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Extranuclear Inheritance
Dr.Shivani Gupta,
PGGCG-11, Chandigarh
Commonly defined as transmission
through the cytoplasm (or things in
the cytoplasm, including organelles)
rather than the nucleus
Generally only one parent
contributes
Extranuclear Inheritance
• Organelle heredity
– Organelles that contain chromosomes
• Chloroplasts and mitochondria
• Infectious heredity
– Involves a symbiotic or parasitic association with a
microorganism
• Maternal effect
– Nuclear gene products are stored in ooplasm and
distributed to cells as the fertilized egg divides to
form developing embryo
Chloroplasts and Mitochondria
• These organelles contain DNA
– First explanation for [some] maternal inheritance
patterns
• Endosymbiont theory
• Analysis of mutant alleles in organelles can be
complex because many genes for organelle
components are nuclear-encoded
– And even subunits of a multicomponent enzyme
may be partially encoded in both locations
– Heteroplasmy makes things even worse…
Chloroplasts
• Carl Correns
– A codiscoverer of Mendel‘s work
– Worked with four o‘clock plants (Mirabilis jalapa)
• Had branches with either white, green or variegated leaves
• Type of offspring dependent only upon the phenotype of the
branch from which the ovule was derived—not the pollen
F(figure 9-1)
– Concluded that leave color was dependent upon the
chloroplasts and that these or other factors were
contributed through the ovule cytoplasm
Four O‗Clocks
Saccharomyces petite Mutations
• petite mutations give rise to small colonies
– Aerobic respiration blocked
– Live anaerobically
• S. cerevisiae is a facultative anaerobe
• Two types
– Segregational petites encoded by nuclear genes
showing Mendelian inheritance
– cytoplasmic transmission pattern petites
• Neutral petites demonstrate (give all wt offspring when
crossed to wt)
• Suppressive petites (behave like poky in Neurospora)
petite Mutant Crosses
Mitochondrial/Chloroplast Evolution
• Endosymbiont theory – Lynn Margulis
• Mitochondria and chloroplasts arose independently
about 2 billion years ago as free-living prokaryotes
• Primitive eukaryotes without these abilities
engulfed the prokaryotes as endosymbionts
– Relationship ultimately changed to that of an organelle
– Organelles have circular DNA
– Most genes moved to ―nucleus‖ (<10% remain)
• Targeting peptides added
– Organelle genes/expression still ―prokaryotic‖
Chloroplast Genes/Expression
• Chloroplasts have circular DNA and a complete
gene expression system
– Components derived from cpDNA and nuclear DNA
encoded genes
– cpDNA commonly 100-225 kbp in size
• No nucleosomes, but has introns and large intergenic regions
• Multiple copies/organelle (75 in Chlamydomonas) and
recombination can occur
• Encode rRNAs, tRNAs, rproteins (~70S ribosome) and other
proteins/enzymes (92 encode thylakoid proteins in the
liverwort)
Mitochondrial Genes/Expression
• mtDNA is circular, generally relatively small
– 16-18 kbp in mammals, 75 kbp in yeast, but 367 kbp in
Arabidopsis (a mustard plant)
• 5-10 copies/organelle in vertebrates, 20-40 in plants
• Introns generally absent, small intergenic spaces in
small mtDNAs, reverse in larger ones such as yeast
• Genetic code similar but modified
• Encodes rRNAs, tRNAs and 13 polypeptides in
humans (portions of electron transport chain)
Mitochondrial
Genes/Expression
• Protein synthetic apparatus combination of
mtDNA and nuclear-encoded
– But nuclear-encoded proteins distinct from their
cytoplasmic or nuclear counterparts
• RNAP is single polypeptide and is inhibited by
rifampicin/rifamycin
– But sensitive to antibiotics targeted normally against
prokaryotes
– Ribosomes range from 55-80S
Many proteins
encoded by nuclear
genes have products
transported to
mitochondria
and RNAs ….
mtDNA Mutations and Human
Genetic Disorders
• Human mtDNA is 16,569 bp
– Encodes 13 proteins, 22 tRNAs and 2 rRNAs
• Heteroplasmy
– Variable mixture of genetically distinct
mitochondria/mtDNAs
• Properties of mtDNA-encoded traits
– Maternal inheritance pattern
– Deficiency in bioenergetic function of organelle
– Specific mutation in an mtDNA gene
Human mtDNA Disorders
• Myoclonic epilepsy and
ragged red fiber disease
(MERRF)
– Fibers from proliferation
of aberrant mitochondria
– Mutation in mtDNA
tRNA gene
Human mtDNA Disorders
• Leber‘s hereditary optic neuropathy (LHON)
–
–
–
–
Sudden bilateral blindness 9average age 27 yrs)
Most mutations in NADH dehydrogenase gene
Maternal transmission to all offspring
Many cases appear to be ―new‖ mutations
• No family history
Infectious Heredity
• Cytoplasmic transmitted phenotypes in
eukaryotes due to an invading
microorganism or particle (e.g. virus)
Kappa in Paramecium
• Certain strains of P. aurelia are called killer
strains because they release paramecin, a
substance toxic to sensitive strains
– Paramecin produced by kappa particles (100200 per cell) that replicate in cytoplasm
– Kappa particles contain DNA and protein and
require a nuclear gene (K, ―little k‖ strains are
sensitive) for maintenance
– Kappa particles are bacterialike and may
contain temperate phage
Infective Particles in Drosophila
• CO2 sensitivity
– Flies fail to recover from CO2 anesthetization
(permanently paralyzed)
– Sensitivity due to presence of virus called sigma
• Transfer to other insect species unsuccessful, suggesting
Drosophila genes essential for its continued
propagation/function
• Sex ratio in D. bifasciata and D. willistoni
– Some flies produce offspring at an altered sex ratio
• Mostly female at below 21 degrees Celsius
• Trait transmitted only to daughters
• Agent shown to be a protozoan that is lethal only to males
– And protozoan may have a virus that is actually responsible…
Maternal Effect/Maternal Influence
• Offspring phenotype under control of
nuclear gene product present in the egg
– Genetic information of mother used to produce
products present in the egg cytoplasm
– Snail Limnaea peregra shell coiling is an
example
Snail Limnaea peregra Shell Coiling
• Hermaphroditic snails
• Some shells have right-handed (DD or Dd)
coiling while others have left-handed
(dd)coiling
• Reciprocal crosses (reverse mail and female
genotypes) of true-breeding snails
– Offspring phenotype depends upon maternal
genotype—not maternal phenotype