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11–4 MeiosisThe making of sex cells!

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Making Sex Cells Meiosis is the

process of making sex cells(gametes)

Gametes are sex cells

Ex: sperm, egg, pollen, etc…

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Female Gametes - Eggs

Human Eggs

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Male Gametes - Sperm

Human Sperm

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Chromosomes A body cell in an

adult fruit fly has 8 chromosomes.

4 of the chromosomes came from the fruit fly's male parent, and 4 came from its female parent.

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Homologous Chromosomes

Chromosomes that each have a corresponding chromosome from the opposite-sex parent Example: Fruit-Fly (Drosophila) Chromosomes -

8 chromosomes (4 from mom and 4 from dad)

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Different Types of Cells

Diploid cell = a cell that contains both sets of homologous chromosomes (“two sets”)

Represented by symbol 2N

Drosophila - the diploid number is 8, which can be written 2N = 8

Diploid cells - body cells (somaticcells)

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Haploid Cell = a cell that contains only a single set of chromosomes (only a single set of genes)

Represented by the symbol N

Drosophila - the haploid number is 4, which can be written N = 4

Haploid cells - sex cells (gametes)

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How are haploid (N) gamete cells

produced from diploid (2N) cells?

Meiosis

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Meiosis

Process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

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Phases of Meiosis

2 distinct divisions: meiosis I and meiosis II

By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

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What is the diploid number for these cells?

2

What is the haploid number for these cells?

1

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Meiosis IBefore meiosis I, each chromosome is

replicated.

Division looks similar to mitosis

Prophase I, Metaphase I, Anaphase I, Telophase I

Prophase of meiosis I - each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad

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Tetrads

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Crossing OverAs homologous chromosomes pair up

and form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing-over

Results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

Alleles = alternative forms of the same gene (ex: blue eyes vs. brown eyes)

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After Crossing Over…Homologous chromosomes separate

2 new cells are formed

New cells:

each pair of homologous chromosomes was separated

neither of the daughter cells has the two original sets of chromosomes (they have been shuffled and sorted)

The new cells are DIFFERENT from each other

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Meiosis IITwo cells produced by meiosis I

now enter a second meiotic division

NO DNA replication before Meiosis II

Prophase II, Metaphase II, Anaphase II, Telophase II

Paired chromatids separate

Produces: haploid cells (N) Haploid cells are DIFFERENT from each

other

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Gamete FormationMales gametes = sperm (pollen in plants)

All sperm the same size at the end of Meiosis

4 sperm produced for each round of meiosis

Female gametes = eggs

One egg produced for each round of meiosis (and 3 polar bodies which can’t be fertilized)

The one egg receives the most cytoplasm = is much larger

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Fertilization of a human egg by sperm.

Notice the HUGE size difference.

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Comparing Mitosis and Meiosis

Mitosis and Meiosis sound alike but are VERY different!

Mitosis produces two genetically identical diploid somatic cells

Meiosis produces four genetically different haploid sex cells

Animated comparison

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MitosisMakes body cells (somatic cells)

Diploid cell two diploid (2N) daughter

cells.

Daughter cells are identical to each other & the original parent cell

Purpose:

Allows an organism's body to grow and replace cells.

In eukaryotic unicellular organisms, it is a form of reproduction.

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Meiosis

Makes sex cells (gametes)

Diploid cell four haploid (N) cells

These cells are genetically differentfrom the diploid cell and from one another

Purpose: Sexual reproduction