miniPCR bio
TM
Chopped! Using CRISPR/Cas9 to cut DNA - Student's Guide
Version: 1.0 - Release: May 2022 - © 2022 by miniPCR bio™
Student's Guide
P./32
2. Let's use our gRNA1, which targets the 20-base sequence GCTAGTCATGCTACCCTAGT. What
are the chances of this specific 20-base DNA sequence matching any random 20-base-pair
stretch of DNA? You should use a calculator but show your work.
Number of times you can expect to find a specific DNA sequence
Now that we know the probability of a specific DNA sequence occurring, we can use that
information to calculate the number of times you can expect to find that sequence within the entire
genome.
To estimate the number of times that a given DNA sequence that is n bases long will appear within
a longer DNA molecule that is b bases long, you can multiply the probability of the sequence
occurring by the length of the DNA molecule. And because DNA is double stranded, you will
multiply this number by 2 because the sequence can occur on either strand of the DNA. This gives
us a final formula of 2b x (1/4)
n
.
3. Again, let's assume we have a restriction enzyme that targets the 6-base sequence CGATCG.
How many times could you expect this restriction enzyme to find and cut the CGATCG
sequence in the 3.2 billion (3,200,000,000) base-pair long human genome? You should use
a calculator but show your work.*
4. Based on your answer to the previous questions, explain whether you think a restriction
enzyme with a six-base-pair recognition sequence would make a good genome editing tool.
* Because most restriction enzymes are palindromes, they are typically found simultaneously on both strands of DNA. This
means that for most restriction enzymes, you should divide your final answer by 2. For simplicity in this exercise, you may
ignore this fact.
