1. Presence & function Of Mitochondrial & Plastid DNA ( Extra nuclear inheritance)
Dr. Madhumita Bhattacharjee
Assiatant Professor
Botany Deptt.
P.G.G.C.G. -11,Chandigarh

2. Organelle heredity Chloroplasts and mitochondria
Organelles that contain chromosomes
Chloroplasts and mitochondria

3. 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.

4. 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

5. Nuclear DNA vs. Mitochondrial 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

6. Nuclear DNA vs. Mitochondrial DNA

7. 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

8. Maternal Inheritance of mtDNA

9. 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

10. 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 little non-coding DNA (“junk” DNA, or introns)”

11. Mitochondrial Inheritance

12. 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 wild- type) and the spore tetrad contained 2 pet- and 2 pet+ spores.

13. Saccharomyces (Yeast) 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
Suppressive petites

14. 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-

15. Inheritance of Segregational petites

16. cytoplasmic transmission pattern petites
Neutral petites
Suppressive petites

17. neutral petites ([rho-N])
When 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.

18. Inheritance of neutral petites

19. 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.

20. 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 wild- type mtDNA.

21. Inheritance of suppressive ([rho-S])

22. 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 ,500 cpDNA molecules per chloroplast.

23. 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).

24. 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.

25. 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.

26. Variegation in the four o’clock

27. Model for the inheritance of shoot color in the four o’clock