Meiosis Phases

Why This Matters

Meiosis is the foundation of sexual reproduction and genetic diversity—two concepts that appear throughout every unit of biology, from Mendelian genetics to evolution and population dynamics. When you understand meiosis, you understand why siblings look different from each other, how genetic disorders arise from nondisjunction, and what makes natural selection possible in the first place. This isn't just cell division; it's the mechanism that shuffles the genetic deck every generation.

You're being tested on more than just the sequence of phases. Exam questions will ask you to explain where genetic variation originates (crossing over, independent assortment), how chromosome number changes (diploid to haploid), and what distinguishes meiosis from mitosis. Don't just memorize that "chromosomes line up"—know what's lining up (tetrads vs. individual chromosomes), what's separating (homologs vs. sister chromatids), and why it matters for the resulting cells.

---

Meiosis I: The Reduction Division

The first division is where the magic happens for genetic diversity. Meiosis I separates homologous chromosomes, cutting the chromosome number in half and introducing variation through crossing over and independent assortment.

Prophase I

• Crossing over occurs—homologous chromosomes exchange segments of DNA at points called chiasmata, creating new allele combinations not found in either parent
• Synapsis forms tetrads (bivalents) as homologous pairs physically connect, allowing the cell to align and later separate them accurately
• Nuclear envelope breaks down while spindle fibers form, preparing the cell for chromosome movement

Metaphase I

• Tetrads align at the metaphase plate—homologous pairs (not individual chromosomes) line up with one homolog facing each pole
• Independent assortment occurs here: each tetrad's orientation is random, producing $2^n$ possible chromosome combinations ($2^{23}$ in humans)
• Spindle fibers attach to kinetochores on homologous chromosomes, not sister chromatids, ensuring homologs will separate

Anaphase I

• Homologous chromosomes separate—pulled to opposite poles while sister chromatids remain joined at their centromeres
• Reduction division happens here—this is the moment the cell transitions from diploid (2n) to haploid (n)
• No centromere splitting distinguishes this from Anaphase II and mitotic anaphase; you're separating chromosome pairs, not copies

Telophase I

• Two haploid nuclei form—each containing half the original chromosome number, though chromosomes still consist of sister chromatids
• Cytokinesis divides the cytoplasm, producing two daughter cells ready for the second division
• Nuclear envelope may or may not reform—this varies by species and doesn't affect the outcome

---

Meiosis II: The Equational Division

Meiosis II closely resembles mitosis—it separates sister chromatids into individual chromosomes. No new genetic variation is introduced here; the diversity was already created in Meiosis I.

Prophase II

• Chromosomes re-condense and the nuclear envelope breaks down again (if it reformed), preparing for the second round of division
• No crossing over occurs—genetic recombination is exclusive to Prophase I when homologs are paired
• Spindle apparatus forms with fibers attaching to kinetochores on sister chromatids, not homologs

Metaphase II

• Individual chromosomes align at the metaphase plate—this looks like mitotic metaphase, with sister chromatids attached at centromeres
• Spindle fibers attach to opposite sides of each centromere, ensuring sister chromatids will be pulled apart
• Random alignment of chromosomes continues to contribute to variation in the final products

Anaphase II

• Sister chromatids finally separate—centromeres split and each chromatid becomes an independent chromosome
• Movement to opposite poles ensures each resulting cell gets one copy of each chromosome
• Resembles mitotic anaphase—this is why Meiosis II is called the "equational division"

Telophase II

• Four haploid nuclei form—each with a unique combination of alleles due to crossing over and independent assortment
• Nuclear envelopes reform and chromosomes de-condense as the cell prepares to complete division
• Cytokinesis produces four daughter cells—genetically distinct gametes ready for fertilization

---

Sources of Genetic Variation

Understanding where diversity comes from is essential for connecting meiosis to evolution and inheritance. Three mechanisms work together to ensure offspring are genetically unique.

Crossing Over (Prophase I)

• Recombination creates new allele combinations—segments of maternal and paternal chromosomes are physically exchanged
• Chiasmata are the visible evidence—these X-shaped structures mark where crossing over occurred
• Increases variation beyond independent assortment—without crossing over, you'd only get parental chromosome combinations

Independent Assortment (Metaphase I)

• Random orientation of tetrads—each homologous pair aligns independently of all others
• Produces $2^n$ combinations—in humans, this means over 8 million possible gamete types from one parent
• Explains Mendel's second law—this is the physical basis for why genes on different chromosomes sort independently

---

Quick Reference Table

---

Self-Check Questions

1. Which two phases are most responsible for generating genetic diversity, and what specific mechanism occurs in each?

2. A student claims that chromosome number is reduced during Anaphase II. Explain why this is incorrect and identify when reduction actually occurs.

3. Compare and contrast Metaphase I and Metaphase II—what structures are present at the metaphase plate in each, and why does this difference matter?

4. If crossing over failed to occur during Prophase I, would the resulting gametes still be genetically diverse? Explain what source(s) of variation would remain.

5. An FRQ asks you to explain how meiosis contributes to evolution. Which phases would you reference, and what would you say about each?