1 11–4 Meiosis The making of sex cells!. 2 Making Sex Cells Meiosis is the process of making sex cells (gametes) Gametes are sex cells Ex: sperm,

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11–4  Meiosis

The 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, as shown in the drawing on the right.

Four 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

A cell that contains both sets of homologous chromosomes is said to be diploid (“two sets”)

a diploid cell is sometimes represented by the symbol 2N for Drosophila, the diploid number is

8, which can be written 2N = 8 Diploid cells = body cells (somatic

cells)

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Haploid Cells = a cell that contains only a single set of chromosomes Therefore only a single set of genes

a haploid cell is sometimes represented by the symbol N for 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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Meiosisa 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 Meiosistwo distinct divisions, called

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?

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What is the haploid number for these cells?

1

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Meiosis I Before 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 Over As 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 two new cells are formed New cells:

each pair of homologous chromosomes was separated

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

The new cells are DIFFERENT from each other

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Meiosis II  two cells produced by meiosis I

now enter a second meiotic division NO DNA replication before Meiosis II Prophase II, Metaphase II, Anaphase

II, Telophase IIpaired chromatids separate Produces: haploid cells (N)

Haploid cells are DIFFERENT from each other

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Gamete Formation

Males 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 and 3 polar bodies (egg is

MUCH larger in size) The one egg receives the most cytoplasm One egg produced for each round of meiosis

(and 3 polar bodies which can’t be fertilized)

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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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Mitosis Makes 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) a diploid cell four haploid (N)

cells These cells are genetically different

from the diploid cell and from one another.

Purpose: Sexual reproduction.