Linear chromosomes are found only in some Actinomycetes. Streptomyces is a Gram-positive bacterium with a high GC-content. It has a linear chromosome varying from 6 to 10 Mb in size, and several circular plasmids with a total size of 1-10 % of the chromosomal DNA. Streptomycetes exhibit a complex life cycle involving asexual and sexual phases of growth and are known for their extensive production of chemically diverse bioactive secondary metabolites.

It is necessary to linearize the chromosome to get individual replicons resolved out in the pulse field gel. The replicons are separated by CHEF on the basis of size. It is usually run for a long period of time, often 20-30 hours, to get good resolution.

Here are the steps on how to fold a linear chromosome of Streptomyces:

1. Prepare the agarose gel. Make a 1% agarose gel in 0.5X TBE buffer.

2. Load the DNA sample. Mix the DNA sample with 6X loading buffer and load it onto the agarose gel.

3. Run the gel. Run the gel at 14°C for 20-30 hours at 2 V/cm.

4. Visualize the DNA. Stain the gel with ethidium bromide and visualize the DNA under UV light.

5. Fold the chromosome. The linear chromosome of Streptomyces will appear as a single large band on the gel. To fold the chromosome, cut the band out of the gel and place it on a piece of Parafilm.

6. Fold the Parafilm over the chromosome several times until it is folded into a compact size.

7. Store the chromosome. The folded chromosome can be stored at -20°C or -80°C.

Note: Be sure to wear gloves and eye protection when working with ethidium bromide. Ethidium bromide is a mutagen and carcinogen.