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Chapter Introduction
The Genetic Code: Using Information
9.1 Genetic Material
9.2 Importance of Proteins
Transcription
9.3 RNA Synthesis
9.4 RNA Processing
Protein Synthesis
9.5 Translation
9.6 Transport and Modification of Proteins
9.7 Translation Errors
Viruses
9.8 Genetic Information and Viruses
9.9 Impact of Viruses
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Explain the connection between DNA and RNA in
protein synthesis; describe the genetic code and
its role in protein synthesis.
B Explain why proteins are important to biological
systems.
C Identify the stages of transcription and explain
what occurs during each stage.
D Summarize the events that occur in RNA
processing.
Learning Outcomes
By the end of this chapter you will be able to:
E Identify the stages of translation and explain what
occurs during each stage.
F Describe posttranslational modification and
transport of proteins.
G Infer the consequences of RNA translation errors.
H Explain the relationship between viruses and
host cells and describe the impact of viruses
on living systems.
Expressing Genetic Information
 How does an organism use
the information stored in its
genetic material?
 Does a cell express all of
its genetic information all
the time?
A colored scanning electron micrograph of
a group of human chromosomes (x6,875)
Expressing Genetic Information
• Living organisms store
information in their genetic
material.
• In a process called gene
expression, organisms read
and use the encoded
information by directing the
synthesis of proteins.
• When a virus infects a cell,
the virus takes control of
gene expression in the cell.
A colored scanning electron micrograph of
a group of human chromosomes (x6,875)
The Genetic Code: Using Information
9.1 Genetic Material
• Genetic material consists of two nucleic acids—
DNA and RNA—that are involved in gene
expression.
• Gene expression depends on two features of
their molecular structure:
1. nucleic acids consist of a long strand of
repeating subunits that act as letters in a code
2. the subunit bases of one strand pair with the
bases of another strand
The Genetic Code: Using Information
9.1 Genetic Material (cont.)
• Living cells store genetic information in DNA which
specifies the primary structures of proteins.
• By determining the primary structure of each protein,
DNA indirectly dictates protein function.
• Proteins, in turn, carry out important cell activities.
• When a gene becomes active, an enzyme makes
a temporary RNA copy of the information the
DNA contains.
The Genetic Code: Using Information
9.1 Genetic Material (cont.)
• Messenger RNA (mRNA) is the temporary copy
of a gene that encodes a protein.
• The process of making an mRNA molecule is called
transcription.
• In translation, the mRNA molecule provides the
pattern that determines the order in which amino
acids are added to the protein being made.
• Protein synthesis takes place on ribosomes which
are made of proteins and ribosomal RNA (rRNA).
• Each amino acid that will be used in making the
protein is attached to transfer RNA (tRNA).
Information stored in DNA is
copied to mRNA, which in
turn directs the synthesis of
a particular protein.
The Genetic Code: Using Information
9.1 Genetic Material (cont.)
• The genetic code describes how a sequence of
bases in DNA or RNA translates into the sequence of
amino acids in a protein.
• The nucleotides serve as the four “letters” of the
DNA “alphabet.”
• A genetic code requires at least 20 different code
words—one for each amino acid.
The Genetic Code: Using Information
9.1 Genetic Material (cont.)
• Three nucleotides are grouped at a time allowing 64
triplet combinations, such as CTG, TAC, and ACA.
• Each nucleotide triplet in DNA directs a particular
triplet to be formed in mRNA during transcription.
• In translation, a second base-pairing step is essential
for reading the genetic code.
The Genetic Code: Using Information
9.1 Genetic Material (cont.)
• A triplet in mRNA, called a
codon, pairs with a triplet on
a tRNA molecule, called an
anticodon, carrying the
correct amino acid.
A molecule of transfer RNA (tRNA) with a specific
amino acid attached reads each codon of a messenger
RNA (mRNA) during protein synthesis (translation).
The genetic code is
written in nucleotide
triplets, or codons, in a
strand of mRNA. Each
triplet codon specifies an
amino acid. For example,
UGG codes for the amino
acid tryptophan. Several
amino acids have more
than one codon. Some
triplets are “punctuation”
telling the system to start
or stop translation.
The Genetic Code: Using Information
9.2 Importance of Proteins
• Many proteins, such as
keratin, collagen, and
myosin, serve as the
material that makes up cell
structures or tissues.
The feathers responsible for the appearance of this
Raggiana bird of paradise, Poradisaea raggiana,
are composed mostly of the protein keratin.
The Genetic Code: Using Information
9.2 Importance of Proteins (cont.)
• Some proteins are enzymes, essential catalysts
that make the chemical reactions of living systems
happen fast enough to be useful.
• Proteins, such as hemoglobin, bind to specific
molecules.
The Genetic Code: Using Information
9.2 Importance of Proteins (cont.)
• Protein hormones, such as insulin, play a key role in
communication within an organism.
• Hormones are chemical signals given off by cells in
one part of an organism that regulate behavior of
cells in another part of the organism.
The Genetic Code: Using Information
9.2 Importance of Proteins (cont.)
• A protein’s structure determines its function, and
information expressed from the code in DNA
determines the structures of proteins.
• Collagen exists as long
fibers that bind cells together
in tissues.
• Many enzymes, such as
lysozyme, have cavities or
pockets that bind only
specific substrate molecules.
A scanning electron
micrograph of human
pancreatic connective
tissue (collagen), x39,000.
Transcription
9.3 RNA Synthesis
• Gene expression begins with RNA synthesis—
when the transcription enzyme RNA polymerase
joins RNA nucleotides according to the base
sequence in DNA.
• Prokaryotes have one type of RNA polymerase.
• Eukaryotes have three RNA polymerases, each
responsible for making different types of RNA.
Transcription
9.3 RNA Synthesis (cont.)
• In eukaryotes, protein synthesis takes place outside
the nucleus; however, mRNA, tRNA, and rRNA are
built in the nucleus.
• During protein synthesis, two ribosomal subunits
bind to each other and an mRNA to form an intact
ribosome.
Each type of RNA
carries out a different
function in protein
synthesis. This figure
uses a linear symbol
for mRNA to
emphasize that its
sequence
corresponds to the
linear sequence of
amino acids in a
protein. In reality, the
mRNA is folded and
twisted rather than
being straight or rigid.
Transcription
9.3 RNA Synthesis (cont.)
• Only one strand of the DNA, the coding or template
strand, directs the synthesis of RNA.
Each DNA nucleotide pairs with a particular RNA nucleotide. This pairing
is the basis of the genetic code. Note that in RNA, uracil (U) replaces the
thymine (T) of DNA.
Transcription
9.3 RNA Synthesis (cont.)
• Transcription takes place in three stages:
1. Initiation—the enzyme RNA polymerase attaches
to a specific region of the DNA
2. Elongation of the RNA—RNA polymerase
partially unwinds the DNA, exposing the coding
strand of the gene
3. Termination—RNA polymerase reaches the
terminator region, or the end of the DNA to be
transcribed and the enzyme and primary
transcript are released from the DNA
The three stages in transcription of RNA from a DNA template
Transcription
9.4 RNA Processing
• In prokaryotes, new mRNA is translated and broken
down by enzymes within a few minutes.
• In eukaryotes, mRNA can last from minutes to days,
depending partly on how the primary transcript is
processed.
A transmission electron micrograph
of an unidentified operon of the
bacterium Escherichia coli, x72,600.
Ribosomes attach to mRNA, and
protein synthesis begins even before
transcription is complete.
Transcription
9.4 RNA Processing (cont.)
• All three types of RNA are processed in the nucleus
of eukaryotes before they leave the nucleus.
• Enzymes add additional nucleotides and chemically
modify or remove others.
Transcription
9.4 RNA Processing (cont.)
• Enzymes attach a cap
of chemically modified
guanine nucleotides
(methyl-guanine, or
mG) to the starting end
of the mRNA molecule.
Transcription
9.4 RNA Processing (cont.)
• Other enzymes then
replace part of the
opposite end with a
tail of 100–200
adenine nucleotides
called a poly-A tail.
Transcription
9.4 RNA Processing (cont.)
• The final step in mRNA
processing involves
removal of some internal
segments of the RNA
that do not code for
protein called introns.
• The parts of the
transcript that remain
(and code for protein)
are called exons.
Transcription
9.4 RNA Processing (cont.)
• The process of removing
introns and rejoining cut
ends is called splicing.
• If introns are left in RNA,
the consequences can
be serious.
Transcription
9.4 RNA Processing (cont.)
• An important step in the processing of tRNA is the
chemical modification of several nucleotides and
folding into a cloverleaf shape.
Mature tRNA resembles a
cloverleaf (a), with the aminoacid binding site at the end of
a stem and the anticodon at
the loop on the opposite end.
Base pairing between parallel
parts of the tRNA molecule
stabilizes the cloverleaf shape.
The three-dimensional
structure of the molecule is
roughly L-shaped (b).
Transcription
9.4 RNA Processing (cont.)
• Ribosomal RNA is not involved in coding.
• The primary rRNA transcript is spliced and modified
to produce mature rRNA molecules.
Protein Synthesis
9.5 Translation
• On ribosomes, protein synthesis translates the
codon sequence of mRNA into the amino-acid
sequence of a protein.
• tRNA anticodons pair with the mRNA codons that
encodes a particular amino acid.
• Attachment of the correct amino acid to its tRNA
molecule is called tRNA charging.
• A molecule of ATP provides the energy to form
this bond.
Protein Synthesis
9.5 Translation (cont.)
• Charged tRNA, mRNA, and the growing polypeptide
chain come together at specific binding sites on a
ribosome.
• At these sites, tRNA anticodons base-pair with
mRNA codons, positioning the amino acids they
carry so that they can bond to the growing
polypeptide chain.
Protein Synthesis
9.5 Translation (cont.)
• One of the binding sites, the P site, holds the tRNA
carrying the growing polypeptide chain.
• The A site holds the tRNA carrying the next amino
acid to be added to the chain.
• Next to the P site is the exit site, or E site.
• An uncharged tRNA leaves the E site after its amino
acid is added to the growing chain.
Protein Synthesis
9.5 Translation (cont.)
A charged tRNA sits in the A site of the
ribosome, bound to the correct mRNA
codon by base pairing. A second tRNA,
carrying a growing polypeptide, is in the
P site, bound to the previous mRNA
codon. The E site is not shown.
A groove between the large and small
subunits of the ribosome accommodates
mRNA and the growing polypeptide
chain.
Protein Synthesis
9.5 Translation (cont.)
• Translation involves initiation, elongation, and
termination, the same three stages as transcription.
• Initiation and elongation require energy supplied by
GTP (guanosine triphosphate), a molecule closely
related to ATP.
Protein Synthesis
9.5 Translation (cont.)
• During initiation of translation, the ribosome attaches
at a specific site on the mRNA.
Protein Synthesis
9.5 Translation (cont.)
• During elongation,
peptide bonds join
each amino acid
with the next in the
sequence.
• A charged tRNA
whose anticodon
matches the next
codon on the
message enters
the A site of the
ribosome.
Protein Synthesis
9.5 Translation (cont.)
• This positions the
amino acid it
carries to form a
peptide bond with
the amino acid
attached to the
tRNA at the P site.
Protein Synthesis
9.5 Translation (cont.)
• When the bond
forms, the
polypeptide chain
transfers to the
tRNA at the A site
Protein Synthesis
9.5 Translation (cont.)
• The entire ribosome
moves down the
mRNA to position
the next codon at
the A site and the
uncharged tRNA
leaves the E site.
• The A site is now open
and available for the
next matching tRNA to
bring in an amino acid.
Protein Synthesis
9.5 Translation (cont.)
• Translation terminates when a stop codon reaches
the A site of the ribosome.
• A special protein known as a release factor binds
to the stop codon in the A site.
• At this point, the ribosome lets go of the mRNA, the
tRNA, and the release factor.
Protein Synthesis
9.5 Translation (cont.)
Transcription produces
mRNA, tRNA, and rRNA.
All three participate in
translation.
Protein Synthesis
9.6 Transport and Modification of Proteins
• Many proteins must be chemically modified and
folded into an active tertiary structure to be
functional.
• Helper, or “chaperone,” proteins often help stabilize
the polypeptide as it is folded.
• After translation, the protein must be transported to
where it will function.
Protein Synthesis
9.6 Transport and Modification of Proteins (cont.)
• Transport can start while the protein is still
being translated.
• The process uses a signal that is part of the protein
sequence, called the signal sequence.
• When translation is complete, the new protein is
released from the ribosome into the inner ER.
• Proteins to be released from the cell pass from the
ER to the vesicles of the Golgi apparatus.
Synthesis of proteins for secretion or insertion in a membrane
Protein Synthesis
9.7 Translation Errors
• Errors sometimes occur during translation although
most are caught and corrected.
• The most common translation error results from
misreading the nucleotide sequence.
• A frame shift occurs when the start of translation is
shifted by one or two nucleotides in either direction.
• The frame changes causing a different sequence of
codons and amino acids will result.
Protein Synthesis
9.7 Translation Errors (cont.)
Each time the reading frame shifts, a different amino-acid
sequence results.
Protein Synthesis
9.7 Translation Errors (cont.)
• Some errors are due to splicing mistakes or changes
in the DNA.
• Insufficient amounts of a particular amino acid also
can disrupt translation.
• In some cases, translational frame shifts or alternate
initiation sites appear to be normal ways in which
one mRNA can specify more than one polypeptide.
Viruses
9.8 Genetic Information and Viruses
• Viruses are tiny particles that have no cells, yet
they replicate and evolve.
• Discovered in 1892 by Russian botanist Dmitri
Ivanovsky, viruses depend on the gene-expression
machinery of the host cells they infect.
Viruses
9.8 Genetic Information and Viruses (cont.)
• Most viruses consist of little more than a small
amount of genetic material and a protective
protein coat.
• Some, such as the familiar viruses that cause colds
and the T2 bacteriophage that infects bacterial cells,
contain DNA.
• Other viruses, such as the influenza virus,
contain RNA.
Viruses
9.8 Genetic Information and Viruses (cont.)
Bacteriophage T2, which infects
bacterial cells, contains DNA
surrounded by a protein coat. The
elongated structure attaches to
bacterial cells and injects DNA.
HIV (human immunodeficiency virus),
which infects human cells, is surrounded
by a protein and lipid membrane envelope.
The genetic material is RNA. HIV also
carries two molecules of the enzyme
reverse transcriptase, ready to copy the
RNA after entry into a host cell.
Viruses
9.8 Genetic Information and Viruses (cont.)
• The method of replication varies among types of
viruses, but the general principle of copying stored
genetic information is the same as for cells.
• Viral replication falls into two patterns:
– In lytic infections, the host cell’s enzymes
replicate the viral DNA.
– In lysogenic infections, the viral DNA (or a DNA
copy of the viral RNA) inserts into the cellular
DNA which is then copied when the cell replicates.
Lytic and lysogenic viral reproduction
Viruses
9.9 Impact of Viruses
• Viruses live at the expense of the host organism and
pose a serious threat to cellular life.
• Antibiotics are useless against viruses.
• Modern technologies such as air travel have, in
some cases, made the threat of viral diseases
much greater.
The Ebola virus (x26,400). This deadly virus
occurs in isolated parts of East Africa, but air
travel and human migration may cause it to
spread to new regions of the world.
Viruses
9.9 Impact of Viruses (cont.)
• Mechanical harvesting and international shipment of
agricultural products can spread viruses that infect
valuable crops and animals.
• Disabled viruses are exploited by advanced
technologies, such as for delivering DNA in
cloning experiments.
Summary
• Genetic information serves as a master program to direct
cell activities.
• Much of the genetic information encodes the primary structure
for proteins.
• Proteins carry out numerous functions, including structural
roles, cell signaling as hormones or cell-surface receptors,
regulators of gene activity, and many catalytic functions.
• Genetic information is stored in DNA or, in the case of some
viruses, as RNA.
• As the information is needed, it is expressed through
transcription and translation.
• Regulation of gene expression is essential for different cells
to carry out their particular activities.
• In transcription, the coding strand of DNA is read as a template
by RNA polymerases to build matching RNA molecules.
Summary (cont.)
• Primary RNA transcripts are processed into tRNA, rRNA, or
mRNA in the nucleus.
• Proteins combine with rRNA to form ribosomes.
• Amino acids are carried by their matching tRNAs to the
ribosomes.
• Protein synthesis occurs as the sequence of codons in mRNA
is translated into the sequence of amino acids in a protein.
• Newly transcribed proteins must fold into the appropriate
three-dimensional structure in order to be functional. Often
they are chemically modified, too.
• Proteins must travel to the appropriate location in order to do
their job.
• Errors in transcription, RNA processing, or translation can
result in poor function or absence of a particular protein.
Summary (cont.)
• A special exception to the usual flow of genetic information is
found in RNA viruses which use RNA as the long-term storage
of information.
• One group of RNA viruses, the retroviruses, enter the host cell
and make a DNA copy of their RNA genes.
• Viruses pose a serious threat to cellular life.
• They are exploited in biological research and for their potential
as agents of gene therapy and vaccination.
Reviewing Key Terms
Match the term on the left with the correct description.
___
transcription
a
___
translation
d
___
codons
b
___
introns
c
___
exons
f
___
RNA polymerase
e
a. the enzyme-catalyzed
assembly of an RNA molecule
b. the basic unit of the genetic
code
c. a segment of RNA that is
removed before mRNA leaves
the nucleus
d. the assembly of a protein on
ribosomes using mRNA
e. an enzyme that catalyzes the
assembly of an RNA molecule
f.
a segment of RNA that remains
after mRNA leaves the nucleus
Reviewing Ideas
1. Describe the lytic viral reproductive cycle.
In lytic infections, the host cell’s enzymes replicate
the viral DNA. Viral genes are transcribed and
translated on the host’s ribosomes to make
proteins for the outer capsule. New viral particles
assemble. When there are many new viruses, the
cell lyses (breaks open) and releases them to
infect other cells.
Reviewing Ideas
2. How will the complementary segment of RNA be
coded if the DNA is coded: GCT TGA AAT GAC?
Which amino acids do these codons represent?
The RNA codons would be:
CGA ACU UUA CUG
These codons represent the following
amino acids (in order):
arginine, threonine, leucine, leucine
Using Concepts
3. What could happen if an intron is left in RNA?
If introns are left in RNA, the consequences can
be serious. For example, a change in one splice
site of an intron in betaglobin, a component of the
oxygen-carrying blood protein hemoglobin, results
in defective hemoglobin.
Using Concepts
4. Why are viruses considered nonliving?
Among the most important basic properties of life is
the ability to replicate and to evolve which viruses
cannot do without help. Viruses depend on the
gene-expression machinery of the host cells they
infect. Most viruses consist of little more than a bit
of DNA or RNA and a protective protein coat. Some
viruses that infect animal cells have a membrane
envelope, but they do not carry out metabolism or
respond to stimuli, as cells do.
Synthesize
5. What makes viruses, particularly lysogenic
infections, attractive for genetic research?
Viruses are designed to insert DNA or RNA into
host cells. Scientists can disarm viruses by
removing the genes that cause disease.
Lysogenic infection, since it inserts the viral DNA
into the host’s DNA is useful in cloning
experiments as well as in vaccine research.
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Chapter Animations
The three stages in transcription of RNA
from a DNA template
Synthesis of proteins for secretion or
insertion in a membrane
Lytic and lysogenic viral reproduction
The three stages in transcription of RNA from a DNA template
Synthesis of proteins for secretion or insertion in a membrane
Lytic and lysogenic viral reproduction
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