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Chapter 13: Meiosis and Sexual Life Cycles Essential Knowledge 3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (13.113.3). 3.c.2 – Biological systems have multiple processes that increase genetic variation (13.4). Heredity The transmission of traits from parents to offspring Comment - Humans have been aware of heredity for thousands of years Also known as inheritance Genetics The scientific study of heredity Comment - Genetics is only about 150 years old NEW/MODERN field of science DNA was only discovered in 1950s Only 60 years ago! Chromosome Review Chromosomes (23 pairs=46) Inherit one chromosome of each pair from each parent Total of 46 (23 maternal, 23 paternal) 1 pair/2 = Sex chromosomes 22 pairs/44 = Autosomes Genes The DNA for a trait Locus - the physical location of a gene in a chromosome Genes program traits How? Program cells (through protein synthesis) to make specific proteins These proteins become “visible” to us through outside traits Reproduction A method of copying genes to pass them on to offspring Fertilization: male/female gametes unite during reproduction Results in zygote (fertilized egg) Two main types: Asexual reproduction Sexual reproduction Asexual Reproduction Parent passes all of its genes to its offspring Offspring are exact copies of parents Also known as cloning (clones) Uses mitosis Often called budding Comment - many organisms reproduce this way Ex: jellyfish, hydra, sponges Asexual Bud Advantages of Asexual Only need 1 parent Offspring are identical to the parent Good genetic traits are conserved and reproduced Usually requires less energy Why? Disadvantages of Asexual No new DNA combinations for evolution to work on Only genetic differences come from DNA mutations (during DNA replication, etc) Clones may become extinct if attacked by a disease or pest Sexual Reproduction Two parents contribute DNA to an offspring Comment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others Sex Chromosomes Homologous? Mostly no! Only a SMALL part of these 2 are homologous Males: One X chrom, One Y chrom Y chromosomes are smaller (with more genetic disorders) Females: Two X chromosomes X chromosomes are bigger (usually contain more genes) Egg vs. Sperm Eggs contain ONLY X chromosomes Sperm contain EITHER X or Y chromosomes Why? Because boys (when they are made during fertilization) get an X chromo from mom and a Y chromo from dad – making them a MALE! If the cell were to get two X chromos, the fertilized cell would become a FEMALE! Who determines sex of baby? Males determine what sex the offspring will be Why? Their sperm have the “option” of having X or Y X from egg and Y from sperm = XY (male) X from egg and X from sperm = XX (female) Advantages of Sexual Offspring has a unique combination of DNA which may be an improvement over both parents New combination of DNA for evolution to work with Disadvantages of Sexual Need two parents (requires time and energy to look/find the right mate) Good gene combinations can be lost Offspring may not be an improvement over the parents Question ? Do parents give their whole DNA copy to each offspring? What would happen to the chromosome number if they did? Chromosome Number Is usually constant for a species Two of these are usually sex chromosomes XY or XX in humans Examples: Humans – 46 (pair #23 is sex) Corn - 20 Onions - 16 Dogs - 72 Two options for life cycle… 1) Mitotic cell division continues life 2) Meiosis cell division continues life WHICH way does life reproduce? Result of Life cycle: if Mitosis 46 in egg + 46 in sperm= 92 total chromosomes Chromosome number would double each generation. Need a method to reduce the chromosome number. Result of life cycle: if Meiosis Egg 23 + sperm 23 = 46 total chromosomes Chromosome number will remain the same with each sexual reproduction event. Meiosis is used to produce the gametes or sex cells. 46 reduced to 23 through meiosis Meiosis - Purpose To reduce the number of chromosomes by half 46 to 23 (in humans) Prevents doubling of chromosome numbers during sexual reproduction Sexual Life Cycle Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species Fertilization: 23 + 23 = 46 total Mitosis: takes original cell made through fertilization and multiplies it to form organs, etc. Meiosis: 46 reduced to 23 (in sex organs) Life Cycle Variations Diploid 2 sets of chromosomes Referred to as 2n Most common number in body or somatic cells Humans 2n = 46 Corn 2n = 20 Fruit Flies 2n = 8 Haploid 1 set of chromosomes Number in the gametes or sex cells Humans n = 23 Corn n = 10 Fruit Flies n = 4 Half of diploid #!!!!!!! Polyploids Multiple sets of chromosomes Examples 3N = triploid 4N = tetraploid Common in plants, but usually fatal in animals Ex: Down Syndrome Ex: Seedless watermelons Triploid Daffodil = more stem produced per bulb Triploid watermelon = no gametes produced (no seeds) Meiosis/Mitosis Preview of differences LOOK at pg. 256 Meiosis Two cell divisions, not one Four cells produced, not two Synapsis and chiasmata will be observed Meiosis/Mitosis Preview of differences Meiosis, cont. 1st division separates PAIRS of chromosomes, not duplicate chromatids (of chromosome) Known as homologous pairs of chromosomes Mitosis vs. Meiosis Interkinesis is present. Animation Meiosis Has two cell divisions (I and II) Steps follow the names for mitosis A “I” or “II” will be added to label Replicated chromatids (dupl. during Inter) Prophase I Basic steps same as in prophase of Mitosis. Synapsis occurs as the chromosomes condense. Synapsis - homologous chromosomes form bivalents or tetrads Synaptonemal complex - proteins which hold chromosomes together Tetrad Prophase I, cont. Chiasmata observed Spot where chromatids cross-over Held together by this until Anaphase Longest phase of division Cell spends 90% of meiosis in this stage Chromatin condenses, nucleolus disappears, nuclear envelope dissolves Metaphase I Tetrads or bivalents align on the metaphase plate Attached to kinetochores of opposite poles Centromeres of homologous pairs point toward opposite poles Anaphase I Homologous PAIRS separate Uses spindle apparatus Duplicate chromosomes are still attached at the centromeres Maternal and paternal chromosomes are now separated randomly Telophase I Similar to Mitosis Chromosomes may or may not unwind to chromatin Cytokinesis separates cytoplasm and 2 cells are formed NOW: Each cell has a haploid set of chromosomes Each chromosome is STILL comprised of 2 identical sister chromatids!!! Interkinesis No DNA synthesis/copying occurs May last for years, or the cell may go immediately into Meiosis II Can appear similar to Interphase of Mitosis Meiosis II Steps are the same as in Mitosis Prophase II = Prophase Metaphase II = Metaphase Anaphase II = Anaphase Telophase II = Telophase Meiosis - Results 4 cells produced Meiosis animation Each cell has ONE sister chromatid Remember: Started with homologous chromosome PAIR Chromosome number reduced Gametes (sex cells) made Genetic variation increased Sexual Sources of Genetic Variation 1. Independent Assortment of Chromosomes during Meiosis 2. Random Fertilization 3. Crossing Over Independent Assortment There are 23 pairs of chromosomes in humans The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2 Random arrangement of homologous pairs during metaphase I This arrangement is totally random! Gamete Possibilities With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are: 23 2 or 8,388,608 (w/ each offspring!) Random Fertilization Choice of which sperm fuses with which egg (random choice) With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the possible combos of offspring is over 64 million (for EACH set of parents) Crossing-Over The exchange of sister chromatid material during synapsis Occurs ONLY in Prophase I Produces recombinant chromosomes Chromosomes with DNA from both parents Chiasmata: The point of contact where two chromosomes are crossing-over Importance Breaks old linkage groups Creates new linkage groups increases genetic variation Very common during meiosis Frequency can be used to map the position of genes on chromosomes Comments Offspring can never be 100% like a parent if sexual reproduction is used Multiple cross-overs are common, especially on large chromosomes Genes near the centromere do not cross-over very often Summary Recognize the general relationships between genes, DNA, and chromosomes. Identify characteristics, advantages, and limitations of asexual and sexual reproduction. Recognize several sexual life cycles differing in the timing of meiosis and fertilization. Recognize the stages and characteristics of the meiosis cell division process. Contrast and compare meiosis to mitosis. Identify sexual sources of genetic variation. Use Chapter 46 to see differences in male/female meiosis (pgs. 1006-1007)