I need a discussion
In coming years, the development in genetics is going to have a big impact on medicine, especially in the areas of diagnosis, prevention,and treatment of diseases. The following link explores some of those applications.
http://web.ornl.gov/sci/techresources/Human_Genome/publicat/primer2001/primer11.pdf
Read about Genetics and DNA technology in chapter 8. Based on what you learned in chapter 8 and from the article on the website, post two paragraphs on
Prompts:
Choose a technology and the applications of DNA technology in medicine.
Discuss the pros and cons of the technology you chose
Please don’t copy sentences out of the book or websites. Please specify the technology you are discussing.
8
Virtually all the microbial traits you have read about in earlier chapters are controlled or influenced by heredity. The
inherited characteristics of microbes include shape,
structural features, metabolism, ability to move, and
interactions with other organisms. Individual organisms
transmit these characteristics to their offspring through
genes.
The development of antibiotic resistance in microorganisms
is often carried on plasmids such as those in the photo,
which are readily transferred between bacterial cells. They
are responsible for the emergence of methicillin-resistant
Staphylococcus aureus and the recent emergence of carbapenem-
resistant Klebsiella pneumoniae. The emergence of vancomycin-
resistant S. aureus (VRSA) poses a serious threat to patient care.
In this chapter you will see how VRSA acquired this characteristic.
Emerging diseases provide another reason why it is important
to understand genetics. New diseases are the results of genetic
changes in some existing organism; for example, E. coli O157:H7
acquired the genes for Shiga toxin from Shigella.
Currently, microbiologists are using genetics to study
unculturable microbes and the relationship between hosts and
microbes.
The Big Picture on pages 206207 highlights key
principles of genetics that are explained in greater detail
throughout the chapter.
In the Clinic
As a nurse at a U.S. military hospital, you treat service members injured in the recent Middle
East conflicts. You notice that wounds infected by Acinetobacter baumannii are not responding
to antibiotics. The Centers for Disease Control and Prevention reports that the antibiotic-
resistance genes found in A. baumannii are the same as those in Pseudomonas, Salmonella, and
Escherichia. Cephalosporin-resistance genes are on the chromosome, tetracycline resistance is
encoded by a plasmid, and streptomycin resistance is associated with a transposon. Can you
suggest mechanisms by which Acinetobacter acquired this
resistance?
Hint: Read about genetic recombination on pages 229235.
Microbial Genetics
Play In the Clinic Video
@MasteringMicrobiology
Plasmids exist in cells separate from
chromosomes.
204
CHAPTER 8 Microbial Genetics 205
Structure and Function
of the Genetic Material
LEARNING OBJECTIVES
8-1 Define genetics, genome, chromosome, gene, genetic code,
genotype, phenotype, and genomics.
8-2 Describe how DNA serves as genetic information.
8-3 Describe the process of DNA replication.
8-4 Describe protein synthesis, including transcription, RNA
processing, and translation.
8-5 Compare protein synthesis in prokaryotes and eukaryotes.
Genetics is the science of heredity. It includes the study of genes:
how they carry information, how they replicate and pass to sub-
sequent generations of cells or between organisms, and how the
expression of their information within an organism determines
its characteristics. The genetic information in a cell is called the
genome. A cells genome includes its chromosomes and plas-
mids. Chromosomes are structures containing DNA that physi-
cally carry hereditary information; the chromosomes contain
the genes. Genes are segments of DNA (except in some viruses,
in which they are made of RNA) that code for functional prod-
ucts. Usually these products are proteins, but they can also be
RNAs (ribosomal RNA, transfer RNA, or microRNA).
We saw in Chapter 2 that DNA is a macromolecule composed
of repeating units called nucleotides. Each nucleotide consists of
a nucleobase (adenine, thymine, cytosine, or guanine), deoxyri-
bose (a pentose sugar), and a phosphate group (see Figure 2.16,
page 45). The DNA within a cell exists as long strands of nucle-
otides twisted together in pairs to form a double helix. Each
strand has a string of alternating sugar and phosphate groups
(its sugar-phosphate backbone), and a nitrogenous base is attached
to each sugar in the backbone. The two strands are held together
by hydrogen bonds between their nitrogenous bases. The base
pairs always occur in a specific way: adenine always pairs with
thymine, and cytosine always pairs with guanine. Because of
this specific base pairing, the base sequence of one DNA strand
determines the base sequence of the other strand. The two
strands of DNA are thus complementary.
The structure of DNA helps explain two primary features
of biological information storage. First, the linear sequence of
bases provides the actual information. Genetic information
is encoded by the sequence of bases along a strand of DNA,
in much the same way as our written language uses a linear
sequence of letters to form words and sentences. The genetic
language, however, uses an alphabet with only four lettersthe
four kinds of nucleobases in DNA (or RNA). But 1000 of these
four bases, the number contained in an average-sized gene, can
be arranged in 41000 different ways. This astronomically large
number explains how genes can be varied enough to provide
all the information a cell needs to grow and perform its func-
tions. The genetic code, the set of rules that determines how a
nucleotide sequence is converted into the amino acid sequence
of a protein, is discussed in more detail later in this chapter.
Second, the complementary structure allows for the pre-
cise duplication of DNA during cell division. Each offspring
cell receives one of the original strands from the parent, thus
ensuring one strand that functions correctly.
Much of cellular metabolism is concerned with translating
the genetic message of genes into specific proteins. A gene is
usually copied to make a messenger RNA (mRNA) molecule,
which ultimately results in the formation of a protein. When
the ultimate molecule for which a gene codes (a protein, for
example) has been produced, we say that the gene has been
expressed. The flow of genetic information can be shown as
flowing from DNA to RNA to proteins, as follows:
RNA ProteinDNA
This theory was called
the central dogma by Francis
Crick in 1956, when he first
proposed that the sequence
of nucleotides in DNA deter-
mines the sequence of amino
acids in a protein.
Genotype and Phenotype
The genotype of an organism is its genetic makeupall its
DNAthe information that codes for all the particular char-
acteristics of the organism. The genotype represents potential
ASM: Although the central dogma is
universal in all cells, the processes differ
in prokaryotes and eukaryotes, as we shall
see in this chapter.
205 224 227 234
CLINICAL CASE Where Theres Smoke
Marcel DuBois, a 70-year-old grandfather of 12, quietly hangs up the phone. His doctor has just called him with
the results of his stool DNA test that he undertook at the
Mayo Clinic last week. Marcels doctor suggested this new,
noninvasive screening tool for colorectal cancer because
Marcel is not comfortable with the colonoscopy procedure
and usually tries to postpone getting one. The stool DNA test,
however, uses stool samples, which contain cells that have
been shed from the colon lining. The DNA from these cells
is tested for DNA markers that may indicate the presence of
precancerous polyps or cancerous tumors. Marcel makes an
appointment to come in to see his doctor the next afternoon.
Once in the office, the doctor explains to Marcel and his
wife, Janice, that the stool DNA test detected the presence
of serrated colorectal polyps. This type of polyp is usually
difficult to see with a colonoscopy because it is not raised and
can be the same color as the colon wall.
How can DNA show whether a person has cancer?
Read on to find out.
206
GeneticsBIG PICTURE
Genetics is the science of heredity. It
includes the study of genes: how they
are replicated, expressed, and passed
on from one generation to another.
The central dogma of molecular biology describes how, typically,
DNA is transcribed to messenger RNA, which, in turn, is translated
into proteins that carry out vital cellular functions. Mutations
introduce change into this processultimately leading to new or
lost functions.
How mutations
alter a genome
DNA
mRNA
Protein
Function
Mutated DNA
Altered mRNA
Altered protein
Altered function
Typical chain of events
described by
central dogma
Mutations can be caused by base substitutions or frameshift
mutations.
In base substitution mutations, a single DNA base
pair is altered.
T A C T T C A
A U G A A G T
T A A T T C A
A U A A G TT
In frameshift mutations, DNA base pairs are added or removed
from the sequence, causing a shift in the sequence reading.
T A C T T C A
A U G A A G T
T A T T C A
A U A A G T
Groups of genes in operons can be inducible or repressible.
Active
repressor
DNA
DNA
Inactive
repressor
Inducer
OFF (gene
not expressed)
ON (gene
expressed)
An inducible operon includes genes that are in the off
mode, with the repressor bound to the DNA, and is turned
on by the environmental inducer.
ON (gene
expressed)
OFF (gene
not expressed)
A repressible operon includes genes that are in the on
mode, without the repressor bound to the DNA, and is turned
off by the environmental corepressor and repressor.
DNA
DNA
Inactive
repressor
Active
repressor
Corepressor
AFM
7 nmAtomic force micrograph showing DNA molecules.
207
Alteration of bacterial genes and/or gene expression may cause disease, prevent
disease treatment, or be manipulated for human benefit.
TEM 0.4 mm
Diseases: Many bacterial diseases are caused by the
presence of toxic proteins that damage human tissue. These
toxic proteins are coded for by genes. Vibrio cholerae, shown
above, produces an enterotoxin that causes diarrhea and
severe dehydration, which can be fatal if left untreated.
Antibiotic resistance: Mutations in the bacterial genome
are one of the first steps toward the development of antibiotic
resistance. This process has occurred with Staphylococcus
aureus, which is currently resistant to beta-lactam antibiotics
such as penicillin. Methicillin was introduced to treat
penicillin-resistant S. aureus. Methicillin-resistant
S. aureus (MRSA), shown in purple above, is now a
leading cause of healthcare-associated infections.
SEM 0.3 mm
Biofilms: Biofilms, such as the one seen here growing on a
toothbrush bristle, are produced by altered bacterial gene
expression when populations are large enough. Various
Streptococcus species, including S. mutans, form biofilms on
teeth and gums, contributing to the development of dental
plaque and dental caries.
SEM 5 mm
Biotechnology: Scientists can alter a microorganisms
genome, adding genes that will produce human proteins used
in treating
disease. Insulin,
used for treatment
of diabetes, is
produced in this
manner.
DNA expression leads to cell function
via the production of proteins.
Genes in operons are turned on or off
together.
Mutations alter DNA sequences.
DNA mutations can change bacterial
function.
KEY CONCEPTS
Play MicroFlix 3D Animation
@MasteringMicrobiology
208 PART ONE Fundamentals of Microbiology
properties, but not the properties themselves. Phenotype refers
to actual, expressed properties, such as the organisms ability to
perform a particular chemical reaction. Phenotype, then, is
the manifestation of genotype. For example, E. coli with the stx
gene can produce the stx (Shiga toxin) protein.*
In a sense, an organisms phenotype is its collection of pro-
teins, because most of a cells properties derive from the struc-
tures and functions of proteins. In microbes, most proteins
are either enzymatic (catalyze particular reactions) or structural
(participate in large functional complexes such as membranes
or flagella). Even phenotypes that depend on structural mac-
romolecules such as lipids or polysaccharides rely indirectly
on proteins. For instance, the structure of a complex lipid or
polysaccharide molecule results from catalytic activities of
enzymes that synthesize, process, and degrade those molecules.
Thus, saying that phenotypes are due to proteins is a useful
simplification.
DNA and Chromosomes
Bacteria typically have a single circular chromosome consist-
ing of a single circular molecule of DNA with associated pro-
teins. The chromosome is looped and folded and attached at
one or several points to the plasma membrane. The DNA of
E. coli has about 4.6 million base pairs and is about 1 mm
long1000 times longer than the entire cell (Figure 8.1). How-
ever, the chromosome takes up only about 10% of the cells
volume because the DNA is twisted, or supercoiled.
The entire genome does not consist of back-to-back genes.
Noncoding regions called short tandem repeats (STRs) occur
in most genomes, including that of E. coli. STRs are repeating
sequences of two- to five-base sequences. These are used in
DNA fingerprinting (discussed on page 258).
Now, the complete base sequences of chromosomes can
be determined. Computers are used to search for open reading
frames, that is, regions of DNA that are likely to encode a
protein. As you will see later, these are base sequences between
start and stop codons. The sequencing and molecular charac-
terization of genomes is called genomics. The use of genomics
to track Zika virus is described in the Clinical Focus box on
page 218.
The Flow of Genetic Information
DNA replication makes possible the flow of genetic informa-
tion from one generation to the next. This is called vertical
gene transfer. As shown in Figure 8.2, the DNA of a cell repli-
cates before cell division so that each offspring cell receives a
chromosome identical to the parents. Within each metaboliz-
ing cell, the genetic information contained in DNA also flows
in another way: it is transcribed into mRNA and then trans-
lated into protein. We describe the processes of transcription
and translation later in this chapter.
Chromosome
1 mm
TEM
Figure 8.1 A prokaryotic chromosome.
How many times longer than the 2-mm cell is the chromosome? Q
CHECK YOUR UNDERSTANDING
8-1 Give a clinical application of genomics.
8-2 Why is the base pairing in DNA important?
DNA Replication
In DNA replication, one parental double-stranded DNA mol-
ecule is converted to two identical offspring molecules. The
complementary structure of the nitrogenous base sequences in
the DNA molecule is the key to understanding DNA replica-
tion. Because the bases along the two strands of double-helical
DNA are complementary, one strand can act as a template for
the production of the other strand (Figure 8.3a).
DNA replication requires the presence of several cellular
proteins that direct a particular sequence of events. Enzymes
involved in DNA replication and other processes are listed
in Table 8.1. When replication begins, the supercoiling is
relaxed by topoisomerase or gyrase. The two strands of parental
DNA are unwound by helicase and separated from each other
in one small DNA segment after another. Free nucleotides
present in the cell cytoplasm are matched up to the exposed
bases of the single-stranded parental DNA. Where thymine
is present on the original strand, only adenine can fit into
place on the new strand; where guanine is present on the
original strand, only cytosine can fit into place, and so on.
Any bases that are improperly base-paired are removed and *Gene names are italicized, but the protein name is not italicized.
CHAPTER 8 Microbial Genetics 209
Genetic information can be
transferred horizontally between
cells of the same generation.
Genetic information can be
transferred vertically to the
next generation of cells.
Genetic information is used
within a cell to produce the
proteins needed for the cell
to function.
New combinations
of genes
Offspring cells
Transcription
DNA
Parent cell
Cell metabolizes and grows Recombinant cell
Translation
expression recombination replication
DNA is the blueprint for a cells proteins, including
enzymes.
DNA is obtained either from another cell in the same
generation or from a parent cell during cell division.
DNA can be expressed within a cell or transferred to
another cell through recombination and replication.
KEY CONCEPTS
The Flow of Genetic Information
FOUNDATION
FIGURE
8.2
replaced by replication enzymes. Once aligned, the newly
added nucleotide is joined to the growing DNA strand by
an enzyme called DNA polymerase. Then the parental DNA
is unwound a bit further to allow the addition of the next
nucleotides. The point at which replication occurs is called
the replication fork.
As the replication fork moves along the parental DNA, each
of the unwound single strands combines with new nucleotides.
The original strand and this newly synthesized daughter strand
then rewind. Because each new double-stranded DNA molecule
contains one original (conserved) strand and one new strand,
the process of replication is referred to as semiconservative
replication.
Before looking at DNA replication in more detail, lets dis-
cuss the structure of DNA (see Figure 2.16 on page 45 for an
overview). It is important to understand that the paired DNA
strands are oriented in opposite directions (antiparallel) rela-
tive to each other. The carbon atoms of the sugar component
of each nucleotide are numbered 1 (pronounced one prime)
to 5. For the paired bases to be next to each other, the sugar
components in one strand are upside down relative to the
other. The end with the hydroxyl attached to the 3 carbon is
called the 3 end of the DNA strand; the end having a phos-
phate attached to the 5 carbon is called the 5 end. The way
in which the two strands fit together dictates that the 5 S 3
direction of one strand runs counter to the 5 S 3 direction
of the other strand (Figure 8.3b). This structure of DNA affects
the replication process because DNA polymerases can add new
nucleotides to the 3 end only. Therefore, as the replication fork
moves along the parental DNA, the two new strands must grow
in different directions.
One new strand, called the leading strand, is synthesized con-
tinuously in the 5 S 3 direction (from a template parental
strand running 3 S 5). In contrast, the lagging strand of the
new DNA is synthesized discontinuously in fragments of about
1000 nucleotides, called Okazaki fragments. These must be
joined later to make the continuous strand.
209
210 PART ONE Fundamentals of Microbiology
A
A
A
A
Parental
strand
Parental
strand
3 end
Daughter
strand
forming
5 end
Daughter
strand Parental
strand
Parental
strand
G
Replication
fork
C
T
C
3 end
3 end
5 end
5 end
Deoxyribose sugar
Phosphate
(a) The replication fork
(b) The two strands of DNA are antiparallel.
The sugar-phosphate backbone of one strand is
upside down relative to the backbone of the
other strand. Turn the book upside down to
demonstrate this.
A
A
C
T A
CG
G
KEY
T
T
C
1
2
2
3
3
1 The double helix of the
parental DNA separates
as weak hydrogen
bonds between the
nucleotides on opposite
strands break in
response to the action
of replication enzymes.
Hydrogen bonds form
between new
complementary
nucleotides and each
strand of the parental
template to form new
base pairs.
Enzymes catalyze
the formation of
sugar-phosphate
bonds between
sequential nucleotides
on each resulting
daughter strand.
O
O
O
P
O
OH
OH
OH
O
O
O
O
O
P
H2C
H2C
H2C
H2C
5 end
3 end
5 end
3 end
O
O
O
P
O
O
O
O
O
O
O
P
CH2
CH2
CH2
CH2
O
O
O
P
O
O
O
P
O
O
O
O
O
O
O
O
O
O
O
P
O
HO
O
P
C G
TA
CG G
G
G C
T AT
T T
Adenine ThymineA T
Guanine CytosineG C
Figure 8.3 DNA replication.
What is the advantage of semiconservative replication? Q
TABLE 8.1 Important Enzymes in DNA Replication, Expression, and Repair
DNA Gyrase Relaxes supercoiling ahead of the replication fork
DNA Ligase Makes covalent bonds to join DNA strands; Okazaki fragments, and new segments in excision repair
DNA Polymerases Synthesize DNA; proofread and facilitate repair of DNA
Endonucleases Cut DNA backbone in a strand of DNA; facilitate repair and insertions
Exonucleases Cut DNA from an exposed end of DNA; facilitate repair
Helicase Unwinds double-stranded DNA
Methylase Adds methyl group to selected bases in newly made DNA
Photolyase Uses visible light energy to separate UV-induced pyrimidine dimers
Primase An RNA polymerase that makes RNA primers from a DNA template
Ribozyme RNA enzyme that removes introns and splices exons together
RNA Polymerase Copies RNA from a DNA template
snRNP RNA-protein complex that removes introns and splices exons together
Topoisomerase or Gyrase Relaxes supercoiling ahead of the replication fork; separates DNA circles at the end of DNA replication
Transposase Cuts DNA backbone, leaving single-stranded sticky ends
CHAPTER 8 Microbial Genetics 211
G C
C
OH
OH
Sugar
Phosphate
OH
OHOH
T
C G C G
P
P
P P
A T A T
A T A
G C
C
New
strand
Template
strand
When a nucleoside
triphosphate bonds
to the sugar, it loses
two phosphates.
Hydrolysis of the
phosphate bonds
provides the energy
for the reaction.
P P i
Figure 8.4 Adding a nucleotide to DNA.
Why is one strand upside down relative to the other strand?
Why cant both strands face the same way?
Q
Energy Needs
DNA replication requires a great deal of energy. The energy is
supplied from the nucleotides, which are actually nucleoside
triphosphates. You already know about ATP; the only differ-
ence between ATP and the adenine nucleotide in DNA is the
sugar component. Deoxyribose is the sugar in the nucleosides
used to synthesize DNA, and nucleoside triphosphates with
ribose are used to synthesize RNA. Two phosphate groups are
removed to add the nucleotide to a growing strand of DNA;
hydrolysis of the nucleoside is exergonic and provides energy
to make the new bonds in the DNA strand (Figure 8.4).
Figure 8.5 provides more detail about the many steps that
go into this complex process.
DNA replication by some bacteria, such as E. coli, goes
bidirectionally around the chromosome (Figure 8.6). Two repli-
cation forks move in opposite directions away from the origin
of replication. Because the bacterial chromosome is a closed
loop, the replication forks eventually meet when replication is
completed. The two loops must be separated by a topoisomer-
ase. Much evidence shows an association between the bacterial
plasma membrane and the origin of replication. After dupli-
cation, if each copy of the origin binds to the membrane at
Enzymes unwind the
parental double
helix.
1
Proteins stabilize the
unwound parental DNA.
2
DNA polymerase
The leading strand is synthesized
continuously from the primer by
DNA polymerase.
3
Replication
fork
The lagging strand is
synthesized discontinuously.
Primase, an RNA polymerase,
synthesizes a short RNA primer,
which is then extended by
DNA polymerase.
4 DNA polymerase
digests RNA primer
and replaces it with DNA.
5
DNA
polymerase
Primase
RNA primer
DNA ligase joins
the discontinuous
fragments of the
lagging strand.
6
DNA polymerase
DNA ligaseOkazaki fragment
Parental
strand
5
3
5
3
REPLICATION
Figure 8.5 A summary of events at the DNA replication fork.
Why is one strand of DNA synthesized discontinuously? Q
212 PART ONE Fundamentals of Microbiology
20 nmSEM(a) An E. coli chromosome in the process of replicating
(b) Bidirectional replication of a circular bacterial DNA molecule
Origin of
replication
Replication
fork
Daughter
strands
Parental
strand
Termination
of replication
Replication
fork
REPLICATION
Replication
fork
Replication fork
Figure 8.6 Replication of bacterial DNA.
What is the origin of replication? Q
opposite poles, then each offspring cell receives one copy of the
DNA moleculethat is, one complete chromosome.
DNA replication is an amazingly accurate process. Typi-
cally, mistakes are made at a rate of only 1 in every 10 billion
bases incorporated. Such accuracy is largely due to the proof-
reading capability of DNA polymerase. As each new base is
added, the enzyme evaluates whether it forms the proper com-
plementary base-pairing structure. If not, the enzyme excises
the improper base and replaces it with the correct one. In this
way, DNA can be replicated
very accurately, allowing
each daughter chromosome
to be virtually identical to
the parental DNA.
Play DNA Replication:
Overview, Forming the
Replication Fork,
Replication Proteins, Synthesis
@MasteringMicrobiology
CHECK YOUR UNDERSTANDING
8-3 Describe DNA replication, including the functions of DNA
gyrase, DNA ligase, and DNA polymerase.
RNA and Protein Synthesis
How is the information in DNA used to make the proteins that
control cell activities? In the process of transcription, genetic
information in DNA is copied, or transcribed, into a comple-
mentary base sequence of RNA. The cell then uses the infor-
mation encoded in this RNA to synthesize specific proteins
through the process of translation. We now take a closer look at
these two processes as they occur in a bacterial cell.
Transcription in Prokaryotes
Transcription is the synthesis of a complementary strand of
RNA from a DNA template. We will discuss transcription in
prokaryotic cells here. Transcription in eukaryotes is discussed
on page 215.
Ribosomal RNA (rRNA) forms an integral part of ribo-
somes, the cellular machinery for protein synthesis. Transfer
RNA is also involved in protein synthesis, as we will see.
Messenger RNA (mRNA) carries the coded information for
making specific proteins from DNA to ribosomes, where pro-
teins are synthesized.
During transcription, a strand of mRNA is synthesized
using a specific portion of the cells DNA as a template. In
other words, the genetic information stored in the sequence of
nucleobases of DNA is rewritten so that the same information
appears in the base sequence of mRNA.
As in DNA replication, a guanine (G) in the DNA template
dictates a cytosine (C) in the mRNA being made, and a C in the
DNA template dictates a G in the mRNA. Likewise, a thymine
(T) in the DNA template dictates an adenine (A) in the mRNA.
CHAPTER 8 Microbial Genetics 213
However, an adenine in the DNA template dictates a uracil (U)
in the mRNA, because RNA contains uracil instead of thymine.
(Uracil has a chemical structure slightly different from thy-
mine, but it base-pairs in the same way.) If, for example, the
template portion of DNA has the base sequence 3-ATGCAT,
the newly synthesized mRNA strand will have the complemen-
tary base sequence 5-UACGUA.
The process of transcription requires both an enzyme called
RNA polymerase and a supply of RNA nucleotides (Figure 8.7).
Transcription begins when RNA polymerase binds to the DNA
at a site called the promoter. Only one of the two DNA strands
serves as the template for RNA synthesis for a given gene.
Like DNA, RNA is synthesized in the 5 S 3 direction. RNA
synthesis continues until RNA polymerase reaches a site on the
DNA called the terminator.
Transcription allows the cell to produce short-term copies
of genes that can be used as the direct source of information
for protein synthesis. Messenger RNA acts as an intermedi-
ate between the permanent
storage form, DNA, and the
process that uses the infor-
mation, translation.
Translation
We have seen how the genetic information in DNA transfers
to mRNA during transcription. Now we will see how mRNA
serves as the source of information for the synthesis of pro-
teins. Protein synthesis is called translation because it involves
decoding the language of nucleic acids and converting it into
the language of proteins.
AU U
U
UG
G
C
U
U
U
G
AA
O
C
A
TT
T T
T
T T T T
A
A A
A
AA
A
T
C C
GT
A
A
C C
G
C
G
G
G G
A
U
A
U
A
U
T A
C
G
A
G
RNA polymerase
binds to the
promoter, and
DNA unwinds at
the beginning of
a gene.
RNA is synthesized
by complementary
base pairing of free
nucleotides with the
nucleotide bases on
the template strand
of DNA.
The site of synthesis
moves along DNA;
DNA that has been
transcribed rewinds.
Transcription reaches
the terminator.
RNA and RNA
polymerase are
released, and the
DNA helix re-forms.
1
3
4
5
2
DNA
mRNA
Protein
TRANSCRIPTION
10 nm
AFMRNA polymerase bound to DNA
DNA
RNA
polymerase
RNA synthesis
Complete
RNA strand
Promoter
(gene begins) RNA polymerase
RNA Terminator
(gene ends)
RNA
RNA nucleotides
RNA polymerase
Template strand of DNA
Promoter
Figure 8.7 The process of transcription. The orienting diagram indicates the relationship
of transcription to the overall flow of genetic information within a cell.
When does transcription stop? Q
Play Transcription:
Overview, Process
@MasteringMicrobiology
214 PART ONE Fundamentals of Microbiology
The language of mRNA is in the form of codons, groups of
three nucleotides, such as AUG, GGC, or AAA. The sequence
of codons on an mRNA molecule determines the sequence of
amino acids that will be in the protein being synthesized. Each
codon codes for a particular amino acid. This is the genetic
code (Figure 8.8).
Codons are written in terms of their base sequence in
mRNA. Notice in Figure 8.8 that there are 64 possible codons
but only 20 amino acids. This means that most amino acids
are signaled by several alternative codons, a situation referred
to as the degeneracy of the code. For example, leucine has
six codons, and alanine has four codons. Degeneracy allows for
a certain amount of misreading of, or mutation in, the DNA
without affecting the protein ultimately produced.
Of the 64 codons, 61 are sense codons, and 3 are nonsense
codons. Sense codons code for amino acids, and nonsense codons
(also called stop codons) do not. Rather, the nonsense codons
UAA, UAG, and UGAsignal the end of the protein molecules
synthesis. The start codon that initiates the synthesis of the pro-
tein molecule is AUG, which is also the codon for methionine. In
bacteria, the start AUG codes for formylmethionine rather than
the methionine found in other parts of the protein. The initiat-
ing methionine is often removed later, so not all proteins contain
methionine.
During translation, codons of an mRNA are read sequen-
tially; and, in response to each codon, the appropriate amino
acid is assembled into a growing chain. The site of translation
is the ribosome, and transfer RNA (tRNA) molecules both rec-
ognize the specific codons and transport the required amino
acids.
Each tRNA molecule has an anticodon, a sequence of three
bases that is complementary to a codon. In this way, a tRNA
molecule can base-pair with its associated codon. Each tRNA
can also carry on its other end the amino acid encoded by the
codon that the tRNA recognizes. The functions of the ribo-
some are to direct the orderly binding of tRNAs to codons and
to assemble the amino acids brought there into a chain, ulti-
mately producing a protein.
Figure 8.9 shows the details of translation. The two ribo-
somal subunits, a tRNA with the anticodon UAC, and the
mRNA molecule to be translated, along with several addi-
tional protein factors, all assemble. This sets up the start
codon (AUG) in the proper position to allow translation to
begin. After the ribosome joins the first two amino acids with
a peptide bond, the first tRNA molecule leaves the ribosome.
The ribosome then moves along the mRNA to the next codon.
As the proper amino acids are brought into line one by one,
peptide bonds are formed between them, and a polypeptide
chain results. (Also see Figure 2.14, page 42.) Translation
Second position
Fi
rs
t p
os
iti
on
Th
ir
d
po
si
tio
n
C A G
C
A
G
U
U
UUU
UUC
UUA
UUG
Phe
Leu
UCU
UCC
UCA
UCG
Ser
Tyr Cys
U
C
A
G
CUU
CUC
CUA
CUG
Leu
CCU
CCC
CCA
