Download Ch. 17 Protein Synthesis

Protein Synthesis
AP Biology
Ch. 17
“One Gene-One Polypeptide”
Theory
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George Beadle and Edward Tatum (late 40’s to
early 50’s)
 used X-rays to induce mutations in Neurospora
crassa (bread mold)
 Auxotrophs—nutritional mutants unable to
synthesize certain amino acid and vitamins.
They traced the defect to the enzymes involved
in their synthesis.
Conclusion: One gene encodes one enzyme.
Gene products encode both protein and RNA
Central Dogma of Molecular
Biology
In eukaryotic cells, a message (mRNA)
carries the genetic information (DNA) from
the nucleus to the cytoplasm
 Transcription
 An adapter (tRNA) translates the nucleic
acid (mRNA) into amino acid (protein)
 Translation
 Information flow is unidirectional

GCTGCTAACGTC
AGCTAGCTCGTA
GCGCTAGCGCTT
GCGTAGCTAAAG
TCGAGCTCGCTT
GCGTAGCTAAAG
TCGAGCTGCTGC
TAACGTCAGCTA
GCTCGTAG
RNA
Proteins
Structure of RNA
Single strand of nucleotides [instead of 2
strands in DNA]
 Nucleotides contain 5-carbon sugar ribose
[instead of deoxyribose in DNA]
 4 different nitrogenous bases
 Adenine (A)
 Uracil (U) [instead of thymine in DNA]
 Cytosine (C)
 Guanine (G)

Forms of RNA

3 Main Types of RNA
 mRNA (messenger RNA) brings DNA message
out of nucleus to the cytoplasm
 Each 3 bases on mRNA is a “codon”
 tRNA (transfer RNA) –The anticodon that
matches with the codon from mRNA to
determine which amino acid joins the protein
chain
 rRNA (ribosomal RNA) – make up the
ribosomes—RNA that lines up tRNA
molecules with mRNA molecules
Transcription: Synthesis and
Processing of RNA

RNA Polymerase is an enzyme that
synthesizes RNA using one strand of the
DNA as a template
 Separates 2 strands of DNA and links
RNA nucleotides as they base-pair along
the DNA template in an overall is 5’ to 3’
direction
Transcription: Initiation

RNA polymerase
attaches to promotor
sequence (TATA box)
of DNA sequence
 Prokaryotes—
enzyme binds
directly to sequence
 Eukaryotes—
enzyme binds after
transcription factors
bind to the site
Transcription: Elongation

RNA polymerase
 Synthesizes in 5’
3’ direction, antiparallel to DNA
template
 Unwinds about 20
base pairs as it reads
and synthesizes
complementary
RNA
Transcription: Termination

RNA polymerase terminates transcription
at special DNA sequences, terminator
 Prokaryote—stops right at the end of
termination signal
 Eukaryote—continues past termination
signal, to a poly(A) tail (AAUAAA) in
the pre-mRNA
Transcription
The code on DNA tells how mRNA is put together.
Example: DNAACCGTAACG
mRNAUGGCAUUGC
Each set of 3 bases is called a triplet or codon
(in mRNA)
UGG CAU UGC
RNA Splicing


Pre-mRNA never
leaves the nucleus
 Introns—noncoding regions
 Exons—coding
regions
snRNPs join with
other proteins to form
spliceosome, which
releases introns
Translation
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Genetic information in
mRNA is read by
codons
64 different codons
Genetic code is
redundant—more than
one codon codes for
many of the 20 amino
acids
AUG—universal start
codon
UAA, UAG, UGA—
stop codons
Structure of tRNA
Aminoacyl-tRNA
Formation
Structure of
a Ribosome
Translation:
Initiation


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Small ribosomal
subunit binds to start
codon (AUG) on
mRNA
Initiator tRNA (UAC)
base-pairs with mRNA
Large ribosomal unit
binds so initiator tRNA
is in the P site
GTP provides energy
for intiation process
Translation:
Elongation
Translation: Termination
Review entire
process
Coupled
Transcription
and Translation
in Bacteria
Point Mutations
Substitution Mutations
Missense mutation: altered codon still codes
for an amino acid, although maybe not the
right one
 Nonsense mutation: altered codon is a stop
codon and translation is terminated
prematurely
 Leads to nonfunctional proteins

Insertions and Deletions

Frameshift mutation: addition or loss of one
or more nucleotide pairs in a gene shifts the
reading frame for translation and incorrect
protein is made
The end
Reading direction
Ribosome
5’
mRNA U U C
3’
UG G
A U G
U A C
Met
U G G
A C C
Trp
U U C
U GG
Incoming tRNA
charged with
amino acid
Ribosome
Reading direction
5’
3’
mRNA U U C
UG G
A U G
U G G
A C C
Outgoing
“empty” tRNA
Trp
Met
U U C
A A G
Phe
U GG
Ribosome
Reading direction
5’
mRNA U U C
3’
UG G
A U G
U G G
U U C
A A G
U GG
A C C
A C C
Phe
Trp
Met
Trp
Ribosome
Reading direction
5’
3’
mRNA U U C
UG G
A U G
U G G
U U C
U GG
A C C
A C C
Phe
Trp
Met
Trp
Reading direction
Ribosome
5’
3’
mRNA U U C
UG G
A U G
Outgoing
“empty” tRNA
Met
Anticodon
A A G
A C C
Phe
Trp
Amino acid
U G G
A C C
Trp
U U C
U GG
Incoming tRNA
charged with
amino acid