Video Transcript
Samples of DNA are taken from two
organisms and mixed with restriction enzyme Bam HI. The restriction enzyme cuts the DNA
from organism A into three sections but cuts the DNA from organism B into only
two. What does this suggest about the
DNA of the organisms? Organism A has fewer Bam HI
recognition sequences in their DNA than organism B. Organism A has more Bam HI
recognition sequences in their DNA than organism B. The DNA of organism B is longer
than that of organism A. Or the sample taken from organism B
was not mixed with enough restriction enzymes.
This question is asking us about
restriction enzymes and how they can cut DNA differently between two organisms. There’s a few key terms in this
question that we’ll need to address. So, let’s clear the answer choices
so we have more room to work with. A restriction enzyme is a special
type of enzyme. You’ll recall that an enzyme is a
protein that acts as a biological catalyst. What this means is it can make a
chemical reaction go much more quickly. It can do this by binding to its
specific substrate and then create conditions for a chemical reaction to occur much
more rapidly, like the cleavage of the substrate into two fragments as shown
here.
Restriction enzymes are enzymes
that catalyze the cleavage of DNA. Restriction enzymes are kind of
like molecular scissors, and they can cut DNA, their substrate, at specific DNA
sequences that they recognize. These specific DNA sequences are
called recognition sequences, and each restriction enzyme has a unique recognition
sequence that it binds to and cleaves.
The restriction enzyme Bam HI in
this question has a recognition sequence of GGATCC. So, if the restriction enzyme that
cut this DNA was Bam HI, then these two cuts would correspond to this sequence. In other words, this DNA has two
Bam HI recognition sequences. After the DNA is cut, it will
produce fragments or sections of DNA. So, in this example where there are
two BAM HI recognition sequences in this linear DNA substrate, there are three
sections that are formed.
Now, let’s look at the question
again. The question states that Bam HI
cuts the DNA from organism A into three sections but cuts the DNA from organism B
into only two. Now, let’s assume the DNA is linear
and work this out. Organism A is cut with Bam HI and
produces three fragments. This implies that there must have
been two recognition sequences for Bam HI, whereas in organism B, there’s only two
fragments that are produced. This implies that there was only
one Bam HI recognition sequence. So, when the DNA is linear, this
suggests that there are more recognition sequences in organism A compared to
organism B.
But what if the DNA is
circular? Bacteria generally have circular
DNA. So, let’s look at this situation
before choosing an answer. Here is organism A being cut into
three sections. In order to produce this fragment,
the circular DNA would have to be cut here and here. And in order to produce the
remaining two fragments, we would need an additional cut here. So, that’s a total of three
recognition sequences for Bam HI in organism A. Here’s the two fragments produced
in organism B. And in order to produce these
fragments, we would need two cuts, one here and one here. So, organism B only has two Bam HI
recognition sequences.
So, when the DNA is circular,
organism A has more recognition sequences than organism B. This is the same result that we
found with linear DNA. Therefore, the correct answer is
that organism A has more Bam HI recognition sequences in their DNA than organism
B.
