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Transcript
MEIOSIS Ch. 8 CELLS FOR SEXUAL REPRODUCTION Meiosis for Sexual Reproduction Sexual Reproduction - two parents a. Offspring are genetic mix of both parents b. Have a NEW combination of genes Advantage – genetic variation in offspring a. Some may have traits that favor survival b. Can pass these traits on to offspring c. Darwin’s theory - “ survival of the fittest” d. Variation in individuals allows a species to evolve Sexual Reproduction in bacteria and protists CONJUGATION a. Recipient cell gets new genes b. Bacteria and protists Complex organisms – make special cells a. gametes – sperm and egg b. Gametes combine in fertilization - make a zygote new organism Chromosome Number: Diploid and Haploid Homologous chromosomes a. matched chromosome pairs b. one member of pair from each parent c. carry genes for the same traits d. 22 autosome pairs; one pair sex chromosomes X, Y Gene for one trait Cells with paired chromosomes are diploid a. 2n (n = number) b. Human: 2n = 46 (23 pairs) b. Somatic (body) cells are diploid 2 sets of chromosomes - 2 of every gene Locus – location of gene on a chromosome Fruit fly 2n = 8 Haploid Cells (n) • Gametes – sperm and egg only • one set of chromosomes, one from each pair • • somatic cell sex cell Meiosis is “Reduction Division” REDUCES chromosome number by half Haploid (n) - one set of chromosomes -human: 2n = 46 n = 23 Meiosis Reduces the Chromosome Number 2n parent cell DNA replicates in interphase First division – pairs separate Second division – sister chromatids separate 4 haploid daughter cells Homologous pairs separate in MEIOSIS TWO cell divisions Diploid cell - Has pairs - Daughter cells have ½ parent (2n = 2) chromosome number Meiosis I - Pairs separate (n = 1) Meiosis II - copies separate (n = 1) Haploid cells - (n = 1) Crossing over – only in meiosis a. In Meiosis I b. Homologous chromatids trade pieces c. Increases genetic variation Meiosis I: homologous pairs separate - makes two daughter cells, but sister chromatids are still attached MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Sites of crossing over Spindle Chromatin 2n parent cell Sister chromatids synapsis Tetrad Microtubules attached to kinetochore ANAPHASE I Metaphase plate Centromere (with kinetochore) pairs line up Sister chromatids remain attached Homologous chromosomes separate pairs separate disjunction Meiosis II: sister chromatids separate 4 haploid cells MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate 2n n two daughter cells one chromosome set each two copies (sisters) chromatids separate disjunction Haploid daughter cells forming 4 daughters one set single copies 8.15 Review: Comparing mitosis and meiosis Mitosis Meiosis 2n copies Parent cell (before chromosome replication) Prophase I Prophase Tetrad formed by synapsis of homologous chromosomes Chromosome replication Chromosome replication Duplicated chromosome (two sister chromatids) 2n Meiosis i 2n = 4 Metaphase Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Metaphase I 2n copies 2n copies copies 2n single 2n Daughter cells of mitosis 2n No further chromosomal replication; sister chromatids separate during anaphase II Anaphase I Telophase I Haploid n=2 Daughter cells of meiosis I n n n Daughter cells of meiosis II copies n single Meiosis ii n n Sources of genetic variation 1. Homologous pairs have different genes •same traits, but may be different forms 2. Crossing over – homologs trade pieces before separating new gene combinations 3. Pairs position in Metaphase I – independent of each other •n pairs 2n possible combinations 4. Random fertilization of eggs by sperm •Any egg or sperm is equally likely to be used 5. Gene or chromosome mutation - Error in replication or cell division 8.16 Chromosomes line up randomly in meiosis – “Independent Assortment” – Many different gene combinations in haploid gametes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16 Making sperm and egg Sperm: 2n parent cell 4 haploid sperm Ovum: 2n parent cell 1 haploid egg + haploid polar bodies Ovum needs all the cytoplasm Ovum and polar body (0.1mm) Sperm needs only DNA - and flagellum - and mitochondria for power - and acrosome to penetrate ovum Fertilization restores diploid What happens after fertilization? Cleavage: mitotic divisions in early embryo reduce cell size, until normal Cells differentiate new organism When meiosis goes wrong Nondisjunction - do not separate correctly In mitosis defective nucleus, cell usually dies In meiosis defective gamete wrong number of chromosomes in zygote 8.21 Accidents during meiosis can alter chromosome number Nondisjunction in meiosis I Normal meiosis I Normal meiosis II Nondisjunctio n in meiosis II Gametes Gametes n1 n1 n1 n 1 n 1 n 1 n Number of chromosomes Number of chromosomes Nondisjunction in meiosis I All gametes abnormal Nondisjunction in meiosis II Some gametes normal n Wrong chromosome number in zygote wrong number in every cell in organism • Fertilization after nondisjunction trisomy in zygote Trisomy = 3 Egg cell n+1 Zygote 2n + 1 Sperm cell n (normal) Abnormal chromosome number = aneuploidy KARYOTYPE picture of a person’s chromosomes Photographed during mitosis - sorted into homologous pairs - largest-to-smallest - sex chromosomes last Abnormalities visible: - missing or extra - pieces broken or moved - pieces added or lost autosomes sex chrom. Trisomy 21 Normal male karyotype Normal female karyotype Down Syndrome Trisomy chromosome # 21 8.22 Abnormal number of sex chromosomes usually do not affect survival in humans • Nondisjunction of large chromosomes is usually lethal • Down Syndrome - # 21 is very small, carries few genes • In sex chromosomes, leads to varying degrees of malfunction, but usually not lethal Turner Syndrome XO Characteristic facial features Web of skin Constriction of aorta Poor breast development Under developed ovaries Figure 8.22B Turner Syndrome Klinefelter Syndrome XXY Klinefelter Syndrome Abnormalities of Sex Chromosomes in Humans Other chromosome changes can cause birth defects or cancer Chromosomes break – pieces lost or rearranged - in somatic cells increases cancer risk - in gametes genetic disorders