A to Z

D is for Drosophila

A whopping six Nobel prizes have been won for research on fruit flies. But what makes these humble little bugs so special?

Drosophila melanogaster, the common fruit fly, has been the superstar of genetic research for over a century. These alien-seeming creatures share more in common with humans than you might think. Around 75% of genes linked to human diseases are also present in drosophila, which means that these little flies can tell us a lot about ourselves. 

Drosophila were first thrust into the limelight by Thomas Hunt Morgan, a geneticist who began working with flies in 1908. At the time, scientists were trying to figure out where the genes passed down from parents to children are hidden within cells. Researchers noticed that chromosomes pair up and split apart in a way that would make them ideal for carrying genes, but they had no proof for this theory.

To solve this conundrum, Morgan exploited the tiny size, speedy life cycle and cheap upkeep of his flies. He bred an entire city population of the creatures in glass jars, scouring them for signs of rare mutations. One day, Morgan spotted a fly with white eyes instead of the normal red. When he mated the white-eyed fly with red-eyed flies, all of the children had red eyes. Bizarrely, white eyes reappeared in the grandchildren — but only in some male flies. Because of this odd pattern and its connection to sex, Morgan realised that the gene for eye colour must be on the fly sex chromosome. In 1933, Morgan won the Nobel prize for demonstrating that genes are carried on chromosomes.

Morgan showed that flies are a simple creature that can be used to unpick complicated biology. Since Morgan’s experiments, scientists have enlisted the help of Drosophila to do Nobel prize-winning work on embryo development, the immune system, circadian rhythms, the sense of smell and much more. Today, flies help us to research life-threatening diseases such as Alzheimer disease and cancer, and to expand our understanding of the intricate inner workings of all living creatures.

C is for CRISPR

Genetic modification of living humans sounds like science fiction, but CRISPR could soon make this idea a reality. This super precise gene-chopping machine, which was stolen from the immune system of bacteria, has the potential to treat hundreds of different genetic diseases.

CRISPR has two main parts: DNA-cutting scissors and a navigation system that tells the scissors where to cut. Scientists can give the navigation system a map to a particular part of the genome — for example, a faulty gene that causes a disease — and the machine will home in and cut the DNA at that place. This ability to edit genetic code in living cells in a very accurate way has sparked a major revolution in medicine in the past few years.

CRISPR was first discovered in the immune system of bacteria. When bacteria survive a virus attack, they save a small part of the virus DNA for future reference. A copy of this DNA is given to a gene-chopping enzyme called Cas9. If the same virus tries to invade the bacteria again, Cas9 uses its scrap of virus DNA to sniff out the virus and cut its DNA.

Scientists realised that you can exploit the homing ability of Cas9 and feed it any piece of DNA to trick it into cutting any gene you want. Researchers have also designed lots of different gizmos that can be attached to Cas9 to make it change the gene in different ways, such as an extra function that adds a new piece of DNA to the place where the scissors cut.

CRISPR treatments are already being tested for dozens of different diseases, including genetic disorders such as Huntington disease, and viruses such as HIV. Fierce patent wars are being fought over the commercial use of the system, but more work is needed to make sure that CRISPR is safe to use in patients. In addition, this futuristic technology raises important ethical questions that sound like science fiction: should CRISPR ever be used to edit genes that affect factors such as appearance, intelligence or physical abilities?

B is for Bacteriophage

Bacteria get sick too! They can catch a special type of virus called a bacteriophage. These nasty little viruses might become our greatest allies in the war against vicious, antibiotic-resistant bacteria known as ‘superbugs’ that are on the rise around the globe.

Bacteriophages are the most common organism on Earth and are found everywhere, with tens of millions in every drop of natural water. These viruses reproduce by latching on to a bacterium and injecting their DNA. Once inside, the virus DNA hijacks the host cell and forces it to create many copies of the virus. An army of newborn viruses grows inside of the doomed bacterium until they explode out of the cell, often destroying it.

Normally, when a person catches a disease caused by bacteria, doctors give them antibiotics — chemicals that kill bacteria. However, bacteria are living creatures that evolve and — slowly but surely — they are becoming immune to every antibiotic available. This resistance is troubling, as illnesses that were once easily curable are becoming deadly again.

The answer could be to enlist the help of our tiny but ferocious allies — the bacteriophages! These viruses don’t infect human cells, so could be injected into patients to wage war against bacteria. However, unlike antibiotics, bacteriophages only attack very specific types of bacteria. On one hand, this precision is helpful because the viruses would only attack illness-causing bacteria and not the ‘good’ bacteria that help out in our gut. On the other hand, we would need to pin point the exact type of bacteria causing each patient’s illness and have a matching phage ready to send into battle, which costs vital time and money.

Nevertheless, major clinical trials are currently underway that are testing the safety and effectiveness of bacteriophages to fight illness caused by bacteria. Injecting millions of viruses into a person might soon become a life-saving treatment.

A is for Agar

Welcome to our new A to Z series in which we explore the inner world of biology one letter at a time!

Agar is a wobbly jelly-like substance that scientists grow bacteria on. But did you know that it was inspired by pudding?

Agar plates are like little greenhouses for microbes. Scientists use these iconic tools to grow bacteria and to isolate particular types. But in the late 1800s, microbiologists were frustrated because the gelatin-based jelly that they grew microbes on would melt into goop the moment they put the dish into a warm incubator.

One summer day, two microbiologists — Fanny Hesse and her husband Walther Hesse — went on a picnic. Fanny noticed that the jellies she had made didn’t melt in the sun, and wondered whether agar, the secret ingredient that helped her desserts to set, might be the answer to their problems. The couple tested out their new agar-based jelly in the lab and found that it worked like a charm even when warmed up — no more microbial goop! Walther took the idea to their lab leader, Robert Koch, who began to use agar in his own experiments. Soon enough, Koch’s use of agar helped him to isolate the bacteria that cause Tuberculosis.

Today, agar is a vital part of every cell biologist’s toolkit. But helping us to learn more about bacteria is not all it can do. Scientists can also use bacteria grown on these plates as little helpers for genetic engineering. Researchers now can add, remove or edit parts of genes and then persuade microbes to create many copies of the edited gene for use in experiments. In this way, Fanny Hesse’s pudding-based eureka moment is now helping scientists all over the world to understand how genes work and to fight diseases.