Video Transcript
Which of the following statements
is true about the genome of a prokaryote? (A) The majority of DNA in a
prokaryotic genome codes for proteins. (B) The size of the genome
increases linearly with the size of the organism. (C) Over 70 percent of the DNA in a
prokaryotic genome does not code for proteins. (D) All prokaryotic genomes are
smaller in size than any eukaryotic genome. Or (E) the genome of a prokaryote
is comprised of RNA only.
To answer this question, let’s
first review the key terms “genome” and “prokaryote.”
A genome is all of the genetic
material contained in an organism. For instance, the human genome
consists of all the DNA found within our cells and contains around 22,000 genes. If we also include the genetic
material of the microorganisms living on and inside of us, known as the human
microbiome, the number of genes increases to over eight million.
Prokaryotes are single-celled
organisms that do not contain membrane-bound organelles. This means their DNA floats freely
within the cytoplasm instead of being contained in a nucleus. Bacteria, like the one drawn here,
are examples of prokaryotes. Bacterial DNA is typically found on
a single closed chromosome. Many bacteria also have DNA found
on small circular structures called plasmids, which replicate independently from the
chromosome.
Eukaryotes are single- or
multicelled organisms that do possess membrane-bound organelles, including a nucleus
that contains most of their DNA. Organelles called mitochondria also
contain small loops of DNA, which, like bacterial plasmids, replicate independently
from nuclear DNA.
The genome of prokaryotes can vary
in size considerably, with the smallest known genome belonging to the symbiotic
bacteria Carsonella ruddii containing a mere 160,000 base pairs. In contrast, the largest known
prokaryotic genome is found in the bacteria Sorangium cellulosum and has over
13 million base pairs. Eukaryotic genomes also vary in
size. The smallest known eukaryotic
genome belongs to a parasitic fungus known as Encephalitozoon intestinalis,
with around two million base pairs. The largest is found in the
Paris japonica plant, with around 150 billion base pairs. This easily trumps our own genome,
which is around three billion base pairs.
There are some interesting
distinctions that can be made between the genome of eukaryotes and prokaryotes. Eukaryotes tend to have genomes
that are largely made up of noncoding DNA. This means that it contains large
regions of DNA that do not code for, or translate into, proteins. In contrast, the vast majority of
the DNA within prokaryotic genomes does code for proteins. Large regions of noncoding and
repetitive DNA are far less common. In fact, prokaryotic genes are
often separated by just one or two nucleotides, as shown by the small pink arrows
above the prokaryotic DNA strand.
Now that we’ve reviewed our key
terms and some characteristics of prokaryotic genomes, let’s have a look back at our
answer options.
We know that in prokaryotes such as
bacteria, the genome is made up of DNA. So we can go ahead and rule out
option (E). We’ve also learned that the
complexity and size of an organism are not correlated to its genome size and that
prokaryotes can have larger genomes than eukaryotes. So, let’s eliminate options (B) and
(D). We also reviewed the fact that
eukaryotic genomes typically contain a lot of noncoding DNA, while prokaryotic
genomes do not. So we can exclude option (C). Therefore, our correct answer is
(A). The true statement is that the
majority of DNA in a prokaryotic genome codes for proteins.
