Meiosis

Biology

Chapter 11.4

Mr. Hines

Mendel was correct with his approach to genetics, be he did not know where genes were located in living things.

Later discoveries determined that genes were found on chromosomes inside of the nucleus.

Mendel’s principles required 2 things1. Each organism must inherit a single copy

of every gene from each parent.2. When gametes are formed, the genes must

be segregated from each other.

So there must be a process that separates genes – meiosis was discovered.

Chromosome number

Very important termsHomologous chromosomes – term used to

refer to the chromosomes that each have a corresponding chromosome from the opposite sex parent.

Each parent organism has 2 matching chromosomes – one from mom and one from dad – homologous chromosomes.

Diploid – term used to refer to a cell that

contains both sets of homologous chromosomes.

Sometimes the term diploid is written with the symbol 2N.

All of the cells in an organism are diploid except sex cells (gametes)

Haploid – term used to refer to a cell that contains only a single set of chromosomes and therefore only single set of genes.

All gametes are haploid – the chromosomes have been segregated and therefore each gamete will only have one of each chromosome (one chromosome from one parent)

Human Karyotype (photograph of chromosomes cut out and arranged)

So how do the chromosomes segregate into gametes?

Meiosis is a process of a reduction division in which there number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to create a haploid cell.

Meiosis is broken into two distinct phases

Meiosis I

Meiosis II

Meiosis I contains 4 sub phases

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

• PMAT – just like in Mitosis

Meiosis II contains 4 sub phases

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II

Before we begin Meiosis, we must explore the concept of

fertilization firstHumans have 23 homologous chromosomes

(total of 46)

This is too complicated to model – we will only use 2 homologous chromosomes so that the drawings are more understandable.

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The following drawings are not in your book.

Pay close attention.

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At this point, we can begin meiosis as seen in your book on page 276

Prophase 1

1. Each pair of chromosomes pair with their corresponding homologous chromosome to form a tetrad.

2. Crossing Over occurs

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Prophase 1

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Metaphase I

• 1. Spindle fibers attach to chromosomes

Metaphase I

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Anaphase I

• The fibers pull the homologous chromosomes to opposite ends of the cell.

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Telophase I

• Nuclear membrane forms and 2 new cells are a result

Telophase I

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If you have been following the color sequence, mom and dad’s chromosomes have now been separated.

Notice that after crossing over, each homologous chromosome has exchanged genetic material with the other.

So mom and dad’s chromosomes mixed – just a little.

After Telophase I

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Prophase II

• Meiosis I results in 2 daughter cells, each with half the number of chromosomes from the original.

Metaphase II

• Chromosomes line up at the center of the cell – Similar to Metaphase I

Metaphase II

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Anaphase II

• The sister chromotids separate and move to opposite ends of the cell.

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Anaphase II

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Telophase II

• This is the final stage of meiosis

• there are now 4 individual haploid daughter cells

• Notice that none of them are the same as indicated by colors.

• These cells can now be called “gametes.”

Telophase II

For males, these will be sperm

For females, these will become eggs.

Notice that these are haploid cells.

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Crossing over in more detail

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Section 11-4

Crossing-Over

Section 11-4

Crossing-Over

Section 11-4

Crossing-Over