Jones (extranuclear inheritance) Flashcards
In what organisms are chloroplasts present?
- green plants
- photosynthetic protists
- blue-green algae
What are the diff parts of a chloroplast?
- DIAG*
- OM and IM are phospholipid bilayers
- stroma = fluid containing DNA and ribosomes
- thylakoids = membrane bound structures, w/ lumen containing chlorophyll
- grana = stacks of thylakoids
What are the characteristics of cpDNA?
- ds and circular genome
- majority of species 100-225kb
- low GC content (36%), compared to 64% in nucleus
- genes arranged into operons (like proks)
- contains introns (unlike proks)
- copy no. varies –> 10-1000 copies per chloro and up to 50 choros, in all species
- assoc w/ thylakoid membrane or IM
What does cpDNA encode?
- subunits from PS1 and PS11 (also parts encoded by nucleus)
- ribosomal proteins
- pols
- tRNAs and rRNAs
What does cpDNA contain?
- 2 inverted repeats of varying length (10-76kb) = most variable part between species
- long single copy region
- small single copy region
How are chloroplasts inherited, and why?
- during cell division
- highly dynamic and divide by binary fission
- rep before chloro division (mol mechanisms vary between species)
What result would a white leaf/stem mutation have?
- lethal
- no chlorophyll, so can’t photosynthesise
- can germinate, but then dies
How did Correns study variegation in Mirabilis jalapa, and what did he observe?
- some plants white, some green and some mixed (variegated)
- utilised fact plants can have stems of diff colours and carried out self fertilisations to see how inherited
- saw if egg from white branch all progeny white and if egg from green branch all progeny green, regardless of phenotype of branch providing pollen
- when variegated phenotypes provided egg, phenotype of progeny could be white, green or variegated
- random segregation in every cell division, so female gametes can be derived from cells which are homoplasmons or heteroplasmons
What is homoplasmy and heteroplasmy?
- DIAG*
- heteroplasmon = combo of chloros containing WT and mutant DNA (due to foreign pop coming in or mutation
- homoplasmon = all copies in organelle are identical
What was Correns conclusion?
- inheritance of leaf/stem colour is “non-Mendelian” and maternal
What do we know now about Correns experiment?
- cpDNA mutation leads to loss of chlorophyll
- cpDNA inherited from egg only = maternal inheritance
- variegation results from WT and mutant tissue
- indiv cells can contain mixture of WT and mutant chloros = heteroplasmon
What is the maternal effect?
- maternal nuclear gene products (proteins and RNA) stored in egg (large w/ lots of cyto) and get transmitted to offspring following fertilisation
- these maternally encoded proteins can exert phenotypic effect on offspring, regardless of offspring genotype
- in some species, req for successful embryo dev, in others just changes certain aspects of phenotype
How does shell coiling differ in Limnaea to most snail species?
- in most species all individuals share same coiling pattern
- Limnaea shells can be coiled in either direction
- -> dextral = right-handed (DD or Dd)
- -> sinistral = left handed (dd)
How was Limnaea peregra studied to show its an example of the maternal effect?
- DIAG*
- maternal effect observed in gens II and III
- maternal parent genotype controls offspring phenotype
- spiralling of shell happens v early in fertilisation, so permanent phenotype determined due to maternal phenotype
What are 2 other examples of maternal effect?
- Ephestia kuehniella –> maternal effect dictates pigmentation of larvae, but mutation starts to become visible in adults, as cytoplasmic proteins diluted and become spread out
- Drosophila –> maternal effect impacts on genes related to embryonic dev and allows homozygous mutants to dev normally
What is the structure of mito?
- IM, IMS and OM
- IM contains OXPHOS proteins
- cristae = folds in IM
- matrix contains enzymes, DNA and ribosomes
What are the characteristics of mtDNA?
- ds closed circle genome
- generally smaller than cpDNA (16-18kb in mammals, 75kb in yeast, up to 367kb in plants)
- introns rare –> present in some larger genomes, eg. yeast
- encodes ETC proteins (also partly encoded by nucleus)
- D loop is largest noncoding part and part most variable between species
- vertebrates have multiple copies, plants have more, but varies lots between species
How was Neurospora crassa used to look at maternal inheritance of mtDNA?
- slow growing mutant strain (= poky) has impaired mito function
- 2 mating types = A and a, fuse together, either can be maternal, dep on which parent had poky genotype
- results of crosses between WT and poky strains revealed maternal inheritance
- offspring either all poky or all WT, dep on which parent had poky genotype
How is mtDNA inherited?
- generally maternally in most species
- yeast don’t follow this
Why is yeast a useful model organism for looking at mtDNA?
- facultative anaerobe, so can gen energy through glycolysis, so can survive loss of mito function
- can still grow, but smaller, as not respiring as efficiently = petites
What causes petites in S. cerevisiae?
- deficiency in cellular resp due to defective ETC –> ETC proteins encoded in mito and nucleus so could be mutation in either genome
What diff results came from crossing haploid petite w/ haploid normal, and what conclusions can be drawn from these?
All prod diploid zygote, then sporulation and meiosis prod following acrospores:
- segregational –> half petite and half normal, ∴ petites can be from mutations in nuclear DNA
- neutral –> all normal, ∴ inheritance of mtDNA bi-parental in yeast (both passing on mtDNA, so some WT from each is enough to prod WT)
- suppressive –> all petite, ∴ mutations can behave dominantly = preferential rep?
What is the rep of mtDNA dep on?
- nuclear encoded genes
How does rep of mtDNA differ from rep of nuclear DNA?
- no recombination of parental alleles
What is the aim of rep of mtDNA and in what way is this not achieved?
- to make exact copies of mtDNA (maintain homoplasmy)
- but mtDNA has faster mutation rate than nuclear DNA
Why does mtDNA have a much faster mutation rate than nuclear DNA?
- no protective histones
- uses pol gamma, which lacks proofreading activity
- conc of mutagenic free-radicals gen by cellular resp
- decreased levels of repair in mtDNA
Where in mtDNA are particularly high levels of variation found, and how this be withstood?
- D loop region
- as doesn’t encode any proteins
What did Giles et al observe to first evidence maternal inheritance of mtDNA in humans?
- looked at RFLP
- saw femailes always passed it onto progeny, but males didn’t
- clear eg as just looking at single polymorphism
Why is it harder to study mitochondrial diseases than single polymorphism, when trying to show mtDNA is maternally inherited?
-people show symptoms of mito diseases to diff extents
How is it ensured than mtDNA is only contributed maternally?
- oocyte has 100,000 - 200,000 copies mtDNA
- sperm has ≈10 copies mtDNA –> in mid-piece, but only head goes into egg
- after fertilisation paternal mtDNA is marked by ubiquitin and degraded
Why is mtDNA useful for pop and genealogical diseases?
- lack of recombination
When looking at inheritance of mtDNA in a pop, what can we observe after a mutation occurs in oocyte?
- mutation passed on and may initially result in heteroplasmy
- but over time random segregation can create a homoplasmic pop –> through random selection or preferential selection
- polymorphism then stably inherited by all future gens
- can trace polymorphism back to most recent common ancestor and determine which females in pop are related to each other (use mol clock calc and can construct hypothetical seq for this ancestor)
How did analysis of mtDNA lead to the Out of Africa hypothesis?
- mtDNA in African pops more diverse than other pops
- and genetic diversity in non-Africans is subset of that found in Africans
When and where did ‘mitochondrial Eve’ live?
- estimated to have lived in East Africa 120,000 - 200,000 years ago
Who is ‘Y-chromosome Adam’ and when is he estimated to have lived?
- male equivalent to ‘mitochondrial Eve’
- all Y chromosomes derived from him (most recent common ancestor)
- lived ≈100,000 years ago
How accurate is the info we have about ‘mitochondrial Eve’ and ‘Y chromosome Adam’?
- can be fairly sure these hypothetical people existed and geographical locations correct
- but years only estimates –> mol clock calcs make lots of assumptions (eg. gen time, no. progeny per gen, influence of polygamy) so huge error rate
What part of the mtDNA are mitochondrial diseases assoc w/ and why does this prod the symptoms it does?
- all 13 mtDNA encoded ETC subunits
- reflect deficiny in bioenergetic function of mito, so most evident in ‘energy intensive’ tissues w/ highest ATP freq
- other tissues are affected but symptoms no as evident
What are the symptoms of Leber’s Hereditary Optic Neuropathy (LHON)?
- degen of retinal ganglion cells and their axons
- acute onset blindness in young adults
What causes Leber’s Hereditary Optic Neuropathy (LHON)?
- mtDNA point mutations in genes encoding NADH deHAse subunits (transversions)
- also many 2° mutations w/in mtDNA or nuclear DNA that affect mito, further impacting severity of disease
Why is Leber’s Hereditary Optic Neuropathy (LHON) considered a homoplasmic disease?
- need almost 100% genomes mutated before symptoms arise
- severe as no WT copies to compensate for mutations
What are the symptoms of Myoclonic Epilepsy and Ragged Red Fibre Disease (MERRF)?
- predominantly affects muscle and neuronal cells
- myoclonus
- myopathy
- ataxia
- peripheral neuropathy
- dementia
What causes Myoclonic Epilepsy and Ragged Red Fibre Disease (MERRF)?
- A –> G in mtDNA gene MT-TK, encoding tRNA Lys
- so mito unable to make proteins req for functional resp chain
In Myoclonic Epilepsy and Ragged Red Fibre Disease (MERRF), why is there a huge variation in severity between patients and signif variations in tissue distributions of mutant mtDNA?
- severity dep on heteroplasmy levels
- random segregation –> so some tissues get high mutant load and others low, so variation w/in person and in cells which become next gen of person
- threshold levels and below them disease is not evident
What is Kearns Sayre Syndrome (KSS)?
- multi system disorder, particularly affecting CNS and eyes
- 100% = dead
- 20 -90% = disease phenotype
- <20% = asymptomatic
What causes Kearns Sayre Syndrome (KSS)?
- generally NOT maternally inherited, new somatic mutations after conception –> large scale deletions in mtDNA
- DNA pol gamma mutated in nuclear-encoded gene and this results in defects in mtDNA, as more error prone rep
- all mtDNA mutations are 2° mutations
What other diseases can mtDNA mutations have impacts in?
- diabetes
- cancer
- CFS
- ageing
Why do cytoplasmic genomes need to be analysed on a species by species basis?
- genome sizes and no. genes encoded vary vastly between diff organisms
- diffs in modes of inheritance
Why is it not immediately apparent if mtDNA or nuclear DNA mutation is affecting mito?
- traits controlled by cytoplasmic inheritance can also be influenced by nuclear-encoded genes
What evidence is there for endosymbiosis?
- double membrane system similar to those in bacteria
- mito and chloros have own DNA and genomes similar size to bacterial ones
- cpDNA and mtDNA have many similarities w/ DNA in prok cells
- -> circular and ds
- -> free from assoc proteins like histones
- -> cpDNA organised into operons
- -> rep indep of nuclear genome, not governed by cell cycle (= semi autonomous)
- protein coding seqs of organelle genes more like those in bacteria than nuclear genes of euks
- ribosomes present in mito diff to those found in cell cyto
What evidence is there for origin of the chloro from cyanobacterium?
- structural similarities
- both have double membrane, w/ thylakoids inside containing chlorophyll
What is serial endosymbiosis?
- ancestral prok engulfed aerobic heterotrophic prok –> became mito, dev into heterotrophic euks
- further subset took up photosynthetic prok, dev into ancestral photosynthetic euks
How does gene distribution between mito/chloro and nuclear genomes change over time, and how is this demonstrated in land plants?
- genes transferred from mito/chloro to nuclear genome
- also lots of genes lost
- in land plants, 11-14% nuclear DNA originates from chloros
How does chloro genome compare to cyanobacteria?
- chloro = 60-100 genes
- cyanobacteria = 1500+ genes
How does mito genome compare to α-proteobacteria?
- mito = 15-350kb
- α-proteobacteria = 4-9Mb
In what way are mito not autonomous anymore?
- over 1000 nuclear encoded gne products req for mito function
