CRISPR: A Word Processor for Editing the Genome

Top

00:00:14.26 Since the discovery of DNA's fundamental role in building
00:00:18.14 and sustaining life, scientists have dreamed of having the
00:00:21.19 ability to easily edit DNA in very precise ways. But why would
00:00:26.12 they want to do this? Well, making specific changes
00:00:29.12 to DNA sequences can help scientists better understand
00:00:32.00 the function of certain genes, produce specific disease models,
00:00:35.25 or even repair defective genes that cause diseases in humans.
00:00:39.11 This is an exciting prospect, but methods to try and do
00:00:42.29 this weren't practical or widely-applicable. However, a few years
00:00:46.12 ago, a gift from biology came from the basic research of
00:00:49.18 the bacteria immune system, which gave scientists the ability
00:00:53.00 to easily customizable and precisely edit genomes.
00:00:56.20 Bacteria evolved ingenious ways of protecting themselves
00:01:00.04 against pathogens such as viruses by using a system
00:01:03.10 called CRISPR/Cas. CRISPRs are stretches of DNA
00:01:06.20 sequence found in the bacterial genome in close proximity
00:01:09.28 to CRISPR, are the Cas genes, which encode proteins
00:01:13.20 necessary for the CRISPR system. Up until a few years
00:01:17.18 ago, what was known about the CRISPR/Cas system
00:01:20.12 is that bacteria infected by a virus incorporate elements
00:01:24.02 of the virus's DNA into the CRISPR sequence, this protected
00:01:27.29 the bacteria from future infection by this virus. Scientists
00:01:31.18 observed that when a virus invades a bacterium, the CRISPR
00:01:35.28 DNA produces one or two small RNAs called crRNA and
00:01:41.22 tracrRNA. These RNAs bound to Cas proteins and formed
00:01:46.27 complexes that cut the DNA of the invading virus. Thus
00:01:50.24 protecting the bacteria from infection. But many questions
00:01:55.14 still remained, how did the small RNAs and Cas work together
00:01:59.14 to detect and destroy viral DNA? In 2012, a group of scientists
00:02:05.13 made a major breakthrough and discovered not only how the
00:02:09.10 CRISPR RNAs and Cas cut DNA, but also how to create a
00:02:14.00 new technique to specifically change the DNA sequence of
00:02:17.22 any organism with great ease. This discovery came from
00:02:21.22 a group of scientists led by Jennifer Doudna at UC Berkeley,
00:02:25.22 and Emmanuelle Charpentier at Umei University in Sweden.
00:02:29.12 They published their results in Science, in an article titled
00:02:32.21 "A programmable dual-RNA guided DNA endonuclease
00:02:37.00 in adaptive bacterial immunity. So what exactly
00:02:41.06 "did these scientists find, and why is it so important for
00:02:43.27 future biomedical research. First, the scientists dissected
00:02:47.25 how Cas9 and the two RNAs can cut DNA. They found that
00:02:52.12 the two RNAs, crRNA here in red, and tracrRNA here in green,
00:02:57.00 pair up, recruit Cas9 protein, and direct it to bind the target
00:03:01.14 DNA, via the complementary base pairing between the
00:03:04.23 red CRISPR RNA and the target DNA. Once at the proper DNA
00:03:08.29 site, Cas9 cleaves both DNA strands. This cleavage occurs
00:03:13.16 at a very specific and conserved position that is dictated
00:03:17.16 by the sequence in the red CRISPR RNA molecule.
00:03:20.16 This process is similar to finding your plane by matching the gate
00:03:24.20 number to the one on your boarding pass. The scientists then
00:03:29.26 wondered if they could engineer one RNA molecule that mimicked
00:03:34.04 the structure of the CRISPR RNA and tracrRNA bound together.
00:03:37.27 that would guide Cas9 to cut DNA at a specific location.
00:03:42.24 The scientists designed one RNA molecule that consisted
00:03:46.24 of the red and green RNA molecules connected together
00:03:50.01 by a hairpin structure. In this case, they engineered the RNAs
00:03:54.24 to target specific sequences of the gene encoding the green
00:03:58.13 fluorescent protein, GFP. They added the engineered RNA
00:04:02.24 molecules to the GFP DNA sequence, along with the Cas9
00:04:06.16 protein. And asked whether Cas9 would cut GFP DNA
00:04:10.03 at specific sequences. And it did!
00:04:15.25 When the RNA molecules were designed to bind to different
00:04:18.12 regions of the GFP sequence, the GFP DNA sequence
00:04:22.10 was cleaved at that specific location. On a gel that separates
00:04:29.18 DNA according to size, you could see distinctly sized fragments
00:04:33.16 of the GFP DNA molecule resulting from having been cut
00:04:38.00 in a specific location. This was huge, as it meant that scientists
00:04:41.28 could engineer one RNA sequence, introduce Cas9, and
00:04:46.14 cut DNA at a specific location of their choice.
00:04:50.12 By having this simple and easily programmable system,
00:04:54.16 scientists can now induce breaks in the DNA at precise
00:04:57.27 locations. When the cell tries to repair the broken DNA
00:05:01.12 strands by ligating them back together, it often causes
00:05:05.08 a small insertion or deletion that changes the DNA
00:05:08.04 sequence. Scientists can take advantage of this process
00:05:12.17 to add or remove specific DNA sequences at the site
00:05:17.03 of the break. We now have a DNA word processor
00:05:21.08 that can be used to change genome sequences, including
00:05:24.12 our own. CRISPR has a bright future ahead, to advance our
00:05:29.06 understanding of human disease by creating a tool for
00:05:32.02 basic research that is now widely used in the fields of
00:05:36.01 molecular biology and genetics to change the genomes of
00:05:38.28 any organism. But much work needs to be done to make this
00:05:43.10 system more reliable. Use of the CRISPR system to edit
00:05:46.19 human embryos is also a controversial issue. And researchers
00:05:50.22 have already started the conversation about using this technology
00:05:53.28 ethically and safely to advance human knowledge.
00:05:57.25 This video has been provided to you by Youreka Science
00:06:03.07 and iBiology, bringing the world's best biology to you.

This Talk

Speaker: Youreka Science

Audience:

Educators of H. School / Intro Undergrad
Student
Educators of Adv. Undergrad / Grad
Researcher
Educators

Recorded: March 2015

Talk Overview

Since the discovery of DNA’s fundamental role in building and sustaining life, scientists have dreamed of having the ability to easily edit DNA in very precise ways. This Youreka Science video explores how a group of scientists made a major breakthrough in understanding the bacterial immune response, called the CRISPR-cas9 system, and discovered a way to utilize this system to create a new technique to specifically change the DNA sequence of any organism with great ease.

Speaker Bio

Youreka Science

Youreka Science was created by Florie Mar, PhD, while she was a cancer researcher at UCSF. While teaching 5th graders about the structure of a cell, Mar realized the importance of incorporating scientific findings into classroom in an easy-to-understand way. From that she started creating whiteboard drawings that explained recent papers in the scientific literature… Continue Reading