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How an Organism’s Genotype Determines Its
Phenotype
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How an Organism’s Genotype Determines Its
Phenotype
• DNA specifies the synthesis of proteins in two
stages:
• An organism’s genotype is its genetic makeup, the
sequence of nucleotide bases in DNA.
1. transcription, the transfer of genetic information
from DNA into an RNA molecule and
• The phenotype is the organism’s physical traits,
which arise from the actions of a wide variety of
proteins.
2. translation, the transfer of information from RNA
into a protein.
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Figure 10.8-1
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DNA
Nucleus
Cytoplasm
Bioflix Animation: Protein Synthesis
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Figure 10.8-2
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Figure 10.8-3
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DNA
DNA
TRANSCRIPTION
TRANSCRIPTION
Nucleus
Nucleus
RNA
RNA
Cytoplasm
Cytoplasm
TRANSLATION
Protein
How an Organism’s Genotype Determines Its
Phenotype
• The major breakthrough in demonstrating the
relationship between genes and enzymes came in
the 1940s from the work of American geneticists
George Beadle and Edward Tatum with the bread
mold Neurospora crassa.
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How an Organism’s Genotype Determines Its
Phenotype
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• Beadle and Tatum
– studied strains of mold that were unable to grow on
the usual growth medium,
– determined that these strains lacked an enzyme in
a metabolic pathway that synthesized arginine,
– showed that each mutant was defective in a single
gene, and
– hypothesized that the function of an individual gene
is to dictate the production of a specific enzyme.
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© 2013 Pearson Education, Inc.
Figure 10.9
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How an Organism’s Genotype Determines Its
Phenotype
38
• The one gene–one enzyme hypothesis has since
been modified.
• The function of a gene is to dictate the production
of a polypeptide.
• A protein may consist of two or more different
polypeptides.
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From Nucleotides to Amino Acids: An Overview
• Genetic information in DNA is
– transcribed into RNA, then
– translated into polypeptides,
– which then fold into proteins.
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From Nucleotides to Amino Acids: An Overview
• What is the language of nucleic acids?
– In DNA, it is the linear sequence of nucleotide
bases.
– A typical gene consists of thousands of nucleotides
in a specific sequence.
• When a segment of DNA is transcribed, the result
is an RNA molecule.
• RNA is then translated into a sequence of amino
acids in a polypeptide.
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© 2013 Pearson Education, Inc.
Figure 10.10
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Gene 1
Figure 10.10a
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DNA strand
Gene 2
TRANSCRIPTION
DNA molecule
Gene 3
RNA
DNA strand
TRANSLATION
Codon
TRANSCRIPTION
Polypeptide
RNA
TRANSLATION
Amino acid
Codon
Polypeptide
Amino acid
From Nucleotides to Amino Acids: An Overview
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The Genetic Code
• Experiments have verified that the flow of
information from gene to protein is based on a
triplet code.
• The genetic code is the set of rules that convert a
nucleotide sequence in RNA to an amino acid
sequence.
• A codon is a triplet of bases, which codes for one
amino acid.
• Of the 64 triplets,
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– 61 code for amino acids and
– 3 are stop codons, instructing the ribosomes to end
the polypeptide.
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© 2013 Pearson Education, Inc.
Figure 10.11
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Second base of RNA codon
U
UUC
Phenylalanine
UCC
(Phe)
UUA
UCA
First base of RNA codon
Tyrosine
(Tyr)
UGC
Cysteine
(Cys)
U
C
A
Stop
UCG
UAG
Stop
UGG Tryptophan (Trp) G
CUU
CCU
CAU
CUC
CCC
CAC
Leucine
(Leu)
Leucine
(Leu)
CUG
AUU
CCA
Proline
(Pro)
CAA
CCG
CAG
ACU
AAU
Isoleucine
(Ile)
ACC
AAC
ACA
Threonine
(Thr)
AAA
Met or start
ACG
AAG
GUU
GCU
GAU
GUC
GCC
GAC
AUC
AUA
AUG
G
Serine
(Ser)
UAC
GUA
GUG
Valine
(Val)
GCA
GCG
Alanine
(Ala)
GAA
GAG
Histidine
(His)
Glutamine
(Gln)
Aspartic
acid (Asp)
Glutamic
acid (Glu)
U
CGU
CGC
CGA
Arginine
(Arg)
AGA
AGG
Serine
(Ser)
Arginine
(Arg)
GGA
GGG
A
U
C
A
G
• Because diverse organisms share a common
genetic code, it is possible to program one species
to produce a protein from another species by
transplanting DNA.
U
GGU
GGC
C
G
CGG
AGU
Asparagine
AGC
(Asn)
Lysine
(Lys)
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G
UGU
UGA Stop
CUA
A
A
UAU
UAA
UUG
C
C
UCU
Third base of RNA codon
U
UUU
The Genetic Code
Glycine
(Gly)
C
A
G
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Figure 10.12
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Transcription: From DNA to RNA
• Transcription
– makes RNA from a DNA template,
– uses a process that resembles the synthesis of a
DNA strand during DNA replication, and
– substitutes uracil (U) for thymine (T).
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Transcription: From DNA to RNA
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• RNA nucleotides are linked by the transcription
enzyme RNA polymerase.
Animation: Transcription
Right click slide / select “Play”
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Figure 10.13
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RNA polymerase
DNA of gene
Promoter
DNA
1 Initiation
RNA
Terminator
DNA
2 Elongation
RNA nucleotides
RNA polymerase
3 Termination
Growing RNA
Newly
made
RNA
Blast Animation: Transcription
Select “Play”
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Direction of
transcription
Completed RNA
Template
strand of DNA
(a) A close-up view of transcription
RNA
polymerase
(b) Transcription of a gene
Initiation of Transcription
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• The “start transcribing” signal is a nucleotide
sequence called a promoter, which is
RNA Elongation
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• During the second phase of transcription, called
elongation,
– located in the DNA at the beginning of the gene
and
– the RNA grows longer and
– the RNA strand peels away from its DNA template.
– a specific place where RNA polymerase attaches.
• The first phase of transcription is initiation, in which
– RNA polymerase attaches to the promoter and
– RNA synthesis begins.
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Termination of Transcription
• During the third phase of transcription, called
termination,
– RNA polymerase reaches a special sequence of
bases in the DNA template called a terminator,
signaling the end of the gene,
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The Processing of Eukaryotic RNA
• In the cells of prokaryotes, RNA transcribed from a
gene immediately functions as messenger RNA
(mRNA), the molecule that is translated into
protein.
• The eukaryotic cell
– polymerase detaches from the RNA and the gene,
and
– localizes transcription in the nucleus and
– the DNA strands rejoin.
– modifies, or processes, the RNA transcripts in the
nucleus before they move to the cytoplasm for
translation by ribosomes.
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The Processing of Eukaryotic RNA
• RNA processing includes
The Processing of Eukaryotic RNA
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• RNA splicing is believed to play a significant role in
humans
– adding a cap and tail consisting of extra
nucleotides at the ends of the RNA transcript,
– in allowing our approximately 21,000 genes to
produce many thousands more polypeptides and
– removing introns (noncoding regions of the RNA),
and
– by varying the exons that are included in the final
mRNA.
– RNA splicing, joining exons (the parts of the gene
that are expressed) together to form messenger
RNA (mRNA).
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© 2013 Pearson Education, Inc.
Figure 10.14
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Translation: The Players
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
• Translation is the conversion from the nucleic acid
language to the protein language.
Introns removed Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
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Replication
Transcription
Translation
Template
DNA
DNA
RNA
Polymer
synthesized
DNA
RNA
Polypeptide
Monomer
nucleotide
(deoxyribose)
nucleotide
(ribose)
Amino acid
Polymerizing
enzyme
DNA polymerase
RNA polymerase
ribosome
initiation site
origin of replication
promoter
start site
termination site
none
terminator
1 of 3 stop
codons
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Messenger RNA (mRNA)
• Translation requires
– mRNA,
– ATP,
– enzymes,
– ribosomes, and
– transfer RNA (tRNA).
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Transfer RNA (tRNA)
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Figure 10.15
Amino acid attachment site
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• Transfer RNA (tRNA)
– acts as a molecular interpreter,
Hydrogen bond
– carries amino acids, and
– matches amino acids with codons in mRNA using
anticodons, a special triplet of bases that is
complementary to a codon triplet on mRNA.
RNA polynucleotide chain
Anticodon
tRNA polynucleotide
(ribbon model)
tRNA
(simplified
representation)
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Ribosomes
• Ribosomes are organelles that
– coordinate the functions of mRNA and tRNA and
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Ribosomes
• A fully assembled ribosome holds tRNA and
mRNA for use in translation.
– are made of two subunits.
• Each subunit is made up of
– proteins and
– a considerable amount of another kind of RNA,
ribosomal RNA (rRNA).
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Figure 10.16a
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Figure 10.16b
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Next amino acid
to be added to
polypeptide
tRNA binding sites
P site
Growing
polypeptide
A site
Large
Ribosome
subunit
mRNA
binding
site
tRNA
mRNA
Small
subunit
(a) A simplified diagram of a ribosome
Codons
(b) The “players” of translation
Figure 10.16
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Next amino acid
to be added to
polypeptide
tRNA
binding sites
P site
mRNA
binding
site
(a) A simplified diagram
of a ribosome
• Translation is divided into three phases:
1. initiation,
Growing
polypeptide
A site
Translation: The Process
2. elongation, and
tRNA
Large
Ribosome
subunit
mRNA
3. termination.
Small
subunit
Codons
(b) The “players” of translation
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Initiation
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Figure 10.17
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Cap
Start of genetic
message
• Initiation brings together
– mRNA,
– the first amino acid with its attached tRNA, and
– two subunits of the ribosome.
• The mRNA molecule has a cap and tail that help
the mRNA bind to the ribosome.
End
Tail
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Initiation
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• Initiation occurs in two steps.
1. An mRNA molecule binds to a small ribosomal
subunit, then a special initiator tRNA binds to the
start codon, where translation is to begin on the
mRNA.
2. A large ribosomal subunit binds to the small one,
creating a functional ribosome.
Animation: Translation
Right click slide / select “Play”
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© 2013 Pearson Education, Inc.
Figure 10.18
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Elongation
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• Elongation occurs in three steps.
Met
Large
ribosomal
subunit
Initiator
tRNA
P site
A site
– Step 1: Codon recognition. The anticodon of an
incoming tRNA pairs with the mRNA codon at the
A site of the ribosome.
mRNA
2
1
Start codon
Small
ribosomal
subunit
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Figure 10.19
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Amino acid
Elongation
Polypeptide
P site
Anticodon
mRNA
A site
Codons
1 Codon recognition
– The polypeptide leaves the tRNA in the P site and
attaches to the amino acid on the tRNA in the A
site.
– The ribosome catalyzes the bond formation
between the two amino acids.
ELONGATION
Stop codon
– Step 2: Peptide bond formation.
2 Peptide bond formation
New peptide
bond
mRNA
movement
3 Translocation
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Figure 10.19a
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– The P site tRNA leaves the ribosome.
– The tRNA carrying the polypeptide moves from the
A to the P site.
P site
Anticodon
mRNA
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– Step 3: Translocation.
Amino
acid
Polypeptide
Elongation
A site
Codons
Peptide bond formation
Codon recognition
ELONGATION
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Figure 10.19b
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Termination
• Elongation continues until
New peptide
bond
– a stop codon reaches the ribosome’s A site,
– the completed polypeptide is freed, and
– the ribosome splits back into its subunits.
mRNA
movement
Translocation
Stop codon
ELONGATION
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Review: DNA→
→ RNA→
→ Protein
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Figure 10.20-1
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RNA polymerase
1 Transcription
• In a cell, genetic information flows from
DNA
mRNA
– DNA to RNA in the nucleus and
Nucleus
Intron
– RNA to protein in the cytoplasm.
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Figure 10.20-2
87
RNA polymerase
1 Transcription
Nucleus
DNA
mRNA
Figure 10.20-3
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RNA polymerase
1 Transcription
DNA
mRNA
Intron
Intron
2 RNA processing
2 RNA processing
Cap
Tail
Cap
Tail
Intron
Nucleus
mRNA
Intron
mRNA
Amino acid
tRNA
Anticodon
ATP
Enzyme
3 Amino acid attachment
Figure 10.20-4
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RNA polymerase
1 Transcription
Nucleus
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RNA polymerase
1 Transcription
DNA
mRNA
Figure 10.20-5
DNA
mRNA
Intron
Nucleus
Intron
2 RNA processing
Anticodon
2 RNA processing
Codon
Cap
Cap
Tail
Tail
Intron
mRNA
Intron
Amino acid
mRNA
5 Elongation
Amino acid
tRNA
A
tRNA
Ribosomal
subunits
A
Ribosomal
subunits
Anticodon
Anticodon
Enzyme
ATP
3 Amino acid attachment
ATP
4 Initiation of
translation
3 Amino acid attachment
Figure 10.20-6
91
RNA polymerase
1 Transcription
Nucleus
4 Initiation of
translation
Review: DNA→
→ RNA→
→ Protein
• As it is made, a polypeptide
DNA
mRNA
Enzyme
– coils and folds and
Intron
Anticodon
2 RNA processing
– assumes a three-dimensional shape, its tertiary
structure.
Codon
Cap
Tail
Intron
mRNA
5 Elongation
Polypeptide
• Transcription and translation are how genes
control the structures and activities of cells.
Amino acid
tRNA
A
Ribosomal
subunits
Stop
codon
Anticodon
ATP
Enzyme
3 Amino acid attachment
4 Initiation of
translation
6 Termination
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Mutations
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• A mutation is any change in the nucleotide
sequence of DNA.
Figure 10.21
94
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
• Mutations can change the amino acids in a protein.
• Mutations can involve
– large regions of a chromosome or
– just a single nucleotide pair, as occurs in sickle-cell
disease.
Glu
Val
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Types of Mutations
• Mutations within a gene can be divided into two
general categories:
1. nucleotide substitutions (the replacement of one
base by another) and
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Figure 10.22
96
Met
Lys
Phe
Gly
mRNA and protein from a normal gene
Met
Lys
Phe
Ser
Ala
Ala
(a) Base substitution
Deleted
2. nucleotide deletions or insertions (the loss or
addition of a nucleotide).
• Insertions and deletions can
Met
Lys
Leu
Ala
(b) Nucleotide deletion
– change the reading frame of the genetic message
and
Inserted
– lead to disastrous effects.
Met
Lys
Leu
(c) Nucleotide insertion
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Trp
Arg
Figure 10.22a
97
Lys
Phe
Ser
98
Met
Lys
Phe
Ala
Gly
mRNA and protein from a normal gene
Met
Lys
Phe
Ala
Gly
mRNA and protein from a normal gene
Met
Figure 10.22b
Deleted
Ala
Met
(a) Base substitution
Lys
Leu
Ala
(b) Nucleotide deletion
Figure 10.22c
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Mutations
• (1) Wild-type gene
– The big red pig ate the red rag.
Met
Lys
Phe
Ala
Gly
mRNA and protein from a normal gene
Inserted
• (2) Base substitution
– The big res pig ate the red rag.
• (3) Base addition
– The big res dpi gat eth ere dra g.
• (4) Base deletion
Met
Lys
Leu
(c) Nucleotide insertion
Trp
Arg
– The big re-p iga tet her edr ag.
• 3 and 4 are know as frameshift mutations since
everything after the mutation is shifted and
would likely code for a new sequence of AAs
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101
Mutagens
VIRUSES AND OTHER NONCELLULAR
INFECTIOUS AGENTS
105
• Viruses share some, but not all, characteristics of
living organisms. Viruses
• Mutations may result from
– errors in DNA replication or recombination or
– possess genetic material in the form of nucleic
acids wrapped in a protein coat,
– physical or chemical agents called mutagens.
• Mutations
– are not cellular, and
– are often harmful but
– cannot reproduce on their own.
– are useful in nature and the laboratory as a source
of genetic diversity, which makes evolution by
natural selection possible.
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© 2013 Pearson Education, Inc.
Figure 10.24a
106
Protein coat
Animal Viruses
121
DNA
• Viruses that infect animals cells
– are a common cause of disease and
– may have RNA or DNA genomes.
• Many animal viruses have an outer envelope made
of phospholipid membrane, with projecting spikes
of protein.
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