From Gene to Protein
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Molecular Components of Translation
• A cell translates an mRNA message into
protein with the help of transfer RNA (tRNA)
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-13
Amino
Polypeptide acids
tRNA with
amino acid
attached
Ribosom
e
p
Tr Phe Gly
tRNA
Anticodon
5′ Codons 3′
mRNA
The Structure and Function of Transfer RNA
• A tRNA molecule consists of a single
A
RNA
strand that is only about 80 nucleotides
C
C
long
• Flattened into one plane to reveal its base
pairing, a tRNA molecule looks like a
cloverleaf
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Building a Polypeptide
• The three stages of translation:
– Initiation
– Elongation
– Termination
• All three stages require protein “factors” that
aid in the translation process
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
1. a small ribosomal subunit binds with mRNA
and a special initiator tRNA
2. Then the small subunit moves along the mRNA
until it reaches the start codon (AUG)
3. Proteins called initiation factors bring in the
large subunit that completes the translation
initiation complex
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-17
Large
ribosomal
3′ U A C 5′ P site subunit
Met 5′ A U G 3′ Met
Initiato
r GT GD
tRNA P P E A
mRNA
5′ 5′
3′ 3′
Start codon
Small
mRNA binding site ribosomal Translation initiation complex
subunit
Elongation of the Polypeptide Chain
• 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 three steps:
1. codon recognition,
2. peptide bond formation, and
3. translocation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-18-1
Amino end
of polypeptide
E 3′
mRNA
P A
5′ sit sit
e e
Fig. 17-18-2
Amino end
of polypeptide
E 3′
mRNA
P A
5′ sit sit
e e GTP
GDP
P A
Fig. 17-18-3
Amino end
of polypeptide
E 3′
mRNA
P A
5′ sit sit
e e GTP
GDP
P A
P A
Fig. 17-18-4
Amino end
of polypeptide
E 3′
mRNA
P A
Ribosome ready for 5′ sit sit
next aminoacyl tRNA e e GTP
GDP
E E
P A P A
GDP
GTP
P A
Termination of Translation sar r
• 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-19-1
Release
factor
3′
5′
Stop codon
(UAG, UAA, or UGA)
Fig. 17-19-2
Release Free
factor polypeptid
e
3′ 3′
5′ 5′ 2 GTP
Stop codon 2
(UAG, UAA, or UGA) GDP
Fig. 17-19-3
Release Free
factor polypeptid
e
5′
3′ 3′
3′
5′ 5′ 2 GTP
Stop codon 2
(UAG, UAA, or UGA) GDP
Completing and Targeting the Functional Protein
• Often translation is not sufficient to make a
functional protein
• Polypeptide chains are modified after
translation
• Completed proteins are targeted to specific
sites in the cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Variation in protein structure: Basis of Biological
diversity
• chemical structures of the different amino acids
• bonding of amino acids
• four levels of protein structure
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• primary structure – sequence of a.a. in the linar *linear
backbone of the polypeptide; specified by sequence of
nucleotides via mRNA; helps determine higher order of
protein formation
• secondary structure – certain regular or
repeating configurations in space assumed by
a.a. lying close to each other in the polypeptide
chain; alpha helix and beta pleated sheet;
describes a.a. within areas in the polypeptide
chain
• tertiary structure – defines 3-dimensional
conformation of the entire chain with twists, turns
and loops;
Tertiary structure of respiratory
pigment, myoglobin. Bound
oxygen atom is shown in red.
Con’t. – tertiary structure
- stabilizes molecule
a. covalent disulfide bonds between cystine
residues forming a.a. cysteine
b. polar hydrophyllic R groups are located on
surface where they can interact with water,
c. nonpolar hydrophobic R groups located
inside, avoiding water)
- product of primary structure
• quaternary structure – most intricate degree of organization
still considered a single molecule
• a protein must have two or more peptide chains forming
subunits. The subunits can be different or identical, and in most
cases they are arranged symmetrically.
Quaternary level of protein
structure as seen in hemoglobin.
Four chains (two alpha and two
beta) interact with four heme
groups to form functional
molecule.
While gene expression differs among the domains of
life, the concept of a gene is universal
• Archaea are prokaryotes, but share many
features of gene expression with eukaryotes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Comparing Gene Expression in Bacteria, Archaea,
and Eukarya
• Bacteria and eukarya differ in their RNA
polymerases, termination of transcription and
ribosomes; archaea tend to resemble eukarya
in these respects
• Bacteria can simultaneously transcribe and
translate the same gene since they have no nucleus
• In eukarya, transcription and translation are
separated by the nuclear envelope
• In archaea, transcription and translation are
likely coupled
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-24
RNA polymerase
DNA
mRNA
Polyribosome
Direction of 0.25
RNA transcription µm
polymeras
e DNA
Polyribosome
Polypeptide
(amino end)
Ribosom
e
mRNA (5′ end)
What Is a Gene? Revisiting the Question
• The idea of the gene itself is a unifying concept
of life
• We have considered a gene as:
– A discrete unit of inheritance
– A region of specific nucleotide sequence in a
chromosome
– A DNA sequence that codes for a specific
polypeptide chain
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-25
DNA
TRANSCRIPTION
3′ -A
ly
Po
RNA
5′ RNA polymeras
transcript
e
RNA PROCESSING
Exon
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
y-A
Pol synthetase
NUCLEUS
Amin
o AMINO ACID ACTIVATION
CYTOPLASM acid
tRNA
mRNA Growing
polypeptide
p 3′
Ca
A -A
Activated ly
P amino acid Po
Ribosomal
E
subunits
Cap
5′
TRANSLATION
E A
Anticodon
Codon
Ribosome
end
