The Mitochondrial Genome Flashcards
Describe the function of mitochondria in the cell.
Mitochondria are important for ATP synthesis, the energy currency of the cell.
Energy is stored in nutrients (such as carbohydrates, lipids and proteins), is broken down and its reducing equivalents are used to produce ATP.
Hence, mitochondria are energy factories or ‘the powerhouse’ of the cell.
Mitochondria also have many other important and essential functions, such as haem synthesis.
Describe the mitochondrial genome.
- it’s a double stranded circular molecule (16.6kb) (15,000x smaller than chromosome 1)
- it consists of the heavy and light strand
- it’s a multicopy genome (10-100,000 copies per cell)
37 genes: - 13 oxidative phosphorylation protein subunits
- 22 transfer RNAs
- 2 ribosomal RNAs
- it has no introns
- the D-loop is a non coding region where replication and transcription are initiated
- it’s maternally inherited; there is no recombination
How is mtDNA packaged?
MtDNA is packaged into structures called nucleoids.
There are one or two copies of mtDNA per nucleoid.
The transcription factor A (TFAM) acts as histone protein.
How is mtDNA used?
Because mtDNA is maternally inherited, this gives rise to mtDNA haplogroups.
MtDNA does not recombine, and mutations acquired over time subdivide the human population into discrete haplogroups.
This is used by population geneticists to track migration of human populations.
Explain the endosymbiotic theory.
According the endosymbiotic theory, which is based on phylogenetic studies – the tracing of evolutionary ancestry of organisms through their DNA – it appears that a primitive eukaryotic cell ingested a bacterium and the bacterium survived and the two organisms benefited through a symbiotic relationship. The bacterium evolved to be what we now call mitochondria.
Most genes of the bacterium in animals have been transferred to the host nucleus but a few have been retained.
What do you need to replicate mitochondrial DNA?
Mitochondrial DNA replication occurs continuously in cells. To replicate mtDNA, you need:
- Polymerase γ (POLG): mtDNA polymerase
- TWINKLE: mtDNA helicase, unwinds DNA for replication
- single-stranded binding protein (SSBP): keeps DNA unwoound
- TFAM: packages and protects mtDNA.
Describe mtDNA polymerase.
It’s called Polymerase γ (POLG).
It’s a heterotrimer protein. It consists of one catalytic subunit (POLγA) and two accessory subunits (POLγB).
POLγA contains a 3’ – 5’ exonuclease domain to proofread newly synthesized DNA.
POLγB enhances interactions with DNA template and increases activity and processivity of POLγA.
Describe TWINKLE.
It’s a hexamer with six TWINKLE subunits.
It unwinds the double stranded mtDNA template to allow replication by POLγ.
Describe mtSSBP.
It binds to single stranded DNA to:
- protect against nucleases
- prevents secondary structure formation
- enhance mtDNA synthesis by stimulating TWINKLE helicase activity
Describe the strand displacement model of mtDNA replication.
There are many other models, but this is the main one:
- the replication is asynchronous as it replicates the heavy strand first and then the light strand
- the origin of the light chain allows the replication to occur in the opposite direction
- this continues until replication of both strands is completed
Why are mtDNA strands named heavy and light?
Heavy chains have more guanine bases than light chains/ The mass is bigger because of the type of base it has (guanine is a purine, which are bigger than pyramidines).
What types of cells/tissues might have high copied of mtDNA?
Cells that need to do a lot of oxidative phosphorylation will probably have lots of mtDNA.
Generally, describe mitochondrial diseases.
- they are rare monogenic diseases
- they affect highly metabolic organs
- they’re abundant in mitochondria
- they can affect one or several organ systems
- they can start at any age
- they have a wide disease spectrum (e.g. hearing loss, fatal cardiomyopathy in infancy)
- they’re genetically heterogenous
How could you diagnose mitochondrial disease using imaging and biochemical tests?
A common characteristic of Leigh syndrome (a mitochondrial disease) is the white matter regions necrosing in the basal ganglia.
Ragged red fibres occur when clumps of diseased mitochondria accumulate in the subsarcolemmal region of the muscle fibre; they appear as RRFs when the muscle is stained with modified Gomori trichrome stain. The mitochondria aggregate around the muscle cell.
Thus, people suspected of having mitochondrial diseases have a muscle biopsy to see what’s happening with the oxphos complex.
You can measure the activity of all the complexes in a test tube (mitochondria respirometry).
Describe mitochondrial disease inheritance.
Since we know there are nuclear genes which encode mitochondrial proteins, you could expect to see autosomal recessive patterns of inheritance of mitochondrial diseases, dominant, X-linked and de novo patterns.
