Cell Division and Cycle

Cell Division and Cycle

Middle School

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Learning Objectives

• Define the cell cycle and describe its main purposes and phases.

• Explain how DNA is organized for cell division in eukaryotic cells.

• Describe the stages of mitosis and compare cytokinesis in plant and animal cells.

• Describe how the cell cycle is regulated and what happens when it fails.

• Compare the cell cycle in eukaryotic and prokaryotic cells.

Key Vocabulary

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Cell Cycle

A repeating series of events in a cell's life that includes growth, DNA synthesis, and division.

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Chromosome

An X-shaped structure of tightly packaged DNA that helps transport genetic material during cell division.

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Interphase

The longest phase of the cell cycle where a cell grows, functions, and copies its DNA.

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Mitosis

The process where a cell nucleus divides, resulting in two identical nuclei with the same genetic material.

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Cytokinesis

The final step in the cell cycle where the cytoplasm divides to form two new daughter cells.

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Cancer

A disease caused by an unregulated cell cycle, leading to uncontrolled cell division and tumor formation.

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The Cell Cycle

• The cell cycle is the life of a cell, including growth and division.

• Cells divide for growth, to repair damage, and to replace old cells.

• The cycle has two main parts: Interphase and the Mitotic Phase.

• This process results in two new, genetically identical daughter cells.

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Multiple Choice

What are the two main parts of the eukaryotic cell cycle mentioned on this slide?

• Prophase and Metaphase

• DNA Synthesis and Cell Division

• Interphase and the Mitotic Phase

• Growth and Repair

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DNA Organization for Division

• Before division, DNA is uncoiled as chromatin, then condenses into X-shaped chromosomes.

• After copying, chromosomes consist of two identical parts called sister chromatids.

• These identical sister chromatids are joined together at the centromere.

• Homologous chromosomes are pairs that have the same size, shape, and genes.

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Multiple Choice

What are the two identical copies of a replicated chromosome called, and where are they joined?

• Chromosomes, joined at the chromatin

• Sister chromatids, joined at the centromere

• Homologous chromosomes, joined at the centromere

• Chromatin, joined at the sister chromatid

The Three Sub-phases of Interphase

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Gap 1 (G₁) Phase

• The cell grows rapidly and performs its normal metabolic functions.

• It focuses on creating new proteins and other essential molecules.

• The cell also makes complete copies of its important organelles.

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Synthesis (S) Phase

• The cell's primary focus is to copy all of its DNA.

• This important process is also known as DNA replication.

• Each chromosome is duplicated, creating two identical sister chromatids.

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Gap 2 (G₂) Phase

• The cell continues to grow and make more necessary proteins.

• It produces the structures that are needed for cell division.

• This is the final preparation stage before the cell divides.

Multiple Choice

In which sub-phase of Interphase does the cell copy its DNA?

• G₀ Phase

• Synthesis Phase (S)

• Gap 2 (G₂)

• Gap 1 (G₁)

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The Stages of Mitosis

• Mitosis is when the nucleus divides, sorting DNA into two identical, complete sets.

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

• Metaphase: Guided by spindle fibers, chromosomes line up at the cell's center.

• Anaphase: Sister chromatids separate and are pulled by fibers to opposite poles.

• Telophase: Two new nuclear membranes form around the two sets of DNA.

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Multiple Choice

During which stage of mitosis do the sister chromatids separate and move to opposite poles of the cell?

• Anaphase

• Prophase

• Metaphase

• Telophase

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Cytokinesis: Splitting the Cell

Animal Cells

• ​The cell membrane pinches inward, creating a crease known as a cleavage furrow.

• ​​This furrow deepens until the cytoplasm and cell membrane are pinched into two.

• ​This process results in two separate daughter cells that are genetically identical.

Plant Cells

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

• ​​A new structure called a cell plate forms in the middle of the cell.

• ​This plate grows outward to form a new cell wall, dividing the cytoplasm.

Multiple Choice

How does cytokinesis differ between animal and plant cells?

• Plant cells' cell walls are flexible, while animal cells' membranes are rigid.

• Animal cells form a cleavage furrow, while plant cells form a cell plate.

• Animal cells divide their nucleus, while plant cells divide their cytoplasm.

• Plant cells form a cleavage furrow, while animal cells form a cell plate.

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Regulating the Cell Cycle

• The cell cycle is controlled by proteins like cyclins and CDKs.

• ​These proteins act at checkpoints, signaling the cell to proceed or pause.

• The G₁ checkpoint decides if the cell is ready to divide.

• Other checkpoints ensure that DNA is copied and chromosomes are aligned correctly.

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Multiple Choice

What is the main function of the Mitosis checkpoint?

• To check if the DNA has been replicated correctly

• To signal the cell to start growing

• To ensure chromosomes are properly aligned before anaphase

• To decide if the cell should divide

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What is Cancer?

• Cancer is a disease where the cell cycle is not regulated.

• ​This can happen if a cell’s DNA is damaged.

• Cancerous cells divide much faster than normal cells.

• This uncontrolled division can form a mass called a tumor.

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Multiple Choice

According to the text, what is the primary characteristic of cancerous cells?

• They cannot form tumors.

• They are smaller than normal cells.

• They divide much faster and are not regulated by the cell cycle checkpoints.

• They do not contain DNA.

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Prokaryotic Cell Division

• Prokaryotes have a simpler cell cycle called binary fission.

• ​The cell grows and its single circular DNA is replicated.

• The two DNA copies move to opposite ends of the cell.

• The cell splits in the middle, creating two identical cells.

Multiple Choice

What is the name of the process by which prokaryotic cells divide?

• Binary fission

• Mitosis

• Meiosis

• Cytokinesis

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Common Misconceptions About Mitosis

Misconception	Correction
Mitosis and cell division are the same thing.	Mitosis is only the division of the nucleus, a part of cell division.
Cells divide to get bigger.	Cells divide because they become less efficient when they get too large.
Chromosomes are always visible.	Chromosomes are visible only when condensed during prophase.
Mitosis results in one large and one small cell.	Mitosis creates two identical daughter cells of about the same size.

Multiple Choice

A cell is observed to be making additional proteins and organelles after its DNA has already been copied. Which phase of the cell cycle is it in?

• Gap 2 (G₂)

• Synthesis (S)

• Mitosis

• Gap 1 (G₁)

Multiple Choice

A cell in the G2 phase has:

• The same amount of DNA as in a G1 phase cell

• Half the amount of DNA as in a G1 phase cell

• Double the amount of DNA as in a G1 phase cell

• Half the number of chromosomes as in a G1 phase cell

Multiple Choice

If a chemical is developed that specifically prevents the formation of the cell plate, which type of cells would be most affected and at what stage?

• Prokaryotic cells during binary fission

• Plant cells during cytokinesis

• Eukaryotic cells during metaphase

• Animal cells during cytokinesis

Multiple Choice

What would be the most likely result for a cell if the metaphase checkpoint fails, causing it to move into anaphase too early?

• The cell would fail to replicate its DNA in the next S phase.

• Two nuclear membranes would form around a single set of chromosomes.

• The cell would be stuck in metaphase indefinitely.

• The daughter cells would likely receive an unequal and incorrect number of chromosomes.

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Summary

• The cell cycle is a process of growth and division.

• DNA condenses from chromatin into chromosomes for division.

• Interphase involves cell growth, DNA replication, and final preparation.

• Mitosis divides the nucleus in four stages, followed by cytokinesis.

• Cytokinesis splits the cytoplasm, creating two daughter cells.

• Checkpoints regulate the cycle, and their failure can lead to cancer.

Poll

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