Question

Mitochondria and chloroplasts are
(a) Semi-autonomous organelles
(b) Formed by the division of pre-existing organelles and they contain DNA but lack protein-synthesizing machinery.
Which one of the following options is correct?
(a) Both a and b are false
(b) Both a and b are correct
(c) b is true but a is false
(d) a is true but b is false

Answer 1

Hint: Mitochondria and chloroplast are believed to be evolved by a process of endosymbiosis where an ancient symbiotic bacteria capable of carrying out aerobic respiration or photosynthesis was engulfed by an ancestor of modern- day eukaryotic host and developed a symbiotic relationship with the host.

Complete step by step answer:
Chloroplast and Mitochondria are semiautonomous organelles as they contain circular DNA, mRNA, and tRNAs. The electron microscopy and various other studies show that new chloroplast and mitochondria originated by the growth and division of pre- existing organelles. They carry some genetic information in the form of DNA and can replicate on its own. They are capable of synthesizing some proteins as they possess ribosomes ranging from 55S to 75S.
- Mitochondria produces cellular energy in the form of ATP by aerobic respiration, hence, commonly known as ‘powerhouses of the cell’. It is a double membrane- bound organelle with a highly convoluted inner membrane forming characteristic folds known as cristae.
- It houses an enzyme complex known as ATP synthase or oxysome. The number of mitochondria correlates with the metabolic activity of cells. The active cell has more mitochondria compared to the less active.
- Chloroplast is a double- membrane organelle found exclusively in plants. Pigments such as chlorophyll and carotenoid pigments are located in the chloroplast. They play a vital role in photosynthesis. The matrix of chloroplasts bears closed flat bags of membranes known as thylakoids bearing pigments.
- These thylakoids are placed over one another to form a granum. These grana are also linked to one another by tubules known as stroma lamellae.
So, the correct answer is ‘(d) a is true but b is false.’

Note: The DNA molecules found in mitochondria and chloroplasts are small and circular, similar to the DNA of prokaryotes. Also, the presence of 70S ribosomes in chloroplast and mitochondria is similar to prokaryotes. These shreds of evidence support the endosymbiont theory which suggests that mitochondria and chloroplasts originated as free- living prokaryotic cells.