Microbial genomes, mobile elements and horizontal gene transfer

A genome is more than one familiar circle

A chromosome is a major organised DNA molecule carrying inherited information. Many familiar bacterial diagrams show one circular chromosome. Microbial genomes are more varied: some chromosomes are linear, some organisms have several major DNA molecules, and the number of copies within a cell can vary with lineage and growth stage. A circular icon is a useful example, not a definition of Bacteria or Archaea.

Plasmids are DNA replicons that can replicate apart from the main chromosome under suitable cellular conditions. Replicon refers to a unit of DNA replication, not necessarily to a transferable element. A plasmid may carry useful, costly or condition-dependent genes. Not every cell has one, not every plasmid can move between cells by itself, and a plasmid is not defined as an antibiotic-resistance molecule.

Other mobile elements can move between positions in DNA or between cellular contexts. A transposable element can change its location, while some viruses can integrate genetic material into a host genome. Moving within one genome and transferring between cells are distinct events, even though mobile systems can participate in both. Acquiring a sequence does not guarantee stable maintenance or expression.

Inheritance and transfer use different routes

Vertical inheritance passes DNA from a cell into its descendants. Horizontal gene transfer introduces inherited material between cells outside that simple parent-to-offspring route. It can make a gene's history differ from the histories of other genes in the same organism. The recipient keeps its broader ancestry; acquiring one region does not turn its whole genome into the donor's genome.

Three well-studied bacterial examples help distinguish physical routes. Transformation takes up DNA from the surroundings. Transduction involves a virus transferring cellular DNA. Conjugation involves direct cell contact and a DNA-transfer system. In the figure, a selected transferable plasmid is copied and a copy is passed during conjugation; the donor retains its own copy. These examples do not mean that every cell is competent for every route.

Incoming DNA can be degraded, fail to replicate, recombine with resident DNA or persist as a compatible replicon. Host range, sequence compatibility, defence and the conditions of selection matter. Transfer opportunities therefore do not equal successful long-term inheritance. Archaeal lineages also exchange genetic material, but the details of their systems should be examined rather than assumed identical to the three bacterial cartoons.

Count carriers separately from copies

The inventory model contains one hundred invented cells. Each has exactly one chromosome copy carrying one copy of marker M. Ninety cells lack plasmid P; ten cells each contain two copies of P, with one copy of gene G per plasmid. The chromosome-marker inventory is one hundred copies, while gene G totals twenty. Only ten cells carry G.

The figure keeps the chromosome and plasmid separate inside one representative carrier, then compares three counts: total cells, carrier cells and gene copies. Ten carriers with two copies each give a gene-to-marker copy ratio of 20/100 = 0.20, while the carrier fraction is 10/100 = 0.10. These values answer different questions.

A real measurement also has DNA extraction, sequencing and sampling limits. Cells can carry different chromosome counts; markers can occur more than once per genome; extracellular or nonliving-cell DNA can contribute. Without suitable assumptions and calibration, a read ratio need not be either a gene-copy ratio or a cell fraction. The constructed inventory removes those complications explicitly rather than claiming they disappear in real samples.

A gene's presence is a potential, not a measured rate

Metagenomics examines DNA recovered from a community without requiring every member to be isolated. It can reveal possible capabilities and inherited relationships. A detected gene does not by itself show that the cell is alive, that the gene is being transcribed, that a functional protein is present, or that the relevant reaction is occurring at a particular rate.

RNA, proteins, chemical transformations and carefully interpreted experiments can contribute different evidence about activity. Even then, the attribution to a specific cell or lineage may remain uncertain. A community can share metabolites, so the organism supplying a substrate and the organism using it need not be the same.

Gene histories can also complicate classification. A recently transferred region may group a recipient with its donor in that region's genealogy. Combining several independent regions, checking paralogues and recognising transfer helps test the organismal interpretation. Horizontal transfer adds meaningful connections; it does not make every ancestry question unknowable.

Worked example

Check your understanding

1. Does a plasmid necessarily carry a resistance gene or transfer independently to another cell?

   Show reasoning

   No. A plasmid is a separately replicating DNA unit in the stated context. Its genes and transfer ability depend on the particular plasmid and host; replication and horizontal transfer are different capabilities.

2. In the original model, why is G/M equal to 0.20 while the carrier fraction is 0.10?

   Show reasoning

   Ten of one hundred cells carry G, but each carrier has two plasmids with one G copy each. There are twenty G copies and one hundred marker copies, compared with only ten carrier cells.

3. Does detecting a carbon-fixation gene in community DNA establish an observed carbon-fixation rate?

   Show reasoning

   No. It indicates possible inherited capacity, subject to identification and sampling limits. Expression, functional machinery, suitable conditions and evidence of transformation are needed to establish activity and its rate.

What to carry forward

Trace the physical DNA route and define the counted entity before interpreting inheritance, abundance or function.