Minichromosome
From Wikipedia, the free encyclopedia
A minichromosome is a small chromatin-like structure resembling a chromosome that contains centromeres, telomeres, and replication origins,[1] but relatively little additional genetic material. They replicate autonomously in the cell during cellular division.[2] Minichromosomes may be created by natural processes as chromosomal aberrations or by genetic engineering.[1]
Minichromosomes can be either linear or circular pieces of DNA.[2] By minimizing the amount of unnecessary genetic information on the chromosome and including the basic components necessary for DNA replication (centromere, telomeres, and replication sequences), molecular biologists aim to construct a chromosomal platform which can be utilized to insert or present new genes into a host cell.[2]
Production
Producing minichromosomes by genetic engineering techniques involves two primary methods, the de novo (bottom-up) and the top-down approach.[1]
De novo
The minimum constituent parts of a chromosome (centromere, telomeres, and DNA replication sequences) are assembled[3] 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[4]) into a functional chromosome. This approach has been attempted for the introduction of minichromosomes into maize for the possibility of genetic engineering, but success has been limited and questionable.[5] In general, the de novo approach is more difficult than the top-down method due to species incompatibility issues and the heterochromatic nature of centromeric regions.[4]
Top-down
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.[2] 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-numerary chromosomes for the use of genetic engineering of plants. In particular it is useful because their stability during cell division has been demonstrated.[6] The limitation of this approach is that it is labor-intensive.