AIIMS 2005 Botany Organelles Assertion Reason Question

Explanation of the Question

Assertion:

Mitochondria and chloroplasts are semi-autonomous organelles.

Mitochondria and chloroplasts are termed semi-autonomous because they possess their own DNA, ribosomes, and can replicate independently of the cell. They are involved in energy production (mitochondria) and photosynthesis (chloroplasts), suggesting a level of autonomy in their functions. However, they still rely on the nuclear DNA of the host cell for some proteins essential for their functions.

Reason:

They are formed by division of pre-existing organelles as well as contain DNA but lack protein synthesizing machinery.

This statement has two parts:

1. Formation by division: Mitochondria and chloroplasts replicate by a process similar to binary fission, which is indeed a division of pre-existing organelles.
2. Contain DNA but lack protein synthesizing machinery: While it is true that they contain their own DNA, this part of the reason is misleading. Mitochondria and chloroplasts possess ribosomes and can synthesize some proteins, although they are limited in comparison to the protein synthesis capabilities of the cell's cytoplasm.

Correct Answer: C

Justification for Correct Answer:

• The assertion is true as both mitochondria and chloroplasts are semi-autonomous organelles.
• The reason is partially true in that they are formed by division of existing organelles and do contain DNA. However, the statement that they "lack protein synthesizing machinery" is false because they do have ribosomes and can synthesize some proteins, albeit not all.

Thus, the assertion is true while the reason is false, making option C the correct choice.

Why Other Options are Incorrect:

• Option A: This option would imply that both the assertion and reason are true, and that the reason explains the assertion correctly. Since the reason is partially false (specifically regarding the lack of protein-synthesizing machinery), this option is incorrect.

• Option B: Similar to option A, this option suggests that both statements are true but that the reason does not correctly explain the assertion. However, since the reason is false, this option is also incorrect.

• Option D: This option states that both the assertion and reason are false. This is not the case, as the assertion is true.

Key Concepts:

1. Semi-autonomous organelles: These are organelles that can replicate independently but still rely on the cell for some functions.
2. Mitochondrial and chloroplast DNA: Both organelles have their own circular DNA, similar to prokaryotes, which encodes some proteins necessary for their respective functions.
3. Protein synthesis: Mitochondria and chloroplasts have their own ribosomes (70S type), allowing them to synthesize some proteins using their own DNA.

In summary, the assertion about mitochondria and chloroplasts being semi-autonomous is accurate, but the reason provided fails to fully capture the capabilities of these organelles regarding protein synthesis, which is why the correct answer is C.