Minichromosome

A minichromosome is a small chromatin-like structure consisting of centromeres, telomeres and replication origins and little additional genetic material. They replicate autonomously in the cell during cellular division. The origin of minichromosomes is the result of natural processes (chromosomal aberrations) or genetic engineering.

Minichromosomes can be either linear or circular pieces of DNA. By minimizing the amount of unnecessary genetic information on the chromosome and including the basic components necessary for replication (centromere, telomeres, replication sequence), scientists aim to construct a chromosomal platform which can be utilized to insert/present new genes into a host organism's cell.

Producing minichromosomes involves two primary methods, the (bottom-up) and the top-down approach.

The minimum constituent parts of a chromosome (centromere, telomere and DNA replication sequences) are assembled This is done by using molecular cloning techniques to construct the desired chromosomal contents in vitro. Next, the desired contents of the minichromosome must be transformed into a host which is capable of assembling the components (typically yeast or mammalian cells) into a functional chromosome. This approach has been attempted for the introduction of minichromosomes into maize for the possibility of genetic engineering, but the success was limited and questionable. In general, the de novo approach is more difficult than a top down method due to species incompatibility issues and the heterochromatic nature of centromeric regions.

This method utilizes the mechanism of telomere-mediated chromosomal truncation (TMCT). This process is the generation of truncation by selective transformation of telomeric sequences into a host genome. This insertion causes the generation of more telomeric sequences and eventual truncation. The newly synthesized truncated chromosome can then be altered through the insertion of new genes for desired traits. The top-down approach is generally considered as the more plausible means of generating extra-numary chromosomes for the use of genetic engineering of plants. In particular it is useful because their stability during cell division has been demonstrated. The limitation of this approach is that it is labor-intensive.

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