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Organisms have tens of thousands of genes - Genes determine traits - Genes are lined up on chromosomes - 1 chromosome can contain a thousand or more genes Each species of organisms contains a different number of chromosomes The chromosome numbers of a species is not related to the complexity of the organism The Chromosomes in most plants and animals occur in pairs • • 1 chromosome comes from the male parent 1 chromosome comes from the female parent Diploid – A cell with two of each kind of chromosome Haploid – A cell with one of each kind of chromosome (gametes) (1 from each parent) We say the cell is a diploid cell or 2n *Supports Mendel’s conclusion that organisms have two factors (alleles) for each trait We say the cell is a haploid or n *Supports Mendel’s conclusion that parents give 1 allele for each trait to their offspring Homologous Chromosomes – Paired chromosomes Each pair of homologous chromosomes has genes for the same traits Ex. Pod shape On homologous chromosomes, the genes are arranged in the same order Because there are different possible alleles for the same gene, the two chromosomes in the homologous pairs are not always identical to each other. Meiosis – cell division . -produces haploid gametes Meiosis occurs in the body cells that produce gametes (Male and Female reproductive organs) The process of meiosis produces: male gametes (sperm) - n, haploid female gametes (eggs) – n, haploid When the sperm fertilizes the egg, the resulting cell is a Zygote – 2n diploid The zygote then can develop by mitosis into a multicellular organism This process is called Sexual Reproduction Two parents produce an offspring that is not identical to either parent What are the benefits to sexual reproduction? Gamete Production Meiosis Square Dance •Write down the phases and what is taking place as you watch the video. 9 Meiosis Similar to mitosis However, several differences: Meiosis occurs in 2 phases (2 cell divisions: Meiosis I & Meiosis II Results in 4 cells Vocabulary: Diploid (2N) – Normal amount of genetic material Haploid (N) 1/2 the genetic material. 10 Meiosis Meiosis forms haploid cells. Example In Humans: Ova (egg) and sperm. Ova produced in ovaries (females) Process is called oogenesis Sperm produced in testes (males) Process is called spermatogenesis 11 Spermatogenesis & Oogenesis Sperm formation Egg formation 12 Overview of Meiosis 13 Meiosis I Prior to division (S phase), amount of DNA doubles 14 Metaphase I During Metaphase I homologous chromosomes lineup along the EQUATOR of the cell Areas of homologous chromosomes connect at areas called CHIASMATA Genes are exchanged at these connections 15 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 16 Chiasmata 17 Anaphase I During Anaphase I, each HOMOLOGOUS CHROMOSOME is pulled to opposite sides of the cell. Unlike mitosis, the CENTROMERES DO NOT BREAK. Nuclei MAY OR MAY NOT reform following division. CYTOKENESIS may or may not occur. 18 Meiosis II DNA DOES NOT double Chromosomes randomly line-up along metaphase plate like regular mitosis. During Anaphase II, CENTROMERES BREAK and each SISTER CHROMATID is pulled to opposite sides of the cell. Nuclei reform and cytokinesis usually occurs (although it is often unequal). Sister chromatids 19 Overview of Meiosis 20 21