DNA Replication 2 - Eukaryotes Flashcards
Why is it important to understand the aspects of mitochondrial DNA?
Mitochondrial DNA is associated with disease states related to metabolism and energy production. Recent studies indicate that mitochondrial dysfunction may be more prevalent than previously thought (1/4300 live births lifetime risk).
What are the 3 unique aspects of mitochondrial DNA replication?
- Each mitochondrion contains 2 to 10 copies of the approximately 17,000 base pairs in a circular genome that are not evenly distributed during cellular division.
- Many of the proteins required for mitochondrial function are nuclear encoded, such as the enzyme responsible for mitochondrial DNA replication (DNA polymerase gamma).
- Heteroplasmy
What is Heteroplasmy in relation to mitochondrial DNA?
Heteroplasmy occurs when one cell contains multiple types of mitochondrial DNA with varying levels of each type from one cell to the next. Thus, unlike nuclear DNA (constant from one cell to the next), mitochondrial DNA can vary from one cell to the next, causing a complicated disease pattern when deleterious mutations are present.
There is a link between Heteroplasmy and mutations with strong dysfunction. What are some of the diseases and why are they rare?
- Diseases include myoclonic ataxia and epilepsy with ragged red fibres (MERRF), mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS), and Kearn-Sayre syndrome.
- Most are de novo mutations and the afflicted individual does not live beyond childhood so they do not pass on the mutation.
Where do mutations that cause weak or moderate dysfunction tend to occur?
What is an example of each?
- Mutations that cause weak or moderate dysfunction tend to occur in a Homoplastic state when exerting an effect on disease. Weak dysfunction alleles tend to only be linked with disease and may exert their effects in combination with environmental factors like smoking.
- Weak = Parkinson’s
Moderate = Leber hereditary optic neuropathy (LHON)
What is mitochondrial DNA depletion syndrome?
Mitochondrial DNA depletion syndrome is progressive disease affecting children and those treated for HIV (AZT drug is nucleotide analog that targets viral replication but may also interact with DNA polymerase gamma).
Metabolic defects in replication of mitochondrial DNA result in deletion of this DNA and associated pathology.
What is the pathology of mitochondrial DNA depletion syndrome?
- 13 members of the ETC are encoded in mitochondrial DNA. Their loss inhibits oxidative phosphorylation.
- Prominent hepatic abnormalities may be explained by the dramatic postnatal developmental changes and mitochondrial adaptation that occurs in this organ in the first few months and years of life.
- Postnatal adaptive changes in skeletal muscle mitochondria occur only later.
What do the non-hepatic (more encephalomyopathic) forms of mitochondrial DNA depletion reflect?
Encephalomyopathic forms may reflect early disturbances in the myogenic program that result in either destabilization of the muscle cell membrane or physical muscle cell breakdown with measurable increases in creatine kinase (CK) from muscle.
Muscle, liver, and brain all undergo encounters with the postnatal environment.
What is the significance of the high ratios of NADH to NAD+ found in the severe deficiencies of complexes I, III, and IV in a patient with mitochondrial DNA depletion?
This should decrease mitochondrial beta oxidation and provide a mechanism for impaired fatty acid oxidation.
What is the “red flag” clinical presentation of mitochondrial disorders?
The “red flag” is multi-organ disturbance with the highly energy-demanding organs (brain, liver, skeletal muscle) involved.
What diseases have been linked to mitochondrial DNA depletion?
Alpers syndrome, progressive external opthalmoplegia, and ataxia-neuropathy syndrome are linked to mitochondrial DNA depletion. (Aging is also associated with mitochondrial DNA damage.)
The domains of the mitochondrial targeting sequence are associated with what functions?
- Exonuclease domain = proofreading
- Linker region = lower protein activity
- Polymerase domain = elongation
(See schematic on page 21 of Notes)
What is the major difference between eukaryotic and prokaryotic DNA replication in regards to time and space?
Eukaryotic DNA synthesis occurs from numerous origins of replication, spaced about every 30,000 to 300,000 base pairs and the replication forks proceed at 1/20th the pace of prokaryotic replication forks.
What is the major difference between eukaryotic and prokaryotic polymerases?
Eukaryotic polymerases are denoted with Greek letters rather than Roman numerals.
What is the function of eukaryotic DNA polymerase alpha?
DNA polymerase alpha contains Primase (lays down primer) and initiates DNA synthesis. (Other enzymes are then recruited to continue synthesis.)
