Jul/Aug 2006 Nonfiction

When Genes Move Sideways

by Anju Kanumalla

Art by Victor Ehikhamenor

Genes are the instructions for life, or at least the proteins that make most life possible. These proteins do all the work of life—the building things up and breaking things down. In addition, proteins help provide living things with a lot of their shape.

When scientists think about how organisms get their genes, they think about two things: vertical gene transfer and lateral gene transfer. When genes move vertically, copies of the genes are passed on from parent to offspring. When genes move laterally, however, they are being passed to cells that may not be related—basically sideways. Scientists also call this kind of movement "horizontal gene transfer."

Your genes and the genes of many other animals probably aren't moving sideways, but in bacteria and other organisms, genes move sideways a fair amount. This works out to be a good thing for bacteria, since they often get genes that let them survive hardship or use new food sources. Unfortunately, this can also be a bad thing for humans, especially humans that want to stay healthy.

Before we talk about moving sideways, however, let's talk about what genes usually do: move vertically.


Moving Vertically

Genes are written into DNA the way sentences are written into books. With DNA, the books are called chromosomes, which are like instruction manuals or perhaps cookbooks. Humans have 46 chromosomes, which come in 23 matching pairs. Yeast have four chromosomes in two matching pairs.

Bacteria usually have just one chromosome, but they can also have lots of smaller pieces of DNA called plasmids. The plasmids are like magazines. Some of these plasmids have instructions that aren't in the chromosome, like the way Family Circle may have recipes that aren't in your cookbook. Bacteria can have plasmids like newsstands have magazine racks: a lot of plasmids, and a varying number of each kind.

Most of the time, genes are passed on from parent to child through chromosomes. A human child gets copies of 23 random chromosomes from her mother and another 23 random chromosomes from her father. Most animals and plants and even some fungi also get half their genes from one parent and half from another. This two parent reproduction is called sexual reproduction. In species that reproduce sexually, the offspring are each genetically different from either parent and from their "siblings."

Other species, however, reproduce asexually. Their offspring are clones, or genetic copies, of their parents. This happens with bacteria, many complex single celled organisms (not that bacteria aren't complex), and even simple animals like hydras and sponges if the conditions are right.


Moving Sideways

Genes can move sideways in a number of different ways. Sometimes they move to another organism, sometimes into another piece of DNA in the same organism. Both have important consequences, but this article only talks about the former.

A few species of bacteria can just pick up DNA that's floating around in their environment, but most can't. Instead, most bacteria either use conjugation or get some help from bacteriophages.

Conjugation is like bacteria sex, only there are no baby bacteria produced. For conjugation to take place, one bacterium has to have a plasmid called a fertility factor or sex factor. This plasmid contains the instructions for building a toolkit to transfer DNA to a recipient bacterium. Usually, the recipient just gets a copy of the conjugation plasmid, but sometimes DNA from other plasmids or the bacteria's chromosome goes along for the ride.

Bacteriophages, often just called phages, are viruses that infect bacteria. For the most part, they operate just like other viruses. They pump their genes into a cell, forcing the cell to make more viruses. Eventually the cell pops, sending out dozens of new viruses to infect other cells.

Some phages, however, don't do that right away. Instead their genes live in the cell for a while, sometimes sneaking into the bacteria's chromosome. Depending on how long the phage DNA stays, generations of the bacteria's offspring may carry whatever genes were received from the phage. When the new phages are finally released, they can take some of the bacteria's DNA—sometimes instead of the phage DNA that they're supposed to have. This DNA can then be transferred to the next bacteria the phage infects.


Gene Movement and Disease

It's hard to catch DNA moving, but the effects of this movement can be seen in the world around us.

Take cholera, for example. The disease is caused by a bacterial species known as Vibrio cholerae. This species of bacteria produces the cholera toxin, which leads to diarrhea and dehydration, abdominal cramps, nausea, and vomiting. Scientists have found and studied the section of the V. cholerae chromosome that contains the toxin. The genes in this section, including the gene for the toxin, probably were carried in by a phage.

Equally important to human health is the resistance some bacteria have to antibiotics. In many cases, bacteria become resistant to antibiotics through mutations that are passed on to offspring. At least some of the time, however, bacteria get the genes for antibiotic resistance through horizontal gene transfer by phages, conjugation, or just sucked up from the environment.

Because genes can cluster together on phage DNA or plasmids, bacteria can sometimes become resistant to several types of antibiotics at once. This resistance usually spreads between bacteria of the same species, but it can cross species and even genus lines. Take, for example, an experiment that scientists conducted with Clostridium difficile, which also causes diarrhea in humans, and Butyrivibrio fibrisolvens, which lives in the gut of cows and other animals that chew cud. Even though these two bacteria are not in the same genus, C. difficile was able to pass on erythromycin resistance to B. fibrisolvens through conjugation.

How much moving?

No one knows how much horizontal gene transfer there really is. It probably happens most often in bacteria, but how much depends on the species. It probably doesn't happen much in animals, but may have been very important in their earliest single cell ancestors.

When genes move, it can cause big health problems. It can also confuse scientists who are trying to figure out how all these life forms are related. Still, humans will try and take advantage of any natural phenomena. Horizontal gene transfer has been used in genetic engineering, both for commercial purposes and research. In studying the way genes move, scientists have learned a lot about how DNA in many species, including humans, is maintained, repaired, and changed.


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