Chromosome Ultrastructure, Karyotype Analysis and Specialized Chromosomes

विस्तृत व्याख्या:

Eukaryotic Cell Cycle Phases (यूकेरियोटिक कोशिका चक्र की प्रावस्थाएं):

• The cell cycle comprises Interphase ($G_1$, S, $G_2$) and the M phase (Mitosis/Meiosis).
• $G_1$ Phase (Gap 1): This is the first growth phase post-mitosis. Cells actively synthesize proteins, enzymes, RNA, and duplicate organelles to prepare for DNA replication. In a typical 24-hour human cell cycle, $G_1$ occupies approximately 10–12 hours, making it the single longest phase of the cell cycle.
• S Phase: Synthesis phase where DNA replication occurs (~6–8 hours).
• $G_2$ Phase: Pre-mitotic preparation (~3–4 hours).
• M Phase: Mitosis and cytokinesis (~1 hour).

विस्तृत व्याख्या:

Geneticists and Their Landmark Discoveries (प्रमुख आनुवंशिकीविद् एवं खोजें):

• T.H. Morgan: Provided direct experimental proof for the Chromosomal theory of inheritance using Drosophila melanogaster and mapped sex-linked genes to chromosomes $\rightarrow$ (IV).
• Calvin B. Bridges: Discovered Chromosomal non-disjunction (1916) in Drosophila, providing conclusive cytological proof that genes are physical components of chromosomes $\rightarrow$ (I).
• Clarence P. Oliver: In 1940, demonstrated intra-genic recombination (crossing over within the lozenge locus of Drosophila), establishing the Divisibility of the gene in Drosophila $\rightarrow$ (II).
• Seymour Benzer: Used overlapping deletion mutants of the rII locus of phage T4 to develop Deletion mapping to pinpoint sub-genic mutations $\rightarrow$ (III).

Matching sequence: (A) - (IV), (B) - (I), (C) - (II), (D) - (III).
Correct Answer: Option 3.

विस्तृत व्याख्या:

Comparison Between Eukaryotes and Prokaryotes (प्रोकैरियोट बनाम यूकेरियोट):

• Eukaryotic-exclusive traits:
• Mitosis: True spindle-mediated nuclear division with chromosome condensation is unique to eukaryotes (prokaryotes lack a true nucleus and divide by binary fission).
• Phagocytosis / Endocytosis: Engulfment of large particles via cytoskeletal and membrane remodeling occurs only in eukaryotic cells; prokaryotes have rigid cell walls and lack the requisite endomembrane machinery.
• Nucleolus: The specialized subnuclear ribosome biogenesis factory exists inside the membrane-bound eukaryotic nucleus (prokaryotes lack both a nucleus and nucleoli).
• Prokaryotic traits:
• 70S ribosomes: Standard in prokaryotes (though also found inside eukaryotic mitochondria/chloroplasts, eukaryotic cytoplasm contains 80S ribosomes).
• Binary fission: Primary mode of prokaryotic asexual division.

विस्तृत व्याख्या:

Chromosome Condensation in the Cell Cycle (गुणसूत्र संघनन एवं दृश्यता):

• During Interphase ($G_1$, S, and $G_2$ phases), chromosomes exist in a decondensed, diffuse chromatin state to allow active gene transcription and DNA replication, making individual chromosomes indistinguishable under a light microscope.
• During the M phase (Mitosis), condensin complexes and histone phosphorylation drive maximal chromosome supercoiling and condensation.
• At Metaphase of M phase, chromosomes achieve their peak state of structural condensation and are arranged along the equatorial plate, making them most distinct, measurable, and ideal for karyotyping and cytogenetic analysis.

Correct Answer: Option 4 (M phase (mitosis)).