AP Biology · Unit 5 · Topic 5.1
Meiosis
Suppose each gamete carried the same two chromosome sets as the cells that produced it. Fertilization would combine two sets with two more, and chromosome sets would keep doubling across generations. Meiosis solves this problem before fertilization: a diploid cell distributes its chromosomes into haploid products. To understand how, we need to distinguish copying a chromosome from adding another chromosome set.
One chromosome can contain two chromatids
A homologous pair consists of corresponding chromosomes inherited from the two parents. They generally carry the same genes at corresponding positions, but may carry different alleles. Sister chromatids, in contrast, arise when one chromosome is replicated. These are two different relationships: matching chromosome types versus copies of one chromosome.
Consider an imaginary organism with \(2n=6\): three homologous pairs. Before replication, its cell has six chromosomes and six nuclear DNA molecules. After S phase, it still has six replicated chromosomes, now containing twelve chromatids and twelve DNA molecules. It has not become tetraploid. Both chromosome sets were copied; no extra set of homologs was added.
An X-shaped drawing therefore represents one replicated chromosome, not a homologous pair. Count a joined sister pair as one chromosome until sisters separate. Once separated, each former chromatid is a chromosome in its own right.
The first division separates homologs
During prophase I, homologs pair by synapsis. Because each has already replicated, a paired unit contains two chromosomes and four chromatids—a tetrad. Crossing over exchanges corresponding DNA segments between nonsister chromatids. The resulting connections, seen as chiasmata together with chromatid cohesion, help maintain the association needed for segregation.
At metaphase I, homologous pairs align at the cell's equator. The two homologs face opposite poles; sister chromatids of a homolog are oriented together toward the same pole. In anaphase I, homologs separate while centromeric cohesion keeps the sisters together. This is the decisive reduction: each pole receives one member of every pair.
After telophase I and cytokinesis in the standard model, each daughter cell has one chromosome set, even though every chromosome still has two chromatids. That is why a cell can be haploid and still contain replicated chromosomes. Nuclear-envelope reformation and the timing of cytokinesis vary among organisms; the chromosome separation is the essential event.
The second division separates sisters—not another pair of homologs
There is no second S phase between meiosis I and II. In prophase II the division apparatus is established; at metaphase II, individual replicated chromosomes align with sisters connected toward opposite poles. At anaphase II, remaining sister cohesion is released and the chromatids separate. Telophase II and cytokinesis partition the resulting nuclei and cells.
For our \(2n=6\) organism, each of the four final nuclei has three unreplicated chromosomes. Meiosis I changed two sets into one; meiosis II distributed the copies within that one set. Calling both divisions “halving the chromosome sets” would incorrectly predict half of a complete set at the end.
Keep chromosome sets and DNA amount in separate accounts
Let \(n\) denote one chromosome set and \(C\) the DNA amount in one unreplicated haploid genome. Use the following values per cell after each division has completed—not totals pooled across all descendants.
| State | Chromosomes per cell | Nuclear DNA molecules per cell | Sets and DNA amount |
|---|---|---|---|
| Before S phase | 6 | 6 | 2n, 2C |
| After S phase, before meiosis I | 6 | 12 | 2n, 4C |
| After meiosis I | 3 | 6 | n, 2C |
| After meiosis II | 3 | 3 | n, 1C |
The post-S cell contains 12 DNA molecules. Following meiosis I, two cells each contain 6; after meiosis II, four each contain 3. The total remains 12 across the products of this idealized division. Reduction per cell is achieved by partitioning DNA, not by destroying half of it.
What changes during anaphase II, before cytokinesis?
In one dividing cell of this example, the three sister pairs become six separate chromosomes when sisters separate. Three move toward each pole. DNA amount in that still-undivided cell remains 2C during separation. After partitioning, each daughter has three chromosomes and 1C. Always specify whether you mean the whole dividing cell, one pole, or one completed daughter cell.
Why a DNA reading cannot identify a stage by itself
Imagine a measurement system in which one haploid unreplicated genome contains 5 pg of nuclear DNA. The expected amounts are 10 pg before S phase, 20 pg after S phase, 10 pg in each post-meiosis-I cell, and 5 pg in each final product.
A cell measures 10 pg. Has it completed meiosis I?
Not necessarily. Both a diploid cell before S phase and a haploid cell after meiosis I contain 2C, or 10 pg in this example. Chromosome configuration or an independent stage marker is needed. Equal DNA amounts do not imply equal numbers of chromosome sets.
To investigate this experimentally, combine calibrated DNA measurements with chromosome imaging and use cells at known stages as references. Do not mistake two attached cells measured together for one nucleus with twice the DNA.
Where the variation enters
Crossing over changes allele combinations along chromatids; independent orientation of homologous pairs changes which homologs travel together. After crossing over, sister chromatids need not remain genetically identical. Neither mechanism should be described simply as creating new alleles: they primarily rearrange existing variation.
As a counting model, ignore crossing over and distinguish the two parental homologs of each pair. Our three pairs each provide two alternatives in a gamete. Independent assortment gives \(2\times2\times2=2^3=8\) possible parental-origin combinations across many meiotic events. One meiotic event still produces only four nuclei; it does not produce all eight combinations. Linked loci on the same chromosome do not automatically follow this independence model. Topic 5.2 develops these sources of diversity further.
Mitosis uses similar machinery for a different distribution
Both processes replicate DNA beforehand and use spindle-mediated chromosome movement. The difference is what aligns and what separates. Typical mitosis aligns chromosomes individually and separates sisters in one division, preserving the starting ploidy. Meiosis pairs homologs and separates them before a second division separates sisters.
Consequently, meiosis II resembles mitosis in its separation event but starts with the products of meiosis I. The chromosomes may already be recombinant, and homologous partners are no longer together in each nucleus. “Meiosis is mitosis twice” misses precisely the first division that makes reduction possible.
Use a segregation error to test the model
For a short extension, suppose just one homologous pair fails to separate in meiosis I, while all other movements occur normally. Both homologs enter one daughter; neither enters the other. Normal meiosis II then produces two \(n+1\) products and two \(n-1\) products for chromosome number.
How would a failure in only one meiosis-II cell differ?
If meiosis I was normal and one sister pair fails to separate in just one of its daughter cells during meiosis II, the four products are two normal n, one n+1, and one n−1. The normal first-division daughter still divides normally. Trace each branch rather than memorizing an outcome without its assumptions.
Return to the life cycle
Fertilization combines two haploid sets: \(n+n=2n\). In animals, meiosis contributes directly to gamete formation, but four meiotic nuclei do not always become four equivalent functional gametes; egg formation involves unequal partitioning. In plants, meiosis produces haploid spores, and the haploid generation later makes gametes by mitosis. Reduction of chromosome sets is the common principle, not an identical life cycle in every organism.
If you are given an unfamiliar cell diagram, first ask what is separating. Paired homologs at the equator or separating with sisters still joined point to meiosis I. Individual chromosomes with sisters facing opposite poles point to meiosis II or mitosis; ploidy and prior cell history distinguish those possibilities.
Try the reasoning with a different chromosome number
A species has \(2n=10\). A normal meiotic cell has finished S phase. Predict its counts now, after meiosis I, and after meiosis II before opening the explanation.
Follow the counts
After S phase there are 10 chromosomes, 20 chromatids, and 4C DNA. Meiosis I gives each daughter one homolog from each of five pairs: 5 replicated chromosomes, 10 chromatids, and 2C. Meiosis II gives each final product 5 unreplicated chromosomes and 1C. The number of sets falls in the first division, while DNA amount per cell falls in both divisions. Fertilization of two normal products restores 10 chromosomes in the zygote.
Concept references: OpenStax Biology 2e: The Process of Meiosis, Sexual Reproduction, and College Board AP Biology Course and Exam Description, Topic 5.1. The explanations, chromosome model, numerical examples, and questions are independently authored for Insomnia Studyroom; source figures and exercises are not reproduced.
Official curriculum reference: College Board AP Biology course page. The explanation and worked example are independently written for this study site.