CRISPR Science: DNA, RNA, and Gene Editing


Gene editing. You may have heard it linked to talk of GMOs, genetic enhancements, or even designer babies, but what is gene editing? And how does gene editing work to let us modify plants, animals, or possibly people? To answer these questions, let’s take a closer look at DNA, RNA, and one of the best tools for
editing genes: CRISPR. So what is DNA? Well, all of your genes are made of DNA. It is the instruction manual that tells your body how to develop and how to work. All of the DNA in a cell is called the cell’s genome. A copy of the genome
is found in most of your cells, in a part called the nucleus. It’s your DNA that controls many of your traits, like your eye color or how many toes you have. To control those traits and to keep your body working, the information in DNA needs to reach other parts of the cell. But DNA can’t leave the nucleus. So how
can those instructions be used? There’s another molecule that helps, called
RNA. RNA is similar to DNA, but it can send those DNA instructions out of the nucleus.
Working as a sort of copy of the instructions, it can move to other parts of the cell. This
lets your cells use DNA instructions to create proteins that keep your body working. Now, as you grow, your cells divide. Each
of these new cells needs a copy of your DNA instructions. But the copying system isn’t
perfect. Sometimes, mistakes are introduced during copying. How do these mistakes happen? If we look at
DNA at a microscopic level, it has four different types of bases. Billions of these bases
are all connected in very long pairs of strings. When cells divide, these paired strings need
to be copied so a new set of DNA can be included in both cells. While copying these long strings of DNA, it
isn’t unusual for a cell to make a mistake. DNA can also be hurt or broken, by things
like exposure to too much sunlight or to certain chemicals. The good news is that our bodies have molecules
that look for and help fix those mistakes. There are special proteins called enzymes
that are always looking for mistakes or for damaged DNA. If they find it, they cut out
that tiny part of the string of DNA. Then they replace it with a new, correct piece. Another important feature of DNA is that it
has two strings that aren’t the same, but that are a matching set. So if you have one
string, you would know what the other string is supposed to look like. If only one string of DNA has a mistake, the
other can be used to let the enzymes know what needs to be added. But if both strings
have problems, both need to be replaced. A situation like this can be used to introduce
new DNA. How did scientists figure this all out? For
the last century or so, people have studied how DNA works. We learned that it passes on
genetic information and we know what it looks like. We also learned which genes are involved
in certain diseases and which genes cause specific traits. There are even kits you can
get to gather a sample of your own DNA to send in to be tested. You can see your risks
of disease, or learn about your ancestors. Once scientists knew the basics of how genes
worked, the next step was to figure out how to change (or modify) specific genes. You
may not know it, but humans have been modifying the genes of other organisms for thousands
of years. But in the past, we’ve done this by picking which animals or plants to breed,
by exposing eggs or seeds to radiation, or by adding genes to DNA at random. In the late
1900s, scientists discovered ways to carefully insert specific genes in exact locations in
DNA. But this was very difficult and expensive to do, until recently. A newer method, called
CRISPR, has made gene editing much faster and cheaper. CRISPR has two main parts: a piece of RNA,
called guide RNA or gRNA, and an enzyme that cuts DNA. The gRNA can be made to find and
connect to a specific place on a strand of DNA. Once the gRNA has lined up the CRISPR
system on the DNA, the enzyme cuts the DNA. The enzyme used most often as a part of CRISPR
is called Cas9. Once the DNA has been cut, the cell’s own
response of trying to fix it takes over the cell will start to repair the cut. It uses
either a remaining strand of DNA, a recent copy made from the DNA (if it is still in
the cell), or it will just rejoin cut ends of DNA. But, if scientists introduce a strand
of DNA that they’ve designed, it could be used as the instructions to repair the damage.
This means a new gene that wasn’t there before could be made and added to the DNA.
In this way, CRISPR-Cas9 has been used to fix genes, remove genes, and add genes. It
can also make genes stop working temporarily (which we call gene silencing), or stop working
altogether (which we call a gene knockout). There are many ways the CRISPR system can
be used. It could help avoid or cure disease, repair DNA damage, or even make existing genes
work better. But, taken further, those tools could also be used to make designer babies, or to make microscopic weapons like drug-resistant bacteria. CRISPR technology is here to stay,
and for good reason. But there is an ongoing debate about what should be off limits for
gene editing. In the comic Spider Man, after Peter Parker’s
genes are modified, giving him superpowers, his uncle tells him something that rings true
in real life now: With great power comes great responsibility.

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