Добавлен: 13.02.2019
Просмотров: 7480
Скачиваний: 3
28
CHAPTER 4
b.
Level of gene expression (Figure 4-5).
If the transposon (T) moves to the
target DNA near an active gene, the
transposon may cause a change in
gene expression. Although most of
these changes in gene expression
would be detrimental to the organ-
ism, some of the changes over time
might be beneficial and then spread
through the population.
c.
Gene inactivation (Figure 4-6). If the
transposon (T) moves to the target
DNA in the middle of an active gene,
the active gene will be mutated and
become an inactive gene.
d.
Gene transfer (Figure 4-7). If two
transposons happen to be close to one
another, the transposition mechanism
may cut the ends of two different
transposons. This mechanism is espe-
cially important in development of
antibiotic resistance in bacteria. If the
bacterial DNA between to the two
transposons (T) contains the gene for
tetracycline resistance (Tet
R
), the Tet
R
gene may insert into phage DNA. The
phage DNA with the Tet
R
gene infects
other bacteria and confers tetracycline
resistant.
● Figure 4-7 Gene Transfer.
● Figure 4-6 Gene Inactivation.
● Figure 4-5 Level of Gene Expression.
Change in
gene
expression
LWBK771-c04_p22-28.qxd 9/29/10 6:52PM Page 28 aptara
The Human Mitochondrial Genome
29
A. The human mitochondrial genome consists of mitochondrial DNA (mtDNA) arranged
as a circular piece of double-stranded DNA
(
H strand and L strand) consisting of
16,569 base pairs and is located within the mitochondrial matrix.
B. In contrast to the human nuclear genome,
mtDNA is not protected by histones (i.e., his-
tone free).
C. The human mitochondrial genome codes for
37 genes which make up
93% of the human
mitochondrial genome.
D. There are 13 protein-coding genes and 24
RNA-coding genes.
E.
The fact that the 37 genes make up
93% of
the human mitochondrial genome means that
93% of the human mitochondrial genome
consists of coding DNA and
7% of the human
mitochondrial genome consists of noncoding
DNA (compare with the human nuclear
genome, Chapter 4).
● Figure 5-1 Pie chart indicating the or-
ganization of the human mitochondrial
genome.
Coding DNA
Noncoding DNA
~93%
37 genes
~7%
noncoding
F.
All human mtDNA genes contain only exons (i.e., no introns are present).
G. The human mitochondrial genome is maternally inherited (i.e., both males and
females inherit mtDNA from the mother, but males do not transmit their mtDNA to
subsequent generations). During fertilization and zygote formation, the sperm con-
tributes its nuclear genome but not its mitochondrial genome because all sperm mito-
chondria degenerate. Consequently, the mitochondrial genome of the zygote is determined
exclusively by the mitochondria found in the cytoplasm of the unfertilized secondary
oocyte.
The 13 Protein-Coding Genes (Figure 5-2, Table 5-1).
The protein-coding
genes encode for 13 proteins that are not complete enzymes but are subunits of multimeric
enzyme complexes used in electron transport and ATP synthesis. These 13 proteins are
synthesized on mitochondrial ribosomes.
The 24 RNA-Coding Genes (Figure 5-2, Table 5-1).
The RNA-coding genes
encode for 24 RNAs.
LWBK771-c05_p29-32.qxd 9/29/10 6:52PM Page 29 aptara
30
CHAPTER 5
13 Protein-coding genes
2 rRNA genes
22 tRNA genes
Leu
Va
l
Phe
Ile
Gln
Asn
Gys
Tyr
Ala
Ser
Met
Trp
Asp
Ly
s
Thr
Pro
Glu
Leu
Ser
His
Arg
Gly
16s
ND4
ND4L
ND3
CO3
CO2
CO1
23s
ND1
ND2
CYB1
ND5
ND6
ATPsyn6
ATPsyn8
H
s
tr
a
n
d
L
s
t
r
an
d
● Figure 5-2 Location of mtDNA genes and their gene products. ND1, ND2, ND3, ND4L, ND4, ND5, ND6
genes
for the seven subunits of the NADH dehydrogenase complex; CO1, CO2, CO3
genes for the three subunits of the cy-
tochrome oxidase complex; ATP synthase 6, ATP synthase 8
genes for the two subunits of the F
0
ATPase complex;
CYB1
gene for the one subunit (cytochrome b) of ubiquinone-cytochrome c oxidoreductase complex; Phe pheny-
lalanine tRNA gene; Val
valine tRNA gene; Leu leucine tRNA gene; Ile isoleucine tRNA gene; Met methionine
tRNA gene; Trp
tryptophan tRNA gene; Asp asparagine tRNA gene; Lys lysine tRNA gene; Gly glycine tRNA
gene; Arg
arginine tRNA gene; His histidine tRNA gene; Ser serine tRNA gene; Thr = threonine tRNA gene; Pro
proline tRNA gene; Glu
glutamic acid tRNA gene; Tyr tyrosine tRNA gene; Cys = cysteine tRNA gene; Asn
asparagine tRNA gene; Ala
alanine tRNA gene; Gln glutamine tRNA gene; 16S 16S rRNA gene; 23S 23S rRNA
gene; rRNA
ribosomal RNA; tRNA transfer RNA.
MITOCHONDRIAL PROTEIN-CODING AND RNA-CODING GENES
TABLE
5-1
13 Protein-coding genes
Seven subunits of the NADH dehydrogenase complex (i.e., ND1, ND2,
ND3, ND4L, ND4, ND5, and ND6; Complex I); three subunits of the
cytochrome oxidase complex (i.e., CO1, CO2, and CO3; Complex IV);
two subunits of the F
0
ATPase complex (i.e., ATP synthase 6 and ATP
synthase 8); one subunit (cytochrome b) of ubiquinone-cytochrome c
oxidoreductase complex (i.e., CYB1; Complex III)
24 RNA-coding genes
2 rRNAs (16S and 23S) 22 tRNAs (corresponding to each amino acid)
LWBK771-c05_p29-32.qxd 9/29/10 6:52PM Page 30 aptara
31
THE HUMAN MITOCHONDRIAL GENOME
A. All other mitochondrial proteins (e.g., enzymes of the citric acid cycle, DNA poly-
merase, RNA polymerase) are encoded by nuclear DNA, synthesized on cytoplasmic
ribosomes, and then imported into the mitochondria.
B. The importation of proteins into mitochondria is assisted by chaperone proteins
(cytoplasmic hsp70, matrix hsp70, and hsp60), which keep the protein in an unfolded
state during importation.
C. The unfolded proteins enter the mitochondria through an import channel called ISP42.
In general, mitochondrial diseases show a wide degree of
severity among affected individuals. This variability is caused, in part, to the mixture of
normal and mutant mtDNA present in a particular cell type (called heteroplasmy). When
a cell undergoes mitosis, mitochondria segregate randomly in the daughter cells. This
means that one daughter cell may receive mostly mutated mtDNA and the other daughter
cell mostly normal mtDNA. Mitochondrial disorders show a threshold level where a criti-
cal level of mutated mitochondria must be reached before clinical symptoms appear. A
woman who has not reached the threshold level can still have affected children. In addi-
tion, mitochondrial diseases affect tissues that have a high requirement for ATP (e.g., nerve
and skeletal muscle). Mitochondrial diseases include the following:
A. MYOCLONIC EPILEPSY WITH RAGGED RED FIBERS SYNDROME (MERRF)
1.
MERRF is a mitochondrial genetic disorder caused by a mutation in the tRNA
Lys
gene whereby a A S G transition occurs at nucleotide position 8344 (A8344G).
2.
The mutated tRNA
Lys
causes a premature termination of translation of the amino
acid chain (the amount and the aminoacylation activity of the mutated tRNA
Lys
is
not affected).
3.
Mitochondrial enzymes with large number lysine residues will have a low proba-
bility of being completely synthesized. In this regard, NADH dehydrogenase (Com-
plex I) and cytochrome oxidase (Complex IV) both of which have a large num-
ber of lysine residues have been shown to be synthesized at very low rates.
4.
Heteroplasmy is common and expression of the disease is highly variable.
5.
Clinical features include
myoclonus (muscle twitching), seizures, cerebellar
ataxia, dementia, and mitochondrial myopathy (abnormal mitochondria within
skeletal muscle that impart an irregular shape and blotchy red appearance to the
muscle cells, hence the term ragged red fibers).
B. LEBER’S HEREDITARY OPTIC NEUROPATHY (LHON)
1.
LHON is a mitochondrial genetic disorder caused by three mtDNA missense mu-
tations which account for 90% of all cases worldwide and are therefore designated
as primary LHON mutations.
2.
The primary LHON mutations include the following:
a.
A mutation in the ND4 gene (which encodes for subunit 4 of NADH dehy-
drogenase; Complex I) whereby a A S G transition occurs at nucleotide po-
sition 11778 (A11778G). This is the most common cause (
50% of all LHON
cases) of LHON.
b.
A mutation in the ND1 gene (which encodes for subunit 1 of NADH dehy-
drogenase; Complex I) whereby a G S A transition occurs at nucleotide po-
sition 3460 (G3460A).
c.
A mutation in the ND 6 gene (which encodes for subunit 6 of NADH dehy-
drogenase; Complex I) whereby a T S C transition occurs at nucleotide posi-
tion 14484 (T14484C).
LWBK771-c05_p29-32.qxd 9/29/10 6:52PM Page 31 aptara
32
CHAPTER 5
3.
All three primary LHON mutations decrease production of ATP such that the de-
mands of a very active neuronal metabolism cannot be met and suggest a common
disease-causing mechanism.
4.
Heteroplasmy is rare and expression of the disease is fairly uniform. Consequently,
the family pedigree of LHON demonstrates a typical of mitochondrial inheritance
pattern.
5.
Clinical features include
progressive optic nerve degeneration that results clini-
cally in blindness, blurred vision, or loss of central vision; telangiectatic microan-
giopathy; disk pseudoedema; vascular tortuosity; onset occurs at
20 years of age
with precipitous vision loss; and affect males far more often than females for some
unknown reason.
C. KEARNS-SAYRE SYNDROME (KS)
1.
KS is a mitochondrial genetic disorder caused by partial deletions of mitochondr-
ial DNA (delta-mtDNA) and duplication of mitochondrial DNA (dup-mtDNA).
The partial deletions of mtDNA have been associated with a marked reduction in
the enzymatic activity of NADH dehydrogenase (Complex I), succinate dehydro-
genase (Complex II), ubiquinone-cytochrome c oxidoreductase (Complex III), and
cytochrome oxidase (Complex IV).
2.
Heteroplasmy is common and expression of the disease is highly variable.
3.
Clinical features include
chronic progressive external ophthalmoplegia (degener-
ation of the motor nerves of the eye), pigmentary degeneration of the retina (“salt
and pepper” appearance), heart block, short stature, gonadal failure, diabetes mel-
litus, thyroid disease, deafness, vestibular dysfunction, and cerebellar ataxia and
onset occurs at
20 years of age.
D. MITOCHONDRIAL MYOPATHY, ENCEPHALOPATHY, LACTIC ACIDOSIS, AND
STROKE-LIKE EPISODES SYNDROME (MELAS)
1.
MELAS is a mitochondrial genetic disorder caused by a mutation in the tRNA
Leu
gene whereby a A S G transition occurs at nucleotide position 3243 (A3243G).
2.
The mutated tRNA
Leu
causes a reduction in the amount and the aminoacylation of
the mutated tRNA
Leu
, a reduction in the association of mRNA with ribosomes, and
altered incorporation of leucine into mitochondrial enzymes.
3.
Mitochondrial enzymes with a large number of leucine residues will have a low
probability of being completely synthesized. In this regard, cytochrome oxidase
(Complex IV) has been shown to be synthesized at very low rates.
4.
Heteroplasmy is common and expression of the disease is highly variable.
5.
Clinical features include
mitochondrial myopathy, encephalopathy, lactic acido-
sis, and stroke-like episodes.
LWBK771-c05_p29-32.qxd 9/29/10 6:52PM Page 32 aptara