lecture 27: organelle inheritance

four-o-clocks have green or variegated leaves, where did this trait come from?

trait is inherited from the mother (maternal inheritance), due to genes found on the chloroplast genome

mitochondria and chloroplasts are nonnuclear organelles with their own small genomes MEANING

they don’t follow the same pattern and organelle genomes lead to non-Mendelian inheritance

mitochondria (main generators of ATP)

membrane bound cytoplasmic organelles

• many in each eukaryotic cell

• outer membrane surrounds wrinkled inner membrane

• produce energy packets (ATP) trough the Krebs cycle and oxidative phosphorylation

mitochondrial genomes:

• some have introns and space between genes; some are circular, others linear

• trypanosomes have a kinetoplast - a single mitochondrion with interlocking circles (minicircles and maxicircles) of mtDNA

human mitochondrial genome is…

compact gene arrangement

• 37 genes encoding tRNAs, rRNAs, and proteins for oxidative phosphorylation

• no introns—- only have exons so no need for splicing

mitochondrial genetic code varies in different organisms in which

there are 5 differences between the universal and mitochondrial genetic codes

chloroplasts

membrane bound cytoplasmic organelles in plants

• internal structures called thylakoids contain chlorophyll

• captures solar energy and store it in chemical bonds of carbohydrates

chloroplast genomes

• more than one copy in each chloroplast

• compact gene arrangement (do have introns)

• circular, linear, and branched forms

• have more genes than mitochondria

how to make transgenic chloroplasts by biolistic transformation

use a gene g*n:

•Coat small metal particles (bullets) with DNA

•Shoot DNA bullets at cells

•DNA can enter the chloroplast and recombines into the genome.

•Transgenic cells identifies using selectable marker.

•Transgenic cell cultured to produce a transplastomic plant.

introducing a transgene into cpDNA when transforming the chloroplast genome

altering the properties of commercially important crop plants

• for example, can produce herbicide-resistant plants

• transgenes will be maternally inherited, so low risk of spreading to other plants via pollen transfer

determine the function of open reading frames

mitochondria and chloroplasts require nuclear gene products to assemble & function

true

• functions in mitochondrial electron transport

• 7 subunits: 3 encoded by mitochondrial genome, 4 by nuclear genes

nuclear genes

cytochrome oxidase c

nuclear genes encode majority of protein required for gene expression in mitochondria and chloroplasts

true

idea that mitochondria and chloroplasts are descended from bacteria that fused with nucleated cells

endosymbiont theory

• have their own DNA

• like in bacteria, mtDNA & cpDNA are not arranged into nucleosomes

• mitochondria use N—formyl methionine and tRNA in translation

• inhibitors of bacterial translation block mitochondrial and chloroplast genomes derive from a common ancestor of nonsulfur and cyanobacteria, respectively

organelles must function inside the cell for gene transfer

• the organelle gene incorporated into nuclear genome

• organelle copy of the gene becomes redundant

• changes may make it non-functional

Different organelle genes transferred to nucleus in different lineages leading to organelle diversity

gene transfer likely the driving force for establishing the symbiotic relationship

inheritance of mitochondria and chloroplasts is non-mendelian

true

mutations in organelle genes often produce….

whole-organism phenotypes

may result in slow cell growth leading to small cell colonies or weak tissues

mtDNA mutations

may decrease chlorophyll production leading to a change in leaf color

cpDNA mutations

DNA polymorphisms can be followed by

DNA sequencing

when progeny obtain organelles from both parents

biparental inheritance

when progeny obtain organelles from just one parent

uniparental inheritance

when uniparental inheritance occurs, progeny most often inherit their organelles from the _______

maternal parent

what leads to maternal inheritience

• female gamete are mostly larger

• zygote receives many female organelles and few parental organelles

Paternal organelles may be actively excluded or destroyed.

Paternal organelles may segregate into non-embryonic cells.

Fertilization may prevent organelles from entering the egg.

Because mtDNA is inherited maternally, it allows exact matches between a child and a maternal grandmother (or other maternal distant relatives); so it can be used to match distantly related ppl. how is it more easily tracked.

bc mtDNA exists in many more copies than nuclear DNA, so it is more easily extracted from ancient samples where DNA might be degraded!

who do ppl say that all mtDNA of every human originated from who lives 200,000 yrs ago

mitochondrial eve

no matter what the father is, progeny had what the maternal had. if she was varigated, progeny was…

variegated or green or white

wildtype cpDNA makes

chlorophyll

mutant cpDNAs have a mutation that

prevents chlorophyll production

cells contain a mixture of organelle genomes

heteroplasmic

cells contain only one type of organelle genome

homoplasmic

mitotic progeny of homoplasmic cells are all homoplasmic

mitotic progeny of herteroplasmic cells can be:

• heteroplasmic

• homoplasmic wild-type

• homoplasmic mutant

uneven distribution of organellar genomes has distinct phenotypic consequences

the relationship between cytoplasmic segregation and variegation is the theshold effect. threshold effect:

a certain fraction of wild-type organelles is sufficient for the normal phenotype

in four-o-clocks, heteroplastic cells make enough chlorophyll to be green

Biparental inheritance of mitochondria in yeast

several diseases of the human nervous system are caused by mutations in the

mitochondrial genome

• mutations are passed from mothers to children

• symptoms vary due to heteroplasmy

tissues with higher energy requirements are less tolerant of mutant mitochondria

true

tissues with low energy requirements are affected only when the proportion of wild-type mitochondria is greatly reduced

true

oxidative phosphorylation system in the mitochondria generates free radicals, which can damage DNA

accumulation of mtDNA mutations overtime may result in age-related decline in oxidative phosphorylation

evidence in support of mtDNA and aging:

• brain cells of people with Alzheimer’s disease (AD) have abnormally low energy metabolism

• 20% to 35% of mitochondria in brain cells of most AD patients have mutations in cytochrome c oxidase genes, which may explain the low energy metabolism

ooctye nuclear transplantation can sidestep transmission of mitochondria disease

oocyte nuclear transfer is successful in primates

cytoplasmic transfer was legal for a litle; 50 babies were born with 3 biological parents

2017- UK was the first country to legalize a similar “spindle transfer” procedure for parents with known mitochondrial disease