1. Introduction: Vertical vs. Horizontal gene transfer

As sexually reproducing eukaryotes, the type of gene transfer that we’re used to is vertical. Vertical gene transfer is the movement of genes from parents to offspring. For example, the diagram on the right from AP Bio Unit 5 shows the transmission of the gene for normal hemoglobin (HB^A) and the mutated hemoglobin that causes sickle cell anemia (HB^S) from parents to offspring.

Vertical gene transfer also happens during asexual reproduction. This can occur in bacteria, or in asexually reproducing eukaryotes. For example, the mutation indicated by the letter A in the diagram on the left is passed from the parent in generation 1 to its offspring in generation 2. These bacteria, in turn, pass on the same mutated gene to generation 3.

In addition to vertical gene transfer, bacteria and other prokaryotes can also transfer genes horizontally. Horizontal gene transfer is also known as lateral gene transfer. It’s the movement of genetic material from one organism to another that occurs outside of reproduction. In other words, horizontal gene transfer does not include the creation of a new organism. That makes it very different from the vertical, parent-to-offspring transmission of genes that we, as sexually reproducing eukaryotes, are accustomed to.

An example of horizontal gene transfer involves the spread of genes through plasmids.

In addition to their main chromosome (“a,” in the diagram to the left) bacteria can have extrachromosomal pieces of DNA known as plasmids (letter “b” in cell “I”). Like any piece of DNA, plasmids can contain genes. In the example below, the plasmid contains genes for creating a structure called a pilus (at “c”).

A pilus can have a variety of functions, but in this case, what it does is create a bridge to a cell that lacks a pilus (cell II). Once the pilus is in place, the plasmid (using enzymes such as DNA polymerase, at “e”) replicates itself and sends a copy of itself to cell II. At the end of the process (row 4), we have two cells that contain plasmids and which are capable of producing a pilus.

That’s horizontal gene transfer.  The recipient cell (II) picked up genes giving it a new genotype and a new phenotype. No reproduction took place. We started with two cells, and we ended with two cells. But one cell (II) took up genes from the other.

The specific type of horizontal gene transfer shown above is called conjugation. Conjugation involves plasmids and pili. At a minimum, conjugation will convert a bacterium without a pilus into one with a pilus. But additional genes can be transferred as well. A typical plasmid contains a few thousand base pairs and can have dozens of genes, including genes for antibiotic resistance: the ability of bacteria to grow even when exposed to antibiotics (substances that inhibit bacterial growth). The transfer of antibiotic resistance genes through conjugation is a key mechanism by which resistance to antibiotics spreads throughout bacterial populations, Antibiotic resistance is a major public health concern, and it’s a topic we’ll address again in AP Bio Unit 7 (evolution).

Horizontal gene transfer occurs mostly in prokaryotes. Let’s look at a few more mechanisms.

2. During Transformation, Bacteria Pick Up Genes from their Environment

Transformation is a type of horizontal gene transfer in which bacteria take up DNA from their environment and incorporate that DNA into their genomes. Note that bacteria can’t just pick up any piece of DNA. It has to be DNA that’s compatible with the DNA they already have. If the DNA is compatible, enzymes called recombinases will integrate the new DNA into the bacterial chromosome.

In the diagram to the right, letter “A” represents a dead bacterial cell that’s releasing its DNA (1) into the environment. “B” is a nearby bacterial cell. Letter “C” shows that same cell absorbing the DNA released from cell “A.” At “2,” you can see the DNA now incorporated into the main chromosome.

The cell at “D” is said to be transformed.

Transformation is also used to describe the technique of forcing bacteria cells to take up entire plasmids during genetic engineering, a topic we’ll address in the next tutorial.

3. Checking Understanding: Conjugation and Transformation

4. Transduction

Viral transduction is another type of horizontal gene transfer in which viruses transfer genes from one bacterial cell to another one. Here’s how it works.

Cell 1 in the diagram on the left is being attacked by a bacteriophage. The phage, as part of the lytic cycle, starts its attack by injecting its DNA into its victim. As the attack proceeds, the phage uses the cell’s molecular machinery to reproduce its own phage DNA (at “d”) while simultaneously breaking down the victim’s chromosome (“b”) into smaller pieces of DNA (at “c”).

Letter “e” shows the assembly of viral proteins, the self-assembly of new phage, and lysis. But during the process of self-assembly, a mistake occurred. The phage labeled “g,” instead of taking up phage DNA (colored blue in this diagram), took up bacterial DNA (colored red). For a comparison, look at phage “f,” which contains phage DNA.

If phage “g” bumps into another bacterial cell, such as cell number 2, it will commence another lytic attack. However, because it’s carrying DNA from cell number 1, it will inject cell number 1’s DNA into cell number 2. When recombinase enzymes detect that DNA, they’ll integrate it into cell number 2’s genome. The result is a gene from cell number 1 (at “j”) integrated into cell number 2.

5. Viral Recombination, Antigenic Shift, and Antigenic Drift

As far as viruses go, since 2020 the world’s attention has mostly focused on SARS-CoV-2 and the COVID-19 pandemic. But in 1918  a far deadlier pandemic caused by the influenza virus swept the world. This pandemic infected as many as 500 million people and killed between 17 and 100 million people. To get a sense of the magnitude of this pandemic note 1) that the world’s population at the time was just under 2 billion (compared to 8 billion today), 2) the COVID-19 pandemic has killed, worldwide, about 6.6 million people.

Influenza (commonly called “the flu”) is caused by a family of RNA viruses. The RNA is divided into segments (represented by “1” in the diagram below). Among the proteins that the RNA codes for are surface proteins (represented by 2). These proteins are the antigens that our immune system responds to when fighting off the virus. In other words, in order to fight off these viruses, our immune cell receptors need to be able to bind to these proteins, allowing for recognition, immune mobilization, and ultimately, neutralization of these viruses (see the diagram to your right).

During influenza’s replication cycle, its RNA (1) can mutate. One such mutation is represented by the red dot (3) in the white bars in cell B. These mutations can change the shape of influenza’s surface proteins (4), with the new protein represented by the red dot on the protein sticking out of virus B.

This slight change in surface proteins in influenza viruses caused by mutation is called antigenic drift. It’s an ongoing process. This shift in protein structure makes it harder for our immune system to recognize and fight off the virus, and it’s part of the reason why we don’t remain immune to the flu from year to year, whether or not we’ve been infected or vaccinated during the previous year.

A much bigger change in viral genes, viral surface proteins, and the potential for the virus to cause infection involves what’s called antigenic shift.

Here’s how antigenic shift works. In addition to replicating in humans (A), influenza can also infect birds (B) and pigs (C). If a human strain and a bird strain wind up in the same pig cell (D), then that cell acts as a kind of mixing vessel in which the genetic material of the human and bird strain can recombine. That causes a new strain of virus (E) in which the surface proteins are significantly different from other strains. That strain will be much harder for the human immune system to recognize and overcome. creating a potential for a worldwide influenza pandemic (such as the ones which occurred in 1918, 1957, 1968, and 2009)

6. Checking Understanding: Viral Transduction; Viral Mutation and Recombination Causing Antigenic Drift and Shift

7. Horizontal Gene Transfer Flashcards

What’s Next?

This tutorial ends AP Bio Topic 6.7: Viruses, Mutation, and Horizontal Gene Transfer. You can access the last topic in AP Bio Unit 6 (Genetic Engineering) from the Unit 6 Main Menu.