AP Biology

## Learning Outcomes

- Describe the purpose of meiosis in producing haploid gametes from diploid cells.
- Identify the major events of prophase I, metaphase I, anaphase I, and telophase I.
- Identify the major events of prophase II, metaphase II, anaphase II, and telophase II.
- Explain why meiosis I is called reduction division and why meiosis II resembles mitosis.
- Track chromosome number and chromatid number using centromeres.
- Connect crossing over and independent assortment to genetic variation.
- Distinguish homologous chromosomes from sister chromatids in diagrams and exam questions.

## AP Biology Syllabus

In AP Biology Unit 5, Heredity, meiosis is tested as a mechanism for transmitting genetic information from one generation to the next. Unit 5 is typically about 8–11% of the AP Biology exam, and meiosis connects strongly to Mendelian genetics, chromosomal inheritance, nondisjunction, and genetic variation. You should be ready to explain stages, interpret diagrams, compare meiosis with mitosis, and justify claims using chromosome behavior.

- Explain how meiosis reduces chromosome number from diploid to haploid.
- Describe the events of meiosis I and meiosis II in correct sequence.
- Identify when homologous chromosomes separate and when sister chromatids separate.
- Explain how crossing over and independent assortment produce genetic variation.
- Use chromosome diagrams to determine whether a cell is diploid or haploid.
- Predict the effect of nondisjunction during meiosis I or meiosis II.
- Compare meiotic division with mitotic division in terms of products and chromosome behavior.

## 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. During which stage of meiosis do homologous chromosomes pair and exchange DNA?
2. Why does meiosis produce haploid cells while mitosis usually maintains diploid chromosome number?
3. What separates during anaphase I, and what separates during anaphase II?
4. If a replicated chromosome has two sister chromatids joined at one centromere, how many chromosomes is it counted as?
5. Why are the four products of meiosis usually genetically different from one another?

## Introduction

Meiosis is the cell division process that produces gametes, such as sperm and eggs in animals. Unlike mitosis, which produces two genetically identical diploid daughter cells, meiosis includes two rounds of division: meiosis I and meiosis II. The end result is four genetically different haploid cells.

This process matters because sexual reproduction combines gametes from two parents. If gametes were diploid, fertilization would double chromosome number in every generation. Meiosis prevents this by halving the chromosome number before fertilization. For AP Biology, you must understand not only the names of the stages, but also what happens to chromosomes during each stage.

> **Key Term: meiosis**
> A two-division cell process that produces four genetically different haploid cells from one diploid starting cell.

A simple overview is:

- DNA replicates once before meiosis begins.
- Meiosis I separates homologous chromosome pairs.
- Meiosis II separates sister chromatids.
- Four haploid cells are produced.

### Preparing for meiosis: chromosome sets and DNA replication

Before meiosis begins, the cell passes through interphase. During the S phase of interphase, DNA is replicated. This means each chromosome is copied, producing two sister chromatids joined at a centromere.

A common AP Biology challenge is keeping chromosome number and chromatid number separate. A replicated chromosome still counts as one chromosome if it has one centromere. After the sister chromatids separate, each chromatid becomes an individual chromosome because each has its own centromere.

> **Key Term: sister chromatids**
> Identical copies of a chromosome produced by DNA replication and joined together at a centromere.

For example, a human diploid body cell has 46 chromosomes before DNA replication. After DNA replication, it still has 46 chromosomes, but each chromosome consists of two sister chromatids. The number of DNA molecules has doubled, but the chromosome number has not doubled because chromosome number is counted by centromeres.

> **Test Tip:**
> When AP Biology asks for chromosome number, count centromeres, not chromatids. A duplicated chromosome with two sister chromatids joined at one centromere is still one chromosome.

Meiosis begins with a diploid cell. Diploid cells contain two sets of chromosomes, one set inherited from each parent. In humans, diploid cells have 46 chromosomes, arranged as 23 homologous pairs. Gametes are haploid, meaning they contain one chromosome from each homologous pair. Human gametes have 23 chromosomes.

> **Key Term: homologous chromosomes**
> A pair of chromosomes with the same genes at the same loci, one inherited from the mother and one from the father.

Homologous chromosomes are not identical. They carry the same genes in the same positions, but they may carry different alleles. For example, one chromosome may carry an allele for brown eyes while its homolog may carry an allele for blue eyes. This distinction becomes very important in meiosis I, when homologous chromosomes pair up and then separate.

### Meiosis I: separating homologous chromosome pairs

Meiosis I is the first meiotic division. It is often called reduction division because it reduces the chromosome number by half. A diploid cell becomes two haploid cells, although each chromosome is still duplicated at the end of meiosis I.

> **Key Term: reduction division**
> A cell division that reduces chromosome number by half, as homologous chromosomes separate during meiosis I.

Meiosis I has four main stages: prophase I, metaphase I, anaphase I, and telophase I.

During prophase I, chromosomes condense and become visible. The nuclear envelope breaks down, and the spindle begins to form. The event that makes prophase I very different from mitotic prophase is that homologous chromosomes pair with each other. This pairing is called synapsis. Each paired set of homologous chromosomes forms a structure called a tetrad because it contains four chromatids: two sister chromatids from one homolog and two sister chromatids from the other homolog.

During prophase I, crossing over can occur. In crossing over, nonsister chromatids of homologous chromosomes exchange corresponding segments of DNA. The visible points where chromatids cross are called chiasmata. This exchange produces recombinant chromosomes with new combinations of alleles.

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

Crossing over is a major source of genetic diversity. Imagine two homologous chromosomes carrying alleles A-B-C and a-b-c. After crossing over, chromatids might carry A-B-c or a-b-C, producing allele combinations not found on the original parental chromosomes.

During metaphase I, homologous chromosome pairs line up along the center of the cell. They do not line up as individual chromosomes in a single file, as they do in mitosis. Instead, each homologous pair lines up together. The orientation of each pair is random: the maternal homolog may face one pole or the other, and the paternal homolog faces the opposite pole.

This random orientation leads to independent assortment. In humans, with 23 homologous pairs, independent assortment alone can produce over 8 million possible chromosome combinations in gametes, even before considering crossing over.

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

During anaphase I, homologous chromosomes separate and move to opposite poles. Sister chromatids remain attached at their centromeres. This is a major difference between anaphase I and anaphase in mitosis. In mitosis, sister chromatids separate. In anaphase I of meiosis, homologous chromosomes separate.

> **Exam Warning:**
> Do not say that sister chromatids separate during anaphase I. Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II.

During telophase I, chromosomes reach opposite poles. New nuclei may form, depending on the organism or cell type. Cytokinesis divides the cytoplasm, producing two haploid cells. Each cell has one chromosome from each homologous pair, but those chromosomes are still duplicated, meaning each consists of two sister chromatids.

The result of meiosis I is two haploid cells that are genetically different from each other. They are haploid because each contains only one chromosome from each homologous pair.

### Meiosis II: separating sister chromatids

Meiosis II is the second meiotic division. It is similar to mitosis because sister chromatids separate. However, meiosis II is not preceded by another round of DNA replication. This detail is often tested. DNA replicates once before meiosis I, then the cell divides twice.

If DNA replicated again before meiosis II, the final products would not be haploid. The absence of DNA replication between meiosis I and meiosis II is one reason meiosis produces four haploid cells.

Meiosis II also has four stages: prophase II, metaphase II, anaphase II, and telophase II.

During prophase II, chromosomes condense again if they had relaxed after meiosis I. The nuclear envelope breaks down if it had re-formed, and a new spindle apparatus forms in each of the two haploid cells.

During metaphase II, chromosomes line up individually along the center of each cell. This resembles metaphase in mitosis. However, the cells entering meiosis II are haploid, not diploid. Each chromosome still consists of two sister chromatids.

During anaphase II, sister chromatids separate and move to opposite poles. Once separated, each chromatid is considered an individual chromosome. This is the stage in meiosis when chromosome number temporarily increases within each dividing cell because each former chromatid now has its own centromere.

During telophase II, chromosomes reach opposite poles, nuclei form, and cytokinesis occurs. The final products are four haploid cells. In animals, these cells can develop into gametes. Each final cell contains one set of chromosomes.

> **Key Term: haploid**
> Having one complete set of chromosomes, symbolized as n.

The final products of meiosis are genetically different for several reasons:

- Crossing over creates recombinant chromatids.
- Independent assortment distributes maternal and paternal homologs in different combinations.
- Sister chromatids may no longer be identical after crossing over.
- Random fertilization later combines gametes from two individuals.

For AP Biology, be able to explain variation using chromosome behavior, not just by saying “meiosis creates diversity.” A stronger answer identifies the stage and mechanism.

> **Test Tip:**
> If a free-response question asks how meiosis increases genetic variation, name the process and stage: crossing over in prophase I and independent assortment in metaphase I.

### Tracking chromosomes, variation, and common errors

Chromosome tracking is a frequent source of errors in meiosis questions. Use three questions whenever you see a diagram:

1. Are homologous chromosomes paired, separated, or absent as pairs?
2. Are sister chromatids still joined at centromeres?
3. Is the cell diploid or haploid?

A diploid cell has both members of each homologous pair. A haploid cell has only one chromosome from each pair. A cell can be haploid even if its chromosomes are duplicated. This happens after meiosis I: each cell has one chromosome from each homologous pair, but each chromosome still has two sister chromatids.

Consider a simplified organism with a diploid number of 4, written as 2n = 4. This means the cell has two homologous pairs. Before DNA replication, the cell has 4 chromosomes. After DNA replication, it still has 4 chromosomes, but 8 chromatids. After meiosis I, each daughter cell has 2 chromosomes, each still duplicated. After meiosis II, each final haploid cell has 2 chromosomes, each no longer duplicated.

This example shows why meiosis I changes ploidy but meiosis II does not. Meiosis I separates homologous pairs, reducing diploid to haploid. Meiosis II separates chromatids, so the cells remain haploid.

Nondisjunction is another exam-relevant meiosis error. It occurs when chromosomes or chromatids fail to separate correctly. If nondisjunction occurs in meiosis I, homologous chromosomes fail to separate. This can produce gametes with an extra chromosome and gametes missing a chromosome. If nondisjunction occurs in meiosis II, sister chromatids fail to separate. Some gametes may be normal, while others may have too many or too few chromosomes.

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

Fertilization involving an abnormal gamete can produce aneuploidy, an abnormal chromosome number. For example, a zygote may have three copies of a chromosome instead of two. AP Biology questions may ask you to infer whether nondisjunction happened in meiosis I or meiosis II based on the chromosome contents of gametes.

> **Exam Warning:**
> Do not assume all meiotic errors happen in meiosis I. Errors can occur in meiosis I when homologous chromosomes fail to separate, or in meiosis II when sister chromatids fail to separate.

Another common mistake is confusing homologous chromosomes with sister chromatids. Homologous chromosomes are two separate chromosomes, one from each parent, that carry the same genes but may have different alleles. Sister chromatids are identical copies produced by DNA replication. Homologs separate during meiosis I. Sister chromatids separate during meiosis II.

You should also avoid describing meiosis II as “the same as mitosis” without qualification. Meiosis II is similar to mitosis because sister chromatids separate, but it occurs in haploid cells and is not preceded by DNA replication. Mitosis usually begins with a diploid cell in growth and repair contexts and produces two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

AP questions often present unfamiliar organisms with different chromosome numbers. Do not rely on human chromosome numbers unless the question gives them. Instead, apply the same logic:

- If 2n = 8, a diploid cell has 8 chromosomes and a haploid gamete has 4.
- If n = 6, a diploid cell has 12 chromosomes.
- After DNA replication, chromosome number is unchanged, but chromatid number doubles.
- After meiosis I, cells are haploid but chromosomes are still duplicated.
- After meiosis II, cells are haploid and chromosomes are unduplicated.

Strong exam answers use precise verbs: homologous chromosomes pair, align, and separate in meiosis I; sister chromatids align and separate in meiosis II. If you can describe the movement of chromosomes through each stage, you can answer most meiosis stage questions even when the diagram looks unfamiliar.

## Key Point Checklist

This article has covered the following key knowledge points:

- Meiosis produces four genetically different haploid cells from one diploid starting cell.
- DNA replicates once before meiosis begins, but not between meiosis I and meiosis II.
- Meiosis I is reduction division because homologous chromosome pairs separate.
- Prophase I includes homolog pairing, synapsis, tetrad formation, and crossing over.
- Metaphase I involves homologous pairs lining up at the cell center.
- Anaphase I separates homologous chromosomes while sister chromatids remain joined.
- Telophase I and cytokinesis produce two haploid cells with duplicated chromosomes.
- Meiosis II resembles mitosis because sister chromatids separate.
- Anaphase II separates sister chromatids, which then count as individual chromosomes.
- Chromosome number is counted by centromeres, not by the number of chromatids.
- Crossing over and independent assortment are major sources of genetic variation.
- Nondisjunction can occur in meiosis I or meiosis II and can produce abnormal chromosome numbers.

## Key Terms and Concepts

- meiosis
- sister chromatids
- homologous chromosomes
- reduction division
- crossing over
- independent assortment
- haploid
- nondisjunction


Meiosis - Meiosis I and II stages