Susmita Majumder
Faculty, Dinabandhu Andrews College
Extrachromosomal inheritance in Paramoecium & snail
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 extrachromosomal inheritance
or cytoplasmic inheritance.
A non-mendelian pattern of inheritance governed by the DNA present in the
cytoplasm is known as extrachromosomal inheritance or cytoplasmic
inheritance.
The extrachromosomal inheritance also is known as cytoplasmic inheritance or non-mendelian
inheritance was first reported by Boris Ephrussi in yeast during 1949.
Cytoplasmic DNA or extrachromosomal 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.
Criteria for extrachromosomal inheritance:
The extrachromosomal DNA follows a non-mendelian pattern of inheritance
Unlike the common Mendelian segregation pattern is not observed in the extrachromosomal DNA
because it does not have the centromere it can not segregate, unlike the normal nuclear DNA.
Their own machinery for protein synthesis:
Unlike nuclear DNA, the organelle DNA or the extrachromosomal DNA has its own replication and
transcription machinery. It synthesised their own DNA.
Maternal inheritance:
The extrachromosomal DNA inherited from the maternal side.
The segregation is observed in somatic cells rather than germ cells, unlike nuclear inheritance.
, Susmita Majumder
Faculty, Dinabandhu Andrews College
Kappa particle in Paramecium
Kappa particles are found in certain killer strains of Paramecium and are responsible for production
of substance paramecin, which is toxic to strains not possessing kappa (sensitive strain).
The production of kappa particles is dependent on a dominant allele K, so that killer strains are KK or
Kk and sensitive strains are ordinarily kk. In absence of dominant allele K, kappa particles cannot
multiply and in absence of kappa particles, dominant allele K cannot produce them de novo.
Consequently sensitive strains with genotypes KK or kk can be obtained. These will not carry any
kappa particles.
However, killer strain with genotype kk cannot be obtained, because even if kappa particles are
present, these would be lost in absence of dominant allele.
If Paramecium clones with genotypes KK or Kk are allowed to multiply asexually at such a fast rate,
that division of kappa particles cannot keep pace with division of cells, kappa particles will be
eventually lost. Consequently sensitive strains with dominant genotype (KK, Kk) having no kappa
particles would be obtained.
If the killer (KK)and sensitive (kk)strains are allowed to conjugate, all exconjugants (the cells
separating after conjugation) will have same genotype Kk. Phenotypes of these exconjugants will,
however, depend upon duration for which conjugation is allowed.
Figure 1 Results of a cross
between a killer (KK)and a
sensitive (kk)strain of
Paramecium, when cytoplasmic
exchange is allowed.
Faculty, Dinabandhu Andrews College
Extrachromosomal inheritance in Paramoecium & snail
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 extrachromosomal inheritance
or cytoplasmic inheritance.
A non-mendelian pattern of inheritance governed by the DNA present in the
cytoplasm is known as extrachromosomal inheritance or cytoplasmic
inheritance.
The extrachromosomal inheritance also is known as cytoplasmic inheritance or non-mendelian
inheritance was first reported by Boris Ephrussi in yeast during 1949.
Cytoplasmic DNA or extrachromosomal 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.
Criteria for extrachromosomal inheritance:
The extrachromosomal DNA follows a non-mendelian pattern of inheritance
Unlike the common Mendelian segregation pattern is not observed in the extrachromosomal DNA
because it does not have the centromere it can not segregate, unlike the normal nuclear DNA.
Their own machinery for protein synthesis:
Unlike nuclear DNA, the organelle DNA or the extrachromosomal DNA has its own replication and
transcription machinery. It synthesised their own DNA.
Maternal inheritance:
The extrachromosomal DNA inherited from the maternal side.
The segregation is observed in somatic cells rather than germ cells, unlike nuclear inheritance.
, Susmita Majumder
Faculty, Dinabandhu Andrews College
Kappa particle in Paramecium
Kappa particles are found in certain killer strains of Paramecium and are responsible for production
of substance paramecin, which is toxic to strains not possessing kappa (sensitive strain).
The production of kappa particles is dependent on a dominant allele K, so that killer strains are KK or
Kk and sensitive strains are ordinarily kk. In absence of dominant allele K, kappa particles cannot
multiply and in absence of kappa particles, dominant allele K cannot produce them de novo.
Consequently sensitive strains with genotypes KK or kk can be obtained. These will not carry any
kappa particles.
However, killer strain with genotype kk cannot be obtained, because even if kappa particles are
present, these would be lost in absence of dominant allele.
If Paramecium clones with genotypes KK or Kk are allowed to multiply asexually at such a fast rate,
that division of kappa particles cannot keep pace with division of cells, kappa particles will be
eventually lost. Consequently sensitive strains with dominant genotype (KK, Kk) having no kappa
particles would be obtained.
If the killer (KK)and sensitive (kk)strains are allowed to conjugate, all exconjugants (the cells
separating after conjugation) will have same genotype Kk. Phenotypes of these exconjugants will,
however, depend upon duration for which conjugation is allowed.
Figure 1 Results of a cross
between a killer (KK)and a
sensitive (kk)strain of
Paramecium, when cytoplasmic
exchange is allowed.
