AP Biology

## Learning Outcomes

- Distinguish homologous chromosomes from sister chromatids using structure, origin, and genetic content.
- Explain how homologous chromosomes behave during meiosis I.
- Explain how sister chromatids behave during meiosis II.
- Count chromosomes and chromatids before and after DNA replication and during meiotic stages.
- Describe how synapsis, crossing over, and independent assortment increase genetic variation.
- Predict the chromosome-number effects of nondisjunction in meiosis I or meiosis II.
- Avoid common AP Biology errors in diagrams of metaphase I, anaphase I, and anaphase II.

## AP Biology Syllabus

In AP Biology Unit 5, Heredity, meiosis is tested as part of the transmission of genetic information from parents to offspring. Unit 5 is commonly assessed at about 8–11% of the AP Biology exam, and meiosis questions often appear through diagrams, chromosome-counting tasks, inheritance patterns, recombination data, or nondisjunction scenarios.

- Explain how meiosis produces haploid gametes from diploid cells.
- Identify homologous chromosome pairs and sister chromatids in diagrams.
- Connect chromosome behavior in meiosis to Mendel’s laws of segregation and independent assortment.
- Describe synapsis and crossing over during prophase I.
- Distinguish meiosis I, which separates homologous chromosomes, from meiosis II, which separates sister chromatids.
- Predict chromosome numbers in cells after meiosis I and meiosis II.
- Explain how errors in chromosome separation can cause aneuploidy, including trisomy and monosomy.
- Use models and data to justify claims about genetic variation or abnormal chromosome number.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What is the difference between a homologous chromosome pair and a pair of sister chromatids?
2. During which meiotic division do homologous chromosomes separate, and during which division do sister chromatids separate?
3. If a diploid cell has $$2n = 6$$ chromosomes, how many chromosomes should each gamete have after normal meiosis?
4. Why does crossing over occur between homologous chromosomes rather than between sister chromatids?
5. How would nondisjunction in meiosis I differ from nondisjunction in meiosis II in its effect on gametes?

## Introduction

Meiosis is the type of cell division that produces gametes, such as sperm and eggs in animals. It begins with a diploid cell and, after two rounds of division, produces haploid cells. In sexual reproduction, this matters because two haploid gametes fuse during fertilization to restore the diploid chromosome number in the zygote.

To understand meiosis, you must be able to track two related but different structures: homologous chromosomes and sister chromatids. Many AP Biology questions test whether you know which structures separate at each meiotic stage. A simple rule is: homologous chromosomes separate in meiosis I, while sister chromatids separate in meiosis II.

> **Key Term: homologous chromosomes**
> A pair of chromosomes in a diploid cell that are similar in size and shape and carry the same genes at the same loci, with one chromosome inherited from each parent.

### Homologous Chromosomes: Matching Pairs with Different Origins

Most body cells in sexually reproducing eukaryotes are diploid, meaning they contain two sets of chromosomes. One set comes from the mother and one set comes from the father. For example, humans have 46 chromosomes in most somatic cells, arranged as 23 pairs. The haploid number is 23, so human gametes normally contain one chromosome from each pair.

A homologous pair consists of two versions of the same chromosome type. The two homologs carry the same genes in the same positions, but they may carry different alleles. For example, both homologs of a chromosome pair may carry a gene affecting pea seed shape at the same locus. One homolog might carry an allele for round seeds, while the other carries an allele for wrinkled seeds.

> **Key Term: allele**
> An alternative version of a gene found at a particular locus on a chromosome.

This is why homologous chromosomes are not usually genetically identical. They match in gene order, but not necessarily in allele sequence. One homolog may contain maternal allele combinations, and the other may contain paternal allele combinations. During meiosis, these homologs pair, exchange genetic material, and are sorted into different gametes.

> **Key Term: diploid**
> Describes a cell with two sets of chromosomes, one set from each parent; represented as $$2n$$.
>
> **Key Term: haploid**
> Describes a cell with one set of chromosomes; represented as $$n$$.

The symbol $$n$$ refers to the number of unique chromosome types in a species. If an organism has $$n = 4$$, then its diploid cells have $$2n = 8$$ chromosomes. A diploid cell has pairs; a haploid cell has one chromosome from each pair.

> **Test Tip:**
> When AP Biology asks for chromosome number, count centromeres, not DNA strands. A replicated chromosome with two sister chromatids still counts as one chromosome until the sister chromatids separate.

### Sister Chromatids: Replicated Copies Joined at a Centromere

Before meiosis begins, the cell goes through interphase. During S phase, DNA is replicated. After replication, each chromosome consists of two sister chromatids joined at a centromere. These sister chromatids are usually identical copies unless crossing over later changes chromatid DNA combinations.

> **Key Term: sister chromatids**
> The two DNA copies of a replicated chromosome, joined at the centromere until they separate during cell division.

A common exam mistake is to confuse a replicated chromosome with a homologous pair. A replicated chromosome has two sister chromatids joined together. A homologous pair has two separate chromosomes: one maternal homolog and one paternal homolog. After DNA replication, each homolog has two sister chromatids, so a homologous pair contains four chromatids in total.

This four-chromatid structure becomes especially clear in prophase I of meiosis, when homologous chromosomes pair during synapsis. The paired homologs form a tetrad.

> **Key Term: centromere**
> The chromosome region where sister chromatids are joined and where spindle fibers attach during cell division.
>
> **Key Term: tetrad**
> A paired set of homologous chromosomes during prophase I of meiosis, containing four chromatids in total.

Here is a useful example. Suppose a diploid cell has $$2n = 4$$. Before DNA replication, it has four chromosomes, each with one chromatid. After DNA replication, it still has four chromosomes because there are still four centromeres. However, it now has eight chromatids because each chromosome has two sister chromatids.

This distinction matters during anaphase. In anaphase I, homologous chromosomes separate, but sister chromatids remain attached. In anaphase II, sister chromatids separate. Once sister chromatids separate, each chromatid has its own centromere and is counted as a separate chromosome.

> **Exam Warning:**
> Do not say that sister chromatids separate during anaphase I. In meiosis I, homologous chromosomes separate while sister chromatids stay together. Sister chromatids separate in anaphase II.

### Meiosis I: Homologous Chromosomes Pair, Recombine, and Separate

Meiosis has two divisions: meiosis I and meiosis II. DNA replication occurs once before meiosis I, but there is no second round of DNA replication before meiosis II. This is why meiosis can reduce chromosome number and produce haploid cells.

Meiosis I is often called the reduction division because it reduces the chromosome number from diploid to haploid. It does this by separating homologous chromosomes into different cells.

During prophase I, homologous chromosomes pair through synapsis. This pairing allows crossing over, also called genetic recombination. Crossing over occurs between nonsister chromatids of homologous chromosomes. A maternal chromatid and a paternal chromatid can exchange corresponding DNA segments. The visible crossover points are called chiasmata.

> **Key Term: synapsis**
> The pairing of homologous chromosomes during prophase I of meiosis.
>
> **Key Term: crossing over**
> The exchange of corresponding DNA segments between nonsister chromatids of homologous chromosomes during prophase I.

Crossing over creates chromatids with new combinations of alleles. For example, a chromosome that began with mostly maternal alleles may gain a segment from the paternal homolog. This increases genetic variation among gametes.

During metaphase I, homologous pairs line up along the cell’s middle. They line up as pairs, not as single chromosomes. The orientation of each pair is random: the maternal homolog may face one pole or the other. Each homologous pair orients independently of the other pairs. This is the chromosomal basis of independent assortment.

> **Key Term: independent assortment**
> The random orientation and separation of homologous chromosome pairs during meiosis I, producing different chromosome combinations in gametes.

For an organism with $$n$$ homologous pairs, independent assortment alone can produce:

$$
2^n
$$

different maternal/paternal chromosome combinations in gametes, before even considering crossing over. In humans, $$n = 23$$, so independent assortment can produce over 8 million combinations. Crossing over adds far more possible allele combinations.

During anaphase I, homologous chromosomes separate and move to opposite poles. Sister chromatids remain joined at their centromeres. By the end of meiosis I, each new cell has one chromosome from each homologous pair. The cells are haploid because they have only one set of chromosomes, but each chromosome is still duplicated.

During telophase I and cytokinesis, two haploid cells form. Each chromosome still consists of two sister chromatids. This is a key stage for AP questions: after meiosis I, the cells are haploid, but the chromosomes are not yet single chromatids.

> **Test Tip:**
> If a question shows chromosomes lined up in homologous pairs at the cell center, it is metaphase I. If chromosomes line up single file in each haploid cell, it is metaphase II.

### Meiosis II: Sister Chromatids Separate to Form Haploid Gametes

Meiosis II resembles mitosis because individual chromosomes line up and sister chromatids separate. However, meiosis II begins with haploid cells, not diploid cells, and it is not preceded by another round of DNA replication.

In prophase II, chromosomes condense again if they had relaxed after meiosis I. In metaphase II, chromosomes line up single file along the metaphase plate. They are no longer arranged as homologous pairs because homologs were separated during meiosis I.

In anaphase II, sister chromatids separate at the centromere and move to opposite poles. After this separation, each former sister chromatid is considered its own chromosome. In telophase II and cytokinesis, the two cells from meiosis I divide again, producing four haploid cells.

For a diploid starting cell with $$2n = 6$$, the expected products of normal meiosis are four haploid cells with $$n = 3$$ chromosomes each. This type of chromosome-counting question appears often in multiple-choice and free-response formats.

Use this sequence to avoid errors:

- Before replication in a $$2n = 6$$ cell: 6 chromosomes, 6 chromatids.
- After replication: 6 chromosomes, 12 chromatids.
- After meiosis I: each cell has 3 chromosomes, 6 chromatids.
- After meiosis II: each gamete has 3 chromosomes, 3 chromatids.

The chromosome number changes when homologous chromosomes separate in meiosis I. The chromatid number changes when sister chromatids separate in meiosis II. The AP exam may ask you to describe both.

> **Key Term: meiosis I**
> The first meiotic division, in which homologous chromosomes separate and the chromosome number is reduced.
>
> **Key Term: meiosis II**
> The second meiotic division, in which sister chromatids separate to produce haploid cells with unduplicated chromosomes.

Meiosis and fertilization work together to keep chromosome number stable across generations:

$$
\text{gamete }(n) + \text{gamete }(n) = \text{zygote }(2n)
$$

Without meiosis, fertilization would double chromosome number in every generation. By halving the chromosome number in gametes, meiosis allows sexual reproduction while preserving the species’ typical chromosome count.

### Nondisjunction: When Chromosomes or Chromatids Fail to Separate

Errors in meiosis can change chromosome number. Nondisjunction occurs when chromosomes or chromatids fail to separate correctly. This can happen in meiosis I or meiosis II, and the resulting gametes may have one extra chromosome or one missing chromosome.

> **Key Term: nondisjunction**
> Failure of homologous chromosomes or sister chromatids to separate correctly during meiosis.

If nondisjunction occurs in anaphase I, homologous chromosomes fail to separate. One daughter cell receives both homologs of a pair, while the other receives neither. After meiosis II, all four gametes are affected: two gametes have an extra chromosome, and two gametes are missing that chromosome.

If nondisjunction occurs in anaphase II, meiosis I is normal, but sister chromatids fail to separate in one of the two cells during meiosis II. This usually produces two normal gametes, one gamete with an extra chromosome, and one gamete missing that chromosome.

> **Key Term: aneuploidy**
> An abnormal chromosome number caused by the gain or loss of one or more chromosomes.

If a gamete with an extra chromosome fuses with a normal haploid gamete, the zygote may have three copies of that chromosome. This condition is called trisomy. If a gamete missing a chromosome fuses with a normal gamete, the zygote may have only one copy of that chromosome. This condition is called monosomy.

> **Key Term: trisomy**
> A condition in which a cell has three copies of a particular chromosome instead of the usual two.
>
> **Key Term: monosomy**
> A condition in which a cell has one copy of a particular chromosome instead of the usual two.

Consider a diploid cell with $$2n = 6$$. Normal gametes should have 3 chromosomes. If nondisjunction of one chromosome occurs during anaphase II, the final products may be two gametes with 3 chromosomes, one gamete with 4 chromosomes, and one gamete with 2 chromosomes. This is a classic AP Biology reasoning pattern.

> **Exam Warning:**
> Nondisjunction changes chromosome number. Crossing over and independent assortment change allele combinations, but they do not normally add or remove whole chromosomes.

AP questions may ask you to identify the stage at which an error occurred based on the gametes shown. If all gametes are abnormal, suspect meiosis I nondisjunction. If two gametes are normal and two are abnormal, suspect meiosis II nondisjunction.

Nondisjunction also connects to inheritance and data analysis. You may be asked to interpret a karyotype, explain why a zygote has an extra chromosome, or justify how an abnormal gamete formed. A strong response should name the failed separation event and identify whether homologous chromosomes or sister chromatids failed to separate.

## Key Point Checklist

This article has covered the following key knowledge points:

- Homologous chromosomes are a maternal and paternal pair carrying the same genes at the same loci.
- Sister chromatids are replicated DNA copies of one chromosome joined at a centromere.
- Diploid cells have two chromosome sets, while haploid cells have one chromosome set.
- DNA replicates once before meiosis I, but not before meiosis II.
- Synapsis occurs in prophase I when homologous chromosomes pair.
- A tetrad contains two homologous chromosomes and four chromatids.
- Crossing over occurs between nonsister chromatids of homologous chromosomes during prophase I.
- Homologous chromosome pairs line up in metaphase I and separate in anaphase I.
- Sister chromatids line up as individual chromosomes in metaphase II and separate in anaphase II.
- Count chromosomes by counting centromeres, not chromatids.
- Nondisjunction in meiosis I affects all four gametes, while nondisjunction in meiosis II often leaves two gametes normal.
- Aneuploidy includes trisomy and monosomy and results from abnormal chromosome separation.

## Key Terms and Concepts

- homologous chromosomes
- allele
- diploid
- haploid
- sister chromatids
- centromere
- tetrad
- synapsis
- crossing over
- independent assortment
- meiosis I
- meiosis II
- nondisjunction
- aneuploidy
- trisomy
- monosomy


Meiosis - Homologous chromosomes and sister chromatids