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CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES • __________ is the study of heredity (transmission of traits from one generation) and variation. • Genes • are segments of _____. • program cells to synthesize specific enzymes and other proteins that produce an organism’s inherited traits • Consist of four __________________ • program specific traits that emerge as we develop from fertilized eggs into adults. • The transmission of hereditary traits has its molecular basis in the precise __________ of DNA • Most DNA in a eukaryotic cell is subdivided into _________________ in the nucleus. • Some DNA exists in ___________ and ____________. • Every living species has a characteristic number of chromosomes. • Humans have 46 in almost all of their cells. • Chromosomes consist of a single ____ molecule in association with various ________. • Each chromosome has hundreds or thousands of genes, each at a specific location, it’s _____. • ________________- occurs mostly in single-cell eukaryotes, by mitotic cell division to produce two identical daughter cells •_________________- results in greater variation among offspring than does _________reproduction. •Offspring of sexual reproduction vary _____________ from their siblings and from both parents. Fig. 13.2 1. Fertilization and ___________ • Humans contain 46 chromosomes in each _____________ (all cells except germ cells) and 23 in the ________ (sperm or ovum). Know these terms! •We have ____________ ________________ pairs that carry genes that control the same inherited characters. •An exception - the _____ ___________ (X and Y). •Females are XX and males are XY _____________ - an ordered displays of an individual’s chromosomes - used to detect large abnormalities Fig. 13.3 ________ - Germ cells with one set of chromosomes (22 __________ and an X or a Y in humans). ________ - Somatic cells with two sets of chromosomes (46 total in humans) _______ - The fertilized egg with two haploid sets of chromosomes- one from mom, one from dad Gametes develop by _________ instead of mitosis in which the chromosome number is halved. Fig. 13.4z 2. Meiosis reduces chromosome number from diploid to haploid In meiosis, there are two consecutive cell divisions, __________ and ___________ which results in four haploid daughter cells. How??- By copying chromosomes once, but dividing twice. Meiosis I – separates ____________________ Meiosis II-separates ______________________ Fig. 13.6 Meiosis I • 4 phases: prophase, metaphase, anaphase, and telophase. ___________ - Previously replicated chromosomes join together at many locations to exchange DNA These exchange points are called _________ Spindles form from ___________ Metaphase, anaphase and telophase are similar to those in __________ End result2 ________ cells Metaphase Anaphase Meiosis II • Meiosis II is very similar to mitosis • BUT- DNA is not ___________ first. Prophase, metaphase, anaphase or telophase occur End result4 _____ cells Fig. 13.7 Comparison of mitosis and meiosis End result Meiosis Mitosis ___ ____ Daughter cells ________ ________ Chiasmata? _____ ____ # of divisions Purpose __ ________ __ _________ are the physical manifestation of crossing over, a form of genetic rearrangement. ____________ • Mitosis produces two _________ daughter cells, but meiosis produces 4 very different cells. Fig. 13.8 3. Sexual life cycles produce genetic variation among offspring • Four mechanisms contribute to genetic variation: 1. _____________________ • A fifty-fifty chance that a particular daughter cell of _______ will get the maternal chromosome of a certain homologous pair • For humans with n = 23, there are about 8 ________ possible combinations of chromosomes. Gametes • Four mechanisms.. (Cont.) 2. ___________________- -homologous portions of two nonsister chromatids trade placesin meiosis I. • produces ____________ ____________ which combine genes inherited from each parent. •For humans, this occurs ______ ________ per chromosome pair. Fig. 13.10 • Four mechanisms.. (Cont.) 3. _____________________ Any sperm can fuse with any egg. • A zygote produced by mating of a woman and man has a unique genetic identity. • A given ovum and sperm each have 223 (8 million) ________________________ • A zygote is composed of 1 in 70 trillion (223 x 223) possible combinations of chromosomes. • __________________ adds even more variation to this. • Four mechanisms.. 4. _ (Cont.) __________________ This results in adaptation, the accumulation of favorable genetic variations. New genetic _________________ that work best in the new conditions will produce more offspring and these genes will increase.