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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 (100-200 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