Reproduction & Cell Types
TYPES OF REPRODUCTION Asexual Relies on MITOSIS All of the parent s DNA goes to the offspring Sexual Relies on MEIOSIS Used to create sex cells
TYPES OF CELLS Body Cells Includes all cells that are not sex cells i.e. skin, liver, muscle, etc Also called somatic cells (soma- means body ) Made by MITOSIS Contain 2 copies of each chromosome Called homologous chromosomes Have a diploid # (2n) Ex. Humans = 46
TYPES OF CELLS Sex Cells Include egg cells and sperm cells Also called gametes Made by MEIOSIS Contain 1 copies of each chromosome Have a haploid # (n) Ex. Humans = 23
Chromosome Numbers Egg (haploid # of 23) Sperm (haploid # of 23) Fertilization Zygote (diploid # of 46)
MEIOSIS
Produces Sex Cells Reduction division Meiosis Starts with 1 diploid cell and ends up with 4 haploid cells Occurs in TWO steps Meiosis I Meiosis II
Interphase I All chromosomes will replicate, much like they do before Mitosis in the Cell Cycle.
Prophase I Homologous chromosomes come together as pairs (may look like a grouping of four; called tetrads) Nuclear envelope and nucleoli disperse Prophase I typically occupies more than 90% of the time required for Meiosis Crossing over can occur
Crossing Over The exchange of material between homologues Occurs during Prophase I Tetrads are formed and DNA is exchanged Produces chromosomes that contain genes from both parents ADVANTAGE: Genetic variation
Metaphase I Tetrads are aligned on the metaphase plate Independent assortment soon follows INDEPENDENT ASSORTMENT: The random distribution of genes located on different chromosomes. Since each homologous pair assorts independently from all others, the process produces 2 n possible combinations of maternal and paternal chromosomes in gametes (where n is the haploid number) In humans, the possible combinations would be 2 23, or eight million there are eight million possible assortments of chromosomes from the parents
Anaphase I Homologues separate & are moved towards the poles by the spindle apparatus Sister chromatids remain attached at their centromeres and move as a unit towards the same pole, while the homologue moves towards the opposite side
Telophase I & Cytokinesis Each pole now has a haploid set of chromosomes that are still composed of two sister chromatids attached at a centromere In some species, nuclear membranes and nucleoli reappear & the cell enters a period of interkinesis before Meiosis II In other species, the daughter cells immediately prepare for Meiosis II
MEIOSIS II This second division separates sister chromatids of each chromosome (virtually identical to Mitosis) Prophase II If the cell entered interkinesis, the nuclear envelope & nucleoli disperse Metaphase II Chromosomes align along the metaphase plate Anaphase II Sister chromatids of each pair move toward opposite poles of the cell Telophase II & Cytokinesis Nuclei form at opposite poles of the cell Cytokinesis occurs producing four haploid daughter cells
Gamete Development in Males SPERMATOGENESIS Meiosis occurs in testes at puberty and then occurs as needed 1 spermatocyte = 4 sperm cells
Gamete Development in Females OOGENESIS Meiosis occurs in the ovaries before birth 1 oocyte = 1 egg & 3 polar bodies
MITOSIS & MEIOSIS ARE NOT THE SAME!!!
Comparing Mitosis & Meiosis How many stages are involved? Mitosis = 1 (PMAT) Meiosis = 2 (P1, M1, A1, T1, P2, M2, A2, T2) How many cells are made? Mitosis = 2 Meiosis = 4 How many chromosomes are present compared to the parent cell? Mitosis = same number Meiosis = ½ the number (Meiosis is a REDUCTION division)
Comparing Mitosis & Meiosis Are the resulting cells genetically the same or different? Mitosis = genetically the same Meiosis = genetically different What types of cells result in the process? Mitosis = body (somatic) cells Meiosis = Gametes (sperm & egg cells) What are the advantages of each process Mitosis = Growth & Repair Meiosis = Formation of sex cells & genetic variation
HUMAN LIFE CYCLE Uses both meiosis & mitosis