Plasmid

Plasmids are small fragments of DNA that can float free in the fluid contents of cells, especially bacterial cells, rather than being incorporated into chromosomes. The book uses their ability to multiply and move between chromosome and cell fluid to illustrate a central implication of the gene-centered view: a replicator’s prospects depend not on loyalty to the organism that contains it, but on whether it can leave copies in the future.

Structure and Movement

A plasmid is described as smaller than a virus and normally consisting of only a few genes. Some plasmids can splice themselves seamlessly into a chromosome, so that the join is indistinguishable from the surrounding chromosomal material. They can also cut themselves out again. This capacity to enter and leave chromosomes makes them a striking example of DNA behaving not only as a stable component of heredity but also as mobile material.

The book notes that these facts about cutting and splicing had come to light since the first edition. It sets aside the question of whether such fragments began as invading parasites or as breakaway rebels: whatever their origin, their likely behavior would be similar. Calling one a rebel fragment is a deliberate framing choice, directing attention to what it might do once it can reproduce outside ordinary chromosomal arrangements.

An Alternative Route Through the Future

The plasmid’s mobility opens a thought experiment about a stretch of human DNA able to detach from its chromosome, float freely in a cell, multiply into copies, and insert itself into another chromosome. Such a replicator would not have to rely exclusively on the conventional route of sperm or egg. The argument is hypothetical; the passage does not establish that this particular human-DNA scenario occurs. It asks what opportunities might follow if a replicator could exploit them.

The proposed route is made concrete through the continual loss and transfer of cells. Skin cells slough off and contribute to household dust; people breathe in one another’s cells; a fingernail drawn across the inside of the mouth can collect hundreds of living cells; and kisses and caresses may transfer many cells between lovers. If mobile DNA could hitch a ride in such cells and reach another body, natural selection would be expected to favor that opportunity and improve it. The plasmid therefore anchors a broader inference: genes able to bypass the usual reproductive channels could pursue different future outcomes from the other genes in the organism.

Significance in the Gene-Centered Argument

The example sharpens the contrast between cooperation within an organism and competition among replicators. Genes ordinarily cooperate in part because they share an outlet into the future through sperm or egg. A replicator that found another route would no longer share precisely the same reproductive prospects as its companions, and could consequently be less cooperative. Plasmids make this possibility materially imaginable, without being presented as proof that every proposed route is used.

The discussion also connects plasmid mobility to the book’s account of genetic influence as distributed rather than independent. Genes interact in intricate ways during development, and no single gene builds a body part on its own; effects depend on other genes and the environment. Against that background, the plasmid is not a miniature, self-sufficient agent. It is an example of a replicating fragment whose possible movement changes the routes by which copies might persist. Its importance lies in widening the question of inheritance from what is transmitted through an organism’s ordinary reproductive role to what any replicator can manage to propagate.