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Chapter 10 Meiosis and Sexual Life Cycles heredity - continuity of biological traits from 1 generation to the next - results from the transmission of genes from parents to offspring - offspring more closely resemble parents and relatives than other members of the same species variation - inherited differences among individuals of the same species genetics - study of heredity and variation Offspring acquire genes from parents by inheriting chromosomes. DNA - a nucleic acid - polymer of 4 different kinds of nucleotides genes - units of hereditary information - made of DNA; located on chromosomes - have specific sequences of nucleotides - most program cells to make specific proteins traits - result from the actions of proteins coded for by genes inheritance is possible because: DNA is replicated producing copies of genes to be passed to offspring sperm & ova carrying each parent’s genes combine chromosome - unit of hereditary material in nucleus of eukaryotic organisms - consists of a single long DNA molecule with proteins - DNA (double helix) is highly folded and coiled - contains genetic information arranged in a linear sequence - each contains 100s to 1000s of genes - each species has a characteristic number of chromosomes locus - specific location of a chromosome that contains a gene Like begets like, more or less: a comparison of asexual versus sexual reproduction. Asexual Reproduction 1 parent all parent’s genes passed to offspring offspring genetically identical to parent results in a clone (genetically identical individual) rarely, genetic difference occur due to mutations (change in DNA) Sexual Reproduction 2 parents each parent passes half its genes to offspring offspring have unique combination of genes inherited from both parents results in greater genetic variation; offspring vary genetically from siblings and parents Role of Meiosis in Sexual Life Cycles Fertilization and meiosis alternate in sexual life cycles. life cycle - sequence of stages in an organism’s reproductive history, from conception to production of its own offspring human life cycle somatic cell - meiosis & fertilization result in alternation between haploid and diploid condition - same basic pattern in all sexually reproducing organisms - any cell other than a sperm or egg cell - contain 46 chromosomes in humans chromosomes - differ in size, position of centromere, and staining/banding pattern - can be matched into homologous pairs to produce a karyotype autosomes sex chromosome karyotype - chromosomes that aren’t sex chromosomes (human cells = 22 pairs) - determine individual’s sex (human cells = 1 pair) - a display or pictomicrograph of an individual’s somatic cell chromosomes arranged in a standard sequence - can be used to screen for chromosomal abnormalities homologous chromosomes - homologues - pair of chromosomes with the same size, centromere position, and banding pattern - they have same genetic loci (except 1 pair) homologous autosomes sex chromosomes - carry same genetic loci on each - carry different loci - X chromosome and Y chromosome females males - cells carry homologous pair of X chromosomes - cells carry one X and one Y chromosome chromosomal pairs in human karyotype: 1 homologue is inherited from each parent the 46 somatic cell chromosomes are actually 2 sets of 23 (1 set from mom, 1 from dad) diploid - cells contain 2 sets of chromosomes (2n) - chromosome number in somatic cells haploid- cells contain 1 set of chromosomes (n) - chromosome number in gametes gamete - haploid reproductive cell - sperm cells and ova - human gametes have 22 autosomes and 1 sex chromosome (X or Y) fertilization - union of 2 gametes to form a zygote - diploid number is restored zygote - diploid cell that results from the union of 2 haploid gametes - contains maternal and paternal haploid sets of chromosomes - as humans develop from the zygote, the genetic information is passed to all somatic cells by mitosis gametes - only cells in body not produced by mitosis - produced in ovaries and testes by meiosis meiosis - special type of cell division - produces haploid cells - produces sperm cells and ova in humans 3 basic patterns of sexual life cycles: animal - gametes are only haploid cells - meiosis results in gamete production - gametes don’t divide further before fertilization - fertilization produces a diploid zygote that divides by mitosis to produce a diploid multicellular animal fungi and some protists - zygote is only diploid stage - meiosis occurs immediately after zygote forms - resulting haploid cells divide by mitosis to produce a haploid multicellular organism - gametes produced by mitosis from already haploid cells plants and some algae - alternate between multicellular haploid and diploid generations - called alternation of generations - sporophyte - multicellular diploid stage - spore producing plant - meiosis produces haploid cells called spores - gametophyte - multicellular haploid stage - gamete producing plant - produced by division of haploid spores by mitosis - produce gametes by mitosis - fertilization produces a diploid zygote which develops into next sporophyte generation Meiosis reduces chromosome number from diploid to haploid: a closer look meiosis - contributes to genetic variation among offspring - steps resemble steps in mitosis some differences between mitosis and meiosis: MITOSIS preceded by replication of chromosomes chromosome replication followed by 1 cell divisions in mitosis 2 daughter cells produced daughter cells have same number of chromosomes as parent cell 2 daughter cells genetically identical to parent cell and each other 1 nuclear division MEIOSIS preceded by replication of chromosomes chromosome replication followed by 2 cell divisions in meiosis 4 daughter cells produced daughter cells have half the number of chromosomes as parent cell creates genetic variation; 4 daughter cells genetically different from parent cell and each other 2 successive nuclear divisions stages of meiosis: Interphase I - precedes meiosis - chromosomes replicate (like in mitosis) - each duplicated chromosomes consists of 2 identical sister chromatids attached at centromeres - in animals, centrioles pairs also replicate Meiosis I - segregates the 2 chromosomes of each homologous pair - reduces chromosome number by half - includes 4 phases: Prophase I - 90% of time needed for meiosis - chromosomes condense - synapsis occurs homologous chromosomes pair up forming a tetrad (complex of 4 chromatids) - chromatids of homologous chromosomes criss-cross at numerous places (called chiasmata) - centrioles pairs move apart; spindle microtubules form between them - nuclear envelope and nucleoli disperse - chromosomes begin moving to metaphase plate Metaphase I - tetrads aligned on metaphase plate - each homologue attached to kinetochore microtubules Anaphase I - homologues separate and moved toward opposite poles by spindle - sister chromatids remain attached at centromeres; move together toward same pole Telophase I / Cytokinesis - chromosomes reach poles - each pole has a haploid set of chromosomes - usually cytokinesis occurs with Telophase I - 2 haploid daughter cells formed (cleavage furrow in animals, cell plate in plants) - some cells enter interkinesis (nuclear membranes and nucleoli reappear) - other cells immediately prepare for meiosis II Meiosis II - separates sister chromatids of each chromosome - includes 4 phases: Prophase II - nuclear membrane & nucleoli disperse (if cell entered interkinesis) - spindle apparatus forms - chromosomes move toward metaphase II plate Metaphase II - chromosomes align singly on metaphase plate - kinetochores of sister chromatids point towards opposite poles Anaphase II - centromeres of sister chromatids separate - sister chromatids of each pair move towards opposite poles Telophase II / Cytokinesis - nuclei form at opposite poles - cytokinesis produces 4 haploid daughter cells comparison of meiosis I and mitosis: MEIOSIS I Prophase Metaphase Anaphase synapsis forms tetrads; chiasmata appear (results in crossing over) homologous pairs align on metaphase plate chromosome pairs separated centromeres don’t divide; sister chromatids stay together; sister chromatids move to same pole MITOSIS no synapsis; no crossing over individual chromosomes align on metaphase plate sister chromatids separated centromere divide; sister chromatids move to opposite poles Sexual life cycles produce genetic variation among offspring. meiosis and fertilization - primary sources of genetic variation among sexually reproducing organisms sources of genetic variation in offspring due to sexual reproduction: independent assortment crossing over (prophase I) random fertilization independent assortment of chromosomes - random distribution of maternal and paternal homologues to the gametes - there is a 50-50 chance a daughter cell will receive the maternal homologue or the paternal homologue after meiosis I (orientation of the pair is random) - each homologous pair assorts independently from all the others (2n possible combinations of maternal and paternal chromosomes in gametes) (n = haploid number) - in humans, there are 223 possible combinations (about 8 million) each human gamete contains 1 of 8 million possible assortments of chromosomes inherited from the parents crossing over - exchange of genetic material (genes) between homologous chromosomes - occurs during Prophase I (synapsis) - homologous portions of 2 nonsister chromatids trade places - X-shaped chiasmata appear at places where this exchange occurs - results in chromosomes with genes from both parents (recombinant chromosomes) - an average of 2 – 3 crossovers occurs per chromosome pair in humans random fertilization - in humans: each ovum has 1 in 8 million possible chromosome combinations each sperm has 1 in 8 million possible chromosome combinations so each resulting zygote has 1 in 64 trillion possible combinations of maternal and paternal chromosomes (without even considering crossing over) Evolutionary adaptation depends on a population’s genetic variation. heritable variation - basis for Darwin’s theory that natural selection is the mechanism for evolutionary change natural selection - increases frequency of heritable variations that favor the reproductive success of some individuals over other - results in adaptation (accumulation of heritable variations that are favored by the environment) - genetic variation increases the likelihood that some individuals in a population will have heritable variations that help them cope with environmental changes 2 sources of genetic variation: sexual reproduction (independent assortment, crossing over, random fertilization) mutation - random and relatively rare structural changes in a gene made during DNA replication