Track homologs and chromatids through meiosis

Mechanism before a label

Meiosis reduces chromosome-set number through one DNA replication followed by two divisions. Homologous chromosomes pair and separate in meiosis I; sister chromatids normally separate in meiosis II. A homologous pair carries corresponding loci but may carry different alleles.

For a stipulated 2n=8 species, the replicated starting cell has eight chromosomes and sixteen chromatids. After meiosis I each nucleus has four duplicated chromosomes: it is haploid despite having replicated DNA. After meiosis II each normal product has four unduplicated chromosomes.

Ploidy counts homologous sets, not DNA mass. Fertilization restores diploidy when two appropriate haploid gametes unite. The four products of a meiotic event need not all become equivalent functional gametes in every organism; the accounting does not specify gamete development.

Follow the two stage sequences

During prophase I, replicated homologs pair (synapsis), crossing over can connect nonsister chromatids at chiasmata, and the spindle develops as the nuclear boundary breaks down. Metaphase I aligns homolog pairs; anaphase I separates homologs while sisters remain joined. Telophase I and cytokinesis partition the products; nuclear-envelope reformation varies among organisms. No new S phase intervenes.

In prophase II the spindle is established again. Metaphase II aligns individual duplicated chromosomes, with sister attachments facing opposite poles. Anaphase II releases sister cohesion and separates sisters. Telophase II reforms nuclei and decondenses chromosomes; cytokinesis uses an animal cleavage furrow or plant cell plate. The standard symmetric model produces four haploid products, not four universally equivalent gametes. OpenStax meiosis mechanism, checked 31 October 2026.

Changed evidence: A drawing shows paired homologs at the equator, then duplicated chromosomes moving apart. Is it metaphase/anaphase I or II? What changes if single former sisters move apart instead?

Check the stage inference

Paired homologs followed by intact duplicated chromosomes separating supports division I. Separation of former sisters supports division II in the stated meiotic sequence, but could also occur in mitosis without that context. A chromosome count alone cannot identify every stage; draw the pairing and attachments.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Stage per nucleus	Value (number)
Before meiosis I chromosomes	8
Before meiosis I chromatids	16
After meiosis I chromosomes	4
After meiosis I chromatids	8

A normal 2n=8 model with no nondisjunction.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Track homologs and chromatids through meiosis

Supplied synthetic evidence: A normal 2n=8 model with no nondisjunction.

Stage per nucleus: reference condition

Quantity	Value	Unit
Before meiosis I chromosomes	8	number
Before meiosis I chromatids	16	number
After meiosis I chromosomes	4	number
After meiosis I chromatids	8	number

Task: For 2n=12, draw one post-meiosis-I nucleus and state chromosomes, chromatids and ploidy.

No recombination rate or organism-specific gametogenesis is modeled.

Compare with the conditional interpretation

Set number halves at the first division although chromosomes remain duplicated.

Four chromosomes with two chromatids each after meiosis I gives eight chromatids but only one homologous set. Counting eight chromatids as eight chromosomes would erase the distinction between replication and segregation.

Case index for this investigation

0. Track homologs and chromatids through meiosis
1. Explain crossing over and assortment without mixing mechanisms
2. Derive cross probabilities and distinguish expectation from outcome
3. Use a testcross to estimate recombination and retain limits
4. Test an inheritance null with chi-square reasoning
5. Distinguish dominance, codominance and multiple alleles
6. Interpret chromosome-linked and cytoplasmic inheritance
7. Separate genotype, environment and their interaction

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

Four chromosomes with two chromatids each after meiosis I gives eight chromatids but only one homologous set. Counting eight chromatids as eight chromosomes would erase the distinction between replication and segregation.

Set number halves at the first division although chromosomes remain duplicated.

Apply to a new case

For 2n=12, draw one post-meiosis-I nucleus and state chromosomes, chromatids and ploidy.

Compare your reasoning with the worked response

It contains six duplicated chromosomes, twelve chromatids and one set, so it is haploid. Label homolog separation before drawing sister separation.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

No recombination rate or organism-specific gametogenesis is modeled. Totals across products must not be confused with ploidy of one product.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 5.1. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.