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Meiosis

High schoolBiologyA deterministic diploid meiosis with one prior DNA replication, independent homolog orientation and an optional two-chromatid crossover

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About this experiment

Orient homologous chromosome pairs and compare the products of two consecutive divisions. The first division separates homologs; the second separates sisters. An optional crossover between two linked loci shows how recombinant chromatids can appear alongside parental ones without an additional round of DNA replication.

Learning goals

  • Distinguish homolog separation from sister separation.
  • Account for DNA across one replication and two divisions.
  • Relate homolog orientation and a defined crossover to gamete allele strings.

How to use it

  1. 1Inspect prophase I after replication and choose an orientation for each homolog pair.
  2. 2Enable one crossover in the AB and ab pair, then advance through the first division.
  3. 3Continue through meiosis II and inspect all four final products.
  4. 4Repeat with crossover off and a different pair orientation; record the allele strings.

The model

DNA distribution through the two divisions
Independent orientation choices for n homolog pairs

Assumptions

  • DNA replication is already complete at the first snapshot. Total DNA is 4C throughout the tracked system, with 1C per final haploid product.
  • The first pair carries AB and ab. A selected crossover swaps the distal locus between one chromatid from each homolog, yielding two recombinant chromatids. Other pairs carry C/c and D/d.
  • The orientation pattern is deterministic, with two choices per pair. Chromosome loss, mutation, gene conversion and multiple crossovers are excluded. The selected crossover is an event, not a measured recombination probability.

Try this in class

  1. Q1Why is there no S phase between the first and second meiotic divisions?
  2. Q2Why does one crossover involve only two of the four chromatids in a homolog pair?

Frequently asked questions

Does 2ⁿ mean this one meiosis produces that many cells?

No. It counts possible homolog-orientation patterns across runs. One modeled meiosis produces four products.

Why are two parental and two recombinant AB products present after one crossover?

Only two non-sister chromatids exchange the selected segment. The other two retain their original combinations.