As introduced in my previous post, this is the second of a 3-part series on genetic engineering. The first post was a brief overview and introduction to genetic engineering; while this one will focus on the tools used, and explain the difference between artificial selection and genetic modification. The third and final post will cover some of the more controversial topics, such as personalized medicine and genetically modified organisms (GMOs).
Artificial Selection: How We Created Man’s Best Friend
In order to explain the differences between genetic modification and artificial selection, it’s easiest to look at specific examples. A prominent example of artificial selection is the animal that has touched so many hearts it’s been named “man’s best friend”- dogs. Dogs are especially interesting because they are believed to be the first animal ever domesticated, occurring even before agricultural animals such as cattle, and there are now several studies suggesting that dogs were independently domesticated twice.
All dog breeds come from a common ancestor – the gray wolf. Genetic sequencing studies show how closely they are related. So how exactly did we get the chihuahua from the gray wolf? Through thousands of years of selective breeding, even as long as 30,000 years. Let’s start with the gray wolf meeting humans: it’s unknown whether humans sought out a relationship with wolves, if it was vise versa, or if it was a mutual “agreement”. However it happened, humans began to interact with wolves to the point where they were breeding them for specific qualities.
From the beginning of domestication, people recognized that if you cross two wolves that are docile than their offspring will have a high chance of being docile as well (versus breeding more aggressive wolves). If you continue this process, while also eliminating the aggressive wolves – let’s be honest, this occurred – than you would eventually, through several generations, have only docile wolves. If you continue this process while concentrating on a single characteristic at a time, such as size or coat-thickness, than eventually the wolves become so genetically different from their descendants that they became a new species: dogs.
For dogs, several characteristics were altered over thousands of years by repeatedly altering a trait at a time. In more modern times, such as with cattle and other agricultural animals, only a single trait or small set of traits is targeted for alteration, not allowing for speciation, or the separation of one species into two.
This same process is used for the domestication of plants. Although the exact date when wheat was first domesticated is still debated today, it is known that the wheat we consume today varies genetically from wild wheat due to years of selective breeding. Two traits in particular allowed humans to consume and to grow wheat: an increase in grain size and the development of non-shattering seeds. The former allows for easier cultivation of the seedlings and the latter prevents natural seed dispersal, so that humans can harvest the seeds at the optimal time. It’s believed that both of these traits occurred naturally, and humans took advantage of them by only selecting for plants with those qualities. Today, common wheat differs even greater from the wild ancestor through actual breeding programs. Wheat breeding programs around the world have artificially selected for traits that mostly confer a resistance to either a pest, disease or other environmental factor, creating a much more modern version of wheat.
For a further understanding of artificial selection, check out these links:
- Artificial Selection in Plants: A Presentation
- An overview of Artificial Selection & Selective Breeding in Animals and Plants
Genetic Modification: The Simple-ish Version
As mentioned in my previous post, genetic engineering, or modification, has been in practice since the 1970’s, when Boyer and Cohen first successfully recombined DNA into E. coli. Although the procedure Boyer and Cohen used, known as recombinant DNA technology, is still used today along with several other methods, there is a current shift into using CRISPR technology as the main tool for genetic alteration. Since this shift is occurring and there has been a focus on CRISPR, both by those that support genetic engineering and those that don’t, the rest of this post will have a major emphasis on CRISPR technology.
CRISPR stands for Clustered Regularly Inter-spaced Short Palindromic Repeats. Palindromic repeats are prokaryotic DNA containing short repetitions of base sequences, and in between the sequences are short segments of non-coding DNA (“spacer DNA”). One of the main components of CRISPR technology are CRISPR associated proteins, or Cas proteins, which are endonuclease enzymes – enzymes that will cut double-stranded DNA (dsDNA) – that are guided by RNA. CRISPR and Cas proteins are naturally found in several species of bacteria, and work as mechanisms of immunity by cleaving the DNA of invading pathogens, such as viruses. Although there are multiple Cas proteins, the Cas9 endonuclease found in Streptococcus pyogenes is currently the most efficient Cas protein, thus the current technology is known as CRISPR/Cas9.
CRISPR/Cas9 technology is progressively getting more complicated with the addition of new information and modifications to the current methodology. However, the basic procedure that CRISPR/Cas9 performs remains the same. As mentioned, one of the main components of the technology is the Cas9 protein. The other component is a guiding RNA (gRNA). The gRNA is created so that it matches, or compliments, a DNA sequence within a genome. The gRNA will find that sequence and will hybridize to it. Next, the Cas9 protein uses that gRNA as a guide to find and bind to both the gRNA and to the DNA it’s hybridized to – binding to both strands of the DNA, not just the strand the gRNA is hybridized to. Finally, Cas9 will cleave both strands of the DNA. When the dsDNA is cleaved, the cell will detect there is a cut and when repairing the area, mutations will be introduced in a natural repair process. If the DNA encoded a gene, the introduction of a mutation will alter or silence the expression of that gene.
After gaining an idea of what the CRISPR/Cas9 system does, it makes sense that scientists are trying to use this tool as a way to silence and edit genes. The system can be modified to target specific genes by creating specific sequences of gRNA. Current studies are using the system in several new ways, including studying epigenetics – the study of how gene expression is effected by the environment, and the potential to target genes associated with cancer and Alzheimer’s, among other diseases. The first cancer-targeted CRISPR human trial has been green-lighted by the National Institute of Health, and may mark the beginning of a future where diseases are edited out of the genome.
Stick around for the next post, I’ll be covering further controversies associated with gene editing. And if you would like any further information on CRISPR/Cas9 technology, please check out the following links:
- CRISPR/Cas9 and Targeted Genome Editing: A New Era in Molecular Biology
- What is CRISPR/Cas9 – YourGenome.Org
- CRISPR: gene editing is just the beginning
- Genome Editing with CRISPR/Cas9 – YouTube Video from the McGovern Institute for Brain Research at MIT
Author’s note: in an attempt to reach a wider audience, the processes of both artificial selection and genetic modification have been simplified. Before forming any opinions of either subject, I highly recommend doing further research on both topics. Thank you for reading.