Mini Chromosomes - An Overview

Introduction

Chromosomes are the thin fiber-like structure that forms the most important part of the genetic material and is mostly confined inside the nucleus in eukaryotic cells due to the presence of cell membrane and are scattered in prokaryotic cells. During cell division chromatin condense to form rod-like structures containing centromere and chromatids.

Mini chromosomes are thread-like structures similar to chromosomes that contain a centromere, chromatids, telomere, and the origin of replication. But they differ from chromosomes in the fact that they contain a little extra genetic material, and they generally divide on their own.

Structure of Mini chromosomes

A minichromosome needs all the necessary components that is centromere, chromatids, and telomere to function properly and for smooth cell division. Centromere is needed for the formation of kinetochore during cell division, the absolute number of chromosomes is maintained by the origin of the replication site and telomeres which are found at the end of the chromosomes help to prevent chromosomes from any kind of degradation. The following are the components of s typical mini chromosome −

Centromere

It is the central and densest part of the minichromosome. It helps in the separation of the chromosome during cell division.

• This part employs the kinetochore proteins that help in the attachment of the microtubules to the centromere and eventually their movement toward the poles during cell division.

• For the formation of the centromere complex sequences are required which code for Histon H3 protein.

Origin of Replication

Reproduction in cells will take place when the cells divide at the right time. Duplication of DNA takes place during the replication process which ensues the equal division of genetic material in the daughter cells.

• The size of the chromosome mostly determines the number of replication origin sites. Generally, eukaryotes have many origins of replication that help in the duplication of large linear chromosomes. But prokaryotes have only one origin of replication site present in their circular DNA.

• In the case of mini chromosomes there is no need to specifically identify the origin of the replication constitution.

Telomeres

Telomeres are found at the ends of the chromosomes and are sequence repeats along with associated proteins. They help to protect the ends of the chromosome from end-to-end fusion or any degradation.

• This protective function can be attributed to the sequence present at the terminus called as G- loop.

• During the construction of mini chromosomes telomeric sequences with desired transgene is introduced.

Methods to Produce Mini Chromosomes

There are two methods that are generally used to produce Mini chromosomes by genetic engineering. They are −

• Bottom-up

• Top-down

Bottom-up Method

Molecular cloning technique is used to produce the desired mini chromosome by assembling the basic constituents namely centromere, telomere, and origin of replication in laboratory conditions or in vitro.

• In the next step the constructed mini chromosome is transformed into a suitable host like yeast which can assemble all the components of the mini chromosome and make it a functional one.

• Since most of the species are incompatible and their centromere is heterochromatic in nature, that is why this method is difficult as compared to the top-down method.

Top-down Method

This method involves the transformation of a sequence of the telomere of the host cell itself. This whole process is known as telomere truncation.

• New genes of interest can be inserted in these truncated areas. This method is preferred over the de novo method as most of the species are compatible and the number of mini chromosomes thus generated is also more. These mini chromosomes can be used in genetic engineering.

• Although this process is laborious, still it is preferable because mini chromosomes remain stable during cell division.

Applications of Mini Chromosomes Technology

• In conventional genetic engineering one or two genes at a time but with mini chromosomes multiple genes can be transferred and expressed. Also, traditional genetic engineering methods can cause damage to the host cell by disrupting the genes of the host cells by the insertion of new genes.

• It is difficult to assess the inheritance of newly inserted genes from one generation to another in a traditional method. But this can be easily controlled with mini chromosomes.

• There are less chances of separation of the new traits as mini chromosome technology allows gene stacking on the same chromosome.

• The process of telomere truncation and subsequent generation of mini chromosomes was first done in maize. This technique was used for the insertion of the gene of interest into the plant genome. Not only in maize but this technique has been successfully used in other plant species also.

• One of the major advantages of this technology is that it is compatible with the host genome hence causing less damage to the host cell and does not interrupt the normal process of cell division and gene expression. Hence it can be used to increase the productivity and yield of many crop varieties.

Conclusion

Mini chromosome technology has been in use since 2006 when a set of genes carrying desired traits were transformed into maize plants. Since then this technique has dominated the traditional genetic engineering technique as it offers greater advantages and less damage to host cells. But this technique is yet to be implemented in animal cells and extensive work has been going on in this field.