Which of the following statements about DNA is correct? (A) The majority of DNA in humans codes for proteins. (B) The DNA of every human, apart from identical twins, is unique. (C) Siblings born to the same parents share over 75 percent of their DNA. (D) Each cell in the human body contains 46 genes.
Our DNA is stored in the nucleus of most of our cells. We have a lot of DNA in our cells, and it is highly compacted. In fact, if we were to take the DNA out of a single cell and stretch it out, it would
be about two meters in length. This isn’t one, long, continuous molecule of DNA. Instead, it is organized into 46 highly compacted structures called chromosomes.
Let’s look at one of these chromosomes in more detail. Each chromosome is a very long molecule of compacted DNA. For example, chromosome one is about 2.5 million base pairs long and contains about
2000 genes. This is just on one chromosome. We have about 25000 genes in total. So we can eliminate answer choice (D) because there’s a lot more than 46 genes in one
These genes are scattered throughout our chromosomes. Here’s a few indicated as orange bands on this chromosome. In fact, the majority of our DNA doesn’t contain genes at all. And not all genes code for proteins. Some can provide instructions for regulatory elements like special RNA molecules. Only about one percent of our DNA codes for proteins, so we can eliminate answer
Even though we have a lot of DNA in our cells, it’s mostly the same between
humans. About 99.9 percent of our DNA is the same. So siblings, who are even more related to one another, would have an even higher
amount of similar DNA. So answer choice (C) is incorrect.
Even though we share so much DNA in common, this 0.1 percent difference accounts for
millions of differences in our DNA. These are mostly single-nucleotide differences. And we have one different nucleotide for every 1000 nucleotides on average. Therefore, answer choice (B) is the correct statement regarding DNA. The DNA of every human, apart from identical twins, is unique.