IMAT 2016 Q23 [Cell Organelles]

Which organelle contains RNA but not DNA?

A. nucleolus
B. mitochondrion
C. chloroplast
D. smooth endoplasmic reticulum
E. ribosome

The correct answer is E.

Let’s review each item individually :

What is a nucleolus ?

The nucleolus is where the genetic information is stored. Its purpose is to serve as the ribosome production factory.

It is made of proteins, DNA and RNA, which makes item A incorrect.

What is a ribosome ?

Ribosomes are essential structures in protein synthesis : it contains and 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 only contains RNA, making item E correct.

What is a mitochondrion ?

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 through a process of oxidative phosphorylation (ADP into ATP).

It contains circular DNA, which makes item B incorrect.

What is a chloroplast ?

A chloroplast is a plant organelle that is composed of different structures :

  1. the stroma is the inside of the chloroplast
  2. it has 2 membranes : one outer and one inner
  3. thylakoids are compartiments inside the chloroplast
  4. a stack of thylakoids is a granum

The thylakoid is a structure in which light dependent reactions occur, also known as the electron transport chain. It results in the production of an ATP molecule.

It contains circular DNA, which makes item C incorrect.

What is a smooth endoplasmic reticulum (SRE) ?

A SRE is an organelle in the shape of disks surrounding the nucleolus. Its purpose is to synthesize steroids, lipids, and phospholipids.

As opposed to the rough endoplasmic reticulum (RER), it does NOT have ribosomes bound to it.

Therefore, it only contains DNA, which makes item D incorrect.