1. Overview: The Flow of Genetic Information
The information content of DNA is in the form of specific sequences of nucleotides
The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins
Proteins are the links between genotype and phenotype
Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation
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2. Basic Principles of Transcription and Translation
RNA is the bridge between genes and the proteins for which they code
Transcription is the synthesis of RNA using information in DNA
Transcription produces messenger RNA (mRNA)
Translation is the synthesis of a polypeptide, using information in the mRNA
Ribosomes are the sites of translation
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3. In prokaryotes, translation of mRNA can begin before transcription has finished
In a eukaryotic cell, the nuclear envelope separates transcription from translation
Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA
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4. A primary transcript is the initial RNA transcript from any gene prior to processing
The central dogma is the concept that cells are governed by a cellular chain of command: DNA RNA protein
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5. DNA Pre-mRNA mRNA DNA mRNA Nuclear envelope TRANSCRIPTION
Figure 17.3
Nuclear envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
DNA
TRANSCRIPTION
mRNA
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information.
Ribosome
TRANSLATION
Ribosome
TRANSLATION
Polypeptide
Polypeptide
(a) Bacterial cell
(b) Eukaryotic cell

6. The Genetic Code
How are the instructions for assembling amino acids into proteins encoded into DNA?
There are 20 amino acids, but there are only four nucleotide bases in DNA
How many nucleotides correspond to an amino acid?
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7. Codons: Triplets of Nucleotides
The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA
These words are then translated into a chain of amino acids, forming a polypeptide
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8. DNA template strand 5 DNA 3 molecule Gene 1 5 3 TRANSCRIPTION
Figure 17.4
DNA template strand 5
DNA 3 A C C A A A C C G A G T
molecule T G G T T T G G C T C A
Gene 1 5 3
TRANSCRIPTION
Gene 2 U G G U U U G G C U C A
mRNA 5 3
Codon
TRANSLATION
Figure 17.4 The triplet code.
Protein
Trp
Phe
Gly
Ser
Gene 3
Amino acid

9. The template strand is always the same strand for a given gene
During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript
The template strand is always the same strand for a given gene
During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction
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10. Codons along an mRNA molecule are read by translation machinery in the 5 to 3 direction
Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide
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11. Cracking the Code All 64 codons were deciphered by the mid-1960s
Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation
The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced
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12. First mRNA base (5 end of codon) Third mRNA base (3 end of codon)
Figure 17.5
Second mRNA base U C A G
UUU
UCU
UAU
UGU U
Phe
Tyr
Cys
UUC
UCC
UAC
UGC C U
Ser
UUA
UCA
UAA Stop
UGA Stop A
Leu
UUG
UCG
UAG Stop
UGG
Trp G
CUU
CCU
CAU
CGU U
His
CUC
CCC
CAC
CGC C C
Leu
Pro
Arg
CUA
CCA
CAA
CGA A
Gln
CUG
CCG
CAG
CGG G
First mRNA base (5 end of codon)
Third mRNA base (3 end of codon)
AUU
ACU
AAU
AGU U
Asn
Ser
AUC
Ile
ACC
AAC
AGC C A
Thr
Figure 17.5 The codon table for mRNA.
AUA
ACA
AAA
AGA A
Lys
Arg
AUG
Met or start
ACG
AAG
AGG G
GUU
GCU
GAU
GGU U
Asp
GUC
GCC
GAC
GGC C G
Val
Ala
Gly
Gly
GUA
GCA
GAA
GGA A
Glu
GUG
GCG
GAG
GGG G

13. Evolution of the Genetic Code
The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after being transplanted from one species to another
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14. Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look
Transcription is the first stage of gene expression
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15. Molecular Components of Transcription
RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides
The RNA is complementary to the DNA template strand
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
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16. Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look
Genetic information flows from mRNA to protein through the process of translation
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17. Molecular Components of Translation
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
tRNAs transfer amino acids to the growing polypeptide in a ribosome
Translation is a complex process in terms of its biochemistry and mechanics
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18. tRNA with amino acid attached
Figure 17.14
Amino acids
Polypeptide
tRNA with amino acid attached
Ribosome
Trp
Phe
Gly
Figure Translation: the basic concept.
tRNA C C C C A C C G
Anticodon G G A A A U G G U U U G G C 5
Codons 3
mRNA

19. The Structure and Function of Transfer RNA
Molecules of tRNA are not identical
Each carries a specific amino acid on one end
Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
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20. Ribosomes
Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)
Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes
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21. P site (Peptidyl-tRNA binding site) Exit tunnel
Figure 17.17b
P site (Peptidyl-tRNA binding site)
Exit tunnel
A site (Aminoacyl- tRNA binding site)
E site (Exit site) E P A
Large subunit
Figure The anatomy of a functioning ribosome.
mRNA binding site
Small subunit
(b) Schematic model showing binding sites

22. Next amino acid to be added to polypeptide chain
Figure 17.17c
Growing polypeptide
Amino end
Next amino acid to be added to polypeptide chain E
tRNA
mRNA 3
Figure The anatomy of a functioning ribosome.
Codons 5
(c) Schematic model with mRNA and tRNA

23. A ribosome has three binding sites for tRNA
The P site holds the tRNA that carries the growing polypeptide chain
The A site holds the tRNA that carries the next amino acid to be added to the chain
The E site is the exit site, where discharged tRNAs leave the ribosome
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24. Building a Polypeptide
The three stages of translation
Initiation
Elongation
Termination
All three stages require protein “factors” that aid in the translation process
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25. Ribosome Association and Initiation of Translation
The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits
First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
Then the small subunit moves along the mRNA until it reaches the start codon (AUG)
Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
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26. 3 U 5 A C 5 A 3 U G Initiator tRNA GTP GDP mRNA 5 5 3 3
Figure 17.18
Large ribosomal subunit 3 U 5 A C
P site
Met 5 A 3
Met U G P i
Initiator tRNA 
GTP
GDP E A
mRNA 5 5
Figure The initiation of translation. 3 3
Start codon
Small ribosomal subunit
mRNA binding site
Translation initiation complex

27. Elongation of the Polypeptide Chain
During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain
Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation
Translation proceeds along the mRNA in a 5′ to 3′ direction
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28. Amino end of polypeptide
Figure
Amino end of polypeptide E 3
mRNA
Ribosome ready for next aminoacyl tRNA
P site
A site 5
GTP
GDP  P i E E P A P A
Figure The elongation cycle of translation.
GDP  P i
GTP E P A

29. Termination of Translation
Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome
The A site accepts a protein called a release factor
The release factor causes the addition of a water molecule instead of an amino acid
This reaction releases the polypeptide, and the translation assembly then comes apart
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30. Release factor Free polypeptide 5 3 3 3 5 5 Stop codon
Figure
Release factor
Free polypeptide 5 3 3 3 5 5 2
GTP
Stop codon 2
GDP  2 P i
Figure The termination of translation.
(UAG, UAA, or UGA)

31. Completed polypeptide Growing polypeptides
Figure 17.21
Completed polypeptide
Growing polypeptides
Incoming ribosomal subunits
Polyribosome
Start of mRNA (5 end)
End of mRNA (3 end)
(a)
Ribosomes
Figure Polyribosomes.
mRNA
(b)
0.1 m

32. Completing and Targeting the Functional Protein
Often translation is not sufficient to make a functional protein
Polypeptide chains are modified after translation or targeted to specific sites in the cell
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33. Protein Folding and Post-Translational Modifications
During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape
Proteins may also require post-translational modifications before doing their job
Some polypeptides are activated by enzymes that cleave them
Other polypeptides come together to form the subunits of a protein
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34. Polypeptide synthesis always begins in the cytosol
Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER
Polypeptides destined for the ER or for secretion are marked by a signal peptide
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