Chlorplast and mitochondrial genomes Flashcards
The origins of mitochondria and chloroplasts
Endosymbiosis
Mitochondria: ancestral host engulfing an aerobic alpha proteo-bacteria
chloroplast: Ancestral eukarote engulfing a photosynthesising cyanobacteria
Engulfed DNA underwent reductive evolution as they lost genes need for free living and DNA was transferred to the host nuclei.
-> Most genes within these organelle are encoded by the nuclear genome. (>90%)
Uniparental inheritance of organelles
Chloroplast and mitochondria tend to undergo maternal inheritance as the egg contributes the bulk of the cytoplasm to the zygote.
Study proving this: Correns and the 4 o’clock plant
- This plant has green/ white or mixed stems and the colour is determined by the plastids (chloroplasts)
- When flowers on white stems are pollinated they produce white plants and when flwoers on green plants are pollinated they produce green plants (the genotype of the pollen does not effect the progeny)
Further proved by study of mitochondrial inheritance in yeast
Comparison of mt/ cpDNA size in plants and animals
Organellar DNA is considerbly smaller than nucelar DNA
mt/cpDNA are also much smaller than their ancestors due to reductive evolution. (example: malaria parasite (Plasmodium) mtDNA is highly reduced, and encodes only 3 proteins)
Comparison:
mtDNA plants > ctDNA > mtDNA animals
However, the size of the genome does not correlate with the number of genes encoded.
e.g.) A.thaliana
- mtDNA is larger than cpDNA yet the cpDNA encoded 87 proteins compared to 34 proteins
Nuclear genome size also does not correlate with organellar genome size
Characteristics of organelles genomes and similarities to prokaryotes
They have certain prokaryotic features due to their prokaryotic ancestry
Small, gene-dense, circular DNA when mapped (not always circular in vivo)
Exist as nucleoids (lack centromere, telomeres, histones)
-> TFAM bind mtDNA into packages in animals by bridging DNA
Many DNA copies per organelle + multiple organelles per cell
Prokaryotic transcription / translation machineries (uses POLRMT)
Some genes transcribed together in polycistronic RNAs (2+ separate proteins encoded on single mRNA molecule)
Genetic code / codon usage may deviate from standard
Organelle transcripts subject to RNA editing (C-to-U editing) – changes coding sequence creating a start / eliminating a stop codon
AT – rich
Similarity of mitochondrial and prokaryotic expression in mammals
Machinery has evolved away from bacteria expression machinery and is similair to bacteriophage machinery.
Similarities:
- Uses the same RNA polymerase (POLRMT) as bacteriophage but uses TF unique to the organelle.
-> e.g. TFAM which is also responsible for packaging DNA into nucleoides
- Single origin of transcription and transcription proceeds in both directions.
Why is the plant mtDNA genome larger than animal?
Plants have larger mtDNA genomes which vary.
Even though they vary in size, the number of genes coded remains relatively constant.
This is due to non-coding, ‘extra’ DNA.
- Some is derived from chloroplast, nuclear or viral DNA
- Some seems to have been acquired by horizontal transfer from other plants
- However, most non-coding mtDNA is of unknown origin
Mutantion rates in plant vs animal organelles
Lower mutation rate in plant organelles due to MSH1 which mediates efficient recognition and correction of DNA sequence errors
Structure of plant mtDNA
Circles are rare or absent
Subgenomic circles of overlapping linear fragments. (still maps as circular)
Many repeats which enables homologous recombination leading to variability in structural organisation.
Loss of mtDNA
- mtDNA can be lost creating mitosomes
Structure of plant cdDNA
Most cpDNAs have the following organization:
* A long single-copy region, LSC
* A short single-copy region, SSC
* Two inverted repeats (IRs) of ~20-25 kb
Tipically encode 100 genes are the identical and sequence of these genes is higly conserved between species.
Loss of CdDNA
- Plastid genome can be lost just leaving the compartment which has important metbaolic functions
- Occassionally the compartment can also be lost
organellar disease
Mutations in mtDNA and cdDNA can lead to disease.
- cdDNA: alinism in plants
- mtDNA: Mcolic epilepsy
Solution: Mitochondrial replacement therapy where nuclear genome is inserteed into donor egg leading to 3 parent offspring
Overview
Organelles have similarities and unique characteristics compared to prokaryotes.
The are maternally inherited and have undergone reductive evolution.
The organelles vary in size but this does not correlate to the number of genes.
plant mtDNA > cdDNA < animals mtDNA
mtDNA and cdDNA have different structures
