AS and A Level Biology 13 Views 1 Answers
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Sourav PanNovember 1, 2024

Explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes

Explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes

Sourav Pan
Sourav PanNovember 1, 2024

Answered step-by-step

Crossing over and random orientation (also known as independent assortment) are two crucial mechanisms during meiosis that contribute to genetic diversity in gametes. Here’s a detailed explanation of how each of these processes works and how they produce genetically different gametes:

1. Crossing Over

  • Definition: Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. This process occurs when homologous chromosomes align closely together, forming structures called tetrads.
  • Mechanism:
    • Formation of Tetrads: Each homologous pair consists of one chromosome from the mother and one from the father. During prophase I, these chromosomes undergo synapsis, where they pair up closely.
    • Exchange of Segments: At certain points along their lengths, chromatids from homologous chromosomes can break and rejoin, effectively swapping segments of genetic material. This results in new combinations of alleles on each chromosome.
  • Result:
    • The outcome of crossing over is that each chromosome produced during meiosis contains a mix of alleles from both parents. This process creates recombinant chromosomes, which are genetically different from both the parental chromosomes. As a result, the gametes formed at the end of meiosis will have unique genetic combinations, contributing to genetic diversity.

2. Random Orientation (Independent Assortment)

  • Definition: Random orientation, or independent assortment, refers to the random arrangement of homologous chromosome pairs along the metaphase plate during metaphase I of meiosis.
  • Mechanism:
    • Alignment at the Metaphase Plate: During metaphase I, the tetrads align at the equatorial plane of the cell. Each pair of homologous chromosomes can orient themselves in two possible ways: one homolog may face one pole of the cell while its partner faces the opposite pole.
    • Random Distribution: The orientation of each homologous pair is independent of the others. This means that the maternal or paternal chromosome can go to either daughter cell, resulting in a random assortment of chromosomes.
  • Result:
    • Because there are many homologous chromosome pairs (23 pairs in humans), the number of possible combinations of chromosomes that can result from independent assortment is 2^n, where n is the number of homologous pairs. For humans, this results in 2^23 possible combinations, which equals over 8 million different combinations of chromosomes in the gametes.

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