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Meiosis

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Learning Objectives

Contrast the number of chromosomes in body cells and in gametes.
Summarize the events of meiosis.
Contrast meiosis and mitosis.
Describe how alleles from different genes can be inherited together.

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Chromosome Number

Homologous: chromosomes with the same genes, one originally from each of the organism’s parents

chromosome number = 4

Diploid: containing both sets of homologous chromosomes; 2N

Haploid: containing only a single set of chromosomes; 1N

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Reviewing Mitosis

Prophase

Metaphase

Anaphase

Telophase

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Meiosis

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Prophase I

Centrioles

Spindle forming

Chromosomes

Nuclear envelope

Centromere

Tetrad

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Metaphase I

Pair of homologous chromosomes

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Anaphase I

Sister chromatids

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Telophase I

Nuclear envelopes reforming

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Prophase II

Explain that chromosomes—those strands of DNA and protein inside the cell nucleus—are the carriers of genes. The genes are located in specific positions on chromosomes. Remind students that almost every cell in their bodies has two copies of each chromosome, one from their fathers and one from their mothers. These chromosomes have the same genes; they are homologous. Humans have 23 pairs of chromosomes, or 46 total. The human chromosome number is 46. Ask students to identify the chromosome number for the organism this cell is from.

Click to reveal the chromosome number.

Discuss terminology from diploid/haploid.

Ask: Are most cells in our body haploid or diploid?

Answer: diploid

Ask: Before mitosis occurs, is the parent cell haploid or diploid? At the end of mitosis, are the daughter cells haploid or diploid?

Answer: diploid; diploid

Ask for a student to identify a single chromosome in the picture. Then ask the student to identify a pair of homologous chromosomes. The student should identify both large or both small chromosomes as a pair.

Ask: Why is each chromosome here duplicated?

Answer: The cell has just replicated its DNA so it can enter the process of cell division.

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Metaphase II and Anaphase II

Anaphase II

Metaphase II

Lead a discussion about the significance of a cell being haploid or diploid. Ask students to consider what kind of cells would need to be haploid and why. Guide them to conclude that gametes (eggs and sperm) need to be haploid so that, when fertilization occurs, the offspring do not end up with two full sets of chromosomes from each parent.

Point out that the process that produces sex cells (meiosis), therefore, needs to divide nuclei and shuffle chromosomes so that resulting cells have half the normal chromosome number.

Emphasize that before meiosis begins every chromosome is copied, so the cell has four copies of each chromosome. Have students identify those structures. Then use the figure to walk students through a brief overview of meiosis I. Emphasize here that the stages of meiosis are divided into two segments: meiosis I and meiosis II.

Encourage students to think, as they look more closely at each stage of meiosis, about how they are similar to and different from the stages of mitosis. Remind them to use their lesson worksheets to jot down ideas.

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Telophase II and Cytokinesis

The end product of meiosis is daughter cells that

have the normal chromosome number.

half

four

Review the structures shown in the Prophase I cell and in the close-up of crossing-over.

Have volunteers come to the board to point out the major structures of a nucleus in prophase in this order: centrioles, spindle forming, nuclear envelope, chromosomes, centromere.

Click to reveal the labeled structures one by one.

Explain that in prophase I of meiosis each replicated chromosome pairs with its corresponding homologous chromosome. This pairing forms a structure called a tetrad, which contains four chromatids.

Have a volunteer come to the board to point out a tetrad on the diagram.

Click to reveal a labeled tetrad.

Explain that, as the homologous chromosomes form tetrads, they undergo a process called crossing-over. First, the chromatids of the homologous chromosomes cross over one another. Then, the crossed sections of the chromatids—which contain alleles—are exchanged. Crossing-over, therefore, produces new combinations of alleles in the cell.

Ask: How does crossing-over affect the alleles of a chromosome?

Answer: During crossing-over, the alleles can be exchanged between chromatids of homologous chromosomes to produce new combinations of alleles.

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Summary of Meiosis

Explain that during metaphase I of meiosis paired homologous chromosomes line up across the center of the cell.

Ask for a volunteer to go to the board to point out a pair of homologous chromosomes.

Click to reveal label for one pair of homologous chromosomes.

Ask: How is metaphase I of meiosis similar to metaphase I of mitosis?

Answer: Chromosomes line up along the center of the cell.

Ask: What do the pink and yellow chromosomes and pieces of chromosomes represent in the diagram?

Answer: The yellow pieces are originally from one of the organism’s parents, and the red pieces are originally from the other parent.

Ask: How many chromatids are in this cell? Are the sister chromatids identical?

Answer: 8; they are not identical any longer, because crossing over exchanged alleles.

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Comparing Mitosis and Meiosis

prophase (I)

metaphase (I)

anaphase (I)

telophase (I)

2N = 4

N = 2

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Gene Linkage

Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

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Gene Maps