Transcript Meiosis
How many chromosomes do we have in body cells? How many chromosomes do we have in sex cells? If your parents have 46 chromosomes in their cells, do you have 92 chromosomes in your cells? Why? • • • Hereditary information is contained in genes, located in the chromosomes of each cell. An inherited trait of an individual can be determined by one or by many genes, and a single gene can influence more than one trait. A human cell contains many thousands of different genes in its nucleus. Each organism must inherit a single copy of every gene from each “parent” 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. • • • Chromosomes come from both the male parent and female parent Homologous: each of the chromosomes coming from one parent have corresponding chromosomes from the other parent Diploid: a cell that contains both sets of homologous chromosomes = “two sets” (2N) – Diploid cells contain two complete sets of chromosomes and two complete sets of genes – Human somatic (body) cells have 46 chromosomes or 23 homologous pairs • • • • Haploid: contain only one set of chromosomes and one set of genes = “one set” (N) Gametes of sexually reproducing organisms are haploid containing one complete sets of chromosomes and one complete sets of genes Human gametes (sex cells) have 23 chromosomes Sperm (23) + Egg (23) = Zygote (46) How are haploid gamete produced from diploid cells? • Gametogenesis is the process in which GAMETES are produced. It involves MEIOTIC cell division and cell maturation. This process occurs in specialized organs called GONADS. ◦ Male gonads are called TESTES perform spermatogenesis ◦ Female gonads are called OVARIES perform oogenesis GAMETES Zygote The process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Meiosis consists of 2 phases: ◦ Meiosis I ◦ Meiosis II ◦ To begin, you have one diploid cell ◦ In the end, you have four haploid cells • • • Cells begin to divide in a way that looks similar to mitosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad – there are 4 chromatids in a tetrad The homologous pairs may exchange portions of their chromatids in a process called crossing-over which results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles Homologous chromosomes separate and two new cells are formed Each new cell has equal numbers of chromatids but neither have the same complete set of chromosomes or alleles as each other or the original diploid cell Figure 11-15 Meiosis Section 11-4 Meiosis I Interphase I Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section: • • • • • The two cells enter a second meiotic division Neither cell goes through chromosomal replication, so each chromosome has 2 chromatids Pairs of chromatids separate Each daughter cell produced receives equal amounts of chromatids The four daughter cells contain haploid (N) chromosomes Figure 11-17 Meiosis II Section 11-4 Meiosis II Prophase II Metaphase II Meiosis I results in two haploid The chromosomes line up in a (N) daughter cells, each with half similar way to the metaphase the number of chromosomes as stage of mitosis. the original. Go to Section: Anaphase II Telophase II The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Male: sperm (4 equal sized gametes produced) Female: egg/polar bodies (1 egg cell produced which contains most of the cytoplasm, and 3 polar bodies which are usually not involved in reproduction) Mitosis – produces 2 genetically identical diploid cells Meiosis – produces 4 genetically different haploid cells MITOSIS: • THE PROCESS BY WHICH A DIPLOID CELL(2n) PRODUCES TWO DIPLOID (2n) CELLS • INCLUDES ONE CELL DIVISION • ASEXUAL REPRODUCTION MEIOSIS: • THE PROCESS BY WHICH A DIPLOID CELL (2n) PRODUCES FOUR HAPLOID (1n) CELLS (GAMETES) • INCLUDES TWO CELL DIVISIONS • SEXUAL REPRODUCTION Compare mitosis and meiosis on the graphic organizer with your seat partner… We know that genes on different chromosomes are independently assorted… But what about genes on the same chromosome? Would they be inherited together? Thomas Hunt Morgan – discovered that genes located on the same chromosome would generally be inherited together or “linked” Chromosomes can be thought of as groups of genes. The chromosomes can assort independently, but all the genes on that chromosomes will be linked! • • • However, crossing-over sometimes separates gene that are usually found on the same chromosome, so genes may not be linked together forever! Crossing-over is soooo important because it helps generate genetic diversity – new combinations of allele are constantly produced Increasing the variability of a species increases the possibility that some individuals of that species will be better adapted than others to survive both short-term and long-term changes in the environment. Show the relative locations of known genes on chromosomes Exact location on chromosomes Chromosome 2 Comparative Scale of a Gene Map Section 11-5 Mapping of Earth’s Features Mapping of Cells, Chromosomes, and Genes Cell Earth Country Chromosome State Chromosome fragment City People Go to Section: Gene Nucleotide base pairs