Cell Division: Meiosis Egg Cell Cell Division: Meiosis Sperm Cell

Meiosis The process reduces the number of chromosomes by ½ to produce sex gametes. Sex gametes = egg or sperm cells When they unite during sexual reproduction, they form a diploid zygote.

2 Types of Cells Diploid = duplicate, paired chromosomes found in somatic (body) cells. They are said to have (2n) for the number of chromosomes. Haploid = single chromosomes found in sex gametes. They are said to have (n) for the number of chromosomes.

Pollination and Fertilization Pollination = occurs in a flower, when the male pollen grains get transferred to the female ovary. Fertilization = occurs when a sperm and egg cell unite to form a zygote.

Phases of Meiosis There are 2 stages of meiosis – Meiosis I and II. Interphase: Chromosomes are replicated. Same as mitosis

Prophase I: Chromosomes coil Spindle fibers form Homologous chromosomes come together to form a four part structure called a tetrad. Homologous chromosomes = have genes for the same trait arranged in the same order. However, they are not identical to each other

Prophase I cont.: Tetrad = 2 homologous chromosomes each made up of 2 sister chromatids. Crossing over can occur now. Crossing over = exchange of genetic material between homologous chromosomes which leads to variation in genes.

Metaphase I: Tetrads line up in the middle together Anaphase I: Homologous chromosomes split and move to opposite ends of the cell The centromeres DO NOT split!

Telophase I: Two new cells are formed, cytoplasm splits Each new cell has a set of sister chromatids still attached

Another cell division is needed – each cell still has 46 chromosomes. Too Many How many chromosomes should a sex gamete have? 23

Meiosis II – the steps are the same as mitosis Prophase II: Spindle fibers form Same as mitosis but happening on two cells at once

Metaphase II: Sister chromatids line up in the middle Anaphase II: Separating sister chromatids; each go towards opposite ends

Telophase II: Nuclei reform and cytoplasm divides 4 new unique cells formed

At the end of Meiosis II: Four haploid cells have been formed from the original diploid cell Each haploid cell contains one chromosome from each homologous pair These haploid cells become gametes (sperm and egg cells), which will pass on their genetic information to future offspring.

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