| MadSci Network: Cell Biology |
There are two types of nuclei in paramecium:
Macronucleus: The larger nucleus which performs normal cell functions like respiration, protein synthesis and digestion.
Micronucleus: The smaller nucleus which is responsible for cell division and used only during reproduction.
The nuclei of Paramecium differ in both structure and function. The diploid micronucleus, which is present in two copies in the P. tetraurelia species, represents the germ line and is completely silent in terms of transcription. This is the nucleus which undergoes meiosis and fertilization during sexual events.
Table: Comparative properties of the micronucleus and macronucleus
| Characteristic | Micronucleus | Maconucleus |
| Polidy | 2n | ~1000n |
| Genome size | 100-120 megabases | 80-90% micronuclear complexity |
| Number of chromosomes | ~50 | ~350 |
| Chromosome size | ~2 megabases | ~300(50-1000) kilobases |
The micronucleus of Paramecium plays an essential role in the development of the oral apparatus in both asexual and sexual cycles.
During conjugation, however, the micronuclei undergo a form of meiosis; creating two haploid micronuclei. The conjugating cells swap one of these, so each partner ends with one original and one new micronucleus. These micronuclei fuse and direct the production of a new macronucleus, while the old macronucleus disintegrates. The new macronucleus is rejuvenated both in size and in the possible compensation for bad genes by new genes from conjugation.
It is observed that the micronucleus is active only during the reproductive phase in Paramecium. The transcription phase is also silent and hence it may be said that the smaller nucleus in paramecium is not fully functioning.
I must appreciate the question raised by the students, Hope the following explanation satisfies their query Prof. Eric.
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Devendra
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