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Transcript
+ 46 CHROMOSOMES + = 46 CHROMOSOMES 1. What is this process called? 2. Is this scenario reasonable? Why or why not? = 92 CHROMOSOMES + 23 CHROMOSOMES + 23 CHROMOSOMES = = = 46 CHROMOSOMES • The purpose of meiosis is to create gametes (sperm and eggs) with half the original number of chromosomes. • Meiosis begins with one diploid cell (2 of each type of chromosome) and ends with four haploid cells (one of each chromosome). • 4 sperm cells in males • 1 egg cell with 3 polar bodies in females • Chromosome: A structure that carries genes. • Each human cell has 46 chromosomes (23 pairs) • Each human gamete has 23 chromosomes (0 pairs) Meiosis: • occurs in testes of males • occurs in ovaries of females • Involves two divisions with one DNA replication. • Before meiosis begins, DNA must replicate itself so enough copies will be available for the four new cells being created. This happens in Interphase. • Chromosomes in the replicated state are made of two SISTER CHROMATIDS that are connected by a CENTROMERE. Occurs in Four Phases • Prophase I- Homologous chromosomes pair up. Crossing over may occur in this phase. Metaphase I • Homologous Chromosomes line up at the spindle equator. • The random arrangement of pairs on either side of the spindle equator results in variation in the genotypes found in the gametes. Anaphase I • Homologous chromosomes are pulled to opposite ends of the cell by the spindle fibers. Telophase I • The cell begins to pinch together and form two distinct haploid cells. • How many chromosomes were in the original cell? • A short Interphase occurs but no DNA replication Meiosis II • Prophase II- No pairing of homologues (they are in separate cells). • Metaphase II- Sister chromatids line up at the spindle equator. • Anaphase II- Sister chromatids are pulled to opposite ends of the two cells. • Telophase II- Cytokinesis occurs in both cells resulting in 4 haploid • The purpose of meiosis is to create gametes. • The production of male gametes is called spermatogenesis • The production of female gametes is called oogenesis. • Definition: the union of two gametes (fertilization), thereby combining the genes from two different organisms. • It is responsible for the abundant variation that can be seen in organisms that reproduce in this way. • The gametes that are formed by meiosis may randomly combine to form offspring that contain a variety of traits. • Meiosis results in the production of gametes which contain a variety of combinations of the parental genes. • Variation in gametes results from meiosis I when: • 1) In prophase I when crossing over may occur between homologues. • 2) During metaphase as homologues randomly line up on either side of the spindle equator. • Fertilization contributes to variation when the winner of the sperm marathon hits the target egg and claims its prize!