Organellar Genome and OrganellarInheritance

History of Organellar Genome

1962

Ris and Plaut

First demonstrated presence of DNA in

chloroplast

1963Nass and Co

worker

Proved the existence of DNA in

mitochondrion

Organellar Genome

CHLOROPLAST DNAMITOCHONDRIAL DNA

Important features of Organellar DNA

Replicates both in chloroplast and mitochondria in a

semi-conservative fashion.

Liable to mutation.

They are expressed and inherited separately from

nuclear genes.

All aerobic animal and plant cells.

They are transcribed and translated within the

organelles.

Uni-parental inheritance is observed.

Present in multiple copies in each organelle. cp-DNA-

20-40 copies per chloroplast. ~4 copies per

mitochondria (E.g.: Yeast).

End

osymbiotic

Theor

yOrigin

Why Endosymbiotic Theory considered true…..

New mitochondria and plastids

are formed only through a

process similar to binary fission.

Both mitochondria and plastids

contain DNA that is similar to that

of bacteria (both in their size and

their having a circular form).

Continue…..

These organelles' ribosomes are like those found in

bacteria (70S).

Proteins of organelle origin, like those of bacteria,

use N-formyl methionine as the initiating amino acid.

Much of the internal structure and biochemistry

of plastids, for instance the presence of thylakoids

and particular chlorophylls, is very similar to that of

cyanobacteria.

Mitochondria have several enzymes and transport

systems similar to those of bacteria.

1) Found in chloroplast and

mitochondria.

2) Usually circular except in

ciliate protozoa, where it is

linear.

3) Synthesis continues

throughout cell cycle.

1) Found in chromosomes.

2) Linear in eukaryotes and

circular in prokaryotes.

3) Synthesis occurs onlyduring interphase.

Difference Between Organellar DNA and Nuclear DNA

Organellar DNA Nuclear DNA

U C A G

U PhePheLeuLeu

SerSerSerSer

TyrTyrStopStop

CysCysStopTrp

UCAG

C LeuLeuLeuLeu

ProProProPro

HisHisGlnGln

ArgArgArgArg

UCAG

A IleIleIleInitiation Codon/Met

(f-Met during initiation)

ThrThrThrThr

AsnAsnLysLys

SerSerArgArg

UCAG

G ValValValVal

AlaAlaAlaAla

AspAspGluGlu

GlyGlyGlyGly

UCAG

Ambiguity in the Genetic Code in chloroplast and mitochondria

U C A G

U PhePheLeuLeu

SerSerSerSer

TyrTyrStopStop

CysCysStop(Trp)Trp

UCAG

C LeuLeuLeu(Thr)Leu

ProProProPro

HisHisGlnGln

ArgArgArgArg

UCAG

A IleIleIleInitiation Codon/Met

ThrThrThrThr

AsnAsnLys(Asn)Lys

SerSerArg(Ser/Termination)Arg(Termination)

UCAG

G ValValValVal

AlaAlaAlaAla

AspAspGluGlu

GlyGlyGlyGly

UCAG

Changes found in mitochondrial genetic code

Met-Initiation/Met-elongation

Mitochondrial Genome

Mitochondria DNA Characteristics

May be circular or linear.

Size: varies from ~16.5kb to ~100kb.

mt-genome shows larger variation in

size compared to that of chloroplast.

In higher plants, mt-DNA has short

homologous sequences, recombination

within which generates smaller, sub-

genomic circles present with the full

genome.

In most animals the two strands of mt-DNA have different

densities, so they are called the H (heavy) and L (light) strands.

Both strands encode genes, although more are on the H

strand.

mt-DNA codes for rRNAs, tRNAs and protein molecules

(mainly protein subunits for respiration)

No structural DNA proteins are associated with mt-DNA.

Within mitochondria are nucleoid regions, each of which

contains several copies of the mitochondrial chromosome.

Yeast has between 4 and 5 mtDNA molecules per nucleoid,

and each mitochondrion has 10 to 30 nucleoids.

Continue…..

A short region (1121 bp), called

the D loop (D = “displacement”), is

a DNA triple helix: there are 2

overlapping copies of the H

strand.

The D loop is the site where

most of replication and

transcription is controlled.

Genes are tightly packed, with

almost no non-coding DNA outside

of the D loop.

Continue…..

Human mitochondrial genes contain no introns, although introns

are found in the mitochondria of other groups (plants, for instance).

Generally, mitochondrial DNA encodes:

tRNAs

rRNAs

Protein subunits, including:

Cytochrome C biogenesis

Polypeptides of cytochrome oxidase.

NADH dehydrogenase.

ATPase.

Continue…..

semi-conservative

relies on mitochondrion-specific

DNA polymerases.

The origin of replication for the

H strand is in the D loop, and it

is initiated by an RNA primer

generated from the L strand

transcript.

After the new H strand is about

2/3 complete, the L strand origin

of replication is uncovered.

Replication of mt-DNA

The L strand origin is on the old H

strand; it is “uncovered” when the

old H strand is displaced by the

DNA polymerase synthesizing the

new H strand.

The L strand origin folds into a loop

structure, which acts as a primer,

and replication of the L strand

begins.

It involves no proof-reading and

DNA replication occurs throughout

the cell cycle.

Diversity of mitochondrial genomes

Diversity Animal Fungi Plants

Size 14kb - 42kb 17kb - 180kb 184kb - 2,400kb

Non-coding DNA Very Low Variable Very High

Mutation rate High Low Very Low

Recombination /

Introns

Universal genetic code

Mostly

Map

of

the

ge

nes

in m

ito

cho

nd

rial

DN

A

of

hu

man

Characteristics of Chloroplast DNA

Double-stranded

Chloroplasts contain naked circular DNA.

Circular, super coiled, and generally not

associated with any proteins.

Size: Varies. 85kb (Codium fragile) to

2000kb (Acetabularia sp.).

CHLOROPLAST GENOME

Larger than mitochondrial genomes (80-600 Kb in length),

Each chloroplast contains 10-60 (sometimes 100) copies of its

genome.

cp-DNA contains mostly inverted repeats.

In higher plants cp-DNA 10-24kb long,

two identical copies per genome as an

inverted repeat.

Machinery for cp-DNA replication

provided by nuclear genes.

Many chloroplasts contain introns which

form 2 classes.

1)Introns of tRNA located on anti-

codon loop.

2)Introns present in protein encoding

genes.

Continue…..

GC content of cp DNA differs

greatly from that of the nuclear

and mitochondrial

cpDNA (36%)

nuclear(64%) and

mitochondrial DNA (71%),

This allows cpDNA to be

isolated by CsCl equilibrium

density gradient

centrifugation.

The number of copies of cpDNA varies from species to species.

In all cases there are multiple copies per chloroplast, and these

copies are found in nucleoid regions that are also present in multiple

copies.

The mechanisms of cpDNA replication are unknown.

Chloroplast ribosomes are distinct from mitochondrial ribosomes.

Translation proceeds in a manner similar to prokaryotes.

The chloroplast genome contains genes for

Chloroplasts.

rRNAs.

tRNAs.

Some ribosomal proteins.

RNA polymerase subunits.

Translation factors.

Proteins involved in photosynthesis.

Continue…..

Org

aniz

atio

ns

of

the

ch

loro

pla

st g

en

om

e

of

rice

(O

ryza

sa

tiva

)

Diagram depicts communication between the nucleus, chloroplast and mitochondrion

History of Organellar Inheritance

1908

Correns

First presented the evidence for

cytoplasmicinheritance in

Mirabilis jalapa

1924

Jenkins

Described the

iojap leaf

variegation in

maize

1933

Rhodes

Described CMS in Maize

1908

Baur

First presented the evidence for

cytoplasmicinheritance in Pelargonium

zonale

ORGENELLAR INHERITANCE

Difference Between Organellar Inheritance and Mendelian inheritance

Organellar Inheritance Mendelian inheritance 1) Governed by Nuclear genes 1) Governed by orgenellar genes

2) Exhibits distinct segregation pattern

2) Does not exhibits distinct segregation pattern

3) Reciprocal differences are not observed

3) Reciprocal differences areobserved

4) Does not show maternal effects 4) Show maternal effects

5) Genes can be easily mapped 5) Mapping is difficult

6) Nuclear genes are associated with chromosomes

7) Plasma genes are associated with either cpDNA or mtDNA.

Important features of Organellar Inheritance

Reciprocal

differences-

As orgenellar

genome from

only one parent,

generally the

female parent

are transmitted

(Uni-parental

inheritance)

Important features of OrganellarInheritance

Lack of Segregation

Irregular Segregation in Biparental Inheritance

Somatic Segregation

Association with Organellar DNA

Mutagenesis

Lack of Association with A Parasite, Symbiont

or Virus

Carl Correns (1908) studied the inheritance of

leaf variegation called ‘albomaculatus’ in the

four O clock plant Mirabilis jalapa.

PLASTID INHERITANCE

Mixed chloroplasts

White/green

Mutant chloroplast

White

non-photosynthetic

Normal chloroplast

Green

photosynthetic

Variegated-shoot phenotypes in four o’clocks

Plas

tid S

tudy

Crosses between Oenothera muricata and

Oenothera hookeri.

Iojap in maize- three types of leaves.

Green, Iojap (green and white stripes) and

white.

Zebrina- white and green sectors in leaves.

Some other examples of plastid inheritance are

Inheritance pattern governed by mt-DNA is termed as

mitochondrial inheritance.

Examples of Mitochondrial inheritance are as follows:-

Cytoplasmic Male Sterility in plants-Maize,

Sorghum..etc

Pokyness in Neurospora.

Petite in Yeast.

MITOCHONDRAL INHERITANCE

Role cytoplasmic organelles in transmission of

characters.

Role in mapping of chloroplast and mitochondrial

genome.

Role of cytoplasmic male sterility.

Mutation of chloroplast-DNA and mitochondrial-DNA

leads to generation of new variants.

mt-DNA is used for the study of human evolution

SIGNIFICANCE IN PLANT BREEDING