Extra chromosomal genetics Flashcards
What is cpDNA?
Chloroplast DNA, genes encode photosynthetic proteins.
What is mtDNA?
Mitochondrial DNA, genes encode components of the cellular aerobic respiration apparatus.
How do chloroplast and mitochondrial genomes get passed to offspring?
Separately to the nuclear genome. Most commonly maternal, but varies between organisms.
What does organelle maintenance require?
Cooperation between nuclear and organelle genomes. Organelle genomes are rather prokaryotic.
Mitochondrial DNA?
Replicates and expresses all of the genes in its own DNA, but this genome only encodes some of the proteins required for their activities.
MtDNA is in highly condensed structures - nucleoids. Circular DNA.
Variable number of mitochondria per cell.
In general, mitochondria duplicate and segregate in each cell generation independently of nuclear DNA.
Human mtDNA genome?
37 genes including for oxidative phosphorylation, tRNA, rRNA, but no introns.
How is the mitochondrial DNA transcribed and translated?
Genes are transcribed in the mitochondria, using RNA polymerase imported from the cytoplasm (nuclear genome origin).
Translation is carried out using mitochondrial rRNAs and tRNAs. Doesn’t follow universal genetic code.
Mitochondrial genetic codes probably diverged from the universal code by mutations arising after the organelles became established components of eukarytotic cells.
Features of chloroplast genomes?
Genomes are more uniform in size than mitochondria. Many more genes, but also introns.
Circular.
92 protein encoding genes - for photosynthesis proteins, chloroplast gene expression, RNA polymerase.
36 tRNA genes
What cooperation is needed with organelle genomes?
Cytochrome is composed of 7 subunits, 3 of which are mtDNA encoded, the others nuclear encoded.
How did organelles arise?
By endosymbiosis.
The cell which engulfed mitochondrion/choroplast like bacterial cell became more productive in terms of enerygy production.
Eventually get a stable symbiotic stage. The engulfed cell could lose some genes critical for independent life. This loss could occur through transfer of genes to the nucleus or the deletion of redundant sequences.
What is the hydrogen hypothesis?
Proposed as an explanation for how the mitochondrion may have arisen as an endosymbiont within a prokaryote.
Host - hydrogen dependent archaeon. Similar to modern methanogenic archaea which use hydrogen and carbon dioxide to produce methane.
Future mitochondrion produced hydrogen and carbon dioxide as by products of anaerobic respiration.
Mutation in mtDNA?
High rate of mutation. The mtDNA of vertebrates evolved 10 times faster than the nuclear DNA of those same organisms.
Probably due to less efficient DNA replication and repair, coupled with generation of free radicals as by products of oxidative phosphorylation.
Tissues with high energy demands are least tolerant of mutant mitochondria.
Mitochondrial disease?
Leber’s hereditary optic neuropathy:
Defect in mitochondrial electron transport chain leads to optic nerve degeneration and blindness which is always passed on through the mother. Nearly always caused by a mutation in the human mitochondrial genome.
However, in affected families, not all offspring show signs of the disease, and not all siblings with the condition are affected to the same effect – due to random allotment of mitochondria during mitosis.
Maternal gamete is much larger, therefore will donate most of the organelles. In some organisms, cells degrade any organelles which are donated by male gametes or events of fertilisation may only allow the sperm nucleus to enter the egg.
Different mixtures of mitochondria found in cells?
Heteroplasmic – contain a mixture of organelle genomes
Homoplasmic – contain only one type of organelle DNA
Due to random mitotic assortment of organelles, the mitotic progeny of heteroplasmic cells may be heteroplasmic, homoplasmic wildtype or homoplasmic mutant
