Prophase I: Homologous chromosomes pair up to form bivalents. During this stage, crossing over occurs where non-sister chromatids exchange segments of DNA at points called chiasmata.
Metaphase I: Homologous pairs line up randomly along the cell equator. This independent segregation means the orientation of one pair does not influence another, leading to possible combinations.
Anaphase I: Spindle fibers pull whole homologous chromosomes to opposite poles. Crucially, the centromeres do not divide here; sister chromatids remain attached.
Telophase I: The cell divides into two haploid cells. Although each cell has half the number of chromosomes, each chromosome still consists of two sister chromatids.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, tissue repair, asexual reproduction | Production of gametes for sexual reproduction |
| Divisions | One single division | Two successive divisions (I and II) |
| Daughter Cells | Two diploid () cells, genetically identical | Four haploid () cells, genetically different |
| Pairing | No pairing of homologous chromosomes | Homologous chromosomes pair (bivalents) in Prophase I |
| Crossing Over | Does not occur | Occurs during Prophase I |
| Outcome | Maintains chromosome number | Halves chromosome number |
Identify the Stage: If you see homologous pairs (bivalents) or crossing over, it is always Meiosis I. If chromosomes are in a single file and the cell is haploid, it is Meiosis II.
Chromosome Counting: Remember that the number of centromeres equals the number of chromosomes. In Meiosis I, the chromosome number is halved, but each chromosome still has two chromatids.
Terminology Precision: Use the term 'independent segregation' when referring to the random alignment of homologous pairs in Metaphase I to describe how variation is generated.
Variation Sources: Always link 'crossing over' to Prophase I and 'independent segregation' to Metaphase I when explaining why offspring are genetically different from parents.
Interphase Confusion: A common error is assuming DNA replicates again before Meiosis II. DNA only replicates once, during the S-phase of Interphase before Meiosis I begins.
Chromatid vs. Chromosome: Students often confuse sister chromatids (identical copies) with homologous chromosomes (one from each parent). Meiosis I separates homologs; Meiosis II separates sister chromatids.
Haploid Timing: Many believe cells only become haploid at the very end. In reality, the cells are considered haploid immediately after Meiosis I because they no longer contain pairs of homologous chromosomes.
