AP Biology

## Learning Outcomes

- Define cytokinesis and explain how it differs from mitosis.
- Describe how cytokinesis divides the cytoplasm and cellular contents between daughter cells.
- Compare cytokinesis in animal cells and plant cells.
- Identify cleavage furrows and cell plates in diagrams or descriptions.
- Explain why cytokinesis must follow accurate chromosome separation.
- Avoid common AP Biology mistakes involving mitosis, interphase, apoptosis, and cytokinesis.

## AP Biology Syllabus

In AP Biology Unit 4, cytokinesis is tested as part of the cell cycle. You are expected to understand where cytokinesis fits relative to interphase and mitosis, what structures form in animal and plant cells, and how cytokinesis contributes to producing two daughter cells after DNA has already been replicated and chromosomes have been separated.

- Cytokinesis is the division of the cytoplasm and its contents.
- It occurs after nuclear division by mitosis and is associated with the end of the M phase.
- Animal cells form a cleavage furrow during cytokinesis.
- Plant cells form a cell plate that contributes new cell wall material.
- Cytokinesis is not the same process as DNA replication, chromosome alignment, or chromosome separation.
- In exam questions, cleavage furrow and cell plate are strong clues that the answer is cytokinesis.
- Cytokinesis helps produce two separate daughter cells after the genome has been distributed.

## 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 exactly is divided during cytokinesis?
2. Why do animal cells form a cleavage furrow, while plant cells form a cell plate?
3. How is cytokinesis different from mitosis?
4. If a cell completes mitosis but fails to complete cytokinesis, what unusual cell structure might result?
5. In an AP Biology multiple-choice question, why would “cleavage furrow” usually point to cytokinesis rather than telophase?

## Introduction

Cytokinesis is the final physical separation step of cell division. Before cytokinesis, a eukaryotic cell has already grown, replicated its DNA during S phase, and separated duplicated chromosomes during mitosis. Cytokinesis then divides the cytoplasm and cellular contents so that two daughter cells form.

For AP Biology, the most common challenge is not memorizing every molecular detail. The key is to separate the jobs of different parts of the cell cycle. Interphase prepares and replicates DNA. Mitosis distributes the genetic material into two nuclei. Cytokinesis divides the cytoplasm and produces two separate cells.

> **Key Term: cytokinesis**
> The division of the cytoplasm and cellular contents between two daughter cells after nuclear division.

### Cytokinesis completes cell division after mitosis

The cell cycle is usually described as a sequence of interphase, mitosis, and cytokinesis. During interphase, the cell spends most of its time growing, carrying out normal functions, and preparing for division. Interphase includes G1, S, and G2. DNA replication occurs during S phase, so each chromosome is copied before mitosis begins.

Mitosis then distributes the replicated chromosomes. In prophase, chromosomes condense and spindle fibers form. In metaphase, chromosomes align at the metaphase plate. In anaphase, sister chromatids separate and move to opposite poles. In telophase, new nuclear membranes form around the separated chromosome sets.

Cytokinesis follows this nuclear division. Its role is different: it divides the cytoplasm rather than the chromosomes. The cytoplasm includes cytosol, organelles, enzymes, structural proteins, vesicles, ribosomes, and many other cellular contents. Each daughter cell must receive enough cellular material to survive and continue normal functions.

> **Key Term: cytoplasm**
> The region of the cell inside the plasma membrane but outside the nucleus, including the cytosol and cellular structures suspended in it.

A useful way to remember the difference is:

- S phase copies DNA.
- Mitosis separates genetic material into daughter nuclei.
- Cytokinesis separates the cell body into daughter cells.

This distinction appears often in AP Biology questions. If a question asks which process divides cytosol or cytoplasmic contents, the answer is cytokinesis. If it asks which process separates replicated chromosomes into two nuclei, the answer is mitosis. If it asks when DNA replication occurs, the answer is S phase of interphase.

> **Test Tip:**
> On AP Biology questions, do not choose “mitosis” when the question asks about division of the cytoplasm. Mitosis distributes chromosomes; cytokinesis divides cytoplasm.

Cytokinesis usually occurs after mitosis and is closely associated with telophase. Some descriptions show cytokinesis beginning while telophase is finishing, especially in animal cells as the cell membrane starts to pinch inward. For exam purposes, the safest phrasing is that cytokinesis occurs after the chromosomes have been separated and new nuclei are forming or have formed.

Failure of cytokinesis can produce cells with more than one nucleus. For example, if mitosis occurs normally but the cytoplasm does not divide, the result may be one larger cell containing two nuclei. This shows why cytokinesis is not simply a minor detail: without it, chromosome separation would not lead to two physically separate daughter cells.

### Animal cell cytokinesis: the cleavage furrow

Animal cells do not have rigid cell walls, so their plasma membrane can be pulled inward during cytokinesis. The visible indentation that forms is called the cleavage furrow. This furrow deepens until the cytoplasm is divided and two daughter cells separate.

> **Key Term: cleavage furrow**
> The indentation that forms in an animal cell during cytokinesis as the cell membrane pinches inward.

The cleavage furrow forms around the cell roughly where the metaphase plate had been. This placement matters because the two chromosome sets have moved to opposite ends of the cell during anaphase. Dividing through the middle helps place one nucleus in each daughter cell.

At a molecular level, the cleavage furrow is associated with a contractile ring made largely of actin and myosin proteins. These proteins interact to tighten the ring beneath the plasma membrane. As the ring contracts, the membrane is drawn inward. You do not usually need to name every protein in the ring for AP Biology, but knowing that the membrane pinches inward can help you interpret diagrams.

A common visual sequence in animal cells is:

1. Chromosomes separate during anaphase.
2. Two nuclei begin forming during telophase.
3. A shallow groove appears at the cell surface.
4. The groove deepens into a cleavage furrow.
5. The cytoplasm is split into two daughter cells.

In diagrams, the cleavage furrow may look like a waist forming in the middle of the cell. If the cell looks like it is being squeezed into two lobes, you are probably seeing animal cell cytokinesis.

> **Exam Warning:**
> A cleavage furrow is not a chromosome structure. It forms at the cell surface during cytokinesis, not at the centromere during chromatid separation.

This warning matters because many students mix together the idea of “splitting” chromosomes and “splitting” the whole cell. During anaphase, sister chromatids separate at their centromeres and move apart. During cytokinesis, the entire cell divides its cytoplasm. These are linked events, but they are not the same process.

Another common mistake is to think cytokinesis creates genetic differences between daughter cells. In mitotic cell division, the goal is to produce genetically identical daughter cells. Cytokinesis does not reshuffle genes or reduce chromosome number. It simply separates the cellular material after the chromosomes have already been sorted.

### Plant cell cytokinesis: the cell plate

Plant cells carry out cytokinesis differently because they have rigid cell walls outside their plasma membranes. A plant cell cannot simply pinch inward like an animal cell. Instead, plant cells build a new partition inside the dividing cell. This structure is called the cell plate.

> **Key Term: cell plate**
> A structure that forms in a dividing plant cell during cytokinesis and provides material for the new cell wall between daughter cells.

During plant cell cytokinesis, vesicles carrying cell wall material move toward the center of the cell. These vesicles fuse to form the cell plate. The cell plate expands outward until it reaches the existing cell membrane and cell wall. It then helps form the new cell walls that separate the two daughter cells.

The key comparison is simple:

- Animal cells pinch inward by forming a cleavage furrow.
- Plant cells build outward from the center by forming a cell plate.

This difference is high-yield for AP Biology because it connects cell structure to cell function. Plant cells have cell walls, so they need a different cytokinesis method from animal cells. A rigid wall resists the inward pinching mechanism used by animal cells.

> **Key Term: cell wall**
> A rigid extracellular structure found in plant cells that provides support and affects how plant cells divide.

If you see a diagram of a rectangular cell with a line or plate forming across its center, that is likely a plant cell undergoing cytokinesis. If you see a rounder cell pinching in from the sides, that is likely an animal cell undergoing cytokinesis.

> **Test Tip:**
> In image-based questions, identify the cell type first. A cell plate suggests a plant cell; a cleavage furrow suggests an animal cell.

Plant cytokinesis also explains why the new boundary between daughter plant cells contains wall material. The cell plate is not just a temporary line. It contributes to the new cell wall and separates the two daughter cells into distinct compartments.

Do not confuse the cell plate with the metaphase plate. These names sound similar but refer to different things.

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

The metaphase plate is not a physical wall or membrane. It is a location where chromosomes align during mitosis. The cell plate is a real structure formed during plant cytokinesis. If the question mentions chromosomes lined up at the cell equator, think metaphase. If it mentions new cell wall material forming between daughter cells, think cytokinesis.

### Cytokinesis in AP Biology data and exam questions

AP Biology often tests cytokinesis through applied questions rather than simple definitions. You may be asked to interpret a diagram, classify a cell cycle stage, or distinguish among similar answer choices such as apoptosis, mitosis, DNA replication, and cytokinesis.

A classic multiple-choice pattern is:

> **Question:**
> During which process are the cytoplasm and its cellular contents divided between daughter cells?

The answer is cytokinesis. Apoptosis is programmed cell death. DNA replication occurs during S phase. Mitosis distributes genetic material into daughter nuclei. Cytokinesis divides the cytoplasm.

Another common pattern is:

> **Question:**
> During which phase of the cell cycle is a cleavage furrow or cell plate formed?

The answer is cytokinesis. The words “cleavage furrow” and “cell plate” are direct clues.

When answering free-response questions, be precise. If asked to describe cytokinesis, do not write only “the cell divides.” A stronger answer says that the cytoplasm and cellular contents are divided between two daughter cells. If asked to compare plant and animal cytokinesis, name both the cleavage furrow and the cell plate and connect the difference to the plant cell wall.

A strong AP-style comparison might be:

“Animal cells form a cleavage furrow as the plasma membrane pinches inward, while plant cells form a cell plate that supplies new cell wall material between the daughter cells.”

This answer earns more credit than simply saying “plants are different because they have cell walls.” The exam rewards clear cause-and-effect explanations.

Cytokinesis also connects to cell cycle regulation. Checkpoints help ensure that cells proceed through the cell cycle only when conditions are suitable. Cyclins and cyclin-dependent kinases help regulate progression. If chromosome separation fails, cytokinesis may produce daughter cells with abnormal genetic content, or regulatory pathways may pause the cycle before division continues. You do not need to know every checkpoint protein for this subtopic, but you should understand that cytokinesis is part of a regulated sequence.

> **Key Term: cell cycle**
> The ordered sequence of events by which a cell grows, replicates its DNA, and divides.
>
> **Key Term: mitosis**
> Nuclear division in which replicated chromosomes are distributed into two daughter nuclei.

Cytokinesis also differs from apoptosis. Apoptosis is programmed cell death and can be used during development, such as removal of tissue between developing fingers. Cytokinesis is not cell death; it is part of cell division. Both may occur during development, but they have opposite outcomes: apoptosis removes cells, while cytokinesis helps produce cells.

> **Key Term: apoptosis**
> Programmed cell death, a regulated process that removes cells during development or when damage cannot be repaired.

You may also see experimental questions involving chemicals that affect mitosis. For example, a drug that inhibits spindle fiber formation would interfere with chromosome movement during mitosis. That is not the same as directly blocking cytokinesis. However, because cell cycle events occur in sequence, disruption of mitosis can prevent normal completion of cell division. In a free-response answer, specify the direct target of the disruption, then explain the likely effect on later events.

For example:

- If spindle fibers fail to form, chromosomes may not separate correctly.
- If chromosomes do not separate correctly, the cell may not produce normal daughter nuclei.
- If normal daughter nuclei do not form, normal cytokinesis may be delayed, prevented, or produce abnormal cells.

This style of answer shows the cause, mechanism, and effect, which is useful across Unit 4.

> **Exam Warning:**
> Avoid saying that cytokinesis “replicates organelles.” Organelles and cellular materials are produced mainly during interphase; cytokinesis distributes cytoplasmic contents between daughter cells.

AP Biology may also ask you to calculate the percentage of cells in interphase or mitosis from a data table. Cytokinesis may be included in descriptions of the cell cycle, but many simplified datasets group cells into interphase and M phase. If cytokinesis is listed separately, treat it as its own category. If it is not listed separately, use only the categories provided and read the question carefully.

A simple cell-counting calculation uses:

$$\text{Percentage in a phase}=\frac{\text{number of cells in that phase}}{\text{total number of cells counted}}\times 100$$

If a table lists G1, S, G2, and M, then interphase includes G1 + S + G2. Mitosis or M phase should not be included in interphase. If the question asks specifically about cytokinesis and lists it as a separate row, use that row only.

A final exam skill is recognizing that cell division must keep chromosome number stable in mitotic daughter cells. Cytokinesis itself does not decide chromosome number, but it completes the production of two daughter cells after mitosis has distributed equal genetic information. In a normal mitotic division, each daughter cell receives the same genetic information as the parent cell.

## Key Point Checklist

This article has covered the following key knowledge points:

- Cytokinesis is the division of the cytoplasm and cellular contents between daughter cells.
- Cytokinesis occurs after nuclear division and is closely associated with the end of mitosis.
- Mitosis distributes replicated chromosomes into two daughter nuclei.
- Cytokinesis is not the same as DNA replication, chromosome alignment, or chromatid separation.
- Animal cells form a cleavage furrow during cytokinesis.
- The cleavage furrow pinches the animal cell membrane inward until two cells separate.
- Plant cells form a cell plate during cytokinesis.
- The cell plate provides material for the new cell wall between plant daughter cells.
- Plant cells use a cell plate because their rigid cell walls prevent animal-style pinching.
- A cleavage furrow or cell plate in an AP Biology question usually signals cytokinesis.
- The metaphase plate is where chromosomes line up; it is not the same as the plant cell plate.
- If mitosis occurs without cytokinesis, one cell may contain more than one nucleus.

## Key Terms and Concepts

- cytokinesis
- cytoplasm
- cleavage furrow
- cell plate
- cell wall
- metaphase plate
- cell cycle
- mitosis
- apoptosis


Cell cycle - Cytokinesis