Video Transcript
Huntington’s disease is a neurodegenerative disease where symptoms usually become
apparent at 30 to 50 years of age. Why might someone whose parent has just been diagnosed with Huntington’s disease want
their genome sequenced? (A) To alter their DNA sequence and prevent themselves from inheriting the
disease. (B) To determine if they will also inherit the disease and prepare for it. (C) To determine at exactly what age they will experience symptoms. (D) To predict their life expectancy and that of their children.
Before answering this question, let’s first remind ourselves what a genome is and
what sequencing refers to.
Our cells contain a lot of DNA. In fact, if you were to take the DNA out of a single cell and unravel it, it would be
about two meters in length. The total amount of genetic material in our cells is what we call a genome. This two meters of DNA isn’t one long, continuous molecule of DNA; instead it’s
organized into 46 individual structures called chromosomes. Here’s what one of these chromosomes might look like. Each chromosome is a highly packaged molecule of DNA that’s coiled up and wrapped
around special proteins called histones.
You’ll recall that DNA is made up of repeating subunits called nucleotides. And each nucleotide contains a nitrogenous base that can be either guanine, cytosine,
adenine, or thymine. Nitrogenous bases can form base pairs with each other. Contained within this DNA are genes. Genes contain instructions for producing some type of functional unit, for instance,
a protein.
The order of bases in DNA determines the sequence of DNA. So when we talk about getting the genome sequenced, as in this question, we’re
referring to determining the order of these bases in all the chromosomes of our
DNA. By getting the genome sequenced, they can determine the sequence of a particular
gene, including the gene that causes Huntington’s disease.
Let’s imagine that a portion of the gene that causes Huntington’s disease is
indicated here and the disease is caused by a single mutation where a guanine base
is substituted for a cytosine. By sequencing the genome, we would now know if this mutation was inherited or
not.
Now, let’s look at our answer choices to determine which option explains why someone
whose parent has just been diagnosed with Huntington’s disease would want their
genome sequenced.
Answer choice (A) is incorrect because we currently don’t have the ability to modify
our own DNA to prevent disease. Answer choice (B) seems to be correct because the genome sequence will tell us if
someone inherited a mutated version of the gene that causes Huntington’s
disease. But before choosing this as our final answer, let’s first confirm this by eliminating
the other two options.
Answer choice (C) is incorrect because we can’t tell the age that the disease will
start by simply knowing the DNA sequence. Like answer choice (C), answer choice (D) is incorrect because the DNA sequence of
the gene won’t tell us anything about the person’s life expectancy or that of their
children. Therefore, the reason someone would get their genome sequenced is given by answer
choice (B): to determine if they will also inherit the disease and prepare for
it.
