Mitosis and Meiosis

Mitosis and Meiosis

Middle School

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Learning Objectives

• Describe the stages of the cell cycle, including interphase and the mitotic phase.

• Explain the four phases of mitosis and the process of cytokinesis.

• Detail the stages of Meiosis I and Meiosis II, including the process of crossing over.

• Compare and contrast the purposes, processes, and outcomes of mitosis and meiosis.

Key Vocabulary

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Mitosis

The process where one cell divides into two identical daughter cells with the same chromosome number.

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Meiosis

Cell division that reduces chromosome number by half, creating four unique gamete cells for sexual reproduction.

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Chromosome

A coiled structure of DNA and proteins that contains the genetic information of a cell.

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Cytokinesis

The final stage of cell division where the cell's cytoplasm divides to form two daughter cells.

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Homologous Chromosomes

A pair of chromosomes, one from each parent, that are the same size, shape, and contain the same genes.

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The Eukaryotic Cell Cycle

• The cell cycle has two main phases: interphase and the mitotic phase.

• Interphase is the longest stage, where the cell grows and prepares to divide.

• It includes Growth 1 (G₁), Synthesis (S) for DNA replication, and Growth 2 (G₂).

• A cell may also enter a non-dividing resting state called G₀.

Multiple Choice

During which phase of the cell cycle is the cell's DNA replicated?

• Growth Phase 1 (G1)

• Synthesis Phase (S)

• Growth Phase 2 (G2)

• Mitotic Phase (M)

DNA Packaging: Chromatin, Chromosomes, and Chromatids

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Chromatin

• This is the form DNA takes when the cell is not dividing.

• It consists of uncoiled DNA wrapped around proteins called histones.

• It appears like grainy material inside the cell’s nucleus.

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Chromosome

• After DNA replicates, chromatin condenses into this coiled structure.

• This condensed form is easily transportable during cell division.

• Each chromosome consists of two identical copies of the DNA.

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Sister Chromatids

• These are the two identical copies of a replicated chromosome.

• They are joined together at a region called the centromere.

• The two copies separate from each other during cell division.

Multiple Choice

What are the two identical copies of a replicated chromosome that are joined at a centromere called?

• Chromatin

• Homologous Chromosomes

• Sister Chromatids

• Histones

The Four Phases of Mitosis

• Chromosomes become visible, and the nuclear membrane breaks down.

• Chromosomes align at the center and are then pulled apart.

• Two new nuclei form around the separated chromosomes.

Multiple Choice

In which phase of mitosis do the sister chromatids separate and move to opposite poles of the cell?

• Prophase

• Metaphase

• Anaphase

• Telophase

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Cytokinesis: Dividing the Cytoplasm

Animal Cells

• The cell membrane pinches inward, forming a groove called a cleavage furrow.

• This furrow deepens until the cytoplasm is completely divided into two parts.

• This process, known as cleavage, results in two separate daughter cells.

Plant Cells

• A rigid cell wall prevents the cell membrane from pinching inward.

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

• The cell plate grows outward, developing into a new cell wall to separate the cells.

Multiple Choice

What structure forms during cytokinesis in plant cells but not in animal cells?

• Cleavage furrow

• Cell plate

• Spindle fibers

• Centromere

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The First Stage of Meiosis: Meiosis I

• In Prophase I, homologous chromosomes pair up and exchange genetic material (crossing over).

• During Metaphase I, these chromosome pairs line up at the middle of the cell.

• In Anaphase I, homologous chromosomes are pulled to opposite poles of the cell.

• In Telophase I, the cell divides to form two unique haploid daughter cells.

Multiple Choice

What is the key event in Prophase I that leads to genetic variation?

• DNA replication

• Crossing over

• Formation of a cell plate

• Separation of sister chromatids

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The Four Phases of Meiosis II

• In Prophase II, a new spindle forms in each of the two cells.

• During Metaphase II, chromosomes line up in the middle of each cell.

• In Anaphase II, the sister chromatids are pulled apart to opposite poles.

• Finally, four unique haploid daughter cells are formed through cytokinesis.

Multiple Choice

What is the final outcome of Meiosis II?

• Two diploid daughter cells

• Two haploid daughter cells

• Four diploid daughter cells

• Four haploid daughter cells

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Comparing Mitosis and Meiosis

Mitosis

• Its purpose is for the growth of an organism, to repair tissues, and for asexual reproduction.

• Occurring in somatic (body) cells, it involves only one round of cell division.

• The result is two diploid (2n) cells that are genetically identical to the parent cell.

Meiosis

• Its purpose is to produce gametes (sperm and egg) for the process of sexual reproduction.

• Occurring in germ (reproductive) cells, it involves two full rounds of cell division.

• The result is four haploid (n) cells that are genetically unique from the parent cell.

Multiple Choice

Which statement accurately describes a key difference between mitosis and meiosis?

• Mitosis produces four unique cells, while meiosis produces two identical cells.

• Mitosis occurs in germ cells, while meiosis occurs in somatic cells.

• Mitosis involves one cell division, while meiosis involves two cell divisions.

• Mitosis produces haploid cells, while meiosis produces diploid cells.

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Common Misconceptions

Misconception	Correction
Mitosis and meiosis are the same process.	Mitosis produces two identical cells, while meiosis produces four unique haploid cells.
Cells are always in the process of dividing.	Cells spend most of their time in interphase, a period of growth.
Chromosomes are always condensed and visible.	Chromosomes are only visible during cell division; otherwise, they exist as chromatin.
Meiosis produces two daughter cells.	Meiosis involves two divisions, resulting in four unique haploid daughter cells.

Multiple Choice

How does the process of mitosis ensure that new cells for growth and repair are identical to the parent cells?

• By undergoing two rounds of division to reduce errors.

• By accurately replicating the DNA in the S phase and then separating the identical sister chromatids.

• By allowing crossing over to swap genetic information.

• By producing haploid cells with half the DNA.

Multiple Choice

Why is the reduction in chromosome number during meiosis essential for sexual reproduction?

• To make the gametes smaller and faster.

• To ensure that when a sperm and egg fuse, the resulting offspring has the correct diploid number of chromosomes.

• To make all offspring genetically identical to the parents.

• To eliminate the need for DNA replication.

Multiple Choice

Predict what would happen if the spindle fibers failed to properly pull the sister chromatids apart during anaphase of mitosis.

• The cell would divide into three daughter cells.

• One daughter cell would have an extra chromosome, and the other would be missing one.

• The daughter cells would be genetically unique.

• The cell would immediately enter Meiosis II.

Multiple Choice

How does crossing over in Prophase I contribute to the genetic variation seen in sexually reproducing organisms?

• It doubles the number of chromosomes in the cell.

• It prevents the sister chromatids from separating.

• It ensures the daughter cells are identical to the parent cell.

• It creates new combinations of genes on chromosomes by exchanging segments between homologous pairs.

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Summary

• The cell cycle includes interphase for growth and the mitotic phase for division.

• Mitosis produces two identical diploid (2n) somatic cells in one round of division.

• Meiosis produces four unique haploid (n) gametes through two rounds of division.

• Crossing over during meiosis is a key source of genetic diversity.

Poll

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