IMAT 2017 Q33 [Cell Structures]

Four students, 1, 2, 3 and 4, recorded different structures that they thought were found in a healthy animal kidney cell and a typical bacterial cell.

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Which students gave totally correct answers?

A. students 2 and 3 only
B. students 1 and 2 only
C. students 1, 2, 3 and 4
D. students 2 and 4 only
E. students 1 and 4 only

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The correct answer is E.

To tackle this question, we have to review each item individually :

  • DNA can found in both prokaryotes and eukaryotes. It can reside in the nucleus in eukarytic cells and, on a much smaller scale, in the mitochondria.

    The difference between prokaryotic organisms like bacterias and eukaryotic ones is the fact that the latter’s DNA is enclosed in the nucleus. Therefore, DNA will be found in the nucleus in animal cells, but not in bacteria cells.

  • Circular DNA , as opposed to linear DNA, in the form of a loop. It can be found in :

    1. Plasmids
    2. Circular bacterial chromosomes
    3. Mitochondrial and chloroplast DNA
    4. Extrachromosomal circular DNA (eccDNA)

    More specifically, eccDNA 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.

  • Plasmids are, like we just said, a small circular DNA molecule. It is found mainly inside bacteria but can occur in some eukaryotes. Its purpose is to provide the bacteria with genetic advantages, and can be transferred to other prokaryotes.

  • Ribosomes are essential structures in protein synthesis : it leads the mRNA that translates the genetic sequence. Prokaryotic and eukaryotic organisms don’t have the same type of ribosomes :

    1. Eukaryotic ribosomes are called 80S
    2. Prokaryotic ribosomes are called 70S : it is composed of 2 subunits, a big one (50S) and a small one (30S).

But how is 30S + 50S the 70S ribosome and not 80S ? First, the letter S refers to the sedimentation under the influence of density gradient sedimentation. Second, the extra 10S is the overlapping part of the subunits (as they are connected).

  • The mitochondria is an organelle that can only be found in eukaryotes. Its purpose is to be the “powerhouse of the cell”, by producing ATP. On the other hand, prokaryotes produce their ATP on their cell surface membrane.

  • The cell wall is the external layer of the cell and is a non-living component. Its purpose is to protect the cell from osmotic and mechanical stress, and to give support to the cell. It is absent in animal cells.

Now, let’s do a recap, shall we ?

Elements that can be found in animal cells are :

  1. DNA inside the nucleus
  2. circular DNA
  3. mitochondria

Elements that can be found in bacterias are :

  1. 70S ribosomes
  2. plasmids

Therefore, the answer is E.