Chapter 13 Meiosis & Sexual Life Cycles 13.1 Offspring acquire genes from parents by inheriting chromosomes Genes Segments of DNA that code for heredity Transmitted from generation to generation Gametes are the reproductive cells that do this A locus (plural loci) is the location of a gene on a chromosome Asexual A single parent is the sole parent Passes copies of all its genes to its offspring New offspring arise by MITOSIS & are virtually exact copies of the parents genome Offspring is called a clone (genetically identical) Sexual Reproduction 2 parents contribute genes Results in greater genetic variation than asexual 13.2 Fertilization and meiosis alternate in sexual life cycles Life Cycle Generation-to-generation sequence of stages in the reproductive history of an organism From conception to production of its own offspring Somatic Cells Any cells that are NOT gametes Each somatic cell in humans has 46 chromosomes Karyotype A picture of its complete set of chromosomes Arranged in pairs of homologous chromosomes from the largest pair to the smallest pair The 46 chromosomes will be paired to make 23 homologous chromosomes Homologous Chromosomes Both chromosomes of each pair carry genes that control the same inherited characteristic Similar in length & centromere position Have same staining pattern One from each parent Sex Chromosomes vs. Autosomes Sex chromosomes are exceptions to the homologous pair rule In humans, X & Y Females = XXMales = XY Nonsex chromosomes are called Autosomes Gametes Sperm & Ova (eggs) are HAPLOID cells Contain the number of chromosomes of somatic cells Humans gametes contain 22 autosomes plus a single sex chromosome A haploid of 23 symbolized by n Meiosis & Fertilization are the key events in sexually reproducing life cycles Fertilization = combination of sperm + egg One haploid from each parent fuse to form a ZYGOTE The zygote is DIPLOID & represented by 2n Meiosis Type of cell division that reduces the number of sets of chromosomes from 2 to 1 Fertilization restores the diploid number as the gametes are combined F & M alternate in the life cycles of sexually reproducing organisms Plants and some algae exhibit an alternation of generations Plants and some algae exhibit an alternation of generations This life cycle includes both a diploid and haploid multicellular stage This life cycle includes both a diploid and haploid multicellular stage The diploid organism, called the sporophyte, makes haploid spores by meiosis The diploid organism, called the sporophyte, makes haploid spores by meiosis Each spore grows by mitosis into a haploid organism called a gametophyte Each spore grows by mitosis into a haploid organism called a gametophyte A gametophyte makes haploid gametes by mitosis A gametophyte makes haploid gametes by mitosis Fertilization of gametes results in a diploid sporophyte Fertilization of gametes results in a diploid sporophyte 13.3 Meiosis reduces diploid to haploid Looks similar to mitosis: Both are preceded by the replication of DNA Difference: In meiosis it is followed by 2 stages of cell division Meiosis I & II Final result = 4 daughter cells (1/2 the chromosomes as original cell) Meiosis Interphase Chromosomes make a copy of themselves Replicates DNA (doubles amount of DNA in cell) Centrosome divides Meiosis I Prophase I (critical to understand meiosis) Chromosome condense results 2 sister chromatids attached at centromere Synapsis occurs Joining of homologous chromosomes along their length Called a tetrad & aligns the HC gene by gene Prophase I cont. Crossing over DNA from one homologue is cut & exchanged with an exact portion of DNA from the other homologue A small part of DNA from one parent is exchanged with the DNA from another INCREASES GENETIC VARIABILITY Happens 2-3 times per pair forms criss-crossed regions termed CHIASMATA Prophase I cont. After crossing over Centrioles move away from each other Nuclear envelope disappears Spindle microtubules attach to the kinetochores forming on the chromosomes that begin to move to the metaphase plate Organize thoughts 1) Synapsis occurs, forming tetrads 2) Tetrads undergo crossing over 3) Crossing over increases genetic variation 4) Areas of crossing over form chiasmata 5) Nuclear envelope disappears, allowing spindle to attach Metaphase I Homologous pairs are lined up on the metaphase plate Microtubules from each pole attach to each member of the pairs Preparing to pull to opposite poles Anaphase I Spindle apparatus helps to move chromosomes to opposite poles Sister chromatids stay connected & move together Telophase I HC move until they reach opposite poles Each pole contains a haploid set of chromosomes Still consisting of 2 sister chromatids Cytokinesis Division of cytoplasm & occurs during telophase A cleavage furrow in animals & cell plates in plants Will form 2 haploid cells Meiosis II 2 cells move together Prophase II Spindle apparatus forms Sister chromatids move toward metaphase plate Metaphase II Chromosomes are lined up on the plate Kinetochores of each sister chromatid prepare to move to opposite poles Anaphase II Centromere of the sister chromatids separate Individual chromosomes move to opposite ends Telophase II & Cytokinesis Nuclei reappear & cytokinesis occurs 4 daughter cells have a haploid of the original parent cell All 4 are genetically different from one another Mitosis vs Meiosis The following occur in Meiosis, NOT in mitosis 1) Synapsis & crossing over 2) At metaphase I, paired HC (tetrads) align on the metaphase plate 3) At anaphase I, the centromere stays intact 13.4 Genetic variation produce in sexual life cycles contributes to evolution 1) Crossing over 2) Independent assortment of chromosomes 3) Random fertilization 1) Crossing over Exchange of genetic material in Prophase I Results in all 4 chromatids of the tetrad being different In metaphase II, sister chromatids separate increasing variation 2) Independent assortment Metaphase I allows the HC to pair up in any combination There is a chance that a daughter cell will get a maternal chromosome or paternal chromosome from the pair 3) Random fertilization Each egg & sperm is different Each combination of the 2 is unique due to #1 & #2