Transcript Protein Synthesis 06-07

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 10
Protein Synthesis
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
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 the sequence of bases in
DNA
• The flow of information is from DNA to RNA
to protein
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• A specific gene specifies a polypeptide
– The DNA is transcribed into RNA, which is
translated into the polypeptide
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DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Figure 10.6A
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• Studies of inherited metabolic disorders first
suggested that phenotype is expressed
through proteins
• Studies of the bread mold Neurospora crassa
led to the one gene-one polypeptide
hypothesis
Figure 10.6B
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Mutate wild type fungus
*Supply all mutant isolates with
complete media
*Grow purified mutants
with minimal media
to find nutritional mutants
*Determine what is the nutritional limitation
 find mutation
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There for the gene used to produce an enzyme
that helps cells manufacture Arginine amino acid
was mutated in that fungal strain
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Transcription produces genetic messages in the
form of RNA
RNA
polymerase
RNA nucleotide
Direction of
transcription
Template
strand of DNA
Figure 10.9A
Newly made RNA
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RNA Transcription
• Process in which the genetic information on
DNA is transferred to RNA
• During transcription only 1 DNA stand serves
as the template or pattern from which RNA is
formed.
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RNA polymerase
• In transcription, the
DNA helix unzips
– 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
DNA of gene
Promoter
DNA
Initiation
Elongation
Terminator
DNA
Area shown
in Figure 10.9A
Termination
Growing
RNA
Completed RNA
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Figure 10.9B
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RNA
polymerase
RNA Transcription
1. Initiation
•
The enzyme RNA polymerase attaches to the
promoter site on the DNA
•
Promoter – a sequence of nucleotides that is
found on one of the DNA strands
– tells RNA polymerase to start transcription
and which of the two DNA strands to
transcribe
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RNA Transcription
2. Elongation
•
RNA nucleotides attach to the free DNA
nucleotides by hydrogen bonds one at a time
•
As RNA synthesis continues the growing RNA
strand peels away from the DNA and the DNA
strands rejoin
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RNA Transcription
3. Termination
•
RNA polymerase reaches the terminator.
•
Terminator – a sequence of bases on DNA
that signals the end of the gene
•
The RNA polymerase detaches from the DNA
and the RNA molecule is complete
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10.10 Eukaryotic RNA is processed before leaving
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the nucleus
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• Noncoding
segments called
introns are
spliced out
Exon Intron
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
• The coding
segments called
exons are
joined together
• A cap and a tail
are added to
the ends
Exon
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
Figure 10.10
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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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Gene 1
Gene 3
DNA molecule
Gene 2
DNA strand
TRANSCRIPTION
RNA
Codon
TRANSLATION
Polypeptide
Figure 10.7
Amino acid
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The genetic code is the Rosetta stone of life
• Virtually all
organisms
share the same
genetic code
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
Translation
•
The process in which a polypeptide is
synthesized using the genetic information
encoded on an mRNA molecule
•
The following are needed for translation to
occur
1. mRNA
-
Contains the instructions for the assembly of
proteins
-
Codon – a sequence of 3 bases on mRNA that
specifies a specific amino acid that will be added to
the polypeptide chain
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Transfer RNA molecules serve as interpreters
during translation
• 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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Translation
2. tRNA (transfer RNA)
•
Carries an amino acid to the ribosome
•
A tRNA molecule is composed of
– A single strand of RNA (about 80 nucleotides)
– A loop at one end that contains the anticodon
– Anticodon – a sequence of 3 bases on tRNA
that are complementary to the bases on mRNA
– At the opposite end of the loop is a site where
an amino acit can attach
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Translation
3. Amino acids
• Located in the cytoplasm
• Synthesized from other chemicals or obtained
from food
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10.12 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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Translation
4. Ribosomes
•
Organelles where protein synthesis occurs
•
Consists of 2 subunits each made up of
proteins and ribosomal RNA (rRNA)
– Small subunit – has binding site for mRNA
– Large subunit – has binding site for tRNA
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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
Large
ribosomal
subunit
Initiator tRNA
P site
A site
Start
codon
mRNA
Small ribosomal
subunit
1
Figure 10.13B
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2
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.
10.14 Elongation adds amino acids to the
polypeptide chain until a stop codon
terminates translation
• The mRNA moves a codon at a time relative to
the ribosome
– A tRNA pairs with each codon, adding an amino
acid to the growing polypeptide
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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
Steps of Translation
1. Initiation
•
mRNA binds to the ribosome
•
The start codon (AUG) is reached
•
The first amino acid (methionine) is brought
to the ribosome by the tRNA
2. Elongation
•
Amino acids are added one by one to a
growing polypeptide chain
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Steps of Translation
3. Termination
•
The stop codon is reached
•
The completed polypeptide is released
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Modification of the polypeptide
Endoplasmic reticulum
• Collects proteins made by the ribosomes
• Packages them into vesicles which move
to the Golgi apparatus
Golgi apparatus
• Proteins are altered, packaged into
vesicles, and transported to different
parts of the cell or exported out of the cell
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• 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.
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
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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
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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
Types of Mutations
There are 2 general categories of mutations:
1. Base substitution
•
The replacement of one nucleotide with
another
•
Can result in no change in the protein
•
An insignificant change
– The altered amino acid has no effect on the
function of the protein
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Types of Mutations
•
A change that is crucial to life of the organism
– The altered amino acid has an effect on the
function of the protein
2. Base insertions or deletions
•
One or more bases are added or deleted from
the DNA
•
Often have disastrous effects
– The nucleotide sequence following the change
alters the genetic message (reading frame)
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Mutations are Useful
Mutations are useful because they
1. Provide diversity that allows evolution by
natural selection to occur
2. Essential tool for geneticists
•
Create different alleles needed for genetic
research
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