Mitochondrial genome Flashcards
How large is the mitochondrial genome?
Tiny compared to the nuclear genome. 16 000 bp vs 3.2 billion bp
What are 4 physical features of the mitochondrial genome?
- Small, circular dsDNA
- GC content of about 44%, similar to the nuclear genome
- Heavy and light strands, distinct density difference between them
- Small triple stranded region with the D-loop
How are the heavy and light strands different from each other?
Base composition. The H strand has more G and the L strand has more C. They can be separated by density
What physical features does the small triple stranded region have?
2 copies of the heavy strand and one of the light strand. Only one H strand is base pairing to the L strand. The other is displaced into the D-loop
What sequences are found in the triple stranded region?
The sequence on the heavy and light strands base pairing to each other is 7S rDNA. The D-loop contains control sequences like origins and promoters
What does the D-loop do?
We don’t know. It might help with replication or anchor mtDNA to the inner mitochondrial membrane
What’s weird about the replication of mitochondrial DNA?
It’s unidirectional, instead of bidirectional like normal. Each strand has its own ori
What are the steps in mitochondrial DNA replication?
- The ori for the heavy strand, found in the D-loop, goes off
- The H strand gets replicated counterclockwise until its about 2/3 of the way around
- DNA around the ori of the light strand gets separated and it goes off
- The L strand is replicated clockwise
How many genes are in the mitochondrial genome? Are they evenly distributed on both strands?
37 total. 28 of them are on the H strand and 9 are on the L strand
How many of the 37 genes in the mitochondrial genome are protein coding? What do they code for?
- All of them encode some part of the electron transport chain
Does the mitochondrial genome encode every component of the electron transport chain?
Nope. It encodes some, but the rest are encoded in the nuclear genome
What’s encoded by the genes in the mitochondrial genome that do not encode proteins?
RNA. 22 of those 24 encode tRNA, and the other two are rRNA
What is strange about mitochondrial DNA promoters?
There’s only 2 in the whole genome, one on each strand. The mRNA generated is polycistronic like it is in prokaryotes
What 4 traits does the mitochondrial genome share with bacteria?
- Circular
- Polycistronic mRNAs
- No introns
- High gene density with little intergenic space
What is the mutation rate of the mitochondrial genome compared to the nuclear genome?
10X higher than the nuclear genome
Why does the mitochondrial genome have a high mutation rate?
- High concentration of ROS
- No histones, so easier access to DNA by mutagens
- mitochondrial DNA polymerase sucks at proofreading
- Concentrations of dNTPs in the mitochondria
What is the result of this high mutation rate?
Heteroplasmy. Get a mixture of mtDNA molecules within the same mitochondria and within the same cell
How can mitochondrial diseases be shown in certain cell lineages and not others?
Random segregation and a selective advantage of disease carrying mtDNA molecules. If the mutants end up in one cell but not another, then that lineage will keep propagating the diseased mitochondria
Why are some tissues more affected in mitochondrial diseases than others?
They’ll show a worse phenotype if they have a higher demand for ATP and functioning mitochondria
How did the in vitro replication experiment show that the imbalance in dNTP concentrations may be partially responsible for the high mutation rate?
The authors used mtDNA polymerase, both WT and a mutant with defective proofreading. They used the LacZ gene as a reporter, and 1 uM of each dNTP as a baseline. The mutation rate went up even with the WT enzyme when the dNTP concentrations were imbalanced
What are 3 reasons why we typically consider mitochondrial diseases to be maternally transmitted?
- Sperm mitochondria get diluted out by the sheer number of egg mitochondria
- Egg destroys sperm mitochondria
- The sperm mitochondria don’t get into the egg in the first place
