Some features mentioned below are important for the segregation of homologous chromosomes in meiosis I.

A. Synaptonemal complex formation between homologous chromosomes.
B. Degradation of cohesins at the chromosome arms.
C. Retention of cohesins at the centromeres.
D. Bi-orientation of kinetochores of sister chromatids.

Which one of the following options has all correct features?

Correct option is C

Meiosis I is a specialized cell division that ensures the proper segregation of homologous chromosomes. The following features play essential roles in this process:

• (A) Synaptonemal complex formation between homologous chromosomes:

  • The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes during prophase I of meiosis.
  • It facilitates homologous recombination and ensures proper chromosome pairing and alignment.
  • This is a correct feature.

• (B) Degradation of cohesins at the chromosome arms:

  • Cohesins are protein complexes that hold sister chromatids together.
  • In anaphase I of meiosis, cohesins at the chromosome arms are degraded, allowing homologous chromosomes to separate.
  • However, cohesins at the centromeres remain intact, preventing sister chromatids from separating until meiosis II.
  • This is a correct feature.

• (C) Retention of cohesins at the centromeres:

  • During anaphase I, centromeric cohesins remain intact due to the action of Shugoshin proteins, preventing premature separation of sister chromatids.
  • This ensures that only homologous chromosomes separate, while sister chromatids stay together until anaphase II.
  • This is a correct feature.

• (D) Bi-orientation of kinetochores of sister chromatids:

  • In meiosis I, sister chromatids do not bi-orient; instead, their kinetochores are mono-oriented, meaning they attach to the same pole.
  • The bi-orientation of kinetochores occurs in mitosis and meiosis II, ensuring the separation of sister chromatids.
  • This is incorrect for meiosis I.

Since option D is incorrect, the correct answer is (3) A, B, and C only.

Information Booster

1. Meiosis I vs. Meiosis II:

   • Meiosis I is the reductional division where homologous chromosomes separate.
   • Meiosis II is the equational division where sister chromatids separate.

2. Synaptonemal Complex (SC):

   • A tripartite protein structure that forms between homologous chromosomes during prophase I and helps in recombination.

3. Cohesin Degradation:

   • Anaphase I: Cohesins at chromosome arms degrade, allowing homologs to separate.
   • Anaphase II: Centromeric cohesins degrade, separating sister chromatids.

4. Shugoshin Protein:

   • Protects centromeric cohesins during anaphase I, preventing premature sister chromatid separation.

5. Mono-orientation of Kinetochores in Meiosis I:

   • Sister chromatids attach to the same spindle pole, ensuring that homologous chromosomes separate first.

6. Errors in Cohesin Regulation:

   • Failure to degrade cohesins properly can lead to nondisjunction, resulting in aneuploidy (e.g., Down syndrome).