contains an
“address label” indicating where in the cell the
polypeptide belongs.
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Posttranslational Events
• Most proteins are modified after translation.
• These modifications are often essential to the
functioning of the protein.
• Three types of modifications:
 Proteolysis (cleaving)
 Glycosylation (adding sugars)
 Phosphorylation (adding phosphate groups)
Figure 12.16 Posttranslational Modifications to Proteins
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Mutations: Heritable Changes in Genes
• Mutations are heritable changes in DNA—
changes that are passed on to daughter cells.
• Multicellular organisms have two types of
mutations:
 Somatic mutations are passed on during
mitosis, but not to subsequent generations.
 Germ-line mutations are mutations that occur
in cells that give rise to gametes.
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Mutations: Heritable Changes in Genes
• Point mutations result from the addition or
subtraction of a base or the substitution of one base
for another.
• Point mutations can occur as a result of mistakes
during DNA replication or can be caused by
environmental mutagens.
• Because of degeneracy (redundancy) in the
genetic code, some point mutations result in no
change in the amino acids in the protein.
Silent Mutation
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Mutations: Heritable Changes in Genes
• Some mutations cause an amino acid
substitution.
• An example in humans is sickle-cell
anemia, a defect in the b-globin
subunits of hemoglobin.
• The b-globin in sickle-cell differs from
the normal by only one amino acid.
• These mutations may reduce the
functioning of a protein or disable it
completely.
Missense mutation
Figure 12.17 Sickled and Normal Red Blood Cells
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Mutations: Heritable Changes in Genes
• Some mutations are base substitutions that
substitute a stop codon.
• The shortened proteins are usually not functional.
Nonsense mutation
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Mutations: Heritable Changes in Genes
• A frame-shift mutation consists of the insertion or
deletion of a single base in a gene.
• This type of mutation shifts the code, changing
many of the codons to different codons.
• These shifts almost always lead to the
production of nonfunctional proteins.
Frame-shift mutation
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• Spontaneous
mutations are
permanent
changes, caused
by any of several
mechanisms.
• Induced
mutations are
changes caused
by some outside
agent (mutagen).
Mutations: Heritable Changes in Genes