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Difficulty: Very hardCell Biology, Genetics, and Evolution

With reference to cellular organelles and genetic mechanisms in eukaryotic organisms, consider the following statements:

1. Mitochondria and chloroplasts possess their own double-stranded circular DNA and 70S ribosomes, allowing them to synthesize some of their own structural proteins semi-autonomously.
2. Lysosomal hydrolytic enzymes are synthesized primarily by ribosomes on the smooth endoplasmic reticulum and are directly secreted into the cytoplasm without entering Golgi cisternae.
3. Genetic recombination through crossing over during Meiosis I occurs specifically between non-sister chromatids of homologous chromosomes during the pachytene substage of Prophase I.

Which of the statements given above is/are correct?

  1. 1 and 3 onlyAnswer
  2. B
    1 and 2 only
  3. C
    2 and 3 only
  4. D
    1, 2, and 3

Answer

Statements 1 and 3 only are correct.
The correct answer states that statements 1 and 3 only are correct. Mitochondria and chloroplasts contain prokaryotic features (70S ribosomes, naked circular DNA) supporting semi-autonomy. Crossing over occurs strictly between non-sister chromatids of homologous chromosomes during pachytene. Statement 2 is false because acid hydrolases in lysosomes are synthesized by the rough endoplasmic reticulum and targeted to lysosomes by the Golgi apparatus.

Step-by-Step Solution

1
Evaluate Statement 1 regarding semi-autonomous organelles.
Statement 1 is correct. Both mitochondria and chloroplasts have their own circular double-stranded DNA molecule and prokaryotic-type 70S ribosomes, replicating independently of the nucleus via binary fission.
Endosymbiotic theory accounts for the bacterial-like genetic apparatus of these organelles.
2
Evaluate Statement 2 regarding lysosome biogenesis.
Statement 2 is incorrect. Lysosomal enzymes (acid hydrolases) are synthesized by ribosomes on the Rough Endoplasmic Reticulum (RER), transported to the cis-face of the Golgi apparatus, tagged with mannose-6-phosphate, and budded off as primary lysosomes from the trans-Golgi network.
Smooth ER is involved in lipid/steroid synthesis and detoxification, not lysosomal protein synthesis.
3
Evaluate Statement 3 regarding meiotic crossing over.
Statement 3 is correct. Crossing over is an enzyme-mediated process (recombinase) occurring between non-sister chromatids of homologous chromosomes at the pachytene stage of Prophase I, forming chiasmata seen in diplotene.
Reciprocal exchange of genetic material between non-sister chromatids creates genetic variation.

Key Concept

Organelle autonomy, lysosome biogenesis, and meiotic genetic recombination
Estimated Time:2m 0s
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Topics

Cell Biology, Genetics, and Evolution

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