Download Topic 10 (From Genotype to Phenotype)

THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
• The DNA genotype is expressed as proteins,
which provide the molecular basis for phenotypic
traits
• The information constituting an organism’s
genotype
– Is carried in its sequence of its DNA bases
• A particular gene, a linear sequence of many
nucleotides
– Specifies a polypeptide
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• The DNA of the gene is transcribed into RNA
– Which is translated into the polypeptide
DNA
Transcription
RNA
Translation
Protein
Figure 10.6A
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Genetic information written in codons is
translated into amino acid sequences
• The “words” of the DNA “language”
– Are triplets of bases called codons
• The codons in a gene
– Specify the amino acid sequence of a
polypeptide
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DNA molecule
Gene 1
Gene 2
Gene 3
DNA strand
A A A C C G G C A A A A
Transcription
RNA
U U U G G C C G U U U U
Codon
Translation
Polypeptide
Figure 10.7
Amino acid
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The genetic code is the Rosetta stone of life
• Nearly all organisms
– Use exactly the same genetic code
Second base
U
U
C
UUU
Phe
UUC
UCU
UCC
UUA
UCA
UUG
Leu
CUU
C
CUC
First base
CUA
CUG
A
Leu
UGA Stop A
UGG Trp
CCU
CAU
His
CAC
CGU
CGC
CAA
CAG Gln
CGA
CGG
AAU
Asn
AAC
U
AGU
Ser
AGC
C
AAA
AGA
A
AGG Arg
G
U
GGU
C
GGC
Gly
GGA
A
CCC
CCA
CCG
ACA
ACC
GUA
Pro
Thr
AAG
GAU
GCU
GCC
Val
GUG
Figure 10.8A
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UAC
UGU Cys U
UGC
C
UAA Stop
AUA
GUC
G
Ser
Tyr
UAG Stop
ACU
ACC
GUU
UAU
G
UCG
AUU
AUC Ile
Met or
AUG
start
A
GCA
GCG
GAC
Ala
Lys
Asp
GAA
GAG
Glu
GGG
G
U
C
Arg
A
G
G
RNA codon table, with corresponding amino acids
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• An exercise in translating the genetic code
Strand to be transcribed
T
A
C
T
T
C
A
A
A
A
T
C
A
T
G
A
A
G
T
T
T
T
A
G
U
A
G
DNA
Transcription
A
U
G
A
A
G
U
U
U
RNA
Start
condon
Stop
condon
Translation
Figure 10.8B
Polypeptide
Met
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Lys
Phe
Transcription produces genetic messages in the
form of RNA
• A close-up view of transcription
RNA nucleotides
RNA
polymerase
T C C A
A U
T
T
A
C C A
T A G G T
Direction of
transcription
Figure 10.9A
Newly made RNA
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A
Template
Strand of DNA
• In the nucleus, the DNA helix unzips
– And RNA nucleotides line up along one
strand of the DNA, following the base
pairing rules
• As the single-stranded messenger RNA
(mRNA) peels away from the gene
– The DNA strands rejoin
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• Transcription of a gene
RNA polymerase
DNA of gene
Promoter
DNA
Terminator
DNA
1 Initiation
2 Elongation
3 Termination
Completed RNA
Figure 10.9B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Area shown
In Figure 10.9A
Growing
RNA
RNA
polymerase
Eukaryotic RNA is processed before leaving the
nucleus
• Noncoding segments called introns are spliced out
– And a cap and a tail are added to the ends
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Figure 10.10
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Transfer RNA molecules serve as interpreters
during translation
• Translation
– Takes place in the cytoplasm
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• A ribosome attaches to the mRNA
– And translates its message into a specific
polypeptide aided by transfer RNAs
(tRNAs)
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Figure 10.11A
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Anticodon
• Each tRNA molecule
– Is a folded molecule bearing a base triplet
called an anticodon on one end
• A specific amino acid
– Is attached to the other end
Amino acid
attachment site
Figure 10.11B, C
Anticodon
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Ribosomes build polypeptides
• A ribosome consists of two subunits
– Each made up of proteins and a kind of
RNA called ribosomal RNA
tRNA
molecules
Growing
polypeptide
Large
subunit
mRNA
Figure 10.12A
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Small
subunit
• The subunits of a ribosome
– Hold the tRNA and mRNA close together
during translation
tRNA-binding sites
Large
subunit
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
mRNAbinding site
mRNA
Small
subunit
Codons
Figure 10.12B, C
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An initiation codon marks the start of an mRNA
message
Start of genetic message
End
Figure 10.13A
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• mRNA, a specific tRNA, and the ribosome
subunits
– Assemble during initiation
Met
Met
Large
ribosomal
subunit
Initiator tRNA
P site
U
A C
A U G
U
A C
A U G
Start
codon
1
mRNA
A site
Small ribosomal
subunit
Figure 10.13B
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
2
Elongation adds amino acids to the polypeptide
chain until a stop codon terminates translation
• Once initiation is complete
– Amino acids are added one by one to the
first amino acid
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Each addition of an amino acid
– Occurs in a three-step elongation process
Amino
acid
Polypeptide
P site
A site
Anticodon
mRNA
Codons
1 Codon recognition
mRNA
movement
Stop
codon
2 Peptide bond
formation
New
Peptide
bond
Figure 10.14
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3 Translocation
• The mRNA moves a codon at a time
– And a tRNA with a complementary
anticodon pairs with each codon, adding its
amino acid to the peptide chain
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Elongation continues
– Until a stop codon reaches the ribosome’s
A site, terminating translation
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
Review: The flow of genetic information in the cell
is DNARNAprotein
• The sequence of codons in DNA, via the
sequence of codons
– Spells out the primary structure of a
polypeptide
Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings
• Summary of transcription and translation
DNA
Transcription
1 mRNA is transcribed
from a DNA template.
mRNA
RNA
polymerase
Amino acid
Translation
2 Each amino acid
attaches to its proper
tRNA with the help of a
specific enzyme and ATP.
Enzyme
ATP
tRNA
Anticodon
Large
ribosomal
subunit
Initiator
tRNA
Start Codon
mRNA
3 Initiation of
polypeptide synthesis
The mRNA, the first tRNA,
and the ribosomal
subunits come together.
Small
ribosomal
subunit
New peptide
bond forming
Growing
polypeptide
4 Elongation
A succession of tRNAs
add their amino acids to
the polypeptide chain as the
mRNA is moved through the
ribosome, one codon at a time.
Codons
mRNA
Polypeptide
5 Termination
The ribosome recognizes a
stop codon. The poly-peptide
is terminated and released.
Figure 10.15
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Stop codon
Mutations can change the meaning of genes
• Mutations are changes in the DNA base
sequence
– Caused by errors in DNA replication or
recombination, or by mutagens
Normal hemoglobin DNA
C
T
T
mRNA
A
T
G U
A
C
mRNA
G
Figure 10.16A
Mutant hemoglobin DNA
A
A
Normal hemoglobin
Glu
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Sickle-cell hemoglobin
Val
• Substituting, inserting, or deleting nucleotides
alters a gene
– With varying effects on the organism
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Figure 10.16B
Met
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Lys
Leu
Ala
His