11–4  Meiosis The making of sex cells!

Making Sex Cells Meiosis is the process of making sex cells (gametes) Gametes are sex cells Ex: sperm, egg, pollen, etc…

Female Gametes - Eggs Human Eggs

Male Gametes - Sperm Human Sperm

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.

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)

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)

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)

How are haploid (N) gamete cells produced from diploid (2N) cells? Meiosis

Meiosis a 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.

Phases of Meiosis two 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.

What is the diploid number for these cells? 2 What is the haploid number for these cells? 1

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

Tetrads

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)

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

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 II paired chromatids separate Produces: haploid cells (N) Haploid cells are DIFFERENT from each other

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)

Fertilization of a human egg by sperm. Notice the HUGE size difference.

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

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.

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.