Microbiology, 2021

396 10 • Biochemistry of the Genome
CHECK YOUR UNDERSTANDING
• What is the role of noncoding DNA?
Extrachromosomal DNA
Although most DNA is contained within a cell’s chromosomes, many cells have additional molecules of DNA
outside the chromosomes, called extrachromosomal DNA, that are also part of its genome. The genomes of
eukaryotic cells would also include the chromosomes from any organelles such as mitochondria and/or
chloroplasts that these cells maintain (Figure 10.26). The maintenance of circular chromosomes in these
organelles is a vestige of their prokaryotic origins and supports the endosymbiotic theory (see Foundations of
Modern Cell Theory). In some cases, genomes of certain DNA viruses can also be maintained independently in
host cells during latent viral infection. In these cases, these viruses are another form of extrachromosomal
DNA. For example, the human papillomavirus (HPV) may be maintained in infected cells in this way.
Figure 10.26
The genome of a eukaryotic cell consists of the chromosome housed in the nucleus, and extrachromosomal DNA found in
the mitochondria (all cells) and chloroplasts (plants and algae). The cells shown in (b) represent cells obtained from a pap smear. The cells
on the left are normal squamous cells whereas the cells on the right are infected with human papillomavirus and show enlarged nuclei with
increased staining (hyperchromasia).
Besides chromosomes, some prokaryotes also have smaller loops of DNA called plasmids that may contain one
or a few genes not essential for normal growth (Figure 3.12). Bacteria can exchange these plasmids with other
bacteria in a process known as horizontal gene transfer (HGT). The exchange of genetic material on plasmids
sometimes provides microbes with new genes beneficial for growth and survival under special conditions. In
some cases, genes obtained from plasmids may have clinical implications, encoding virulence factors that give
a microbe the ability to cause disease or make a microbe resistant to certain antibiotics. Plasmids are also
used heavily in genetic engineering and biotechnology as a way to move genes from one cell to another. The
role of plasmids in horizontal gene transfer and biotechnology will be discussed further in Mechanisms of
Microbial Genetics and Modern Applications of Microbial Genetics.
CHECK YOUR UNDERSTANDING
• How are plasmids involved in antibiotic resistance?
Case in Point
Lethal Plasmids
Maria, a 20-year-old anthropology student from Texas, recently became ill in the African nation of
Botswana, where she was conducting research as part of a study-abroad program. Maria’s research was
Access for free at openstax.org.