Mitochondrial Genome

Question 1

describe the theorized symbiotic relationship of mitochondria and human cells

Answer 1

mitochondria provide ATP, human cell host provides everything else

Question 2

describe the major features of mtDNA that differentiate it from nuclear DNA

Answer 2

double stranded, circular, multiple copies of mitochondrial genome per mitochondria (vs single copy in nucleus)

far fewer base pairs (bp) than nuclear DNA and only 37 genes

Question 3

name 3 things that mtDNA genes code for

Answer 3

1. oxidative phosphorylation machinery proteins (13 genes)
2. mitochondrial tRNA
3. mitochondrial rRNA

Question 4

____ is the most efficient way for cells to generate ATP

Answer 4

oxidative phosphorylation

Question 5

mutations in mtDNA affect what kind of tissues (name 3 examples)

what kind of symptoms may this cause?

Answer 5

tissues with high metabolic demand - nervous, cardiac, muscle

mitochondrial diseases are often multi-systemic (all tissues contain cells with mitochondria)

symptoms associated with mtDNA mutation: encephalopathy, myopathy, ataxia, retinal degeneration, deafness, lactic acidosis (due to increased anaerobic respiration)

Question 6

name the important enzymes involved in each complex of oxidative phosphorylation in the mitochondrial matrix

Answer 6

Complex I: NADH Dehydrogenase (FMN substrate)

Complex II: Succinate Dehydrogenase (FADH2 substrate)

Complex III: Cytochrome bc1 (Cytochrome B substrate, produces cytochrome c)

Complex IV: Cytochrome C Oxidase (Cytochrome C substrate)

Complex V: ATP Synthase (ADP substrate)

Question 7

which complexes of oxidative phosphorylation produce hydrogen, and in what direction is it pumped (in/out mito matrix)?

Answer 7

Complex I: pumps H+ OUT of mito matrix
Complex III: pumps H+ OUT of mito matrix
Complex IV: pumps H+ OUT of mito matrix

Complex V: pumps H+ IN mito matrix

(complex II is also part of TCA, connects TCA to ETC)

Question 8

T/F: mutations in nuclear genes can cause defects in mitochondrial function?
Explain your answer.

Answer 8

TRUE: majority of proteins found in mitochondria are encoded by nuclear genes

(precursor proteins imported into mitochondria, combined with proteins synthesized in mitochondria)

Question 9

how many chromosomes of mitochondrial genome are there in the mitochondria

Answer 9

one! (vs 46 nuclear chromosomes)

Question 10

which of these statements is TRUE:

a. mitochondrial genome is associated with histones
b. mitochondrial genome replicates dependently of nuclear DNA
c. mitochondrial genome lacks introns

Answer 10

mitochondrial genes LACK INTRONS

mitochondrial genome is NOT associated with histones, replicates INDEPENDENTLY of nuclear DNA

Question 11

why does mtDNA have a higher rate of mutation? provide 4 reasons

Answer 11

1. more exposure to ROS damage - byproduct of cellular respiration [—> mtDNA mutations increase with age due to accumulated ROS damage, and decreased levels of oxidative phosphorylation and therefore energy follows]
2. lack DNA repair mechanisms
3. DNA pol gamma (y) has lower fidelity than nuclear polymerase
4. no introns —> mutations are within mostly coding region

Question 12

what does current evidence suggest regarding maternal inheritance of mtDNA

Answer 12

MITOPHAGY: paternal mitochondria are eliminated from zygote following fertilization

Question 13

mtDNA replicate independently of cell cycle within what portion of the mitochondria? What is a special feature of this replication?

Answer 13

mitochondrial matrix

mitochondria segregate randomly among daughter cells

also, athletes have increased mt replication due to aerobic demand, so their muscles have more mitochondria

Question 14

describe heteroplasmy of mitochondria, its cause, and its clinical implication

Answer 14

mitochondria sort (segregate) randomly among daughter cells, causing heteroplasmy - presence of more than one type of mitochondrial genome in a cell (healthy vs mutated)

proportion of mutant mtDNA molecules determines severity of disease expression —> exceeding the disease threshold causes cellular dysfunction (and if enough cells are dysfunctional —> phenotype of disease)

Question 15

describe the bottleneck effect of mtDNA amplification

Answer 15

mtDNA copies/cell decrease during development, then recovers

supposed that cell is “ridding” itself of nonfunctional/mutant mitochondria, then follows with replication of functional mitochondria

Question 16

T/F: offspring can only inherit mitochondrial defects from female mothers, but not all offspring of females with mitochondrial defects will inherit disease

Answer 16

TRUE: due to random mitochondrial segregation and the threshold effect, only offspring who inherit a large enough proportion of mutant mitochondria to exceed threshold will show phenotype of disease
(and severity of that disease/ variation of symptoms is dependent on how large that proportion is)

Question 17

T/F: different types of tissues harboring the same mtDNA mutation may be affected to different degrees

Answer 17

TRUE: heteroplasmy and threshold effect can lead to selective organ involvement

Question 18

neuroimaging, cardiac evaluation, muscle biopsy, and measuring lactate blood concentration are all diagnostic tests of what type of disease (think about these tissues)

Answer 18

mitochondrial diseases - these are high energy demand tissues (and lactate is a byproduct of anaerobic respiration)

Question 19

what histological finding is characteristic of mitochondrial disease, and what stain is issued to identify it?

Answer 19

ragged-red fibers due to aggregating mitochondria

stain with gomori trichrome

Question 20

describe mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS)

Answer 20

brain dysfunction combined with muscle weakness (hemiparesis)

mitochondrial disease, presents with ragged-red fibers (RRFs)

Question 21

this mitochondrial disease presents with brain dysfunction and hemiparesis. Under a microscope of an affected tissue, red-ragged fibers are visible. What is?

Answer 21

MELAS: mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes

Question 22

contrast IVF mitochondrial replacement with pronuclear transfer after fertilization

Answer 22

IVF mito replacement: put mother’s DNA in donor woman’s egg with healthy mitochondria and fertilize with father’s sperm (then implant)

pronuclear transfer: put healthy DNA from couple’s fertilized egg into donor couple’s fertilized egg which has had its nuclear DNA removed (then implant)