How does a single faulty gene result in
the dramatic appearance of an albino
animal?
The albino deer has a faulty version of a
key protein, an enzyme required for
pigment synthesis, and this protein is
faulty because the gene that codes for it
contains incorrect information.
GENE EXPRESSION: FROM GENE TO
PROTEIN
Chapter 17.1 and 17.2
CONCEPT 17.1: GENES SPECIFY PROTEINS VIA
TRANSCRIPTION AND TRANSLATION
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 and RNA molecules involved in protein
synthesis.
Proteins are the links between genotype (what the genome says)
and phenotype (what traits physically appear).
Gene expression, the process by which DNA directs protein synthesis,
includes two stages: transcription and translation.
EVIDENCE FROM THE STUDY OF
METABOLIC DEFECTS
In 1902, British physician Archibald Garrod
first suggested that genes dictate
phenotypes through enzymes that catalyze
specific chemical reactions.
He thought symptoms of an inherited disease
reflect an inability to synthesize a certain
enzyme.
• “Inborn errors of metabolism.”
Linking genes to enzymes required
understanding that cells synthesize and
degrade molecules in a series of steps, a
metabolic pathway.
Archibald
Garrod
NUTRITIONAL MUTANTS IN NEUROSPORA:
SCIENTIFIC INQUIRY
George Beadle and Edward Tatum exposed bread mold to X-rays,
creating mutants that were unable to survive on minimal media.
Using crosses, they identified three classes of arginine-deficient
mutants, each lacking a different enzyme necessary for synthesizing
arginine.
They developed a one gene–one enzyme hypothesis, which states
that each gene dictates production of a specific enzyme.
THE PRODUCTS OF GENE EXPRESSION: A
DEVELOPING STORY
Some proteins aren’t enzymes, so researchers later revised the
hypothesis: one gene–one protein.
Many proteins are composed of several polypeptides, each of which
has its own gene.
Therefore, Beadle and Tatum’s hypothesis is now restated as the one
gene–one polypeptide hypothesis.
(Multiple polypeptides
can come together
to form a function
protein).
One Gene
One Polypeptide
BASIC PRINCIPLES OF TRANSCRIPTION AND TRANSLATION
RNA is the bridge between genes
and the proteins for which they
code.
Transcription is the synthesis of
mRNA (messenger RNA) using
information in DNA.
Translation is the synthesis of a
polypeptide at a ribosome, using
information in the mRNA.
Prokaryotes
Transcription and translation occur
in the cytoplasm.
In prokaryotes, translation of mRNA
can begin before transcription has
finished.
mRNA is not modified.
Eukaryotes
Transcription occurs in the nucleus.
Transcription directly produces premRNA molecules.
Pre-mRNA transcripts are modified
(before leaving nucleus) through
RNA processing to yield the finished
mRNA.
Translation occurs in the
cytoplasm.
CENTRAL DOGMA
First dubbed by Francis Crick in 1956.
Some exceptions to this rule have emerged
over the years (some enzymes produce DNA
from RNA), but they have not invalidated this
idea.
In a eukaryotic cell,
transcription occurs in
nucleus and
translation occurs in
the cytoplasm.
In a prokaryotic cell,
transcription and
translation occur in the
cytoplasm.
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.
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.

An mRNA molecule is complementary to its DNA template because RNA
nucleotides are assembled on the template according to base-pairing rules.
During translation, the mRNA base triplets, called codons, are written
in the 5 to 3 direction.
 Codons along an mRNA molecule are read by translation machinery in the 5 to 3
direction.
 Each codon specifies one amino acid (one of 20).
CRACKING THE CODE
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.
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.
CONCEPT 17.2: TRANSCRIPTION IS THE
DNA-DIRECTED SYNTHESIS OF RNA
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.
Bacteria have one
type of RNA
polymerase.
Eukaryotes have
three types of RNA
polymerase.
The DNA sequence where RNA polymerase
attaches is called the promoter; in bacteria, the
sequence signaling the end of transcription is called
the terminator.
• The stretch of DNA that is transcribed is called a transcription
unit.
Overview of Transcription
1. RNA polymerase II binds, DNA
unwinds, and polymerase
initiates transcription.
2. Polymerase moves downstream,
and mRNA elongates in the
5’3’ direction.
3. mRNA is releases and
polymerase detaches.
In eukaryotes, mRNA
is modified before
translation.
In prokaryotes,
mRNA moves on
to translation.
INITIATION OF TRANSCRIPTION
Promoters signal the transcriptional start point and usually extend
several dozen nucleotide pairs upstream of the start point.
• A promoter called a TATA box is crucial in forming the initiation complex in
eukaryotes.
Transcription factors mediate the binding of RNA polymerase and
the initiation of transcription.
Transcription
factors allow
RNA
polymerase
to bind in
correct
location and
orientation.
Initiation of Transcription
1. Eukaryotic promoter
includes TATA boxy about
25 nucleotides upstream
from start.
2. Several transcription
factors, one recognizing
the TATA box, must bind to
DNA.
3. RNA polymerase II unwinds
DNA and adds
complementary RNA base
pairs.
ELONGATION OF THE RNA STRAND
As RNA polymerase moves along the DNA, it
untwists the double helix, 10 to 20 bases at a
time.
A gene can be transcribed simultaneously by
several RNA polymerases.
• Increase amount of mRNA produced, which
consequently increase the amount of protein
produced.
 Nucleotides
are added to the 3 end of the
growing RNA molecule.
TERMINATION OF TRANSCRIPTION
THE MECHANISMS OF TERMINATION ARE DIFFERENT IN BACTERIA AND
EUKARYOTES.
In bacteria, the polymerase stops
transcription at the end of the
terminator and the mRNA can be
translated without further
modification.
In eukaryotes, RNA polymerase II
transcribes the polyadenylation
signal sequence.
 This signals the binding of certain proteins.
 Proteins cut the RNA transcript free 0–35
nucleotides past this polyadenylation
sequence.