IMAT 2020 Q23 [Healthy Cells & Circular DNA]

Which of the following healthy cells contain circular DNA?

  1. Escherichia coli
  2. Saccharomyces cerevisiae (yeast)
  3. human liver cell

A. 1 and 2 only
B. 2 only
C. 2 and 3 only
D. 1, 2 and 3
E. 1 only

The answer to this question is D.

Before we answer this question, let’s remind ourselves what circular DNA is :

  • Linear DNA is found in the nucleus of eukaryotes.
  • Like the name suggests, circular DNA is, as opposed to linear DNA, in the form of a loop.
  • Circular DNA can be found in :
    • Plasmids
    • Circular bacterial chromosomes
    • Mitochondrial and chloroplast DNA
    • Extrachromosomal circular DNA (eccDNA)

With that being said, let’s now analyze the items individually :

  1. Escherichia coli is a bacteria that lives, in physiological conditions, inside the intestines of warm-blooded organisms. Being able to identify the fact that E. coli is in fact a bacteria helps you determine that this item is correct, since we’ve established that circular DNA can be found in bacterial chromosomes.

  2. Saccharomyces cerevisiae, also commonly known as yeast, is an eukaryotic single cell organism that belongs to the fungus kingdom. This particular species of yeast enables the conversion of carbohydrates into CO2 and alcohol during fermentation. It is also used as a model organism to study eccDNAs.

But what exactly is eccDNA ? It is a double-stranded circular DNA structure that can be found in the cell nucleus ; its formation is done through a process of replication slippage of the genomic DNA. It has been found in yeast, plants and human cells. Therefore, this item is also correct.

  1. As we just said, DNA in mammalian cells can exist in a circular form, and therefore can also be found in a healthy human liver cell, making item 3 correct as well.

Now that we´ve identified all 3 items to be correct, it makes answer D the right one.