Transcript Meiosis
Meiosis Sexual Reproduction Meiosis: What is the point? • Cell division that produces a sperm cell or an egg cell (Gametes). • Starts with one cell that contains two copies of each chromosome (Diploid). One chromosome from female parent and one from male parent. • Ends with 4 Haploid (only one set of chromosomes) daughter cells. 4 cells all genetically different. Vocabulary • Meiosis- process of reduction division in which the number of chromosomes per cell is cut in half • Homologous- chromosomes that each have a corresponding chromosome from the opposite-sex parent • Diploid – a cell that contains both sets of homologous chromosomes More Vocab • Haploid- a cell that contains only a single set of chromosomes. (single set of genes) • Tetrad- Structure containing 4 chromatids that form during meiosis • Crossing over- homologous chromosomes exchange portions of their chromatids during meiosis Mendel’s principles of genetics require at least two things: 1.Each organism must inherit a single copy of every gene from both its “parents”. 2.When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes. Let’s Look at a Fruit Fly: Drosophila • A fruit fly has 8 chromosomes • 4 of the chromosomes came from the fruit fly’s male parent and 4 came from the female parent. • These two sets of chromosomes are homologous. Meaning that each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent • A cell that contains both sets of homologous chromosomes is said to be diploid. (two sets) Diploid Cell • A diploid cell is sometimes represented by the symbol 2N. • What is the diploid number for the Drosophilia fly? • Your answer should be 8 2N = 8 Gametes (sperm & egg cells) • The gametes of sexually reproducing organisms, contain only a single set of chromosomes and therefore only a single set of genes. • These are called Haploid cells (one set) • Haploid is represented as N. • What is the Haploid number for Drosophilia? • Your answer should be N = 4 Why are all 4 daughter cells different? When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles. Crossing Over Segments of homologous chromosomes break and reattach at similar locations. Results in new genetic combinations of offspring. This is the main advantage of sexual reproduction Why is this an advantage? • Offspring has more variety • If an environment undergoes frequent changes it is better for the offspring to have more variety so the species will not die Meiosis I • Prior to Meiosis I, each chromosome is replicated. • The cells then begin to divide similar to mitosis. • In mitosis, the 4 chromosomes line up individually at the center of the cell. • The 2 chromatids that make up each chromosome then separate from each other. The Difference • In prophase of Meiosis I, however, each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad. • There are 4 chromatids in a tetrad. • This pairing of homologous chromosomes is the key to understanding meiosis. How are Mitosis and Meiosis the same? • Division of cells • DNA replication occurs before the division of the nucleus. Summary Similar in many ways to mitosis Several differences however Involves 2 cell divisions Results in 4 daughter cells with 1/2 the normal genetic information. The 4 cells are genetically DIFFERENT. Gametes or cells resulting from meiotic division will have HALF as many chromosomes. Overview of Meiosis Phases of Meiosis Meiosis II Telophase I and Cytokinesis I Meiosis II Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Gamete Formation In male animals, meiosis results in four equalsized gametes called sperm. In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. • http://www.sumanasinc.com/webcontent/a nimations/content/meiosis.html • video Practice questions Q: What is the cell process that occurs only in organisms that reproduce sexually?___________ Q.A sperm cell of an alligator has 16 chromosomes. What is the total number of chromosomes normally present in a stomach cell of this alligator? _________