Phases of Meiosis

Why This Matters

Meiosis isn't just another cell division process to memorize—it's the foundation of sexual reproduction and the engine driving genetic diversity in populations. On the AP Bio exam, you're being tested on your understanding of how meiosis generates variation (crossing over, independent assortment, random fertilization) and why this matters for evolution, inheritance patterns, and genetic disorders. The College Board wants you to connect chromosome behavior during each phase to bigger concepts like heredity, genetic recombination, and nondisjunction errors that cause conditions like Down syndrome.

Think of meiosis as two back-to-back divisions with completely different purposes: Meiosis I separates homologous chromosomes (the reductional division), while Meiosis II separates sister chromatids (just like mitosis). The phases themselves follow a predictable pattern, but the real exam gold lies in understanding the mechanisms—synapsis, chiasmata formation, monopolar kinetochore attachment—that make meiosis unique. Don't just memorize the sequence; know what biological principle each phase demonstrates and where things can go wrong.

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Meiosis I: The Reductional Division

Meiosis I is where the chromosome number gets cut in half, going from diploid (2n) to haploid (1n). This division separates homologous chromosomes—not sister chromatids—which is why it's called the reductional division. The magic of genetic diversity happens here.

Prophase I

• Longest and most complex phase—this is where crossing over and synapsis occur, generating the genetic recombination that drives variation
• Homologous chromosomes pair up via the synaptonemal complex, forming structures called tetrads or bivalents (four chromatids total)
• Chiasmata become visible as X-shaped structures marking where crossing over exchanged genetic material between non-sister chromatids

Metaphase I

• Homologous pairs align at the metaphase plate—not individual chromosomes like in mitosis, but paired homologs facing opposite poles
• Random orientation of each homologous pair creates independent assortment, contributing $2^n$ possible chromosome combinations ($2^{23}$ in humans)
• Spindle fibers attach to kinetochores with monopolar orientation, meaning both sister chromatids connect to the same pole

Anaphase I

• Homologous chromosomes separate and move to opposite poles—this is the actual reduction event
• Sister chromatids stay together, held by cohesin proteins protected at the centromere by shugoshin
• Separase cleaves cohesin along chromosome arms but not at centromeres, allowing homolog separation while keeping sisters attached

Telophase I and Cytokinesis

• Chromosomes arrive at poles and may partially decondense depending on the organism
• Nuclear envelope may or may not reform—this varies by species and doesn't affect the outcome
• Cytokinesis produces two haploid cells, each containing one chromosome from each homologous pair (but still with sister chromatids attached)

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Mechanisms That Generate Genetic Diversity

These processes are the why behind meiosis's role in evolution and inheritance. The AP exam frequently tests your ability to explain how each mechanism contributes to variation.

Crossing Over

• Exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I
• Spo11 protein induces double-strand breaks that are repaired using the homologous chromosome as a template, creating recombinant chromosomes
• Increases genetic variation by producing new allele combinations not present in either parent—essential for evolution and adaptation

Synapsis

• Precise pairing of homologous chromosomes facilitated by the synaptonemal complex, a protein structure that zippers homologs together
• Required for crossing over to occur accurately between the correct chromosomal regions
• Errors in synapsis can lead to improper segregation and aneuploidy, connecting to nondisjunction disorders

Chiasmata Formation

• Physical connection points where crossing over has occurred, visible as X-shaped structures during late Prophase I
• Hold homologs together until Anaphase I, ensuring proper alignment and segregation at the metaphase plate
• Number and position vary—at least one chiasma per chromosome pair is typically required for accurate segregation

Independent Assortment

• Random orientation of homologous pairs at the metaphase plate during Metaphase I
• Each pair orients independently, meaning maternal and paternal chromosomes sort into gametes randomly
• Creates $2^{23}$ combinations in humans (over 8 million) before even accounting for crossing over

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Meiosis II: The Equational Division

Meiosis II looks almost identical to mitosis—it separates sister chromatids. The key difference? It starts with haploid cells and produces haploid cells. No DNA replication occurs between Meiosis I and II.

Interkinesis

• Brief pause between divisions—not a true interphase because no S phase (DNA replication) occurs
• Chromosomes may partially decondense but remain in the haploid state with sister chromatids attached
• Prepares cells for Meiosis II by reorganizing the cytoskeleton and centrioles

Prophase II

• Chromosomes recondense if they relaxed during interkinesis
• Nuclear envelope breaks down and new spindle fibers form from centrioles
• No synapsis or crossing over—homologs are already in separate cells

Metaphase II

• Individual chromosomes align at the metaphase plate—not homologous pairs like in Metaphase I
• Spindle fibers attach to kinetochores of sister chromatids, with bipolar orientation (each sister connects to opposite poles)
• Resembles mitotic metaphase but occurs in haploid cells

Anaphase II

• Sister chromatids finally separate as separase cleaves the remaining centromeric cohesin (no longer protected by shugoshin)
• Each chromatid becomes an individual chromosome and moves to opposite poles
• Ensures equal distribution of genetic material to daughter cells

Telophase II and Cytokinesis

• Chromosomes decondense and nuclear envelopes reform around each set
• Cytokinesis divides the cytoplasm, producing four cells total from the original parent cell
• Final products are four genetically unique haploid gametes—ready for fertilization

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When Meiosis Goes Wrong: Nondisjunction

Understanding normal meiosis helps you recognize what happens when errors occur. Nondisjunction is the failure of chromosomes to separate properly, and it can happen in either division.

Nondisjunction in Meiosis I

• Homologous chromosomes fail to separate during Anaphase I, sending both homologs to one pole
• All four resulting gametes are abnormal—two with an extra chromosome (n+1) and two missing one (n-1)
• Causes aneuploidy when these gametes fertilize, leading to conditions like trisomy 21 (Down syndrome)

Nondisjunction in Meiosis II

• Sister chromatids fail to separate during Anaphase II in one of the two cells
• Two gametes are normal, one has an extra chromosome, and one is missing a chromosome
• Also causes aneuploidy but affects fewer gametes than Meiosis I errors

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Quick Reference Table

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Self-Check Questions

1. Which two phases involve chromosome alignment at the metaphase plate, and how does what aligns differ between them?

2. A student claims that crossing over and independent assortment both occur during Metaphase I. Identify the error and explain when each mechanism actually occurs.

3. Compare the outcomes of nondisjunction occurring in Meiosis I versus Meiosis II—how many abnormal gametes result from each?

4. If an FRQ asks you to explain how meiosis generates genetic diversity, which three mechanisms should you discuss, and during which phases do they occur?

5. Why do sister chromatids remain attached during Anaphase I but separate during Anaphase II? Identify the proteins involved in this regulation.