AP Biology

## Learning Outcomes

- Describe the purpose of mitosis in growth, tissue repair, and asexual reproduction.
- Identify the main events of prophase, metaphase, anaphase, and telophase.
- Explain how sister chromatids, centromeres, spindle fibers, and the metaphase plate function during mitosis.
- Distinguish mitosis from interphase, DNA replication, and cytokinesis.
- Predict how disrupting spindle fiber formation or chromosome attachment affects mitotic progression.
- Apply chromosome-counting logic to cells before, during, and after mitosis.
- Interpret AP-style data about the number of cells in different cell-cycle phases.

## AP Biology Syllabus

In AP Biology Unit 4, mitosis is assessed as part of the cell cycle: an ordered process that allows eukaryotic cells to grow, replicate DNA, and divide. The exam usually asks you to explain events, interpret diagrams, analyze experimental data, or predict the effect of a disruption rather than simply recite phase names. You should know what happens to chromosomes and chromatids at each stage and how mitosis differs from cytokinesis.

- Unit 4, Cell Communication and Cell Cycle, includes the cell cycle and its regulation.
- The eukaryotic cell cycle proceeds through interphase, mitosis, and cytokinesis.
- Interphase includes G1, S, and G2; DNA replication occurs during S phase, not during mitosis.
- Mitosis distributes replicated genetic material into two daughter nuclei.
- The main mitotic stages are prophase, metaphase, anaphase, and telophase.
- Spindle fibers attach to chromosomes and help move chromatids to opposite poles.
- Cytokinesis divides the cytoplasm after nuclear division.
- AP questions often connect mitosis to checkpoints, cancer, cyclins, CDKs, or drugs that disrupt spindle fibers.

## 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 mitosis do chromosomes line up at the center of the cell?
2. Why does DNA replication have to occur before mitosis begins?
3. At what point do sister chromatids become separate chromosomes?
4. What is the difference between mitosis and cytokinesis?
5. A drug prevents spindle fibers from forming. Which mitotic events would be directly affected?

## Introduction

Mitosis is the part of the eukaryotic cell cycle that separates replicated chromosomes into two new nuclei. Its main purpose is accuracy: each daughter cell must receive a complete copy of the parent cell’s genome. In multicellular organisms, mitosis supports growth, tissue repair, and replacement of worn-out cells. In many single-celled eukaryotes, mitosis also allows asexual reproduction.

For AP Biology, the key is not just naming the stages. You need to connect each stage to what is happening to DNA, chromosomes, spindle fibers, and nuclei. You also need to avoid mixing up mitosis with interphase, where DNA is replicated, or cytokinesis, where the cytoplasm divides.

> **Key Term: mitosis**
> The process of nuclear division in eukaryotic cells that distributes replicated chromosomes into two genetically identical daughter nuclei.

### Preparing for mitosis during interphase

Before mitosis can begin, the cell must pass through interphase. Interphase is the longest part of the cell cycle for most dividing cells. Although it is sometimes described as the time between divisions, the cell is not inactive. It grows, carries out normal metabolism, duplicates organelles and cell structures, and replicates DNA.

> **Key Term: cell cycle**
> The ordered series of events in a cell’s life, including growth, DNA replication, mitosis, and cytokinesis.

Interphase has three main stages: G1, S, and G2. During G1, the cell grows and prepares for DNA replication. Some cellular structures, including centrioles in animal cells, may be duplicated. During S phase, DNA is replicated. This is the only stage in the cell cycle where chromosome replication occurs. During G2, the cell continues to grow and produces materials needed for mitosis, including proteins that will help form the mitotic spindle.

A common AP Biology trap is the relationship between chromosomes, chromatids, and DNA amount. At the start of S phase, each chromosome consists of one chromatid. After S phase, each chromosome consists of two identical sister chromatids joined at a centromere. The cell has twice as much DNA as it had before S phase, but it does not yet have twice as many chromosomes, because each duplicated chromosome is still counted as one chromosome while its sister chromatids remain attached.

> **Key Term: sister chromatids**
> Two identical copies of a replicated chromosome that remain attached to each other until they separate during mitosis.
>
> **Key Term: centromere**
> The region where sister chromatids are held together and where spindle-related structures attach during mitosis.

For example, suppose a diploid somatic cell has 46 chromosomes at the end of G1. After S phase, it still has 46 chromosomes, but each chromosome has two sister chromatids. The amount of DNA has doubled, but the chromosome number has not doubled yet. When sister chromatids separate during anaphase, each chromatid becomes an individual chromosome because it now has its own centromere and is no longer attached to its sister chromatid.

> **Test Tip:**
> If an AP question asks when DNA replication occurs, answer S phase of interphase. If it asks when chromosomes align, answer metaphase. If it asks when sister chromatids separate, answer anaphase.

### Prophase and metaphase: chromosome condensation and alignment

Mitosis begins after interphase, once DNA has been replicated and the cell has prepared the proteins and structures needed for division. The first stage is prophase. During prophase, chromatin condenses into visible chromosomes. Before this stage, the DNA is usually spread out in a less condensed form, which makes individual chromosomes difficult to see under a microscope. Condensation helps chromosomes move without becoming tangled or broken.

During prophase, the nuclear membrane dissolves. This allows spindle fibers to interact with the chromosomes. Spindle fibers begin to form and organize from opposite sides of the cell. In animal cells, centrioles help organize the spindle, though the main AP focus is usually the function of the spindle rather than the details of centriole structure.

> **Key Term: spindle fibers**
> Protein fibers that form the mitotic spindle and move chromosomes during mitosis.

Some textbooks divide late prophase into a separate stage called prometaphase, when spindle fibers attach to protein structures associated with centromeres. In many AP-level summaries, mitosis is described using four stages: prophase, metaphase, anaphase, and telophase. You should be able to recognize the event even if a diagram or question uses the term prometaphase.

After prophase comes metaphase. By metaphase, spindle fibers have attached to each chromosome at the centromere region. The chromosomes line up in a single row along the center of the cell. This central region is called the metaphase plate. It is not a physical plate like a cell wall; it is an imaginary plane through the center of the spindle.

> **Key Term: metaphase plate**
> The plane at the center of a dividing cell where chromosomes line up during metaphase.

Metaphase is highly testable because it is visually distinctive. In microscope images, metaphase cells show condensed chromosomes lined up across the middle. This alignment helps ensure that, when separation begins, one copy of each chromosome goes to each side of the cell.

The cell’s checkpoint systems help monitor whether chromosomes are properly attached before separation occurs. If spindle fibers are not correctly attached, the cell should not proceed into anaphase. This prevents unequal chromosome distribution. Errors in chromosome separation can lead to cells with abnormal chromosome numbers, which may harm development or cell function.

> **Exam Warning:**
> Do not say that chromosomes are “replicated” in prophase or metaphase. Replication has already occurred during S phase. Mitosis separates replicated chromosomes; it does not copy DNA.

A useful way to remember the first half of mitosis is: prophase prepares, metaphase organizes. In prophase, chromosomes condense, the nuclear membrane breaks down, and spindle fibers form. In metaphase, chromosomes align at the middle with spindle fibers attached.

### Anaphase and telophase: chromatid separation and nuclear rebuilding

Anaphase is the stage in which sister chromatids separate. Each duplicated chromosome splits at the centromere, and opposing spindle fibers shorten. The sister chromatids are pulled toward opposite poles of the cell. Once chromatids separate, each one is considered an individual chromosome.

> **Key Term: anaphase**
> The stage of mitosis in which sister chromatids separate and move toward opposite poles of the cell.

This is another major AP Biology counting point. At the start of mitosis in a human somatic cell, there are 46 duplicated chromosomes, each made of two sister chromatids. During anaphase, the sister chromatids separate. For a short time, the cell contains 92 chromosomes, because each separated chromatid is now its own chromosome. This does not mean DNA replication has occurred again; it only means the chromatids have separated and are now counted individually. After cytokinesis, each daughter cell will have 46 chromosomes.

The movement in anaphase depends on spindle fibers. If a drug prevents spindle fiber formation or prevents spindle shortening, chromatids may fail to separate properly. Colchicine is a classic example of a substance that interferes with spindle fiber formation. In an experiment, cells treated with a spindle inhibitor may show fewer cells successfully completing mitosis or may become arrested at a stage before proper chromatid separation.

> **Test Tip:**
> When a question describes a chemical that blocks spindle fibers, connect it to chromosome movement. The most direct effects involve attachment, alignment at metaphase, and separation during anaphase.

After anaphase comes telophase. During telophase, two new nuclear membranes form around the separated sets of chromosomes. Each new nucleus contains the same number of chromosomes and the same genetic information as the original parent cell. Chromosomes begin to decondense, returning toward a less compact chromatin form.

> **Key Term: telophase**
> The stage of mitosis in which new nuclear membranes form around the separated chromosome sets.

Telophase can be thought of as reversing many events of prophase. In prophase, chromosomes condense and the nuclear membrane dissolves. In telophase, chromosomes begin to relax and nuclear membranes reform. By the end of telophase, nuclear division is essentially complete, but the cell may still need to divide its cytoplasm.

Mitosis produces two genetically identical nuclei, not two fully separate cells by itself. The physical division into two cells requires cytokinesis, which usually overlaps with late telophase or follows mitosis.

### Cytokinesis and AP-style applications of mitotic stages

Cytokinesis is the division of the cytoplasm and its contents between the two daughter cells. It is closely linked to mitosis, but it is not the same process. Mitosis distributes genetic material into nuclei; cytokinesis divides the cytoplasm.

> **Key Term: cytokinesis**
> The division of the cytoplasm and cellular contents to form two daughter cells.

Animal and plant cells perform cytokinesis differently because plant cells have rigid cell walls. In animal cells, a cleavage furrow forms. This furrow pinches the cell membrane inward until the cytoplasm is divided. In plant cells, a cell plate forms in the middle of the dividing cell. The cell plate provides new cell wall material that separates the two daughter cells.

> **Key Term: cleavage furrow**
> An indentation that forms in the membrane of an animal cell during cytokinesis and helps divide the cytoplasm.
>
> **Key Term: cell plate**
> A structure that forms in the center of a dividing plant cell and develops into new cell wall material between daughter cells.

A frequent AP Biology question asks which process divides cytoplasm. The answer is cytokinesis, not mitosis. Another common question asks which process distributes genetic material. That answer is mitosis. Keep these roles separate.

Mitosis also connects to cell-cycle regulation. Before mitosis begins, the cell must have replicated DNA and prepared the machinery for chromosome separation. Cyclins and cyclin-dependent kinases help regulate progression through the cell cycle. Cyclin levels rise and fall, while cyclin-dependent kinases are present at more constant levels but are active only when bound to cyclins. A cyclin-CDK complex can help trigger mitosis by phosphorylating target proteins.

Checkpoints can stop the cell cycle if conditions are not suitable. For example, if DNA is damaged, the cell may pause to allow repair. If the damage is too severe, the cell may undergo apoptosis, or programmed cell death. Cancer cells often lose normal regulation, allowing them to divide when ordinary somatic cells would stop.

In exam questions, you may be given data showing numbers of cells in G1, S, G2, and M phase. To find the percentage of cells in interphase, add the cells in G1, S, and G2, then divide by the total number of cells. For example, if a sample contains 73 cells in G1, 89 in S, 68 in G2, and 43 in M, the interphase total is:

$$73 + 89 + 68 = 230$$

The total number of cells is:

$$73 + 89 + 68 + 43 = 273$$

So the percentage in interphase is approximately:

$$\frac{230}{273} \times 100 \approx 84\%$$

This matches the general idea that most noncancerous cells spend much more time in interphase than in mitosis.

> **Exam Warning:**
> Do not assume that the stage with the most cells is always mitosis because it looks more dramatic in diagrams. In real cell populations, interphase usually contains the largest fraction of cells because it lasts the longest.

AP free-response questions may also ask you to predict the effect of a treatment. For example, if lectin increases the number of cells in mitosis compared with a control group, you can describe it as stimulating cell division or increasing entry into mitosis. If colchicine reduces the number of cells completing mitosis, justify your claim by connecting colchicine to spindle fiber disruption and the failure of chromosomes to move correctly.

Strong AP answers use cause-and-effect language:

- “The drug prevents spindle fiber formation.”
- “Without spindle fibers, chromosomes cannot attach or separate normally.”
- “Therefore, cells are less likely to complete mitosis and may arrest before or during chromosome separation.”

You should also be ready to interpret microscope images. Condensed chromosomes appearing but not aligned suggest prophase. Chromosomes aligned in the middle suggest metaphase. Chromatids moving apart suggest anaphase. Two forming nuclei suggest telophase. A cleavage furrow or cell plate suggests cytokinesis.

Finally, remember the biological purpose of mitosis. The process maintains chromosome number from one generation of somatic cells to the next. It produces daughter cells that are genetically identical to the parent cell, assuming no mutation has occurred. That is why mitosis supports body growth and tissue repair: skin cells produce more skin cells, and other somatic cells produce cells of the same type.

## Key Point Checklist

This article has covered the following key knowledge points:

- Mitosis is nuclear division that distributes replicated chromosomes into two daughter nuclei.
- The main stages of mitosis are prophase, metaphase, anaphase, and telophase.
- DNA replication occurs during S phase of interphase, not during mitosis.
- After S phase, each replicated chromosome consists of two sister chromatids joined at a centromere.
- In prophase, chromosomes condense, the nuclear membrane dissolves, and spindle fibers begin to form.
- In metaphase, chromosomes line up at the metaphase plate with spindle fibers attached.
- In anaphase, sister chromatids separate and move toward opposite poles.
- Once sister chromatids separate, each chromatid is counted as an individual chromosome.
- In telophase, two nuclear membranes form around the separated chromosome sets.
- Cytokinesis divides the cytoplasm; mitosis divides the genetic material into nuclei.
- Animal cells form a cleavage furrow during cytokinesis, while plant cells form a cell plate.
- Spindle fiber disruption can prevent proper chromosome alignment or separation.

## Key Terms and Concepts

- mitosis
- cell cycle
- sister chromatids
- centromere
- spindle fibers
- metaphase plate
- anaphase
- telophase
- cytokinesis
- cleavage furrow
- cell plate


Cell cycle - Mitosis stages