Meiosis: Meiosis I, Meiosis II, and How It Differs From Mitosis

Meiosis is the specialized type of cell division that produces gametes — sperm and egg cells. Unlike mitosis, which occurs in somatic (body) cells and produces two identical diploid cells, meiosis occurs only in germline cells and produces four genetically unique haploid cells from a single diploid starting cell.

Key Takeaways

• Meiosis produces four genetically unique haploid gametes from one diploid germline cell, across two rounds of division: Meiosis I (reductional) and Meiosis II (equational).

• Prophase I is where synapsis and crossing over occur, introducing genetic recombination.

• Anaphase I separates homologous chromosomes; Anaphase II separates sister chromatids — this distinction is the single most-tested detail in the topic.

• Meiosis differs from mitosis in ploidy outcome, location, homologous chromosome pairing, and crossing over.

• Nondisjunction — failure of proper chromosome separation in Anaphase I or II — produces gametes with an abnormal chromosome number and can cause conditions like Down syndrome.

Meiosis I: The Reductional Division

A haploid cell carries just one copy of each chromosome — 23 total in humans. A diploid cell carries two copies of each — 46 total. This reduction in chromosome number is essential for sexual reproduction: when a haploid sperm and haploid egg unite, the resulting zygote ends up with the correct diploid number.

Meiosis happens in two sequential rounds, Meiosis I and Meiosis II, each with its own prophase, metaphase, anaphase, and telophase.

Before Meiosis I begins, interphase occurs just as it does before mitosis: the cell’s DNA is copied, so each chromosome now consists of two sister chromatids joined at the centromere. Meiosis I is called the reductional division because it’s the step that cuts the chromosome number from diploid to haploid.

Prophase I — Synapsis and Crossing Over

Prophase I is where meiosis introduces genetic variation into the next generation, and it’s worth slowing down on.

• Homologous chromosomes — the maternal and paternal copies of each chromosome, carrying the same genes but possibly different versions of them — find each other and pair up side by side. This pairing process is called synapsis, and the resulting four-chromatid structure is called a tetrad.

• Once paired, the chromosomes can exchange segments of DNA at points called chiasmata, in a process called crossing over. The result is recombinant chromosomes — hybrids carrying a mix of maternal and paternal genetic material.

• Crossing over increases genetic diversity in offspring and helps unlink genes that would otherwise always be inherited together.

Metaphase I, Anaphase I, Telophase I

• Metaphase I: The tetrads — not individual chromosomes — line up along the metaphase plate, in a random orientation that adds further genetic variation (one chromosome of each homologous pair faces each pole).

• Anaphase I: The homologous chromosomes are separated and pulled to opposite poles. Sister chromatids stay attached to each other at this stage.

• Telophase I and Cytokinesis: A nuclear membrane may begin re-forming around each set of chromosomes, and the cell splits into two haploid daughter cells. Each cell now has one chromosome from every original homologous pair — but each of those chromosomes is still made of two sister chromatids.

Meiosis II: The Equational Division

Meiosis II is called the equational division because the chromosome number doesn’t change — the cell is haploid going in and haploid coming out. What Meiosis II does accomplish is separating the sister chromatids that Meiosis I left intact.

Prophase II Through Telophase II

• Prophase II: The spindle apparatus re-forms in each of the two haploid cells.

• Metaphase II: Individual chromosomes (not pairs) line up at the center of each cell.

• Anaphase II: Sister chromatids are finally pulled apart toward opposite poles.

• Telophase II and Cytokinesis: Nuclear membranes reform and both cells divide again, yielding four haploid daughter cells, each genetically unique.

In sperm production, all four of these cells go on to become functional sperm. In egg production, only one becomes a mature egg — a difference covered in detail in the reproductive system’s spermatogenesis and oogenesis section.

Meiosis vs. Mitosis: Key Differences

Mitosis and meiosis share stage names — prophase, metaphase, anaphase, telophase — which makes them easy to mix up. Their purposes and outcomes, though, are fundamentally different.

In short: mitosis makes identical cells for growth and repair, while meiosis makes genetically unique sex cells for reproduction.

MCAT Callout — Which Anaphase Separates What: This is one of the most commonly confused points on this topic. In mitosis’s single anaphase and in Meiosis II’s Anaphase II, sister chromatids separate. In Meiosis I’s Anaphase I, it’s the homologous chromosomes that separate — sister chromatids stay joined until Anaphase II.

Nondisjunction: When Meiosis Goes Wrong

Nondisjunction occurs when chromosomes fail to separate properly during meiosis. It can happen at either anaphase:

• In Anaphase I: the homologous chromosomes fail to separate. One resulting cell ends up with both chromosomes from the pair, and the other gets none.

• In Anaphase II: the sister chromatids fail to separate. One resulting cell ends up with an extra chromatid, and the other gets none.

Either way, the result is gametes with an abnormal chromosome number — too many or too few. If such a gamete is involved in fertilization, the resulting zygote will also carry the wrong chromosome number. This can lead to genetic conditions such as Down syndrome, caused by an extra copy of chromosome 21 (trisomy 21).

Why Meiosis Matters for the MCAT

• Chromosome-counting questions are a recurring MCAT trap for meiosis, just as they are for mitosis — passages often ask how many chromosomes or chromatids a cell has at a specific stage, which requires tracking synapsis, crossing over, and which anaphase you’re in.

• Genetics passages on inheritance and recombination build directly on Prophase I — crossing over and independent assortment are the mechanistic basis for why offspring aren’t genetic copies of either parent, a link tested throughout the genetics content.

• Nondisjunction and chromosomal disorders are a common passage topic, since they connect meiotic mechanism directly to clinical genetics (e.g., trisomy 21) — knowing which anaphase failed lets you reason about what the resulting gametes look like.

Common MCAT Mistakes

• Mixing up which anaphase separates what. Anaphase I separates homologous chromosomes (sister chromatids stay joined); Anaphase II separates sister chromatids. This is the single most commonly tested distinction in the topic.

• Assuming crossing over happens in mitosis too. Crossing over is exclusive to Prophase I of meiosis — mitosis never involves homologous chromosome pairing or genetic exchange.

• Forgetting that Meiosis I is reductional and Meiosis II is equational. Meiosis I cuts chromosome number from diploid to haploid; Meiosis II doesn’t change chromosome number at all — it separates sister chromatids within an already-haploid cell.

• Assuming nondisjunction only happens in Anaphase I. It can occur at either Anaphase I or Anaphase II, and the resulting gamete abnormality differs depending on which stage failed.

MCAT-Style Concept Check

Question: A diploid germline cell with 46 chromosomes undergoes nondisjunction during Anaphase II, in which one pair of sister chromatids fails to separate. How many chromosomes will the resulting affected gamete contain?

• A) 22

• B) 23

• C) 24

• D) 46

Answer: C

Explanation: By the start of Meiosis II, the cell is already haploid (23 chromosomes), each still made of two sister chromatids following Meiosis I. Normally, Anaphase II separates every pair of sister chromatids, yielding gametes with 23 chromosomes each. If nondisjunction occurs at Anaphase II and one pair of sister chromatids fails to separate, one resulting gamete receives an extra chromatid — 23 + 1 = 24 chromosomes — while the other receives one fewer. Choice B is the normal, unaffected outcome. Choices A and D don't match how Anaphase II nondisjunction redistributes chromosomes.

FAQ

What’s the difference between Meiosis I and Meiosis II?

Meiosis I is the reductional division — it separates homologous chromosomes and cuts chromosome number from diploid to haploid. Meiosis II is the equational division — it separates sister chromatids without changing chromosome number, since the cell is already haploid going in.

When does crossing over happen?

Crossing over happens during Prophase I of Meiosis I, when paired homologous chromosomes (tetrads) exchange DNA segments at chiasmata, producing recombinant chromosomes.

What’s the difference between Anaphase I and Anaphase II?

Anaphase I separates homologous chromosomes, while sister chromatids stay joined. Anaphase II separates sister chromatids. Mixing these up is one of the most common errors on this topic.

What causes Down syndrome in terms of meiosis?

Down syndrome is caused by trisomy 21 — an extra copy of chromosome 21 — which results from nondisjunction during meiosis, when chromosome 21 fails to separate properly at Anaphase I or Anaphase II, producing a gamete with an abnormal chromosome count.