Добавлен: 13.02.2019
Просмотров: 7489
Скачиваний: 3
48
CHAPTER 7
A. The trp operon consists of five genes positioned in sequence all of which encode for
proteins that are involved in tryptophan biosynthesis.
B. The trp repressor gene lies upstream of the trp operon and is expressed separately us-
ing its own trp repressor promoter. The trp repressor gene encodes for a protein called
the trp repressor which blocks the transcription of the five genes of the trp operon.
C. Consequently, the trp operon is under the control of the trp repressor. This is high-
lighted by the response of E. coli to two culture conditions as indicated below:
1.
Tryptophan
culture medium S trp operon OFF.
When E. coli is cultured in
tryptophan
culture medium, there is tryptophan available for metabolism. There-
fore, the trp operon is switched off because the trp repressor is bound to the trp
operator when two molecules of tryptophan attached to the trp repressor and acti-
vate it.
2.
Tryptophan
culture medium S trp operon ON.
When E. coli is cultured in
tryptophan
culture medium, there is no tryptophan available for metabolism.
Therefore, the trp operon is switched on because the trp repressor is not bound
to the trp operator because there is no tryptophan available to activate the trp
repressor.
LWBK771-c07_p39-48.qxd 9/29/10 8:45PM Page 48 aptara
49
A. The size of the human nuclear genome places huge demands on DNA polymerase to
faithfully replicate the precise DNA sequence code every time a cell undergoes mitosis
such that the average nucleotide diversity has been calculated. The average nucleotide
diversity
0.08% (i.e., 1 out of 1250 nucleotides differs on average between allelic
sequences).
B. BASE SUBSTITUTIONS are the most common type of mutation and are divided into
two types:
1.
Transitions
involve the substitution of a purine with a purine (A 4 G) or a pyrim-
idine with a pyrimidine (C 4 T).
2.
Transversions
involve the substitution of a purine with a pyrimidine (A 4 C or T)
or a pyrimidine with a purine (C 4 A or G).
C. Mutations that occur in the
2% of the human nuclear genome consisting of coding
DNA will clearly have the most clinical consequence. Mutations that occur in the
coding DNA are grouped into two classes:
1.
Silent (synonymous) mutations
where the sequence of the gene product is not
changed.
2.
Non-silent (nonsynonymous) mutations
where the sequence of the gene product
is changed.
Silent (Synonymous) Mutations.
Silent mutations are mutations where a change in
nucleotides alters the codon but no phenotypic change is observed in the individual. Silent
mutations produce functional proteins and accumulate in the genome where they are called
single nucleotide polymorphisms. A polymorphism is a DNA variation that is so common
in the population that it cannot be explained by a recurring mutation. Silent mutations may
occur in
● Figure 8-1 Silent Mutation: Spacer
DNA.
Silent
DNA
Spacer DNA
Gene 1
Functional
protein
Transcription
Translation
x
A. SPACER DNA (Figure 8-1). A mutation in
spacer DNA will not alter any genes or
proteins.
LWBK771-c08_p49-57.qxd 9/29/10 8:52PM Page 49 aptara
50
CHAPTER 8
B. INTRONS (Figure 8-2). A mutation in an in-
tron will not alter a protein because introns are
spliced out as mRNA is made.
C. THIRD NUCLEOTIDE OF THE CODON
(Figure 8-3). A mutation in the third nu-
cleotide of the codon will not alter the protein
because one amino acid has several codons.
The third nucleotide can often be mutated
without changing the amino acid for which it
codes. This is called third nucleotide (base)
redundancy.
Non-Silent (Nonsynonymous) Mutations
A. MISSENSE MUTATIONS (Figure 8-4). Mis-
sense mutations are point mutations where a
change in a single nucleotide alters the codon
so that one amino acid in a protein is replaced
with another amino acid. Missense mutations
produce proteins with a compensated func-
tion if the mutation occurs at an active or cat-
alytic site of the protein or alters the three di-
mensional structure of the protein. Missense
mutations are divided into two categories:
● Figure 8-2 Silent Mutation: Introns.
Silent
● Figure 8-3 Silent Mutation: Third
Nucleotide.
● Figure 8-4 Missense Mutation: Loss or
Gain of Function.
1.
Conservative substitutions
occur when the amino acid is replaced with another
amino acid that is chemically similar. The effect of such a replacement is often min-
imal on protein function.
2.
Nonconservative substitutions
occur when the amino acid is replaced with an-
other amino acid that is chemically dissimilar.
B. NONSENSE MUTATIONS (Figure 8-5).
Nonsense mutations are point mutations
where a change in a single nucleotide alters the
codon so that a premature STOP codon is
formed. Nonsense mutations produce unsta-
ble mRNAs which are rapidly degraded or
nonfunctional (truncated) proteins.
● Figure 8-5 Nonsense Mutation: Loss
of Function.
LWBK771-c08_p49-57.qxd 9/29/10 8:52PM Page 50 aptara
51
MUTATIONS OF THE DNA SEQUENCE
C. FRAMESHIFT MUTATIONS (Figure 8-6).
Frameshift mutations are point mutations
where either a deletion or insertion of nu-
cleotides (not a multiple of three) alters the
codon so that a premature STOP codon is
formed or the reading frame is shifted.
Frameshift mutations produce either unstable
mRNAs which are rapidly degraded or non-
functional (“garbled”) proteins because all of
the amino acids after the deletion or insertion
● Figure 8-6 Frameshift Mutation: Loss
of Function.
are changed, respectively. In-frame mutations are point mutations where either a dele-
tion or insertion of nucleotides (a multiple of three) alters the codon but does not shift
the reading frame. In-frame mutations produce compensated proteins. Clinical exam-
ples of frameshift and in-frame mutations are Duchenne muscular dystrophy (DMD)
and Becker muscular dystrophy (BMD).
1.
Duchenne muscular dystrophy
a.
DMD is an X-linked recessive genetic disorder caused by various mutations in
the DMD gene on chromosome Xp21.2 for dystrophin which anchors the cy-
toskeleton (actin) of skeletal muscle cells to the extracellular matrix via a
transmembrane protein (
-dystrophin and -dystrophin), thereby stabilizing
the cell membrane. The DMD gene is the largest known human gene.
b.
DMD is caused by small deletion, large deletion, deletion of the entire gene,
insertion, duplication of one of more exons, or single-based change mutations.
The deletion or insertion of nucleotides (not a multiple of three) results in a
frameshift mutation. These mutations result in either the absence of dys-
trophin protein or a nonfunctional (“garbled”) dystrophin protein which
causes severe clinical features (more severe than BMD).
c.
Serum creatine phosphokinase (CK) measurement. The measurement of
serum CK is one of the diagnostic tests for DMD ([serum CK]
10 times
normal is diagnostic).
d.
Skeletal muscle biopsy. A skeletal muscle biopsy shows histological signs of
fiber size variation, foci of necrosis and regeneration, hyalinization, and depo-
sition of fat and connective tissue. Immunohistochemistry shows almost com-
plete absence of the dystrophin protein.
e.
Clinical features include symptoms appear in early childhood with delays in
sitting and standing independently; progressive muscle weakness (proximal
weakness
distal weakness) often with calf hypertrophy; progressive muscle
wasting; waddling gait; difficulty in climbing; wheelchair bound by 12 years
of age; cardiomyopathy by 18 years of age; death by
30 years of age due to
cardiac or respiratory failure.
2.
Becker muscular dystrophy
a.
BMD is an X-linked recessive genetic disorder caused by various mutations in
the DMD gene on chromosome Xp21.2 for dystrophin which anchors the cy-
toskeleton (actin) of skeletal muscle cells to the extracellular matrix via a
transmembrane protein (
-dystrophin and -dystrophin) thereby stabilizing
the cell membrane.
b.
BMD is caused by the deletion or insertion of nucleotides (a multiple of three)
which results in an in-frame mutation. The in-frame mutation results in a com-
pensated dystrophin protein which causes less severe clinical features com-
pared with DMD.
LWBK771-c08_p49-57.qxd 9/29/10 8:52PM Page 51 aptara
52
CHAPTER 8
D. RNA SPLICING MUTATIONS (Figure 8-7).
RNA splicing mutations are mutations where a
change in nucleotides at the 5
-end or 3-end
of an intron alters the codon so that a splice
site in the RNA transcript is changed which
results either in intron retention (due to com-
plete failure in splicing) or exon skipping. In-
tron retention generally results in a mRNA that
is unable to exit the nucleus to make contact
with the translational machinery and therefore
no protein is produced. Exon skipping may re-
● Figure 8-7 RNA Splicing Mutation.
RNA splicing
● Figure 8-8 Transposon Mutation: Loss
of Function.
Transposon mutation
Loss of function
● Figure 8-9 Translocation Mutation:
Loss or Gain of Function.
Translocation
Loss of function or gain of function
the breakpoint is near the centromere. The short arms (p) of these chromo-
somes are generally lost.
b.
Carriers of an RT are clinically normal because the short arms, which are lost,
contain only inert DNA and some rRNA (ribosomal RNA) genes which occur
in multiple copies on other chromosomes.
c.
One of the most common translocations found in humans is the RT t(14q21q).
d.
The clinical issue in the RT t(14q21q) occurs when the carriers produce ga-
metes by meiosis and reproduce. Depending on how the chromosomes segre-
gate during meiosis, conception can produce offspring with translocation
sult in a frameshift mutation where a premature STOP codon is formed or the reading
frame is shifted. Frameshift mutations produce either unstable mRNAs which are rap-
idly degraded or nonfunctional (“garbled”) proteins because all of the amino acids af-
ter the deletion or insertion are changed, respectively.
E.
TRANSPOSON MUTATIONS (Figure 8-8).
Transposon mutations are mutations where a
transposon alters the codon so that a gene is
disrupted. Transposable element mutations
produce no protein at all because the gene is
completely disrupted. Transposition is a fairly
common event in the human genome. How-
ever, in reality, it is very rare that transposition
disrupts a gene.
F.
TRANSLOCATION MUTATIONS (Figure 8-9).
Translocation mutations are mutations where
a section of a gene is moved from its original
location to another location either on the same
or different chromosome. Translocations result
from breakage and exchange of segments be-
tween chromosomes. Translocation mutations
produce either no protein or fusion proteins
with a novel function. The following are clin-
ical examples caused by translocations.
1.
Robertsonian translocation (RT)
a.
An RT is caused by translocations be-
tween the long arms (q) of acrocen-
tric (satellite) chromosomes where
LWBK771-c08_p49-57.qxd 9/29/10 8:52PM Page 52 aptara