Video Transcript
Which of the following is true about the human genome? (A) The human genome is more complex than that of any other species because it contains more genes relative to its size. (B) Like most eukaryotic genomes, the human genome contains a great deal of noncoding DNA. (C) The human genome has the largest number of nucleotides in the living kingdoms. (D) Most of the human genome codes for proteins.
This question is asking us about the human genome, which is the total set of our genetic information. Let’s first review some details about DNA so we can answer this question.
A molecule of DNA is usually composed of two strands, each of which is a chain of individual subunits called nucleotides. Each nucleotide in DNA contains a nitrogenous base, which can be either guanine, cytosine, adenine, or thymine. You can see these different nucleotides here and the corresponding nucleotide sequence for one of these DNA strands as it coils around the opposing strand.
In humans, if we look at the complete sequence of our genome, it’s over three billion nucleotides long. We discovered this after sequencing the human genome as a part of the Human Genome Project. This was a worldwide effort by multiple scientists that took over 10 years to complete. This sounds like a massive amount of DNA, and it is, but it’s not the biggest. So far, the largest genome belongs to a flowering plant named Paris japonica with a genome of about 150 billion nucleotides, which is about 50 times the size of our genome. So we can eliminate answer choice (C).
From the Human Genome Project, we also learned that we have over 20,000 protein-coding genes. Some organisms, like wheat for instance, have over 100,000 protein-coding genes. And other organisms like the microscopic Trichomonas vaginalis contains about 60,000 genes in its genome. The genome of Trichomonas vaginalis is only about 160 million nucleotides long, which is about one twentieth of the size of the human genome yet contains about three times the number of genes. So answer choice (A) is incorrect.
The human genome can be divided into coding DNA, which consists of DNA sequences that code for proteins, and noncoding DNA, which consists of DNA that doesn’t code for proteins. In humans, about one percent of our genome is made up of coding DNA, so most of our genome doesn’t code for proteins. Therefore, answer choice (D) is incorrect. In fact, about 99 percent of our genome is made up of noncoding DNA, which is similar to most eukaryotic genomes.
Therefore, the option that is true regarding the human genome is given by answer choice (B). Like most eukaryotic genomes, the human genome contains a great deal of noncoding DNA.
