THE EXTRANUCLEAR THE EXTRANUCLEAR INHERITANCE INHERITANCE

Muhammad Rizwan Anwar IUB Bahawalpur

INTRODUCTIONINTRODUCTION• Extra nuclear inheritance is defined as non mendelian

inheritance, usually involving DNA in replicating mitochondria and some other organelles of cell.

• The genes that have been called cytoplasmic genes, extrachromosomal genes, or extranuclear genes are located on a unique kind of chromosome inside cytoplasmic organelle.

• Commonly defined as transmission through the cytoplasm (or things in the cytoplasm, including organelles) rather than the nucleus

• Generally only one parent contributes

• Organelle heredity

Organelles that contain chromosomes• Chloroplasts and mitochondria

• Infectious heredity– Involves a symbiotic or parasitic association with a

microorganism

Criteria for extranuclear inheritance

VARIEGATION IN LEAVES OF HIGHER VARIEGATION IN LEAVES OF HIGHER PLANTSPLANTS

• In 1909, carl correns reported some surprising resuls from his study on four O clock plants ( Mirabilis jalapa).

• The blotchy leaves of these variegated plants showed patches of green and white tissues, but some branches carried only white and some carried only green leaves.

• Variegated-shoot phenotypes in four o’clocks

Normal chloroplastGreenphotosynthetic

Mutant chloroplastWhitenon-photosynthetic

Mixed chloroplastsWhite/green

• They may be intercrossed in a variety of different combinations by transferring pollen from one flower to another

• Two features are surprising

1. There is difference between reciprocal crosses2. Phenotype of maternal parent is solely responsible

for determining the phenotype of all progeny(This phenomenon is called maternal inheritance)

• How such curious results could be explained?

• The difference in leaf color was known to be due to presence of either green or colorless chloroplast

• The inheritance pattern might be explained if these cytoplasmic organelle are somehow genetically autonomous and further are never transmitted via the pollen parent

• For an organelle to be genetically autonomous, it must have its own genetic determinants.

• Thus this organelle has its own genome

• The process of segregation and recombination of organelle genotype is called “cytoplasmic segregation and recombination (CSAR)”

Chloroplasts are inherited via the seed cytoplasm

3 types of eggs (female):

NormalMutantMixed

Assumption:

Pollen (male) contributes no information

POKY NEUROSPORAPOKY NEUROSPORA

• In 1952, Mary Mitchell isolated a mutant strain of Neurospora that she called poky.

• Poky Neurospora is:1. Slow growing2. It shows maternal inheritance3. It has abnormal amount of cytochromes

It is possible to cross some fungi in such a way that one parent contributes the bulk of cytoplasm to the progeny and this cytoplasmic contributing parent is called female even though no true sex is involved

Maternal inheritance for the poky phenotype was established i the following crosses

Poky(female) x wild type (male) → all pokyWild type (female) X poky (male) → all wild type

• Mutant [poky] Neurospora possess altered mtDNA cytochrome complements that lead to slow growth.

• [poky] phenotype is inherited with the cytoplasm.

, Reciprocal crosses of poky and wild-type Neurospora.

protoperitheca (sexual mating type)

conidia(asexual mating type)

• But where in the cytoplasm is the mutation carried?

1. Slow growth suggest lack of ATP energy, which is produced by mitochondria

2. There are abnormal amounts of cytochromes and cytochromes are known to be located in the mitochondria

These clues led to the conclusion that in this case mitochondria are involved and mutation is in the mtDNA.

THE KILLER TRAIT IN PARAMECIUMTHE KILLER TRAIT IN PARAMECIUM

• 1930s,----- sonneborn observed that when two stocks of P.aurelia are mixed together, some of them die.

• Certain strains of P. aurelia are called killer strains because they release paramecin, a substance toxic to sensitive strains

BASIS OF KILLING ACTIONBASIS OF KILLING ACTION

• The killing action was due to the possession by killer cells of a cytoplasmic particle that was named kappa

• A cell lacking kappa is sensitive to the effect of kappa

• Sensitive stocks are immune to kappa’s killing action during conjugation

• For a cell to be a killer, it must posses the kappa particles in cytoplasm

• However, to maintain kappa , the paramecium must posses a dominant allele (K) in nucleus.

• A cell homozygous for recessive (kk) is sensitive

• A cell that carries the dominant (KK) may lack kappa is also sensitive

• Paramecium has diploid micronuclei• At the time of conjugation it contain two haploid

micronuclei that were formed following meiosis• It behaves as gametes• Micronuclei are exchanged between mating cells by

way of conjugation tube.• The micronuclei received from a mate unites with

the stationary one and restore the diploid state of micronucleus.

• Following a mating between killer (KK) and sensitive (kk), both F1 cells become Kk.

• After later divisions and self fertilization, homozygous KK and kk cells arise

• To become a killer a sensitive cell of genotype KK must gain kappa paticles through cytoplasmic exchange at the time of mating

• This comes by prolong mating and cytoplasmic bridge

THE PETITE MUTATION IN YEASTTHE PETITE MUTATION IN YEAST

• Cells carrying a petite mutation grow slowly and form tiny colonies on agar in contrast with the larger ones of the wild phenotype

• These petites posses enzyme defects and are deficient in aerobic respiration

LIFE CYCLE OF YEASTLIFE CYCLE OF YEAST

• It is a unicellular organism

• Haploid cells can be classified into either of two mating types, + or –

• The diploid zygote formed from fusion of a + and a – cell may grow by budding to produce a diploid colony

• The diploid cells can also be stimulated to undergo meiosis

• The cell then enlarge and forms four haploid nuclei, each of which becomes the nucleus of a spore

• The meiotic cells behave like an ascus or sac and thus four spore are called ascospores

• Two of the ascospores will be mating type + and two will be –

• This 2:2 indicates that the genetic determinants are nuclear

TYPES OF PETITE MUTATIONTYPES OF PETITE MUTATION

1. Nuclear petites2. Neutral or recessive petites3. Suppressive petites

NUCLEAR PETITESNUCLEAR PETITES

• These petites behave in the expected mendelian fashion

• A cross of petite with wild produce wild diploid cells

• The ascospore yield a 2:2 segregation of wild type to petite

Chapter 15 slide 27

NEUTRAL OR RECESSIVE PETITESNEUTRAL OR RECESSIVE PETITES

• A cross of neutral petite with a wild produce diploid cells that are normal in phenotype

• When sporulation is induced, the ascus yields spores that produce only wild type cells

• The segregation is thus 4:0• The petite phenotype is disappeared and not appear

in successive generations• Clearly the neutral petite is not behaving in

mendelian way

Chapter 15 slide 29

Further clarificationFurther clarification

• Yeast was treated with acriflavin• Almost whole population of normal cells can be

transformed into petite• No known mutagen can affect nuclear genes to such

an extent that all the population become mutated• This indicates that determination for petite mutation

reside in cytoplasm

SUPPRESSIVE PETITESUPPRESSIVE PETITE

• It also behave in non mendelian fashion

• When it is crossed with wild type , the result depends when the sporulation is induced

• If ascospore formation takes place very soon after the zygote forms, it is found that most of the asci will give a segregation of 0:4, that is all the spores will give rise to petites.

• The zygote if immediately plated out on agar after the mating, form diploid colonies that are also petite

• In contrast to neutral petite, it is as if the wild type were tending to disappear.

• However different results can be obtained from the same cross

• mtDNA of cytoplasmic petites has undergone some sort of alteration

CHLAMYDOMONAS CHLAMYDOMONAS CHLOROPLAST MUTATIONSCHLOROPLAST MUTATIONS

• Unicellular alga– Haploid

• Mating gives diploid cell that immediately undergoes meiosis to form haploid cells

– Single chloroplast with 50 copies of cpDNA• mt+ and mt- strains• strS and strR strains (streptomycin resistance)

Offspring of strOffspring of strSS and str and strRR Crosses Crosses

• always have str phenotype of mt+ parent– Only mt+ parent donates cytoplasm

• But 50% of offspring mt+ and 50% mt- – mt encoded by nuclear gene

strstrSS and str and strRR Crosses Crosses

SHELL COILING IN SNAILSHELL COILING IN SNAIL

• 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

• This happens because the genotype of the mother’s body determines the initial cleavage pattern of the developing embryo

• These Segregation ratios would never appear in organelle genes

• The term maternal effect can be used for the cases like the shell coiling example in order to distinguish them from organelle based inheritance

THE “ENDOSYMBIONT THEORY”THE “ENDOSYMBIONT THEORY”

• This theory was given by lynn margulis in 1970’s• “mitochondria and chloroplast originated more than

a billion years ago when ancient precursors of eukaryotic cells engulfed and established a symbiotic hereditary relationship with some bacteria. The primitive cells carrying a mitochondrion-like or chloroplast-like bacteria would havw gain an edge in the fierce competition for energy and eventually evolved into complex eukaryotes”.

THE MOLECULAR EVIDENCE FOR THE THE MOLECULAR EVIDENCE FOR THE THEORY INCLUDE THE FOLLOWING THEORY INCLUDE THE FOLLOWING

FACTSFACTS1. Both mt DNA and ct DNA have their own DNA, which

replicates independently of the nuclear genome2. Like the DNA of the bacteria, mt DNA and ct DNA are

not organized into nucleosomes by histones3. Mitochondrial genomes use N-formyl methionine and

tRNA-fMet in translation4. Inhibitors of bacterial translation, such as

chloramphenicol and erythromycin, block mitochondrial translation but have no effect on eukaryotic cytoplasmic protein synthesis.

Factors against the theory:

• Mitochondria and chloroplasts only code for a few proteins. Most of the proteins found in the organelles are actually coded for by the nuclear DNA. (Did the organelle DNA jump to the nuclear DNA in evolutionary history?)

• Mitochondrial and chloroplast DNA have introns, a phenomenon never seen in prokaryotes.(Did this characteristic jump from the nuclear DNA to the organelle DNA?)

• In modern symbioses, there is no good evidence for gene transfer between endosymbiont and the host.

Comparisons shows that

•Mitochondrial genomes are derived from a common ancestor of present-day gram-negative non-sulfar purple bacteria

•Chloroplast genomes are derived from cyanobacteria (formerly referred to as blue green algae)

GENE TRANSFER FROM ORGANELLE GENE TRANSFER FROM ORGANELLE TO NUCLEUSTO NUCLEUS

• Some of the genes required for oxidative phosphorylation and photosynthesis reside in the nuclear genome

• These may have been transferred from organelle to nucleus

• Why?• How?

• The deletion of the redundant sequences might have occurred when organelle genes encoding a biosynthetic pathway critical for autonomous growth were discarded or lost from the organelle because the nuclear genome of the host encode its own version of the pathway

• The engulfed cells may have not only lost genes but also contributed coding sequences to the host cell: the sequence could have encoded enzymes that added to or replaced proteins encoded by the genes of the nuclear chromosome

MECHANISM OF GENE TRANSFERMECHANISM OF GENE TRANSFER

• Gene transfer occurs through an RNA intermediate or by movement of pieces of DNA

• In many plants, the mitochondria genome encodes the COXII gene of the mitochondrial electron transport chain

• In other plants, the nuclear DNA encodes that same gene• in several plant spp where the nuclear COXII gene is

functional, mt DNA still contains a recognizable, but non functional, copy of the gene (that is a COXII pseudogene)

• Remarkably the mt DNA gene contains in intron, while the nuclear gene does not.

• Geneticist have interpreted this finding to mean that the COXII gene transferred from mt DNA to nDNA via an RNA intermediate.

• The RNA would have lacked the intron, and when the mRNA was copied into the DNA and integreted into chromosome in the nucleus, the resulting nuclear genome also had no intron

THE STRUCTURE AND FUNCTION OF THE STRUCTURE AND FUNCTION OF MITOCHONDRIA AND CHLOROPLASTMITOCHONDRIA AND CHLOROPLAST

• Mitochondria and chloroplast are the organelle of energy conversion that carry their own DNA

• Mitochondria are sites of the Krebs cycle and an electron transport chain that carries out the oxidative phosphorylation of ADP to ATP

• Chloroplast are the sites of photosynthesis . The capture, conversion and storage of solar energy in the bonds of carbohydrates

MITOCHONDRAMITOCHONDRA

• Powerhouse of cell• In human nerve, muscle and liver cells have

abundant mitochondria• It is two membrane structure (outer and inner)• The inner membrane has many infoldings called

cristae and is studded with enzymes of the electron transport chain and ATP synthesis

• Tightly sequenced inside the inner membrane is a core compartment called matrix

• Matrix contain many enzyme systems

• It also contain one or more circles of mitochondrial DNA

• There are two stages by which mitochondria help to convert food into energy

1. Mitochondria employ Krebs cycle to metabolize pyruvate and fatty acid and produce the high energy electron carriers NADH and FADH2.

2. In the second stage, four multi subunits enzyme complexes (I,II,III, IV) which are embedded in the inner membrane, harness the energy in the high energy electrons carried by NADH and FADH2.

• The process by which they harness this energy is oxidative phosphorylation

• A set of reactions requiring oxygen that create portable packets of energy in the form of ATP.

CHLOROPLASTCHLOROPLAST

• Chloroplast have an outer membrane, inter membrane space, and inner membrane enclosing a core compartment(stroma)

• Stroma contains chloroplasts DNA and various enzyme systems

THE GENOMES OF THE THE GENOMES OF THE MITOCHONDRIAMITOCHONDRIA

• Mitochondrial DNA lies within the matrix of the organelle, where it appears in highly condensed structures called nucleoids

• Variation in the number of mitochondria, nucleoids and mtDNA molecules are regulated by complicated means that researchers don't yet understand

• In general mitochondria double in size and then divide in half in each cell generation

• The replication and division of mtDNA can occur throughout the cell cycle independent of nuclear genome replication

• Some molecules replicate many times and some not replicate• This is decided randomly• This is one cause of mitochondrial segregation• Size of mtDNA varies widely.

• Humans and other vertebrates 16.5 kb– (all of the mtDNA codes gene products)

• Yeast 75 kb• Plants ~100 kb to 2 Mb

HUMAN MITOCHONDRIAL DNA HUMAN MITOCHONDRIAL DNA CARRIES CLOSELY PACKED GENESCARRIES CLOSELY PACKED GENES

• Mt genome size---------- 16.5 kb• 0.3% of cells DNA• Circular DNA molecule• Carries 37 genes• 13 genes code for polypeptide subunits of the

protein complexes for oxidative phosphorylation• 22 are tRNA genes• The remaining 2 genes are for the lager and small

rRNAs found in mitochondrial ribosomes

• A significant feature of human mt genome is the compactness of its gene arrangement

• The genes are adjacent or slightly overlap

• No nucleotides between them or no introns within them

• The reason for this compactness is not yet known

Physical map of the human mtDNA

THE LARGER YEAST MT GENOME THE LARGER YEAST MT GENOME CONTAINS SPACERS AND INTRONSCONTAINS SPACERS AND INTRONS

• The mitochondrial genome of the yeast is more than four times linger than human and other animal mtDNA

It is due to1. Long intergenic sequences 2. Introns

• Long A-T rich sequences called spacers separate the gene in yeast mtDNA and account for the more than half of the additional DNA

• Introns the second DNA lengthening element , form about 25% of the yeast mt genome

THE GENOMES OF THE CHLOROPLASTTHE GENOMES OF THE CHLOROPLAST

• Chloroplast DNA contains many more genes than mt DNA

• These genes are closely packed• They contain introns• Shape is circular• Chloroplast genomes occur in multiple copies and carry

lots of non-coding DNA.

• Complete chloroplast sequences have been determined for several organisms (tobacco 155,844 bp; rice 134,525 bp).

maize mitochondrial genome

COOPERATION BETWEEN THE COOPERATION BETWEEN THE ORGANELLE AND NUCLEAR GENOMESORGANELLE AND NUCLEAR GENOMES• The maintenance and assembly of functional

mitochondria and chloroplast depends on gene products from both the organelle and nuclear genome

• For example cytochrome c oxidase, the terminal protein of the mitochondrial electron transport chain is composed of 7 subunits

• 3 of which are encoded by mt genes, whose mRNAs are translated on mitochondrial ribosomes

• The remaining 4 are encoded by nuclear genes whose mRNAs are translated on ribosome in the cytoplasm

Many proteinsencoded by nucleargenes have productstransported to mitochondria

and RNAs ….

Plant mitochondria contain chloroplast genes - suggesting that genetic transfer occurs between the two organelles

• mtDNA contains genes for:

• tRNAs• rRNAs• cytochrome oxidase, NADH-dehydrogenase, & ATPase subunits.

• Mitochondria’s genetic information also occurs in the nuclear DNA:

• DNA polymerase, replication factors• RNA polymerase, transcription factors• ribosomal proteins, translation factors, Additional cytochrome oxidase,

NADH, ATPase subunits.

• %G and A (2-ring structures) content and densities differ between light (L: CT-rich) and heavy (H: GA-rich) strands.

• mtDNA genes occur on both strands.

• Functions of all human mtDNA ORFs are assigned

TRANSCRIPTION AND TRANSLATION TRANSCRIPTION AND TRANSLATION IN ORGANELLESIN ORGANELLES

• mRNAs from the mtDNA are synthesized and translated in the mitochondria.

• Gene products encoded by nuclear genes are transported from the cytoplasm to the mitochondria.

• Mammalian and other vertebrate mtDNAs are transcribed as a single large RNA molecule (polycistronic) and cleaved to produce mRNAs, tRNAs, and rRNAs before they are processed.

Translation of the mtDNA genome

• Specialized mt initiation factors (IFs), elongation factors (EFs), and release factors (RFs) are used for translation.

• AUG is the start codon (binds with fMet-tRNA like bacteria).

• Only plants use the “universal” genetic code. Codes for mammals, birds, and other organisms differ slightly.

• Extended wobble also occurs in tRNA-mRNA base-pairing (22 tRNAs are sufficient rather than 32 tRNA needed for standard wobble).

• Cyclohexamide can inhibit protein synthesis by ribosomes in the cytosol but does not affect the ribosomes of the organelle

• Chloramphenicol and erythromycin can affect the mitochondrial ribosome and shut off any translation that may result from their activities but not affect the ribosomes of the cytosol

• From such inhibition studies following information was obtained

• Organelle contain information in its DNA coded for specific rRNA of the organelle

• It also have genes for its tRNAs

• Genes for Ribosomal protein and aminoacylsynthetase are present in nucleus

• ATPase which is bound to inner membrane of the mitochondria , is composed of 10 polypeptide subunits and function in oxidative phosphorylation

• Some of ATPase subunits are made in cytosol, others in the organelle

• Cytochrome oxidase--- 7 polypeptide subunits• 3 larger one are made in mt• 4 smaller one in cytosol

HOW MANY COPIESHOW MANY COPIES

• More than one copy of organelle DNA is present• The number of copies varies among species• It may even also vary with in species e.g. leaf cells

have more chloroplast and hence more copies of DNA

• 2-50 copies in yeast• 4-10 copies in human , mouse, rat

• Even with the multiple copies , the amount of mt DNA in a mammalian cell is less than 1% of the total amount of cellular DNA

• In plants, however, with their larger mitochondrial genomes and in which 20-40 copies may exist, an appropriate % of the cellular DNA (15%) may be accounted for by organelle

EXTRANUCLEAR GENOMES EXTRANUCLEAR GENOMES

• Both mitochondrial DNA and the chloroplast DNA are

typically circular molecule• In some protozoans ____ paramecium, for

example_______ the mtDNA is linear• Both the mt DNA and ct DNA are double stranded

molecule• Unassociated with proteins(histones) and

unbounded by any membrane

• Organelle DNA usually differs sufficiently in its G+C contents from nuclear DNA

• One study shows the following results

Mt DNA of yeast Nuclear DNA

Density g/cm31.683 1.699

GC% 21 40

• The replication of mt DNA is unaccompanied by a proof reading mechanism

• Mitochondrial DNA polymerase lacks the capacity to proof read

• Mutations can accumulate much more rapidly• The multiple copies may offset the effect of lack of

DNA repair In organelle

• In addition to DNA organelle also posses components for the synthesis of proteins

• Both kinds of organelle contain ribosomes• Ribosomes of mitochondria are composed of two

subunits of unequal size• The intact ribosome varies in sedimentation value

from 55S to 80S

• The ribosomes of chloroplast are much less variable than mitochondria

• Composed of two unequal subunits• Value of 70S have been obtained from many sources• Smaller subunit____ 30S• Larger subunit_____ 50S• Closely resemble bacterial cell ribosomes• The ribosomes of both organelle contain RNA• These ribosomal RNAs of chloroplast are very similar to

those of bacterial cells

• In addition to DNA and ribosomes, organelle also contain other machinary

• But this is quite distinct from that of present in cytosol• DNA polymerase• tRNAs for different amino acids• Aminoacyl synthetase• RNA polymerase• Both enzymes posses a formylase, an enzyme that

convert methionine to n-formyl methionine, as well as an initiating tRNA, tRNAf-met

RESEMBLANCES BETWEEN ORGANELLE RESEMBLANCES BETWEEN ORGANELLE AND BACTERIAL PROTEIN AND BACTERIAL PROTEIN

SYNTHESIZING SYSTEMSYNTHESIZING SYSTEM1. Occurrence together of transcription and

translation 2. Presence of formylase3. Components of organelle system respond similarly

to those agents that inhibit nucleic acid and polypeptide synthesis in prokaryotes

4. components of organelle and prokaryotic protein-synthesizing system (tRNAs, various enzymes) are interchangeable in in vitro investigation and can substitute for each other in translation

Comparison of codon recognition by Comparison of codon recognition by mammalian and yeast mitochondrial mammalian and yeast mitochondrial

translation systemtranslation systemCode in mitochondrial translation system

Code in mitochondrial translation system

codon Universal codon

mammals yeast

UGA CT trp trpAUG ile met metAGA arg CT argAGG arg CT argCUU leu leu thrCUC leu leu thrCUA leu leu thrCUG leu leu thr

MITOCHONDRIAL DNA HAS A HIGH MITOCHONDRIAL DNA HAS A HIGH RATE OF MUTATIONRATE OF MUTATION

• In 1980s, surveys of DNA sequences variations among individuals of a given species and between closely related species showed that the mt DNA of vertebrates evolves almost 10 times more rapidly than the nuclear DNA of the same organism

• The higher rate of DNA mutation in mitochondrion probably reflects

1. More errors in replication2. Less efficient repair mechanism / No mechansim

HUMAN GENETIC DISEASES AND HUMAN GENETIC DISEASES AND MITOCHONDRIAL DNA DEFECTSMITOCHONDRIAL DNA DEFECTS

1. Mutations in mtDNA can produce human genetic disorders. Examples:a. Leber’s hereditary optic neuropathy (LHON). Optic nerve degeneration results

in complete or partial blindness in mid-life adults.i. LHON is caused by mutations in mtDNA genes for electron transport chain proteins. (These include ND1, ND2, ND4, ND5, ND6, cyt b, COI, COIII, and ATPase 6.)ii. LHON results from defects in the enzymes of oxidative phosphorylation. Without ATP production, the optic nerve dies.

b. Kearns-Sayre syndrome produces three types of neuromuscular defects:i. Progressive paralysis of certain eye muscles.ii. Abnormal pigment accumulation on the retina, causing chronic inflammation and degeneration of the retina.iii. Heart disease.iv. Kearns-Sayre syndrome results from deletions in mtDNA. A model for the disorder is that tRNA genes are removed, disrupting mitochondrial translation.

c. Myoclonic epilepsy and ragged-red fiber disease (MERRF). Symptoms include:i. Microscopic tissue abnormality, “ragged-red fibers.”ii. Myoclonic seizures (jerking spasms).iii. Ataxia (uncoordinated movement).iv. Accumulation of lactic acid in blood.v. Additional symptoms are sometimes present, including:

(1) Dementia.(2) Loss of hearing.(3) Difficulty speaking.(4) Optic atrophy.(5) Involuntary jerking of eyes.(6) Short stature.

vi. Mitochondria have abnormal appearance.vii. The cause is a single nucleotide substitution in the lysine tRNA gene. Mitochondrial protein synthesis is affected, and in some way this phenotype is produced.

2. In most mtDNA disorders, cells of affected individuals have a mix of normal and mutant mitochondria (heteroplasmy).

a. Proportions of the two mitochondrial types vary between tissues, and between individuals.

b. Severity of disease correlates with the relative amount of mutant mitochondria.

• Deletions of mitochondrial genome may be taking place all the times in cells

• Multiple copies may prevent the onset of disorder• However once , mt DNA becomes shorter , it

multiplies more quickly than the larger, intact DNA• Eventually the deleted mt DNA will take over in the

organelle population, leading to a lethal condition

PLASMID AND TUMOR PLASMID AND TUMOR TRANSFORMATIONTRANSFORMATION

• Extrachromosomal DNA molecules that replicate independently and maintain themselves in the cytoplasm of plant cells are called plasmids

• They have much in common with chromosomes of mitochondria, but are not organized into organelles that are vital to their host cells

• A plasmid called Ti (for tumor-inducing) carries a DNA sequence that transforms cells of dicotyledonous plants to tumor cells

• It is caused by viable bacteria that enter a wounded surface of a plant

• The bacteria may be killed after few days but tumor continue to grow

• A fragment of Ti plasmid carried by the bacterium has been combined with a DNA segment of infected plant cell

• Genes carried by the plasmid code for enzymes that promote continuous and uncontrolled tumor growth

MITOCHONDRIAL INHERITANCE IN MITOCHONDRIAL INHERITANCE IN IDENTICAL TWINSIDENTICAL TWINS

• In some pairs of identical twins, one twin manifests symptoms of neurodegenerative disease but other twin does not

• This is because even though their nuclear genomes are identical, their mitochondrial genomes are not identical

MITOCHONDRIAL MUTATION AND MITOCHONDRIAL MUTATION AND AGINGAGING

• Some researchers think that

Accumulation of mutations ( both base substitutions and deletions ) in mt DNA over a life time, and progressive enrichment of deleted mt DNAs through their tendency to replicate more rapidly than longer, non deleted mt DNAs, results in an age-related decline in oxidative phosphorylation

This decline in turn accounts for some of the symptoms of aging, such as decrease in heart and brain function

MITOCHONDRIAL EVEMITOCHONDRIAL EVE

• Mitochondria is maternally derived in humans• Males are Mitochondrial Dead ends• Each individual has a single Mitochondrial Line -- Mother to

Maternal Grandmother to Maternal-Maternal Great Grandmother, etc.

• Each generation, some Mitochondrial Lines die out due to no females being born in that line to carry on the mitochondria.

• Eventually, all lines converge to a single Female who live in Africa some 200,000 years ago.

• Mutation rates can be used to estimate the time of divergence, as well as the relationship among the different subpopulations in the world.

APPLICATIONS OF EXTRANUCLEAR APPLICATIONS OF EXTRANUCLEAR INHERITANCEINHERITANCE

1. mtDNA is useful for forensic examinations

2. molecular anthropologists have been using mtDNA for two decades to examine both the extent of genetic variation in humans and the relatedness of populations all over the world.

3. Since the organelle genome is so highly simplified, mtDNA or cpDNA can be retrieved and analyzed from ancient or poorly preserved samples in which there would be no chance of retrieving a nuclear marker

4. human diseases caused by deleterious mutations in gene-coding regions of the mtDNA molecule, which have been studied by the medical

profession to understand their mode of inheritance5. Chloroplast genes are used for evolutionary studies in plants and algae6. Mitochondrial genes can also be used to trace

the female ancestor of humans, while Y-chromosome genes can be used to trace male ancestors.

APPLICATIONS IN PLANT BREEDINGAPPLICATIONS IN PLANT BREEDING

1. Some time Extranuclear inhetiance is associated with dwarf growth.

2. Affect flower morphology3. Some time tolerance to herbicide is encoded by mt

gene4. Cms is the trait encode by mt genome5. Induction of leaf variegation patterns in

ornamentals

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