Cell Division

Cell Division

Middle School

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Learning Objectives

• Define the cell cycle and its main stages: Interphase, Mitosis, and Cytokinesis.

• Explain the primary reasons cells divide, including growth, repair, and replacement.

• Describe the structure of a chromosome and its role in cell division.

• Outline the four phases of mitosis (PMAT) and the key events in each.

• Compare and contrast cytokinesis in animal and plant cells.

Key Vocabulary

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Cell Cycle

The ordered set of steps a cell repeats as it grows and then divides into new cells.

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Interphase

The preparation stage where the cell grows, functions, and makes a copy of its DNA before dividing.

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Mitosis

The process of nucleus division, which ensures that each new daughter cell gets an identical copy of DNA.

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Cytokinesis

The final stage where the cell's cytoplasm splits, forming two separate and complete daughter cells.

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Chromosome

A structure made of tightly packaged DNA, which makes it easier to move around during cell division.

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Sister Chromatids

The two identical copies of a single chromosome that are joined together before the cell divides.

Key Vocabulary

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Centromere

The specific region where two sister chromatids are attached to one another.

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Daughter Cells

The two genetically identical cells that are the final result of cell division.

Why Do Cells Divide?

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Growth

• Multicellular organisms, like humans, start as a single cell.

• A series of cell divisions allows the organism to grow and develop.

• This process helps the organism to increase in overall size and complexity.

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Repair & Replacement

• When tissues are damaged, cells divide to produce new healing cells.

• This is how a cut on your skin is able to heal.

• Cell division also replaces old or worn-out cells in your body.

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Reproduction

• For single-celled organisms, this is their method of creating new organisms.

• The single cell divides to form two new, independent living organisms.

• This is a common form of reproduction for many simple lifeforms.

Multiple Choice

What is the main purpose of cell division in multicellular organisms like humans?

• To allow an organism to grow, develop, and repair tissues

• To create energy for the organism to use

• To turn a single cell into a new type of organism

• To absorb nutrients and remove waste from the body

Multiple Choice

How does the role of cell division in a single-celled organism differ from its role in a multicellular organism?

• Cell division is for reproduction in single-celled organisms, but for growth and repair in multicellular organisms.

• Cell division helps single-celled organisms grow, while it helps multicellular organisms heal.

• Cell division makes single-celled organisms more complex, but only replaces old cells in multicellular organisms.

• There is no difference; cell division serves the exact same purpose in all living things.

Multiple Choice

Based on the functions of cell division, what would most likely happen if a person got a deep cut, but their cells in that area lost the ability to divide?

• The cut would not heal because new skin cells could not be produced.

• The organism would immediately stop growing taller.

• The organism would create a new, independent organism.

• The damaged cells would simply grow larger to close the wound.

The Main Stages of the Cell Cycle

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1. Interphase

• ​This is the longest phase where the cell primarily grows.

• ​​It performs its normal functions while preparing to divide.

• ​The cell makes a complete and identical copy of its DNA.

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2. Mitosis

• ​In this stage, the cell's nucleus completes its division.

• ​​The duplicated DNA is sorted into two identical, complete sets.

• ​This ensures each new cell has the correct genetic material.

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3. Cytokinesis

• ​The cell's cytoplasm splits down the middle to fully divide.

• ​​This is the final step in the cell division process.

• ​Two new, genetically identical daughter cells are finally formed.

Multiple Choice

What is the main outcome of the cell cycle?

• To create two new, genetically identical daughter cells.

• To make the cell grow larger without dividing.

• To copy the cell's DNA and then stop.

• To divide the cell's nucleus but not the cytoplasm.

Multiple Choice

What is the relationship between the Interphase and Mitosis stages of the cell cycle?

• Interphase prepares the cell for division by copying DNA, which is then sorted during Mitosis.

• Mitosis copies the cell's DNA, which is then split apart during Interphase.

• Interphase is when the cytoplasm divides, while Mitosis is when the nucleus divides.

• Mitosis happens first to divide the nucleus, and then Interphase grows the new cells.

Multiple Choice

If a cell completes Interphase but fails to undergo Mitosis, what would be the most likely consequence for the daughter cells after Cytokinesis?

• The cell would not be able to copy its DNA.

• The cell would divide its cytoplasm but not its nucleus.

• The resulting cells would not have the correct genetic material.

• The cell would grow much larger than normal.

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DNA and Chromosomes

• DNA holds genetic instructions and packs into chromosomes for cell division.

• Before a cell divides, each chromosome makes an exact copy of itself.

• The original chromosome and its new copy are called sister chromatids.

• A centromere is a point that holds the two sister chromatids together.

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Multiple Choice

What is the primary function of a chromosome during cell division?

• To package DNA for cell division.

• To create genetic instructions.

• To hold two cells together.

• To produce energy for the cell.

Multiple Choice

What is the relationship between a chromosome and its sister chromatids?

• The sister chromatids are the original chromosome and its exact copy.

• They are two completely different and unrelated chromosomes.

• One chromatid contains DNA and the other contains the centromere.

• A chromosome is formed only after sister chromatids separate.

Multiple Choice

What would be the most likely result if a chromosome makes a copy of itself, but the centromere fails to form?

• The two sister chromatids would not be held together.

• The chromosome would be unable to make a copy of itself.

• The DNA would lose its genetic instructions.

• The cell would immediately divide without the DNA.

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The Four Stages of Mitosis (PMAT)

• Prophase: DNA condenses into X-shaped chromosomes, and the nuclear membrane breaks down.

• Metaphase: Chromosomes, guided by spindle fibers, line up in the cell’s middle.

• Anaphase: Sister chromatids separate and are pulled to opposite sides of the cell.

• Telophase: Two new nuclear membranes form around the uncoiling chromosomes at each end.

Multiple Choice

What is the correct order of the four stages of mitosis?

• Prophase, Metaphase, Anaphase, Telophase

• Telophase, Anaphase, Metaphase, Prophase

• Metaphase, Prophase, Telophase, Anaphase

• Anaphase, Telophase, Prophase, Metaphase

Multiple Choice

What is a key difference between what happens during prophase and telophase?

• The nuclear membrane breaks down in prophase and re-forms in telophase.

• Chromosomes line up in the middle during prophase and separate during telophase.

• The cell divides in prophase and chromosomes condense in telophase.

• Chromosomes uncoil in prophase and are pulled apart in telophase.

Multiple Choice

A chemical is added to a cell that prevents sister chromatids from separating. At which stage would mitosis be unable to proceed to the next step?

• Metaphase

• Prophase

• Telophase

• Anaphase

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Cytokinesis: Division of the Cytoplasm

Animal Cells

• The flexible cell membrane starts to pinch inward from the outside.

• This pinching creates a crease known as a cleavage furrow.

• The furrow deepens until the cell splits into two new daughter cells.

Plant Cells

• The rigid cell wall prevents the cell from pinching inward.

• Instead, a new structure called a cell plate forms in the middle.

• This plate grows to form a new cell wall, separating the two cells.

Multiple Choice

What is the primary purpose of cytokinesis?

• The division of the cytoplasm into two new cells.

• The duplication of the genetic material.

• The growth of the cell before division.

• The formation of a protective outer layer.

Multiple Choice

How does the process of cytokinesis in animal cells differ from the process in plant cells?

• An animal cell pinches inward to form a cleavage furrow, while a plant cell builds a cell plate.

• An animal cell develops a cell wall, while a plant cell develops a cell membrane.

• An animal cell splits from the inside out, while a plant cell pinches from the outside in.

• An animal cell's membrane disappears, while a plant cell's wall gets thicker.

Multiple Choice

What is the best explanation for why plant cells must form a cell plate during cytokinesis instead of pinching in half?

• The plant cell has a rigid cell wall that prevents the cell from pinching inward.

• The animal cell has more cytoplasm that needs to be divided.

• The plant cell's nucleus is located in the center of the cell.

• The animal cell lacks a cell plate to create a new wall.

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Common Misconceptions

Misconception	Correction
Mitosis and cell division are the same thing.	Mitosis is just one part of the cell cycle (division of the nucleus).
Cells divide because they want to get bigger.	Cells divide because they become less efficient when too large.
Chromosomes are always visible inside the cell.	Chromosomes are only visible when they condense during mitosis.
Mitosis results in one large and one small cell.	Mitosis creates two genetically identical daughter cells of about the same size.

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Summary

• Cells divide for growth and repair, resulting in two identical daughter cells.

• The cell cycle includes Interphase, Mitosis, and Cytokinesis.

• Mitosis ensures each new cell gets an identical copy of DNA.

• Cytokinesis, the final step, is different in animal and plant cells.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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