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Transcript
Chapter 9 Meiosis And Sexual Reproduction Genes and Alleles Heredity • The transmission of traits from one generation to next is called inheritance , or heredity Inheritance of Genes • Genes are segments of DNA that encode heritable information about traits Alleles – Slightly different forms of the same gene – Each specifies a different version of gene product Sexual and Asexual Reproduction • Asexual reproduction (1 parent) – Offspring inherit parent’s genes – Clones (identical copies of parent) • Sexual reproduction (2 parents) – Offspring differ from parents and each another – Different combinations of alleles – Different details of shared traits Sexual Reproduction • Meiosis, gamete formation, and fertilization occur in sexual reproduction • Meiosis and fertilization shuffle parental alleles – Offspring inherit new combinations of alleles Sexual Reproduction Meiosis • Nuclear Division process that halves the chromosome number in half • Basis of sexual reproduction Fertilization • Fusion of a sperm nucleus and an egg nucleus result in the formation a single - celled zygote Gamete • Reproductive cells • Vehicles that transmit genes from one generation to next • Male and female gametes (Sperm and egg) Where Gametes Form Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION • By asexual reproduction, one parent alone transmits its genetic information to offspring • By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles • Alleles are different forms of the same gene; they specify different versions of a trait What Meiosis Does • Meiosis – Nuclear division mechanism that precedes gamete formation in eukaryotic cells – Halves parental chromosome number • Fertilization – Fusion of two gamete nuclei – Restores parental chromosome number – Forms zygote (first cell of new individual) 2n germ cell germ cell each chromosome duplicated during interphase n MEIOSIS I separation of homologues MEIOSIS II separation of sister chromatids gametes gametes 2n diploid number restored at fertilization zygote Fig. 9.12, p.150 Homologues • Sexual reproducers inherit pairs of chromosomes – 1 from maternal parent, 1 from paternal parent • The pairs are homologous (“the same”) – Except non-identical sex chromosomes (X and Y) – Same length, shape, genes • All pairs interact at meiosis – One chromosome of each type sorts into gametes Homologous Chromosomes p.141b Meiosis I • The first nuclear division • Each duplicated chromosome lines up with its homologous partner • The two homologous chromosomes move apart, toward opposite spindle poles p.141c Prophase I • Chromosomes condense and align tightly with their homologues • Each homologous pair undergoes crossing over • Microtubules form the bipolar spindle • One pair of centrioles moves to the other side of the nucleus Prophase I (cont.) • Nuclear envelope breaks up – Microtubules growing from each spindle pole penetrate the nuclear region • Microtubules tether one or the other chromosome of each homologous pair Metaphase I • Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator Anaphase I • Microtubules separate each chromosome from its homologue, moving to opposite spindle poles • Other microtubules overlap midway between spindle poles, slide past each other to push poles farther apart • As anaphase I ends, one set of duplicated chromosomes nears each spindle pole Telophase I • Two nuclei form – Typically, the cytoplasm divides • All chromosomes are still duplicated – Each still consists of two sister chromatids Meiosis I one pair of homologous chromosomes newly forming microtubules of the spindle plasma membrane breakup of nuclear envelope Prophase I spindle equator (midway between the two poles) centrosome with a pair of centrioles, moving to opposite sides of nucleus Metaphase I Chromosomes were Prior to metaphase I, one duplicated earlier, in set of microtubules had interphase. tethered one chromosome of each type to one spindle pole and another set tethered its homologue to the other spindle pole. Anaphase I One of each duplicated chromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole. Telophase I One of each type of chromosome has arrived at a spindle pole. In most species, the cytoplasm divides at this time. Fig. 9.5a, p.142 Meiosis II there is no DNA replication between the two divisions Prophase II In each cell, one of two centrioles moves to the opposite side of the cell, and a new bipolar spindle forms. Metaphase II By now, microtubules from both spindle poles have finished a tug-ofwar. Anaphase II The sister chromatids of each chromosome move apart and are now individual, unduplicated Telophase II A new nuclear envelope encloses each parcel of chromosomes, so there are now four nuclei. Fig. 9.5b, p.142 Haploid Daughter Cells • When cytoplasm divides, four haploid cells result • One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies Key Concepts: STAGES OF MEIOSIS • Diploid cells have a pair of each type of chromosome, one maternal and one paternal • Meiosis, a nuclear division mechanism, reduces the chromosome number • Meiosis occurs only in cells set aside for sexual reproduction Key Concepts: STAGES OF MEIOSIS (cont.) • Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei • Haploid nuclei are distributed to daughter cells by way of cytoplasmic division Introducing Variation in Offspring • Three events cause new combinations of alleles in offspring: – Crossing over during prophase I (meiosis) – Random alignment of maternal and paternal chromosomes at metaphase I (meiosis) – Chance meeting of gametes at fertilization • All three contribute to variation in traits How Meiosis Introduces Variation in Traits Prophase I: Crossing Over • Non sister chromatids of homologous chromosomes undergo crossing over – They exchange segments at the same place along their length • Each ends up with new combinations of alleles not present in either parental chromosome Fig. 9.6c, p.144 Fig. 9.6d, p.144 Fig. 9.6d, p.144 a A maternal chromosome (purple) and paternal chromosome (blue) were duplicated earlier, during interphase. They become visible in microscopes early in prophase I, when hey star to condense to threadlike form. mom’s allele A mom’s allele B c Here is a simple way to think about crossing over. (Chromosomes are still c ondensed and threadlike, and each is tightly aligned with its homologous partner.) b Each chromosome and its homologous partner zipper together, so all four chromatids are tightly aligned. dad’s allele a dad’s allele b d Their intimate contact promotes crossing over at different places along the length of nonsister chromatids. e At the crossover site, paternal and maternal chromatids exchange corresponding segments. mom’s allele A mom’s allele A mom’s allele B dad’s allele b f Crossing over mixes up maternal and paternal alleles on homologous chromosomes. Fig. 9.6, p.144 Fig. 9.6f, p.144 Metaphase I: Chromosome Shuffling • Homologous chromosomes align randomly during metaphase I • Microtubules can harness either a maternal or paternal chromosome of each homologous pair to either spindle pole • Either chromosome may end up in any new nucleus (gamete) Chromosome Shuffling: Random Alignment CHROMOSOME RECOMBINATION AND SHUFFLING Key Concepts: • During meiosis, each pair of maternal and paternal chromosomes swaps segments and exchanges alleles • Pairs get randomly shuffled, so forthcoming gametes end up with different mixes of maternal and paternal chromosomes From Gametes to Offspring • Multicelled diploid and haploid bodies are typical in life cycles of plants and animals • Gamete Formation in Plants – Sporophyte: A multicelled plant body (diploid) that makes haploid spores – Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form) meiosis multicelled sporophyte (2n) zygote (2n) DIPLOID fertilization meiosis HAPLOID spores (n) gametes (n) multicelled gametophyte meiosis Plant life cycle (n) meiosis From Gametes to Offspring Gamete Formation in Animals – Germ cells in the reproductive organs give rise to sperm or eggs – Fusion of a sperm and egg at fertilization results in a zygote meiosis zygote (2n) fertilization multicelled body (2n) DIPLOID HAPLOID gametes (n) Animal life cycle meiosis Gamete Formation in Animals • In male animals • Germ cell develops into a primary sporocyte • After meiosis, 4 haploid cells are formed and develop into spermatids • Then it becomes sperm, a type of mature male gamete Sperm Formation in Animals Gamete Formation in Animals • In Female Animals • Germ cell becomes primary oocyte • Meiosis I followed by unequal cytoplasmic division produces a small polar body and a large secondary oocyte. Both are haploid • Meiosis II and unequal cytoplasmic division produces one large secondary oozyte and three small polar bodies • Secondary oozyte is also called ovum Egg Formation in Animals Comparing Mitosis and Meiosis • Both mitosis and meiosis require bipolar spindle to move and sort duplicated chromosomes • Some mechanisms of meiosis resemble those of mitosis, and may have evolved from them – Example: DNA repair enzymes function in both Differences in Mitosis and Meiosis • Mitosis maintains parental chromosome number – Duplicates genetic information – Occurs in body cells • Meiosis halves chromosome number – Introduces new combinations of alleles in offspring – Occurs only in cells for sexual reproduction Comparing Mitosis and Meiosis Comparing Mitosis and Meiosis no interphase and no DNA replication between the two nuclear divisions Prophase II All chromosomes still duplicated. New spindle forms in each nucleus, tethers chromosomes to spindle poles. Metaphase II All chromosomes aligned at the spindle equator. Anaphase II Sister chromatids of each chromosome moved to opposite spindle poles. Telophase II Four haploid (n) nuclei form. After cytoplasmic division, haploid cells function as gametes or spores.