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From DNA to Protein
Chapter 9
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
9.1 Ricin and Your Ribosomes
 Ricin
• Naturally occurring protein that is highly toxic
• A dose smaller then a few grains of salt can kill an adult
(die of low blood pressure and respiratory failure)
• One of Ricin’s two polypeptide chains is an enzyme
that inactivated ribosomes
• The second polypeptide chain binds to the plasma
membrane allowing the cell to take up Ricin by
endocytosis
 Ribosomes
• Assembly amino acids into proteins
• Proteins are critical to all life processes
Proteins
 All proteins consist of polypeptide chains
• A linear sequence of amino acids
 Each chain corresponds to the nucleotide base
sequence of a gene
The Path From Genes to Proteins
1. Transcription
• Enzymes use the base sequence of a gene in the
DNA as a template to make a strand of RNA
2. Translation
• Information in the RNA strand is decoded
(translated) into a sequence of amino acids
Prokaryotes and Eukaryotes
 In prokaryotic cells (no nucleus)
• Transcription and translation occur in cytoplasm
 In eukaryotic cells
• Genes are transcribed in the nucleus
• Resulting mRNA is translated in the cytoplasm
Key Concepts:
INTRODUCTION
 Life depends on enzymes and other proteins
 All proteins consist of polypeptide chains
 Chains are sequences of amino acids that
correspond to sequences of nucleotide bases in
DNA called genes
 The path leading from genes to proteins has two
steps: transcription and translation
9.2 The Nature of Genetic Information
 Genetic information consists of the nucleotide
base sequence of DNA
 The linear order, or sequence, of the four bases
in the strand is the genetic information
 Genetic information occurs in subsets called
genes
• Genes  part of the DNA sequence that
specifies an RNA or protein production
Transcription: DNA to RNA
 Two DNA strands unwind in a specific region
 RNA polymerase assembles a strand of RNA
• Covalently bonds RNA nucleotides (adenine,
guanine, cytosine, uracil) according to the
nucleotide sequence of the exposed gene
Three Types of RNA
 Messenger RNA (mRNA)
• Carries protein-building codes from DNA to
ribosomes
 Ribosomal RNA (rRNA)
• Forms ribosomes (where polypeptide chains are
assembled)
 Transfer RNA (tRNA)
• Delivers amino acids to ribosomes
RNA and DNA Compared
RNA Base Pairing
phosphate
group
base
(uracil)
sugar (ribose)
Fig. 13.2, p.198
Gene Transcription
Definition: The process by which the information in a gene
becomes converted to an RNA or protein product
RNA polymerase  enzyme that carries out transcription
Promoter  in DNA, a sequence to which RNA
polymerase binds
Fig. 13.3, p.198
Fig. 13.3, p.198
gene region
newly forming
RNA transcript
RNA polymerase, the enzyme
that catalyzes transcription
DNA template
winding up
DNA template
unwinding
Fig. 13.3, p.198
Fig. 13.3, p.198
Fig. 13.3, p.198
RNA Modification: Alternative Splicing
 Before mRNA leaves the nucleus:
• Introns are removed during RNA processing
• Some exons are removed along with introns;
remaining exons are spliced together in different
combinations
• Most are not removed during RNA processing
• Alternative splicing
• RNA processing event in which some exons
are removed or joined in various combinations
• Poly-A tail is added to 3’ end of new mRNA
The Poly-A Tail
 The longer its poly-A tail, the more time an
mRNA transcript (and its protein-building
message) will remain intact in the cytoplasm
Post-Translational RNA Modification
unit of transcription in DNA strand
exon
intron
exon
intron
exon
transcription into pre-mRNA
cap
poly-A tail
5'
3'
snipped out
snipped out
mature mRNA transcript
Fig. 13.4, p.199
Key Concepts:
TRANSCRIPTION
 During transcription, the two strands of the DNA
double helix are unwound in a gene region
 Exposed bases of one strand become the
template for assembling a single strand of RNA
(a transcript)
 Messenger RNA is the only type of RNA that
carries DNA’s protein-building instructions
RNA and the Genetic Code
 Messenger RNA (mRNA) carries DNA’s proteinbuilding information to ribosomes for translation
 mRNA’s genetic message is written in codons
• Sets of three nucleotides along mRNA strand
• The genetic code 
• The concept that a set of three nucleotides
specifies a particular amino acid
Codons
 Codons specify different amino acids
• A few codon signals stop during translation
 Sixty-four possible codons constitute a highly
conserved genetic code
Genetic Code: RNA Triplets
From DNA to Polypeptide
DNA
mRNA
mRNA
codons
amino
acids
threonine
proline
glutamate
glutamate
lysine
Fig. 13.5, p.200
Variation in Genetic Code
 Variant codons occur among prokaryotes,
prokaryote-derived organelles (such as
mitochondria), and some ancient lineages of
single-celled eukaryotes
Key Concepts:
CODE WORDS IN THE TRANSCRIPTS
 The nucleotide sequence in RNA is read three
bases at a time
 Sixty-four base triplets that correspond to
specific amino acids represent the genetic code,
which has been highly conserved over time
tRNA and rRNA
Function in Translation
 Transfer RNA (tRNA)
• Anticodon binds to mRNA codon
• Also binds amino acid specified by codon
 Different tRNAs carry different amino acids
• tRNAs deliver free amino acids to ribosomes
during protein synthesis
tRNA
rRNA
 Ribosomal RNA (rRNA) and proteins make up
the two subunits of ribosomes
Three Stages of Translation
 mRNA-transcript information directs synthesis of
a polypeptide chain during translation
 Translation proceeds in three stages
• Initiation
• Elongation
• Termination
Initiation
 One initiator tRNA, two ribosomal subunits, and
one mRNA come together as an initiation
complex
 Methionine (M)
• tRNA carries M  M is the first amino acid of the
new polypeptide chain
Initiation
Elongation
 tRNAs deliver amino acids to the ribosome in the
order specified by mRNA codons
 Ribosomal rRNA catalyzes the formation of a
peptide bond between amino acids
Elongation
Peptide Bond Forms between Met and Valine
Elongation
Elongation
Termination
 Translation ends when RNA polymerase
encounters a STOP codon in mRNA
• New polypeptide chain and mRNA are released
• Ribosome subunits separate from each other
Termination
Initiation
A mature mRNA
leaves the nucleus
and enters cytoplasm,
which has many free
amino acids, tRNAs,
and ribosome subunits.
An initiator tRNA binds
to a small ribosomal
subunit and the mRNA.
mRNA
initiator small
ribosomal
tRNA
subunit
large
ribosomal
subunit
A large ribosomal
subunit joins, and the
cluster is now called
an initiation complex.
Fig. 13.8, p.202
Elongation
An initiator tRNA
carries the amino acid
methionine, so the first
amino acid of the new
polypeptide chain will be
methionine. A second
tRNA binds the second
codon of the mRNA
(here, that
codon is GUG, so the
tRNA that binds carries
the amino acid valine).
The first tRNA
is released and the
ribosome moves to
the next codon in the
mRNA. A third tRNA
binds to the third
codon of the mRNA
(here, that codon is
UUA, so the tRNA
carries the amino
acid leucine).
A peptide bond
forms between the
first two amino acids
(here, methionine
and valine).
A peptide bond
forms between the
second and third
amino acids (here,
valine and leucine).
Fig. 13.8, p.202
The second RNA
is released and the
ribosome moves to
the next codon. A
fourth tRNA binds
the fourth mRNA
codon (here, that
codon is GGG, so
the tRNA carries
the amino acid
glycine).
A peptide bond
forms between the
third and fourth
amino acids
(here, leucine
and glycine)
Termination
Steps d and e are repeated over and over
until the ribosome encounters a STOP codon
in the mRNA. The mRNA transcript and the
new polypeptide chain are released from the
ribosome. The two ribosomal subunits
separate from each other. Translation is
now complete. Either the chain will join the
pool of proteins in the cytoplasm or it will
enter rough ER of the endomembrane system
(Section 4.8).
Fig. 13.8, p.202
Key Concepts:
TRANSLATION
 During translation, amino acids become bonded
together into a polypeptide chain in a sequence
specified by base triplets in messenger RNA
 Transfer RNAs deliver amino acids one at a time
to ribosomes
 Ribosomal RNA catalyzes the formation of
peptide bonds between the amino acids
Transcription-Translation Concepts
 Many ribosomes may simultaneously translate the
same mRNA, this is called  polysomes
 Transcription and translation both occur in the
cytoplasm
 Compared to DNA, RNA is not very stable
• An mRNA may last only a few minutes before it
gets disassembled by enzymes in the cytoplasm
 Translation is  Energy intensive (use ATP)
Mutated Genes and
Their Protein Products
 Mutations are permanent, small-scale changes
in the base sequence of a gene
 Common mutations include
• Insertions 
• one or more base pairs are inserted into the DNA
• Deletions 
• one or more base pairs are lost
• Base-pair substitutions
• A single base-pair is changed
Common Gene Mutations
Transposable Elements
 Segments of DNA that can insert themselves
anywhere in a chromosome
Some Causes of Mutations
 Natural and synthetic chemicals
• Cigarette smoke
 Environmental agents
• Ionizing radiation
• Nonionizing radiation
Key Concepts:
MUTATIONS IN THE CODE WORDS
 Mutations in genes may result in changes in
protein structure, protein function, or both
 The changes may lead to variation in traits
among individuals
SUMMARY: Protein Synthesis
Assembly of RNA on unwound regions of
DNA molecule
Transcription
mRNA
processing
mRNA
rRNA
tRNA
proteins
mature mRNA
transcripts
Translation
At an intact
ribosome,
synthesis of
a polypeptide
chain at the
binding sites
for mRNA
and tRNAs
ribosomal
subunits
Convergence
of RNAs
mature
tRNA
cytoplasmic
pools of
amino acids,
ribosomal
subunits,
and tRNAs
Final protein
Fig. 13.11, p.206
Animation: Base-pair substitution
Animation: Frameshift mutation
Animation: Gene transcription details
Animation: Genetic code
Animation: Pre-mRNA transcript
processing
Animation: Protein synthesis summary
Animation: Structure of a ribosome
Animation: Structure of a tRNA
Animation: Translation
Animation: Uracil-thymine comparison