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#chromosomal DNA

2 public questions tagged with this topic.

Gene and Genome

Prokaryotic chromosomal DNA is usually:

Prokaryotic chromosomes typically consist of single large DNA molecule occupying nucleoid region without nuclear envelope. Most bacteria including Escherichia coli carry circular, supercoiled chromosome of several megabases organized in loop domains constrained by HU, FIS and H-NS proteins. Circular topology confers replicative advantage with single origin oriC and bidirectional replication terminating at dif site. Exceptions exist such as linear chromosomes in Borrelia, Streptomyces and Agrobacterium, but archetypal pattern remains circular double-stranded DNA often associated with plasmids providing accessory functions under selection pressures.

Ref: Lewin B., Genes XII, Chapter 13: Prokaryotic Chromosomal DNA is Usually Circular, Bacterial Genome

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DNA & RNA isolation

What causes the separation of plasmid DNA from chromosomal DNA?

In alkaline lysis for plasmid isolation, covalent closed circular supercoiled plasmid DNA and linear chromosomal DNA behave differently due to topology. Under alkaline conditions with SDS, both are denatured. Upon rapid neutralization, small supercoiled plasmids renature quickly because their two strands remain interlinked topologically, becoming soluble. Large chromosomal fragments cannot correctly reanneal and form an insoluble network with SDS-protein complexes and precipitate. Thus separation is based on topological differences and renaturation kinetics, not merely size variation or charge differences alone.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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