Gene Expression
Chapter 17: From Gene to Protein
The Central Dogma of
Biology
 DNA (in genes) is a recipe book for polypeptides (proteins)
 RNA is an essential partner in this process

 Proteins are the links between genotype and phenotype

, the process by which DNA directs protein
synthesis, includes 2 stages: transcription and translation
Relationship between
genes and proteins
 George Beadle and Edward Tatum exposed
 This created mutants that were unable to survive on minimal
medium as a result of inability to synthesize certain molecules
 Using crosses, they identified three classes of argininedeficient mutants
Each lacked a different enzyme necessary for synthesizing
arginine
 They developed a
States that each gene dictates production of a specific
enzyme
 Some proteins aren’t enzymes, so researchers later revised the
hypothesis to the
Protein synthesis
 RNA is the intermediate between genes and the proteins
for which they code

is the synthesis of RNA
under the direction of DNA
Occurs in the
(where the DNA is located)
Produces

is the synthesis of a
polypeptide, which occurs under the direction of mRNA
Occurs on the
(cytoplasm)
mRNA transcript language is changed to protein
language
Prokaryotes vs Eukaryotes
 In prokaryotes, mRNA produced
by transcription is immediately
translated without more
processing
 In a eukaryotes, the nuclear
envelope separates transcription
from translation
A
is the initial RNA transcript from
any gene (pre-mRNA)
 Eukaryotic RNA transcripts are
modified through
to yield finished
mRNA
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
 The flow of information from gene to protein is based on a
: a series of three-nucleotide words called
 Example: AGT on a DNA strand results in the placement of the
amino acid serine at the corresponding position of the
polypeptide
 During transcription, one of the two DNA strands called the
provides a template for ordering the
sequence of nucleotides in an RNA transcript
 During translation, the mRNA codons are read in the 5 to 3
direction
 Each codon specifies the addition of one of 20 amino acids
Codons
 The genetic code is
but not
No codon specifies more than one amino acid
But there are many codons that code for the same amino
acid
 Code has start and stop signals
 All 64 codons were deciphered by the mid-1960s
61 code for amino acids
3 are “stop” signals to end translation
 Codons must be read in the correct
(correct groupings) in order for the specified polypeptide to
be produced
Transcription
The stretch of DNA
that is transcribed is
called a
The three stages of
transcription:
Initiation
Elongation
Termination
Initiation

RNA polymerase is an
enzyme that pries the DNA
strands apart and hooks
together the RNA nucleotides
The promoter is the DNA at
the beginning of a gene
sequence
 DNA helix unwinds (10-20 bases
at a time)
Similar to
 RNA synthesis begins at the start
point on the template strand
Eukaryotic Promoters
 Promoters signal the initiation of RNA synthesis

mediate the binding of
RNA polymerase and the initiation of transcription
The completed assembly of transcription factors
and RNA polymerase II bound to a promoter is
called a
 A promoter called a
is crucial in
forming the initiation complex in eukaryotes
Transcription factors bind here and help mediate
the binding of RNA polymerase
 A gene can be transcribed simultaneously by several
RNA polymerases
Elongation
 RNA polymerase moves
 It continues to unwind the
DNA and elongate the
transcript in the 5’ to 3’
direction (40 nucleotides
per second)
 Double helix reforms as
section is passed
 This creates a copy of the
“recipe”
 RNA synthesis follows the
same base-pairing rules as
DNA, except uracil
substitutes for thymine
Termination
 RNA polymerase reaches the
Signals the end of the transcription unit
Involves a
 RNA transcript is released
 RNA polymerase detaches from DNA
 Transcription Animation
mRNA processing
 Enzymes in the eukaryote modify pre-mRNA before the
messages are sent to the cytoplasm
 Each end of a pre-mRNA molecule is modified in a particular
way:
 The 5 end receives a modified
This tells ribosome where to begin translating
 The 3 end gets a
Chain of 150-200 adenines
Helps to get transcript out of nucleus
Protects mRNA from degradation by enzymes
mRNA processing
 Next the transcript must be edited by
 Most eukaryotic genes and their RNA transcripts have long
noncoding regions
of nucleotides that lie
between coding, or expressed regions
 The introns must be cut out to make a mature mRNA transcript
 In some cases, RNA splicing is carried out by
 A variety of proteins and several small nuclear
ribonucleoproteins (snRNPs) that recognize the splice sites
RNA as an enzyme
are RNA
molecules that function as
enzymes (like splicesome)
 3 properties of RNA enable it to
function as an enzyme
It can form a 3-D structure
because of its ability to base
pair with itself
Some bases contain functional
groups
It may hydrogen-bond with
other nucleic acid molecules

Alternative RNA Splicing
 Some genes can encode
more than one kind of
polypeptide, depending on
which segments are treated
as exons during RNA
splicing
 Such variations are called
 Because of alternative
splicing, the number of
different proteins an
organism can produce is
much greater than its
number of genes
Components of Translation
There are 3 different forms of RNA, each with a
specific purpose during translation

is a copy of the
DNA recipe, created in transcription

together with
proteins forms ribosomes where proteins are
made

brings an amino
acid to the ribosome to help create the
polypeptide
tRNA
 A tRNA molecule consists of a
single RNA strand that is about
Bases hydrogen bond to each
other
Cloverleaf shape
 3’ end picks up the amino acid and
carries it to ribosome
 Amino acid is determined by 3
bases opposite the amino acid
called an
Anticodons pair with
complementary codons in
mRNA at a ribosome
tRNA
 Molecules of tRNA are unique since
each carries a specific amino acid
 Accurate translation requires two
steps:
 1. A correct match between a
tRNA and an amino acid, done
by the enzyme
 2. A correct match between the
tRNA anticodon and an mRNA
codon
 Flexible pairing at the third base of
a codon is called
and allows some tRNAs to bind to
more than one codon
rRNA
 Ribosomes consists of two subunits (large and small)
made of RNA & protein
 Two functions
Pair complementary codons with anticodons
Bond amino acids together to form the polypeptide
 A ribosome has three binding sites for tRNA:
The
holds the tRNA that
carries the next amino acid to be added to the chain
The
holds the tRNA that
carries the growing polypeptide chain
The
is where discharged
tRNAs leave the ribosome
rRNA
 Ribosome moves along
mRNA until it reaches
termination sequence
 A number of ribosomes
can translate a single
mRNA simultaneously,
forming a
 Polyribosomes enable a
cell to make many copies
of a polypeptide very
quickly
Steps of translation
There are 3 stages of translation:
Chain initiation
Chain elongation
Chain termination
All three stages require protein “factors”
that aid in the translation process
Chain Initiation
 First, a small ribosomal subunit binds with mRNA and a
special
 Then the small subunit moves along the mRNA until it
reaches the start codon (AUG) in the P site
 Anticodon of the initiator tRNA will complementary
base pair with first codon, which always codes for
methionine
 Proteins called initiation factors bring in the large
subunit that completes the
Chain Elongation
During the elongation stage, amino acids are
added one by one to the preceding amino acid
Each addition involves proteins called elongation
factors and occurs in 3 steps:

(complementary
tRNA comes into the A site)

(amino acid from
the tRNA at the P site is attached to the amino
acid on the tRNA at the A site)

(mRNA moves
the tRNA at the A site to the P site so the next
tRNA can go into the P site)
Chain Termination
Occurs when a stop codon in the mRNA
reaches the A site of the ribosome
The A site accepts a protein called a
and the parts are dissociated

is released

is released
Ribosomal subunits disjoin
Translation Animations
Animation 1

http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.s
wf
Animation 2

http://carbon.cudenver.edu/~bstith/transla.MOV
Practice

http://gslc.genetics.utah.edu/units/basics/transcribe/
Polypeptides vs Proteins
 Often translation is not sufficient to make a functional
protein
 Polypeptide chains are
after translation
 During and after synthesis, a polypeptide chain
spontaneously coils and folds into its
Remember: A protein’s shape determines its
 Some polypeptides are activated by
that cleave them
 Other polypeptides come together to form the subunits
of a protein
Review Questions
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Explain the central dogma of biology.
Define gene expression, including its 2 main parts.
Explain how Beadle & Tatum’s experiment related genes to proteins.
Differentiate between transcription and translation.
Explain how gene expression differs in prokaryotes and eukaryotes.
Define codons and their importance to gene expression.
Name and describe the 3 stages of transcription.
Define the roles of the promoter sequence, RNA polymerase, transcription
factors, and the terminator sequence in transcription.
Describe the 3 main events that occur in mRNA processing.
Differentiate between introns and exons.
Define ribozymes.
Describe alternative RNA splicing.
Differentiate between mRNA, rRNA, and tRNA.
Explain the importance of an anticodon to gene expression.
Name and describe the importance of the 3 sites of the ribosome.
Name and describe the 3 steps of translation.
Name 2 main events that occur during chain initiation of translation.
Explain the 3 steps of translational elongation.
Name 3 events that happen during chain termination of translation.
Differentiate between polypeptides and proteins.