38. Cellular basis of sexual reproduction. Haploid-diploid cycle. Meiosis.

Question 1

Cellular basis of sexual reporduction

Answer 1

1. Asexual Reproduction:

Offspring genetically identical to parent.

2. Sexual Reproduction:
   -Offspring genetically different from parents and each other.

Process:
-Alternates between haploid (gametes) and diploid (zygote) generations.
-Zygote: Diploid cell formed from fusion of gametes.
-Gametogenesis: Zygote’s offspring divide by meiosis to form haploid gametes.

Gametes:
-Egg Cells: Large, immobile.
-Sperm: Small, motile.

Question 2

Haploid- diploid cycle

Answer 2

Gene Combinations: Alternation of haploid and diploid states creates new gene combinations.

Mutation Effect: Lethal mutations in haploids may be unnoticed in diploids if the other gene copy is normal.

Advantage: Sexual reproduction enhances competitiveness in variable environments, driving evolution.

Question 3

Meiosis

Answer 3

Overview:

Reduces diploid chromosome set to haploid.
Part of gametogenesis in eukaryotes.
Involves two divisions: meiosis I (reductional) and meiosis II (equatorial).

Phases of Meiosis I:

-Prophase I: Long, includes leptotene, zygotene, pachytene, diplotene, diakinesis.
-Prometaphase: Centrosomes divide, spindle forms, nuclear envelope breaks.
-Metaphase I: Bivalents align on equatorial plane.
-Anaphase I: Homologous chromosomes move to poles.
-Telophase I: Two haploid nuclei form.
-Interkinesis: No DNA replication.

Phases of Meiosis II:
-Prophase II: Chromosomes condense.
-Metaphase II: Chromosomes align on equatorial plane.
-Anaphase II: Sister chromatids separate.
-Telophase II: Haploid nuclei form, cytokinesis produces 4 gametes.

Advantages:
Genetic variation from random distribution of homologues and crossover.

Disadvantages:
Risk of chromosomal disorders (trisomies, monosomies) due to non-separation in meiosis I.