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Biology - Meiosis and Gamete Formation

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    Study Guide Biology – Meiosis and Gamete Formation 1. Meiosis Most plant and animal cells are diploid . The word diploid comes from the Greek word diplos , meaning “double.” This means diploid cells have pairs of chromosomes . For example, human body cells contain 46 chromosomes , arranged into 23 pairs . Because there are two copies of each chromosome, human body cells are diploid. 1.1 Diploid vs. Haploid Cells Sexual reproduction involves special cells called gametes (sex cells). • In humans, gametes are sperm cells and egg cells • Gametes are haploid , meaning they contain only one set of chromosomes The word haploid comes from the Greek word haplos , meaning “single.” Human gametes contain 23 chromosomes , which is half of the usual 46. When a sperm and an egg join during fertilization, they form a zygote . This restores the diploid number (46 chromosomes) . After that, mitosis allows the zygote to grow into a multicellular organism. 1.2 What Is Meiosis? Meiosis is the type of cell division that produces gametes. Its main purpose is to: • Cut the chromosome number in half • Produce four haploid cells from one diploid cell In humans, this means:

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    Study Guide • One cell with 46 chromosomes • Divides to form four cells , each with 23 chromosomes 1.3 The Two Stages of Meiosis Meiosis happens in two major divisions : 1. Meiosis I 2. Meiosis II Each division includes the same basic phases seen in mitosis: • Prophase • Metaphase • Anaphase • Telophase However, meiosis has important differences that make it unique. 1.4 Homologous Chromosomes Chromosomes exist in pairs called homologous chromosomes . • One chromosome comes from each parent • They are similar but not identical • They may carry different versions of the same gene For example, one chromosome may code for blond hair, while its partner codes for black hair. 1.5 Preparing for Meiosis Before meiosis begins, the cell goes through the S phase of the cell cycle. • Each chromosome is copied • Every chromosome now consists of two sister chromatids • These chromatids are joined at a structure called the centromere

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    Study Guide 1.6 Meiosis I: The Reduction Division At the start of meiosis I, a human cell has: • 46 chromosomes • 92 chromatids 1.6.1 Prophase I This stage is very important. • Chromosomes condense and become visible • Synapsis occurs: homologous chromosomes pair up • Each paired group forms a tetrad (four chromatids total) Another key event here is crossing over : • DNA segments are exchanged between chromatids • This creates new genetic combinations • Crossing over increases genetic variation • It plays a major role in evolution After crossing over, homologous chromosomes are genetically different. 1.6.2 Metaphase I • Tetrads line up at the equatorial plate (center of the cell) • Spindle fibers attach to the centromeres • Each centromere attaches to one spindle fiber 1.6.3 Anaphase I • Homologous chromosomes separate

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