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Meiosis Date __________

Diploid vs. Haploid • Diploid (2n): cells that contain TWO sets of

chromosomes

–All of our somatic cells are diploid – we have one set of chromosomes from EACH parent.

• Haploid (1n): cells that contain ONE set of chromosomes

–All of our gametes are haploid – they have half of the DNA a person should have

• You must join two halves (sperm and egg) to make a whole person!

Homologous Chromosomes • Two chromosomes containing the same TYPE of

genes

– i.e. both chromosomes have the eye color gene at the SAME SPOT, but one gene may be for blue eyes while the other is for brown eyes

• The paternal chromosome comes from dad

• The maternal chromosome comes from mom

• Each chromosome in a tetrad has two sister chromatids attached at the centromere

What is Meiosis? • Cell division used to create gametes

• Splits the DNA in half to make haploid gametes

• Two steps –Meiosis I (1 diploid cell 2 haploid

daughter cells)

–Meiosis II (2 haploid daughter cells 4 haploid gametes)

Phases of Meiosis • Meiosis I –Interphase

–Prophase I

–Metaphase I

–Anaphase I

–Telophase I

• Meiosis II

–Prophase II

–Metaphase II

–Anaphase II

–Telophase II

Meiosis I: Interphase • Cell increases in size

• Makes proteins in preparation for cell division

• Replicates DNA

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Prophase I • Nucleus breaks down

• Centrioles move to opposite sides of the cell

• Spindle fibers form

• Chromosomes pair up into a cluster of FOUR chromatids called a tetrad

–A tetrad is made up of a pair of homologous chromosomes

• Crossing over occurs – the homologous chromosomes trade equal pieces of DNA

Metaphase I

• Tetrads line up in the middle of the cell

Anaphase I

• Tetrads separate, but sister chromatids remain together

• Sister chromatids move to opposite sides of the cell

Telophase I • Chromosomes gather at opposite

ends of the cell

• Nuclear membrane may reform

• Cytokinesis – Cytoplasm divides the cell into

2 daughter cells

• Each daughter cell from meiosis I contains ONE of every chromosome (23 total).

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Prophase II

• Spindle fibers form, centrioles move to opposite sides of cell

• Nuclear membrane breaks down

Metaphase II

• Chromosomes (each with sister chromatids) line up in the center of the cell

Anaphase II

• Sister chromatids separate – one chromatid from each chromosome goes to each pole

Telophase II • Nuclear membrane forms

around each new set of chromosomes

• Spindle fibers break down

• Cytokinesis: cells divide

• Resulting daughter cells are haploid, containing one single chromosome from each pair of chromatids – each from mom OR dad (a combination of each)

Watch a video! http://www.sumanasinc.com/webcontent/animations/co

ntent/meiosis.html

What phase of meiosis?

PROPHASE II

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What phase of meiosis?

TELOPHASE I

What phase of meiosis?

PROPHASE I

What phase of meiosis?

TELOPHASE II

What phase of meiosis?

METAPHASE II

What phase of meiosis?

ANAPHASE I

What phase of meiosis?

ANAPHASE II

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Which phase of meiosis?

METAPHASE I

Meiosis I or meiosis II?

MEIOSIS I – METAPHASE I

Why? Tetrads are lined up in the center of the cell.

Meiosis I or meiosis II?

MEIOSIS II – TELOPHASE II

Why? New nuclei are forming around chromosomes that are NOT in sister chromatid form anymore.

Meiosis I or meiosis II?

MEIOSIS I – PROPHASE I

Why? Tetrads are formed and are undergoing crossing over.

Meiosis I or meiosis II?

MEIOSIS II – ANAPHASE II

Why? The SISTER CHROMATIDS are separating.

Meiosis I or meiosis II?

MEIOSIS II – PROPHASE II

Why? The SISTER CHROMATIDS are still paired and the cell is in prophase. No crossing over is occurring.

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Meiosis I or meiosis II?

MEIOSIS I – ANAPHASE I

Why? The SISTER CHROMATIDS are still paired, but the tetrads are separating.

Meiosis I or meiosis II?

MEIOSIS II – METAPHASE II

Why? The SISTER CHROMATIDS are still paired, but are preparing to be separated.

Meiosis I or meiosis II?

MEIOSIS I – TELOPHASE I

Why? The SISTER CHROMATIDS are still paired – they haven’t been separated in meiosis II yet.

Focus Questions

1. What is the difference between diploid and haploid?

2. What is the purpose of meiosis I? Meiosis II?

3. What are the stages of meiosis?

4. What is crossing over?

5. What is a tetrad?

6. Be able to recognize the stages!