We all owe our existence to a process called gene transfer. Each of us received genes from mom and dad and will pass them on to our children, ensuring genes are constantly transferred from one organism to another. But transmission from parents to children is not the only way to pass on genetic information. In this topic, we will discuss in more detail what horizontal gene transfer is, what it is, and how it differs from traditional gene transfer.
What is the transfer of genes?
The transfer of genes from one organism to another can be classified into two large groups. The first is vertical transfer – from the parent organism to the daughter organism. This is the "standard" transfer that each of us knows.
But there is also a second group – horizontal or lateral transfer. In lateral transfer, the genetic material is transferred not to a descendant, but to a separate unrelated organism. Roughly speaking, imagine that one of your cats has passed on the gene for a very loud meowing to another cat, and one fine day you wake with a chorus of absolutely identical meows.
It seems that this is impossible – how can one organism simply transfer something to another, however, horizontal gene transfer exists. For example, it is thanks to horizontal transfer that bacteria are able to "spread" resistance to antibiotics by passing on the corresponding genes in plasmids to each other. In addition, scientists have learned how to apply horizontal transfer in genetic engineering – the case is that it occurs only in unicellular organisms. Traditionally, horizontal gene transfer is classified according to the mode of transfer into three groups: conjugation, transduction, and transformation.
Conjugation
Conjugation is the unidirectional transfer of genetic material (usually plasmids, but in very rare cases, a chromosome can also be transmitted) from one bacterium to another during their direct contact. The contact is formed by a special organelle called the pilus (pic. 1).
An important point in conjugation is that not the plasmid itself is transferred – but a copy of the plasmid. DNA replication proceeds in parallel with the transfer, and the new strand is "delivered" immediately to the acceptor cell. Not all plasmids can be conjugated.
Bacteria need conjugation primarily to maintain genetic diversity – after all, they do not have sexual reproduction, and a diverse genome is a great step toward survival.
Transduction
Transduction is the transfer of genetic information by viruses (pic. 2). One of the simplest models is the transfer by a bacteriophage of the genetic information of one bacterium to another bacterium. Transduction is, in fact, a mistake in the assembly of the virus – during assembly, both the DNA of the virus itself and the DNA of the bacterium enter the capsid. Such a virus with a cargo, infecting the next bacterium, transfers that DNA to the previous bacterium.
Transduction is practically not used in genetic research and is poorly controlled. In addition, lethal viruses that kill the host cell often act as a carrier viruses, so such a transfer, strictly speaking, does not make any sense.
Transformation and transfection
The process of introducing foreign DNA into a cell by using a special vector molecule (most often an artificially created plasmid acts as a vector) is called transformation when DNA is introduced into prokaryotic cells, and transfection when introduced into eukaryotic cells (pic. 3). DNA is inserted into the vector, after which the vector-DNA complex is transferred into the cell. This process is constantly found in genetic engineering because it allows you to create a colony of bacteria that produce one or another target protein.
The vector-DNA complex can be introduced by various methods. Sometimes the microinjection method is used – the manual introduction of the complex into each individual cell. This is done when the complex is introduced into mammalian cells. Another route of administration is through the use of liposomes, small vesicles made up of fatty acids similar to cell membrane fatty acids. In addition, methods often use that result in the formation of small pores in the cell membrane, such as electroporation, sonication, and so on.
Conclusion
So, the horizontal transfer is the process of transferring genetic material from one organism to another unrelated organism. There are three ways of horizontal transmission – conjugation (occurs with the help of the pilus), transduction (transmission with the help of a virus), and transfection/transformation – artificial transfer of DNA.