“A Non-Mendelian pattern of inheritance governed by the DNA present in the cytoplasm is known as extra-chromosomal inheritance or cytoplasmic inheritance.” The DNA is the genetic material of us and arranged on chromosomes. It helps to store and transfer information (called traits) through the process of replication, transcription and translation. Nuclear DNA is the basis for inheritance of almost all type of phenotype of ours. It inherited in a particular fashion from parents to their offspring. Though all genes are inherited in Mendelian style, some genes present in the cytoplasm of the cell, inherited in a Non-Mendelian pattern. This type of inheritance is called as extra-chromosomal inheritance or cytoplasmic inheritance.
It was first reported by Boris Ephrussi in yeast during 1949. Cytoplasmic DNA or extra-chromosomal DNA is present significantly in some important organelles like chloroplast and mitochondria. It is a big mystery that how actually these organelles created their own genome. One theory which stated that it was a symbiotic relationship. It is believed that mitochondria were once free-living bacteria. Over a period of time, it created a symbiotic relationship with eukaryotic cells and established themselves into the cytoplasm and ultimately evolved as an organelle in living eukaryotic cell. Similarly, the chloroplast in green plants comes from the free-living algae and established a symbiotic relationship with eukaryotic plant cells and settled into cytoplast of green plants. Both types of sub-genome have well-developed DNA machinery which is equipped with all the component required for central dogma. Additionally, chloroplast has antibiotic resistance genes indicate that it was derived from bacteria, previously. The genome is made up of few genes and several thousand base pairs, still it has their own rRNA, tRNA and DNA for replication, translation and transcription.
Definition:
“The extra-chromosomal DNA present in the cytoplasm and not on chromosomes which follows the Non-Mendelian pattern of inheritance is known as extra-chromosomal inheritance.”
Criteria for extrachromosomal inheritance:
The extra-chromosomal DNA follows a Non-Mendelian pattern of inheritance Unlike the common Mendelian segregation pattern is not observed in the extra-chromosomal DNA because it does not have the centromere it cannot segregate, unlike the normal nuclear DNA.
Their own machinery for protein synthesis:
Unlike nuclear DNA, the organelle DNA or the extra-chromosomal DNA has its own replication and transcription machinery. It synthesised their own DNA.
Inheritance through mitochondria
Mitochondria can self-replicate and represent another genetic system in the cell. Of
course, the amount of mitochondrial DNA is so small, representing less than 1% of the nuclear
DNA is mammalian cells and it can code for a part of the protein in the mitochondria. The
synthesis of the cytochrome found in mitochondria for example, is known to be present in
minute amount in cytoplasm under the control of nuclear genes. Therefore, it is suggested that
both mitochondria and chloroplast seem to have a semi-autonomous existence and their DNA
forms the basis for genetic systems separate from that in the nucleus.
Episome in Bacteria
Some hereditary particles have been found to exist in two states, either in an
autonomous state in the cytoplasm, where they replicate independently, of the chromosomes,
or in an integrated state incorporated into the chromosome. Particles with such properties are
known as episomes and include such things as the sex factor. The episomes are apparently not
essential to the life of the bacteria, because they may or may not be present. If they are absent,
they can be acquired only from an external source. In bacteria, E coli, sex is determined by the
presence or absence of the sex factor (F). Male bacterial cells (donor) have the sex factor and
this factor is responsible for the transfer of DNA from male to female bacterial cells (Recipient).
This sex factor is the cytoplasmic particle.
Maternal inheritance:
The extra-chromosomal DNA inherited from the maternal side. The segregation is observed in somatic cells rather than germ cells, unlike nuclear inheritance.
Carl Correns (1908) first reported non-mendelian inheritance in Mirabilis Jalapa plastid DNA. Another extra-chromosomal inheritance was reported by M M. Rhoades (1933). He postulated that inheritance of male sterility in maize is governed by maternal inheritance and it becomes one of the greatest discoveries in science. Another important point that makes extra-chromosomal DNA even unique is maternal inheritance. It inherits from mother to their offspring which means that only female individual from the entire population can inherit cytoplasmic DNA. One theory suggests that female reproductive cell (ovum) is bigger, contain more cytoplasm and more organelles than male reproductive cells. This would be expected to influence Non-Mendelian inheritance or maternal inheritance.
One of the classical examples of maternal inheritance is :
Cytoplasmic male sterility in maize.
Here nuclear genes do not play any significant role rather, the sterility is inherited through egg cytoplasm from generation to generation. When a male sterile plant is crossed with a normal fertile plant, all the F1 plants remain sterile. When all F1 sterile plants are backcross with a normal fertile plant, until all chromosomes from the male sterile line are exchanged to male fertile, the sterility persists in the progeny.
Generally, male-sterile lines are denoted as tcs, T (Texas), C (Cytoplasmic), S (Sterility). It was believed that T (Texas) cytoplasm is associated with susceptibility against several types of disease like leaf blight disease and yellow blight disease in maize. This result indicates that chromosomal nuclear DNA does not have any significant role in male sterility (particularly in maize). Furthermore, most of the cytoplasm and organelles are inherited from the maternal side. From the scientific findings, it is confirmed that the sterility is inherited from the cytoplasm. This discovery becomes a crucial milestone in crop improvement. Hybrid sterile maize plant becomes more popular as the corn of maize developed uniformly. The hybrid seed becomes more popular for mass production of maize. Though maternal inheritance may be extra-chromosomal or chromosomal, it is one of the miracle events in nature. Here genetic compositions of maternal side influence several phenotypes of offspring. In some organism, not only maternal inheritance rather the genotype of the maternal side has great influence on the phenotype of offspring. Here phenotype of mother does not have any role in the development of phenotype in offspring.
The maternal-effect in snail:
The character of coiling in snail is governed by maternal inheritance. Snail, Limnaea peregra, has two types of shell coiling phenotypes: one is dextral shells which coil for the right side and another is a sinistral shell which coils for the left side. Here, the mother’s genotype (not a phenotype) is exclusively responsible for the development of coiling style. Assume that D+ genotype codes for dextral (right side) coiling and D is codes for sinistral coiling. The reciprocal cross of D+ and D is shown in the figure:
The image represents the maternal effect of the snail. Here crossing between dextral female and sinistral male results in dextral offspring in F1, while inbreeding results in all dextral progenies in the F2 generation.
Crossing between D+D+ female and DD male, all the F1, as well as F2 progeny, become dextral as the mother is D+D+ dextral, here the DD recessive phenotype is not expressed and typical Mendelian 3:1 ratio is not obtained (all four are dextral). In another condition when DD sinistral female is crossed with D+D+ dextral male, F1 offspring become sinistral with genotype D+D, here mentioning genotype is important because the inheritance is governed by genotype not by phenotype. When this F1 progeny is inbred (D+D * D+D) all the F2 progeny become dextral and coil for the right side. This result indicated that phenotype of parents do not have any influence on the phenotype of progeny because although all of the F1 progeny are sinistral, all F2 offspring becomes dextral.
The image represents the maternal effect in the snail. Crossing between sinistral female and dextral male results in sinistral F1 progenies. Though all F1 progenies are sinistral, all F2 progenies become dextral.
Detailed investigation shows that spindle formed during the second metaphase division decides the direction of coiling. The spindles of dextral snail are tipped to right and vice verse for sinistral. Interestingly, spindle arrangement in metaphase in controlled by maternal genes. So the actual phenotype of “type of coiling” in snail is governed by maternal genes and it does not depend on the phenotype of any of parent. In some of the organism, the amount of exchange of cytoplasm plays a crucial role in the inheritance of phenotype.
Inheritance of kappa particles in paramaecium:
Paramecin is a substance found in some of the killer strain of paramaecium which kills the sensitive strains. Paramecin production is governed by the kappa particles present in the cytoplasm of the paramaecium.
The image represents inheritance of kappa particles in paramaecium at a shorter period of conjugation.
Here KK gene is responsible for the production of kappa particle which is dominant over kk gene. In case of inheritance of kappa particle, cytoplasmic exchange during conjugation plays a crucial role. When KK killer strains are crossed with kk strains by conjugation, all the progeny obtained are heterozygous with genotype Kk but the phenotype of paramaecium depends on presence or absence of kappa particles and it will be influenced by time of conjugation.
If both are conjugates for a shorter period of time, in F1 generation killer strains remain killer and non-killer remain non-killer in the heterozygous condition. Here only nuclear genes are transferred but the cytoplasm is not exchanged between both strains.
In another condition, if killer and non-killer strains are conjugated for a longer period of time, due to the exchange of kappa particles, sensitive strain receives kappa particles through cytoplasmic exchange and sensitive strains become killer in F1 generation.
Significance of Cytoplasmic Inheritance
1. Development of cytoplasmic male sterility several crop plants like maize. Pearl millet, sorghum, cotton, etc.
2. Role of mitochondria in the manifestation of heterosis.
3. Mutation of chloroplast DNA and mitochondrial DNA leads to generation of new
variation.
Conclusion
The cytoplasm is an important component of the cell, not only for transferring organelles but also for the inheritance of characters. Cytoplasmic inheritance and maternal inheritance are responsible for some of the disease condition in the human. Any defect in the inheritance of extra-chromosomal genes results in serious physical, mental and biochemical problems.
References
-Cell Biology, Genetics, Molecular Biology, Evolution & Ecology by Verma, P. S., & Agrawal, V. K. (2006) . S. Chand and Co.
- Principles of Genetics by Gardner, E.J., Simmons, M.J. and Snustad, D.P. (1991). 8th Edition. John Wiley
-Wikipaedia and internet sites are also consulted.
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