Video Transcript
Which of the following statements
is true about the genome of a eukaryotic organism? (A) Humans have the highest number
of protein-coding genes of any eukaryotic organism. (B) The percentage of the genome
that codes for proteins is lower in a eukaryotic genome than in a prokaryotic
one. (C) Over 90 percent of the DNA in a
eukaryotic genome will code for proteins. (D) The size of the genome
increases linearly with the size of the organism. (E) The genome will not contain any
sections of repeated DNA.
This question is asking us about
the genome of eukaryotes. Let’s talk about the genome of
humans as a model for eukaryotes. You’ll recall that eukaryotes are
characterized by the presence of membrane-bound organelles, including the nucleus
which contains DNA.
As humans, there’s a lot of DNA in
our cells. If you were to take the DNA from a
single cell and stretch it out, it would be about two meters in length. This isn’t just one long molecule
of DNA; instead, it’s organized into structures called chromosomes. In humans, there’s 23 pairs of
chromosomes, or 46 in total. The amount of genetic material in
an organism is called its genome, and these 46 chromosomes in the nucleus of most of
our cells represent the majority of our genome.
Let’s go over some key facts about
our genome and compare it to the genome of other species as we eliminate some answer
choices from this question. Our genome is over three billion
nucleotides long. That is really big, but is it the
largest? Not quite. A species of flowering plant called
Paris japonica has a genome of about 150 billion nucleotides, which is
about 50 times the size of a human genome. Humans are larger organisms than
this plant, so we can eliminate answer choice (D).
Our chromosomes are tightly
compacted DNA. Different segments of DNA, called
genes, can provide instructions to produce different functional units, for example,
proteins. We have over 20,000 protein-coding
genes in our genome. Still, this isn’t the most. The genome of the wheat plant has
over 100,000 genes. So we can eliminate answer choice
(A) because this is false.
You might be surprised to hear that
only about one percent of our genome is made up of coding DNA or genes that code for
proteins. So we can eliminate answer choice
(C).
Over half of our genome is made up
of repetitive DNA sequences. These are regions of DNA with no
clear function that are repeated over and over again. So we can eliminate answer choice
(E).
Just like in eukaryotes,
prokaryotes also contain noncoding DNA, although there is much less of it. This means that the percentage of
our genome codes for less compared to prokaryotes. Therefore, answer choice (B) is
correct. The percentage of the genome that
codes for proteins is lower in a eukaryotic genome than in a prokaryotic one.
