Transcript Transcription & Translation

THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
A specific gene specifies a polypeptide
– The DNA is transcribed into RNA, which is
translated into the polypeptide
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
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CODONS
• 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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Gene 1
Gene 3
DNA molecule
Gene 2
DNA strand
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Figure 10.7
Amino acid
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CODON TRANSLATION
• Virtually all
organisms share
the same genetic
code
• 64 Codons total
(4 x 4 x 4)
• AUG = Start (Met)
• 3 Stops
• Redundant, but
NOT ambiguous
Figure 10.8A
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• An exercise in translating the genetic code
Transcribed strand
DNA
Transcription
RNA
Start
codon
Polypeptide
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Translation
Stop
codon
Figure 10.8B
Where does RNA come from: TRANSCRIPTION
RNA
polymerase
RNA nucleotide
Direction of
transcription
Template
strand of DNA
Figure 10.9A
Newly made RNA
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RNA polymerase
• In transcription, the
DNA helix unzips
– Promoter: turns a
gene on/off
– RNA nucleotides line
up along one strand
of the DNA following
the base-pairing rules
– The single-stranded
messenger RNA peels
away and the DNA
strands rejoin
Figure 10.9B
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DNA of gene
Promoter
DNA
Initiation
Elongation
Terminator
DNA
Area shown
in Figure 10.9A
Termination
Growing
RNA
Completed RNA
RNA
polymerase
RNA Processing: Splicing Exons & Discarding Introns
• Noncoding
segments called
introns are
spliced out
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
• A cap and a tail
are added to
the ends
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
Figure 10.10
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TRANSLATION: tRNA helps assemble the
polypeptide chain (amino acids)
• In the cytoplasm, a
ribosome attaches
to the mRNA and
translates its
message into a
polypeptide
• The process is aided
by transfer RNAs
Amino acid attachment site
Hydrogen bond
RNA polynucleotide chain
Anticodon
Figure 10.11A
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• Each tRNA molecule has a triplet anticodon on
one end and an amino acid attachment site on
the other
Amino acid
attachment
site
Anticodon
Figure 10.11B, C
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Ribosomes build polypeptides
Next amino acid
to be added to
polypeptide
Growing
polypeptide
tRNA
molecules
P site
A site
Growing
polypeptide
Large
subunit
tRNA
P
A
mRNA
mRNA
binding
site
Codons
mRNA
Small
subunit
Figure 10.12A-C
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• mRNA, a specific tRNA, and the ribosome
Large
ribosomal
subunit
Initiator tRNA
P site
A site
Start
codon
mRNA
Small ribosomal
subunit
1
Figure 10.13B
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2
Amino acids are added to the polypeptide chain
until a stop Codon is reached
• The mRNA moves a codon at a time
• A tRNA pairs with each codon, adding an
amino acid to the growing polypeptide
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Amino acid
Polypeptide
A
site
P site
Anticodon
mRNA
1
Codon recognition
mRNA
movement
Stop
codon
New
peptide
bond
3
Translocation
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2
Peptide bond
formation
Figure 10.14
Review: The flow of genetic information in the cell
is DNARNAprotein
• The sequence of codons in DNA spells out the
primary structure of a polypeptide
– Polypeptides form proteins that cells and
organisms use
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Summary of
transcription
and
translation
TRANSCRIPTION
DNA
mRNA
RNA
polymerase
Stage 1 mRNA is
transcribed from a
DNA template.
Amino acid
TRANSLATION
Enzyme
Stage 2 Each amino
acid attaches to its
proper tRNA with the
help of a specific
enzyme and ATP.
tRNA
Initiator
tRNA
mRNA
Figure 10.15
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Anticodon
Large
ribosomal
subunit
Start
Codon
Small
ribosomal
subunit
Stage 3 Initiation of
polypeptide synthesis
The mRNA, the first
tRNA, and the
ribosomal subunits
come together.
New
peptide
bond
forming
Growing
polypeptide
Codons
Stage 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.
mRNA
Polypeptide
Stop Codon
Figure 10.15 (continued)
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Stage 5 Termination
The ribosome recognizes
a stop codon. The polypeptide is terminated and
released.
Mutations can change the meaning of genes
• Mutations are changes in the DNA base
sequence
– These are caused by errors in DNA replication
or by mutagens
– The change of a single DNA nucleotide causes
sickle-cell disease
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Normal hemoglobin DNA
mRNA
Mutant hemoglobin DNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Figure 10.16A
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• Types of mutations
NORMAL GENE
mRNA
Protein
Met
Lys
Phe
Gly
Ala
Lys
Phe
Ser
Ala
BASE SUBSTITUTION
Met
Missing
BASE DELETION
Met
Lys
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Leu
Ala
His
Figure 10.16B