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Biology
A Guide to the Natural World
Chapter 14 • Lecture Outline
How Proteins are Made: Genetic Transcription,
Translation, and Regulation
Fifth Edition
David Krogh
© 2011 Pearson Education, Inc.
14.1 The Structure of Proteins
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The Structure of Proteins
• Proteins are composed of building blocks
called amino acids.
• A string of amino acids is called a
polypeptide chain.
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The Structure of Proteins
• Once a polypeptide chain has folded into its
working three-dimensional shape, it is a
protein.
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glycine (gly)
isoleucine (ile)
(a) Amino acids
The building blocks of proteins
are amino acids such as glycine
and isoleucine, which differ only
in their side-chain composition
(light colored squares).
(b) Polypeptide chain
These amino acids are strung
together to form polypeptide
chains. Pictured is one of the two
polypeptide chains that make up
the unusually small protein insulin.
(c) Protein
Polypeptide chains function as
proteins only when folded into
their proper three-dimensional
shape, as shown here for insulin.
Note the position of the glycine
and isoleucine amino acids in one
of the insulin polypeptide chains
(colored light green).
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Figure 14.1
The Structure of Proteins
• Although there are hundreds of thousands
of different proteins, all of them are put
together from a starting set of 20 amino
acids.
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The Structure of Proteins
• It is the order in which the amino acids are
linked in a polypeptide chain that
determines which protein will be produced.
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The Structure of Proteins
• Proteins often are composed of two or more
linked polypeptide chains.
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The Structure of Proteins
Animation 14.1: Structure of Proteins
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14.2 Protein Synthesis in Overview
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Stages of Protein Synthesis
• There are two principal stages in protein
synthesis:
• Transcription
• Translation
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Stages of Protein Synthesis
• The first stage is transcription, in which
the information encoded in DNA is copied
onto a length of messenger RNA (mRNA).
• In eukaryotes, mRNA moves from the cell
nucleus to a structure in the cytoplasm
called a ribosome.
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Stages of Protein Synthesis
• The second stage is translation, in which
amino acids brought to a ribosome by
transfer RNA (tRNA) molecules are linked
together within the ribosome in the order
specified by the mRNA sequence.
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Transcription
1. In transcription, a section
of DNA unwinds and
nucleotides on it form base
pairs with nucleotides of
messenger RNA, creating
an mRNA chain.
DNA
Transcription
mRNA
2. This segment of mRNA then
leaves the cell nucleus,
headed for a ribosome in
nucleus
the cell’s cytoplasm, where
translation takes place.
Translation
3. Joining the mRNA chain
at the ribosome are amino
acids, brought there by
transfer RNA molecules.
The length of messenger
RNA is then “read” within
the ribosome. The result?
A chain of amino acids is
linked together in the order
specified by the mRNA
sequence.
cytosol
amino
acids
tRNA
mRNA
ribosome
protein
Translation
4. When the chain is finished
and folded up, a protein has
come into existence.
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Figure 14.2
14.3 A Closer Look at Transcription
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Transcription
• The information in DNA is transferred to
messenger RNA through complementary
base pairing.
• Each “C” nucleotide in a segment of DNA
being transcribed results in a “G” nucleotide
being added to a segment of RNA, and so
forth.
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(a) Comparison of RNA and DNA nucleotides
DNA nucleotide
RNA nucleotide
base
thymine
base
uracil
phosphate
group
sugar
ribose
phosphate
group
sugar
deoxyribose
(b) Comparison of RNA and DNA three-dimensional structure
RNA
strand
sugar-phosphate
handrails
DNA
strand
bases:
cytosine (C)
guanine (G)
adenine (A)
uracil (U)
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sugar-phosphate
handrails
bases:
cytosine (C)
guanine (G)
adenine (A)
thymine (T)
Figure 14.3
Transcription
• The enzyme RNA polymerase unwinds the
DNA sequence to be transcribed and then
strings together the chain of RNA
nucleotides that is complementary to it.
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Figure 14.4
1. RNA polymerase unwinds a region of
the DNA double helix.
RNA nucleotides
RNA
2. RNA polymerase begins assembling
RNA nucleotides on the DNA
template.
DNA
RNA
3. The completed portion of the RNA
transcript separates from the DNA.
Meanwhile, RNA polymerase unwinds
more of the untranscribed region of
the DNA.
RNA
4. The RNA transcript is released from the
DNA, and the DNA is rewound into its
original form. Transcription is completed.
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Transcription
• In all eukaryotes (including humans), the
initial RNA chain transcribed from a DNA
sequence is not the finished messenger
RNA chain.
• Instead it is a sequence, called a primary
transcript, that must undergo some editing
before becoming an mRNA chain.
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Transcription
• Each three coding bases of DNA pair with
three RNA bases, but each group of three
mRNA bases then codes for a single amino
acid.
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Transcription
• Each triplet of mRNA bases that codes for
an amino acid is called a codon.
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Transcription
• The inventory of linkages between base
triplets and the amino acids they code for is
called the genetic code.
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14.4 A Closer Look at Translation
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Transfer RNA
• Transfer RNA serves as a bridging
molecule in protein synthesis thanks to its
ability to bind with both amino acids on the
one hand and nucleic acids on the other (in
the form of mRNA).
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Transfer RNA
• A given tRNA molecule binds with a
specific amino acid in the cell’s cytoplasm,
and then transfers that amino acid to a
ribosome in which an mRNA transcript is
being “read.”
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Transfer RNA
1. tRNA and amino acids
float freely in cytoplasm.
2. tRNA links to an amino
acid and transfers it to
the ribosome.
4. A polypeptide
chain is
produced.
3. tRNA links to the
appropriate mRNA
codon at the ribosome.
mRNA
ribosome
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Figure 14.6
Translation
• There, another portion of the tRNA
molecule, called an anticodon, binds with
the appropriate codon in the mRNA chain.
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arg
amino acid attachment site
tRNA
molecule
mRNA attachment site
G C U
anticodon
C G A
codon
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mRNA
Figure 14.7
Ribosomes
• Ribosomes, the complex “workbenches” of
protein synthesis, are composed of proteins
and ribosomal RNA (rRNA).
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Ribosomes
• Each ribosome exists as two subunits in the
cytoplasm that come together only with the
initiation of protein translation.
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Ribosomes
• Each ribosome bears A, P, and E binding
sites to which tRNA molecules bind. Thus,
it facilitates the synthesis of an amino acid
chain.
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(a) Large and small ribosomal units
protein
large
subunit
mRNA
Ribosomes are composed
of two subunits that come
together during translation
E PA
small
subunit
(b) Binding sites in the ribosome
protein
large
subunit
mRNA
E P A
site site site
small
subunit
A simplified cross section of the
ribosome illustrates the E, P, and
A sites where tRNA molecules bind
during translation
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Figure 14.8
Protein Synthesis
Suggested Media Enhancement:
Meiosis
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Translation
• Translation works by means of a succession
of tRNA molecules arriving at a ribosome,
bound to their appropriate amino acids, and
then binding to their appropriate codon in
the mRNA transcript.
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Translation
• As this process takes place, the succession
of amino acids is linked together into a
polypeptide chain.
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Translation
The steps of translation
met
1. A messenger RNA transcript binds to the
small subunit of a ribosome as the first
transfer RNA is arriving. The mRNA codon
AUG is the “start” sequence for most
polypeptide chains. The tRNA, with its
methionine (met) amino acid attached,
then binds this AUG codon.
AUG
mRNA
start codon
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Figure 14.9
Translation continues
2. The large ribosomal subunit
joins the ribosome, as a second
tRNA arrives, bearing a leucine
(leu) amino acid. The second
tRNA binds to the mRNA
chain, within the ribosome’s
A site.
met leu
met
CUG
E P A
site site site
3. A bond is formed between the
newly arrived leu amino acid and
the met amino acid, thus forming
a polypeptide chain. The
ribosome now effectively shifts
one codon to the right,
relocating the original P site
tRNA to the E site, the A site
tRNA to the P site, and moving
a new mRNA codon into the A site.
4. The E site tRNA leaves the ribosome,
even as a new tRNA binds with the A
site mRNA codon, and the process
of elongation continues.
E P A
site site site
met leu
met leu
E P A
site site site
E P A
site site site
polypeptide
chain
met leu
E P A
site site site
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Figure 14.10
14.5 Genetic Regulation
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Genetic Regulation
• Protein production is carefully controlled or
“regulated” in living things.
• Most genes do not simply stay “on,” but
instead are transcribed in accordance with
the needs of an organism.
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Genetic Regulation
• Less than 2 percent of the DNA in the
human genome codes for proteins.
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Genetic Regulation
• Some noncoding segments of DNA may be
“junk” that never had a function.
• Other segments seem to have served an
enabling function; they have enabled
organisms to become more complex.
• Still other segments are regulatory, meaning
they help regulate the production of
proteins.
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Promoters and Enhancers
• All gene transcription requires that RNA
polymerase be properly aligned at a
noncoding sequence of DNA bases, called a
promoter, that lies just “upstream” from a
gene sequence.
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Promoters and Enhancers
• There is usually a second noncoding
segment of DNA, called an enhancer, that
lies at some distance from the promoter
sequence.
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Promoters and Enhancers
• Separate groups of proteins, called
transcription factors, bind to both the
promoter and enhancer sequences. Thus,
they facilitate the alignment of RNA
polymerase at the promoter.
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Genetic Regulation
(a) Chicken
enhancer
proteins
7 thoracic
vertebrae
low transcription rate
Hoxc8 gene
DNA
RNA
polymerase
transcription complex
(b) Mouse
enhancer
proteins
Better alignment of transcription
complex by enhancer proteins…
13 thoracic
vertebrae
high transcription rate
Hoxc8 gene
DNA
RNA
polymerase
…results in a higher transcription rate
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Figure 14.12
Genetic Regulation
• Transcription factors are themselves
produced through normal transcription and
translation.
• They are coded for by DNA, but then feed
back on it, helping control its transcription.
• Thus, the entire system is self-regulating.
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Alternative Splicing
• In all eukaryotes, the initial RNA chain
produced during transcription—the primary
transcript—undergoes editing by means of
some sequences being cut out of it.
• Then, the remaining sequences are spliced
back together.
• The result is a completed messenger RNA
chain.
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Alternative Splicing
• The sequences that are removed from the
primary transcript are called introns, while
the sequences that are retained are called
exons.
• Introns do not code for protein, but most
exons do.
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Alternative Splicing
intron
exon 2
enzyme
intron
primary
transcript
enzymes cut out the
introns
messenger RNA
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Figure 14.15
Alternative Splicing
• Some relatively simple organisms have
nearly as many genes as human beings do
(20,000–25,000).
• Human beings are able to be much more
complex than these organisms, thanks in
part to a form of genetic regulation called
alternative splicing, in which a primary
transcript can be edited in different ways.
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Alternative Splicing
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Figure 14.14
Alternative Splicing
• Through alternative splicing, a single
primary transcript can result in different
messenger RNA chains.
• These in turn can result in different
proteins.
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Alternative Splicing
protein A
primary transcript
edited mRNA transcripts
protein B
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Figure 14.16
RNA in Genetic Regulation
• DNA codes for several different forms of
RNA.
• Of these, only messenger RNA then goes on
to code for proteins.
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RNA in Genetic Regulation
• DNA also codes for several varieties of
regulatory RNA that go under the collective
name of micro-RNAs.
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RNA in Genetic Regulation
• Of the 1,500 micro-RNA sequences
discovered to date, all have the effect of
reducing the production of particular
proteins, usually by targeting their
messenger RNAs for destruction.
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The Importance of Regulation
• A case can be made that it is not genes, but
instead the regulation of genes, that is the
most important factor in bringing about the
differences among organisms.
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The Importance of Regulation
• In a similar vein, there seems to be little
relation between the number of genes an
organism has and the complexity of that
organism.
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The Importance of DNA That Doesn’t
Code for Protein
• There does seem to be correlation between
the complexity of an organism and the
proportion of its DNA that does not code
for protein.
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100
. . . but more than 98 percent
of the human genome
Non-coding DNA
makes up only
about 10 percent
of the prokaryote
genome . . .
75
50
25
0
bacterium
baker’s
yeast
mustard
plant
roundworm
fruit fly
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mouse
Percent of DNA not coding for protein
What Is Central in Genetics?
human
Figure 14.17
What Is Central in Genetics?
• Genetic regulation and noncoding DNA
may be more important to genetics than has
traditionally been assumed.
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What Is Central in Genetics?
• We have no definitive answers about these
questions, however, because research on
genetic regulation and noncoding DNA lies
at the cutting edge of contemporary genetic
research.
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14.6 Genetics and Life
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Genetics and Life
• Life is made possible by the fantastic ability
of genetic systems to store, use, and pass on
information.
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