Transcript Extra nuclear inheritance - PGGCG

Presence & function Of
Mitochondrial & Plastid DNA
( Extra nuclear inheritance)
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany Deptt.
P.G.G.C.G. -11,Chandigarh
Organelle heredity
 Organelles that contain chromosomes
►Chloroplasts
and mitochondria
Organelle Inheritance follow
Non-Mendelian Inheritance
1. Extranuclear genes display non-Mendelian inheritance,
which has following characteristics:
a. Typical Mendelian ratios do not occur, because meiosis-based
segregation is not involved.
b. Reciprocal crosses usually show uniparental inheritance, with all
progeny having the phenotype of one parent, generally the mother
because the zygote receives nearly all of its cytoplasm (including
organelles) from the ovum.
c. If a nucleus with a different genotype is substituted, non-Mendelian
inheritance is unaffected.
Mitochondrial DNA
► The
nucleus is an organelle in eukaryotes which
houses the primary genetic material (DNA)
► Mitochondria are organelles which are responsible
for cellular respiration (ATP production)
► Mitochondria have a double membrane, cristae
(folds), a matrix, and their own DNA
► Mitochondrial DNA (mtDNA) codes for proteins and
enzymes used by the mitochondria
► Nuclear DNA also codes for enzymes used in the
mitochondria
Nuclear DNA vs. Mitochondrial DNA
► Nuclear
DNA
 found in nucleus of the
cell
 2 sets of 23
chromosomes
 maternal and paternal
 double helix
 bounded by a nuclear
envelope
 DNA packed into
chromatin
►
Mitochondrial DNA
 found in mitochondria of
the cell
 each mitochondria may
have several copies of the
single mtDNA molecule
 maternal only
 circular
 free of a nuclear envelope
 DNA is not packed into
chromatin
Nuclear DNA vs. Mitochondrial DNA
Maternal Inheritance of mtDNA
► during
fertilization, the sperm only
contributes its nucleus (23 chromosomes)
► mitochondria of the sperm cell are located
at the mitochondrial sheath which is
destroyed upon fertilization
► the only available mitochondria (mtDNA) is
that of the mother's; this is why mtDNA is
of maternal origin
Maternal Inheritance of mtDNA
Key Facts About mDNA
► mtDNA
of siblings will match each others and that
of their mother
► mtDNA is found as a single, circular chromosome
in the cell
► mitochondrion may contain multiple copies of
mtDNA
► a human cell may contain hundreds or thousands
of mitochondria
► mtDNA may be useful when nuclear DNA is limited
because of its abundance
The Mitochondrial Genome
► 16,569
base pairs (bp) in length
► encodes 37 genes, 13 proteins, 22 tRNAs, and 2
rRNAs
► two general regions:
 coding region: "responsible for the production of various
biological molecules involved in" cellular respiration
 control region: "responsible for the regulation of the mtDNA
molecule"
► “contains
introns)”
little non-coding DNA (“junk” DNA, or
Mitochondrial Inheritance
Yeast petite Mutants
1. Yeast can grow either anaerobically by fermentation (slow
growth) or aerobically using mitochondria (fast growth),
forming colonies from single cells on solid media.
2. Yeast petite colonies are much smaller than those formed by
wild-type cells, due to cytochrome deficiencies that prevent
aerobic respiration.
a. On a medium that supports only aerobic respiration, petite cells are
unable to grow.
b. The spontaneous mutation rate is 0.1–1%, but exposure to an
intercalating agent (e.g., ethidium bromide) raises the rate to 100%.
c. This allows isolation of different petite cell lines, containing different
mutations.
ii. The cross in this case was pet- X pet+. Diploid was pet-/pet+ (hence wildtype) and the spore tetrad contained 2 pet- and 2 pet+ spores.
Saccharomyces (Yeast)
petite Mutations
► petite
mutations give rise to small colonies
 Aerobic respiration blocked
 Live anaerobically
►S.
► Two
cerevisiae is a facultative anaerobe
types
 Segregational petites encoded by nuclear genes
showing Mendelian inheritance
 cytoplasmic transmission pattern petites
►Neutral
petites
►Suppressive petites
Segregational petites encoded by nuclear
genes showing Mendelian inheritance
►
Yeast crosses between petite and wild-type
cells (a X α crosses) determine the mechanism of
inheritance for this phenotype.
 Some petite X wild-type crosses give 2:2
segregation (wild-type:petite).
 This is the same ratio as seen in nuclear genes, so
these petite mutants are nuclear (segregational)
petites, written as pet-
Inheritance of Segregational petites
cytoplasmic transmission pattern petites
►Neutral
petites
►Suppressive petites
neutral petites ([rho-N])
crossed with wild-type ([rho-N] X [rho+N])
produce wild-type diploids ([rho-N]/[rho+N]) and spores
that segregate 0:4 (no petite : 4 wild-type).
►
This is an example of uniparental (not maternal,
since gametes are same size) inheritance.
►
In [rho-N] mutants, nearly 100% of the mtDNA is
missing, and so mitochondrial functions are also
missing.
►
Spores produce only wild-type colonies because
normal mitochondria from the wild-type parent provide
normal mitochondria for the progeny. The petite trait
thus is lost after one generation.
►When
Inheritance of neutral petites
suppressive ([rho-S])
Most petite mutants are of suppressive ([rho-S]) type.
They differ from neutral petites by having an effect on
the wild-type, although both are mutations in mtDNA.
A [rho+/rho-S] diploid has a respiratory-deficient
phenotype, and if it divides mitotically the progeny
will nearly all be petites.
►
Sporulation of the rare wild-type [rho+/rho-S]
diploid produces tetrads with a 0:4 (petite : wild-type)
ratio.
►
suppressive ([rho-S])
►Suppressive
petite mutants start with deletions in
mtDNA., often creating gene deletions and
rearrangements that cause deficiencies in the
enzymes for aerobic respiration.
► The suppressive effect over normal mitochondria
might result from either:
(1) Faster replication of the mutant mitochondria,
outcompeting wild-type, or
(2) Fusion with normal mitochondria and
recombination between [rho-S] mtDNA and wildtype mtDNA.
Inheritance of suppressive ([rho-S])
Chloroplast Genome
1. Chloroplasts have a double membrane, internal lamellar
structure containing chlorophyll, and protein-rich stroma.
Chloroplasts divide and grow in the same way as mitochondria.
2. The chloroplast genome (cpDNA) is not as well characterized
as mtDNA, but some things are known:
a. Structurally, cpDNA is similar to mtDNA. It is dsDNA, a
super- coiled circle lacking structural proteins.
b. The GC content of cpDNA often differs from both nuclear
and mtDNA.
c. The size of cpDNA varies from 80 kb-600 kb. All
chloroplast genomes carry noncoding DNA.
d. Each chloroplast has multiple copies of cpDNA in several
nucleoid regions. An example is Chiamydomonas with 5001,500 cpDNA molecules per chloroplast.
Chloroplast Genome
3. Nuclear genes encode some chloroplast
components, while cpDNA genes (which may
include introns) encode the rest, including
a. Two copies of each chloroplast rRNA (loS, 23S,
4.5S and 5S). The copies are included with other
genes in inverted repeats designated IRA and IRB
that define the short (SSC) and long (LSC) single
copy regions of the cpDNA.
b. The tRNAs (30 in tobacco and rice, 32 in the
liverwort Marchantia).
Chloroplast Inheritance
Shoot Variegation in the Four O’Clock
1. Variegated-shoot phenotype in four o’clocks involves non-Mendelian
inheritance of chloroplasts in the shoots (stem, leaves and flowers).
a. Green shoots have normal chloroplasts.
b. White shoots have only leucoplasts, which lack chlorophyll, and are incapable of
photosynthesis.
c. Variegated shoots received both chloroplasts and leucoplasts, which segregated
during cell division. Progeny cells are therefore green or white, in a variegated
(mixed) pattern
2. Results of crosses between plants with shoots that are variegated illustrate this
phenomenon (Table 15.2):
a. When ova are from green plants, only green progeny result, regardless of pollen
source.
b. When ova are from white plants, only white progeny result (but soon die from lack
of chlorophyll), regardless of pollen source.
c. When ova are from variegated plants, all three types of progeny result, regardless of
pollen source.
3. Shoot color in these plants therefore shows a pattern of maternal
inheritance. There are three assumptions in the model:
a. Pollen contributes no chloroplasts or leucoplasts to the zygote.
b. The chloroplast genome replicates autonomously, so that progeny plastids
retain the same color phenotype as the original plastid.
c. Segregation of plastids during eukaryotic cell division is random, providing
some offspring cells with chloroplasts, some with leucoplasts, and some with a
mixture.
Variegation in the four o’clock
Model for the inheritance of shoot color in the four o’clock