Lecture 22 - Extranuclear inheritance Flashcards
(26 cards)
Where can extranuclear inheritance occur?
- DNA present in the cell, outside of the nucleus.
- Chloroplast DNA
- Mitochondrial DNA
- Maternal RNA
Chloroplasts - present in green plants, photosynthetic protists & blue-green algae; site of photosynthesis.
What is the role of Chloroplast DNA?
Chloroplasts DNA encodes important genes required for photosynthesis. Mutations can impact photosynthesis, and therefore the success & life of the plant
What do chloroplasts?
Outer membrane - phospholipid bilayer on the outside of the chloroplast
Inner membrane - phospholipid bilayer inside the the outer membrane
Stroma - fluid which contains DNA & ribosomes
Thylakoids - membrane-bound structures; lumen containing chlorophyll
Grana: stacks of thylakoids
Photosystem proteins found in thylakoid membranes
What does the Stroma contain?
DNA & proteins
What do stacks of thylakoids contain?
chlorophyll
What is present within thylakoids?
Proteins (for photosynthesis)
What are features of Chloroplast DNA (cpDNA)
- Double-stranded & circular
- Majority of species (100-225kb)
- low GC content (36%)
- Genes arranged into operons, as in prokaryotes
- cpDNA does contain introns
- Contains- 2 inverted repeats of varying length (10-76 kb; 4-150 genes)
- Encodes: subunits from PSI & PSII, Ribosomal proteins, polymerases, tRNAs & rRNAs.
Variable number of bases, due to variation in species
Parallel with prokaryotic genomes
Variation in inverted-repeats causes biggest variation in size.
Where are the subunits required to assemble the photosynthetic apparatus encode?
- in chloroplast DNA
- in nucleus
Subunits coming from 2 different genomes, assembling to make vital cellular apparatus. Intergenomic communication is required.
Dependent on nuclear encoded proteins to function.
Within chloroplast DNA, there are enzymes. Some of the apparatus required to replicate, translate & transcribe genes from chloroplast DNA are encoded in the cpDNA, but most are nuclear encoded.
Chloroplast DNA enzymes - some proteins used to translate & replicate chloroplast DNA in encoded in nucleus.
How does Chloroplast DNA (cpDNA) vary in copies per cell?
Number of copies per cell (copy number) varies:
- in all species, multiple (10-1000) cpDNA per chloroplast
- up to 50 chloroplasts per cell
cpDNA is associated with membranes (thylakoid or chloroplast inner membrane)
Chloroplasts are highly dynamic & divide by binary fission; they must be inherited during cell division.
cpDNA is replicated prior to chloroplast division; molecular mechanisms vary between species.
Cell copy number is cumulative total of all chloroplasts in the cell - up to 50 chloroplasts per cell.
cpDNA associated with other proteins and other membranes - e.g. thylakoid membrane.
In inheritance, as the chloroplast grows, they distribute the DNA before reproduction via binary fission.
Chloroplast DNA has to be replicated before cell division.
Where does variegation come from?
Some variegation comes from mutation in chloroplast DNA. On a plant, there could be some branches all green, some variegation pattern, some all white.
Took eggs & pollen from plant & carried out crossing experiments.
Results from Crossing experiments
Phenotype of progeny, matches phenotype providing egg, regardless of phenotype of pollen.
Variegated eggs leads to all 3 possible phenotype.
How can heteroplasmy lead to homoplasmy?
Through mitotic segregation
Heteroplasmic (combination of chloroplasts containing wild-type & mutant DNA)
Mitotic divisions - random/vegetative/mitotic segregation of organelle DNA.
This can lead to homoplasmy
What is Plasmy?
about cytoplasm of genomes. Heteroplasmic cell is a cell that includes chloroplast or mitochondrial genomes that differ - variability in genomes. Homoplasmic - exactly the same. It is important to remember there is many copies per cell.
Heteroplasmic cell has a mixed population of chloroplast DNA - some with & without mutation.
Process of random segregation (mitotic segregation), each cell division, each chloroplast genomes are separated to separate daughter genomes are separated to separate daughter cells at random.
This means occasionally, this can lead to a cell only having wild-type/mutation genome.
Most daughter cells will be heteroplasmic, but homoplasmic can arise.
Copy number important.
What can female gametes from a variegated stem produce?
Homoplasmons or heteroplasmons
During cell division through gamete production, some gametes will be homoplasmic. It is this process of random segregation through gamete production that determines whether we have homoplasmic or heteroplasmic eggs & therefore green, white or variegated progeny.
You can get white branches as some chloroplast will function, but some white will die due to lack pf chloroplast.
Summarise cpDNA inheritance
Conclusion of Correns:
- Inheritance of leaf/stem colour is “Non-Mendelian” & “Maternal”
- chloroplast DNA mutation leads to loss of chlorophyll
- chloroplast DNA is inherited from the egg only = maternal inheritance
- variegation results from wild-type & mutant tissue
- individual cells can contain a mixture of wild-type & mutant genomes = heteroplasmy
In heteroplasmic cell you can get compensation for mutant, there can still be enough chlorophyll for the plant to survive.
What is mitochondria structure?
- outer membrane, inner membrane & intermembrane space
- inner membrane - contains OXPHOS (oxidative phosphorylation) proteins
- Cristae - folds in the inner membrane
- Matrix - contains enzyme, DNA, ribosomes etc.
DNA complexed with different proteins & associated with inner mitochondrial membrane.
Describe features of Mitochondrial DNA (mtDNA)
- Double-stranded closed circle
- Generally smaller than cpDNA (16-18kb in mammals) - 75kb in yeast, up to 367 kb in plants.
- Introns rare; present in some of the larger genomes (e.g. Yeast)
Non-coding control regions:
D-loop - displacement loop (or control region)
HSP - H (heavy)-strand promoter region
LSP - L (light)-strand promoter region
OH - H-strand origin of replication
OL - L-strand origin of replication.
SMALLER THAN CHLOROPLASTS.
What does genome differing in size (between species) indicate?
The extent to which genes have moved from mtDNA to nucleus overt time & also the loss of genes (through redundancy).
No enzymes encoded - they all come from nucleus.
Co-assembly of nucleated products & mitochondrial products.
D-loop non-coding region - contains transcription start sites & origin of heavy replication (also lots of variation in SNPs.
Describe features of the mitochondrial genome.
- MtDNA from different species vary in: size (e.g. 16.6kb in humans vs 75 kb in yeast); presence of & size of introns; genes encoded.
There are multiple copies of mtDNA in each organelle & mutliple organelles per cell; copy number varies.
Copy numbers range from 20-1000 in somatic cells to over 150,000 in a mature oocyte.
Bacterial origins: mitochondrial ribosomes differ; variation in triplet code.
COPY NUMBER IMPORTANT: mitochondria has multiple genomes & multiple mitochondria is cell.
What is mtDNA variation?
- no recombination of parental alleles (unlike nuclear genome)
- mtDNA has a faster mutation rate (estimated 10x to 20x) than nuclear DNA; no protection from histones; poorer repair mechanisms
- particularly high level of variation in the D loop (control region) of mtDNA.
Mitochondrial DNA is replicated without recombination events - which usually introduce variation. Purpose of replication is to make exact copy, but lots of mutations made due to no protection from histones & poorer repair mechanisms - MUCH HIGHER MUTATION RATE.
Describe inheritance of human mtDNA
Conclusion - only affected females passed on polymorphism - therefore inheritance of mtDNA is maternal in humans.
Homoplasmic Polymorphisms that introduces a restriction site. Cuts the sequence of white people - CLEAR MATERNAL INHERITANCE
Mitochondrial DNA is maternally inherited in humans.
What is Maternal inheritance of mtDNA?
- mutations that occur in OOCYTES will be passed on
- this may initially result in heteroplasmy
- over time, random segregation can create a homoplasmic population
- The polymorphisms is then stably inherited by all future generations.
Overtime, we get an introduction of variation in mtDNA through people
What are mitochondrial haplogroups?
MtDNA sequence diversity has increased considerably over time.
However, proportion of SNPs is much smaller than in nuclear genome (e.g. 8-15 nucleotide differences between individual in HV1 & HV2)
Variation with similar variations of mtDNA being put into groups.
Not a high level of variation - 8-15 SNPs in hypervariable regions.
How can we use mtDNA to understand migration patterns?
mtDNA in African population is more diverse than other populations & genetic diversity in non-Africans is a subset of that found in Africans.
Mitochondrial Eve estimated to live in East Africa between 120,000 & 200,000 years ago.
Can be used to understand human migration patterns. Variation from the rest of the world is a subset of variation seen in Africa. Earliest mutations occurred in Africa. This is an example of how mitochondrial inheritance allows it to be a useful tool. Also useful due to high copy number.
