Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Koinophilia wikipedia , lookup
Plant breeding wikipedia , lookup
Animal sexual behaviour wikipedia , lookup
Maternal effect wikipedia , lookup
Parental investment wikipedia , lookup
Developmental biology wikipedia , lookup
Drosophila melanogaster wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Flowering plant wikipedia , lookup
Parthenogenesis wikipedia , lookup
Meiosis and Sexual Reproduction Chapter 12 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011. Why Sex? Why Sex? Sexual reproduction • Reproductive mode by which offspring arise from two parents and inherit genes from both • ½ of each parent’s genetic information is passed to offspring • Variation in the forms and combinations of heritable traits diversity • Increase chance of surviving in a changing environment Asexual reproduction • All offspring are clones of their parents • Adapt same way to an environment equally vulnerable to changes Why Sex? An adaptive trait tends to spread more quickly through a sexually reproducing population than through a asexually reproducing one. Asexual reproduction • New combinations of traits can arise only by mutation and then the mutation is passed along Sexual reproduction • Mixes up the genes of individuals that often have different forms of traits • It generates new combinations of traits in far fewer generations than does mutation alone. Genes and Alleles Genes • Sequences of DNA that encode heritable traits Alleles • Forms of a gene the encode slightly different forms of the gene’s product • 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 Where Gametes Form anther (where sexual spores that give rise to sperm form) ovules inside an ovary (where sexual spores that give to eggs form) Flowering plant Fig. 9.3a, p.140 testis (where sperm originate) Human male ovary (where eggs develop) Human female Fig. 9.3b-c, p.140 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 Basis of sexual reproduction • Nuclear division mechanism • Halves parental chromosome number • Starts in germ cells (immature diploid reproductive cells) AND leads to the formation of gametes • Gametes mature, haploid reproductive cells (egg/sperm) Fertilization • Fusion of two gamete nuclei • Restores parental chromosome number • Forms zygote (first cell of new individual) Meiosis and Fertilization Homologues Sexual reproducers inherit pairs of chromosomes • 1 from maternal parent, 1 from paternal parent The pairs are homologous (“the same”) • Except nonidentical sex chromosomes (X and Y) • Same length, shape, genes All pairs interact at meiosis • One chromosome of each type sorts into gametes Homologous Chromosomes The Process of Meiosis All chromosomes are duplicated during interphase, before meiosis Two divisions, meiosis I and II, divide the parental chromosome number by two Each forthcoming gamete is haploid (n) • Haploid having one of each type of chromosome 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 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 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 Prophase I Metaphase I Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator Metaphase I 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 Anaphase I Telophase I Two nuclei form • Typically, the cytoplasm divides All chromosomes are still duplicated • Each still consists of two sister chromatids Telophase I Meiosis II The second nuclear division Sister chromatids of each chromosome are pulled away from each other Each is now an individual chromosome Prophase II Metaphase II Anaphase II and Telophase II In anaphase II, one chromosome of each type is moved toward opposite spindle poles • Occurs in both nuclei formed in meiosis I By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes Anaphase II Telophase II Meiosis I one pair of homologous chromosomes newly forming microtubules of the spindle spindle equator (midway between the two poles) plasma membrane breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus Prophase I Chromosomes were duplicated earlier, in interphase. Metaphase I Prior to metaphase I, one set of microtubules had 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 I Prophase I Metaphase I Anaphase I Telophase I Stepped Art 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 tugof-war. 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 Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei Haploid nuclei are distributed to daughter cells by way of cytoplasmic division Meiosis Introduces Variation in Traits Two events in meiosis cause variation in traits in sexually reproducing species • Crossing over during prophase I of meiosis • Chromosome shuffling during metaphase I of meiosis Prophase I: Crossing Over Nonsister chromatids of homologous chromosomes undergo crossing over • They exchange corresponding segments during prophase I of meiosis Each ends up with new combinations of alleles not present in either parental chromosome Crossing Over Fig. 9.6d, 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 1 2 3 combinations possible or or or Stepped Art Fig. 9-7, p.145 Key Concepts: CHROMOSOME RECOMBINATION AND SHUFFLING 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 Plants • Sporophyte: A multicelled plant body (diploid) that makes haploid spores • Spores give rise to gametophytes a haploid, multicelled plant body in which gametes form during the life cycle of plants From Gametes to Offspring Animals • Germ cells in the reproductive organs give rise to sperm or eggs • Sperm mature male gamete • Egg mature female gamete, or ovum • Fusion of a sperm and egg at fertilization results in a zygote • Zygote first cell of a new individual Comparing Plant And Animal Life Cycles meiosis zygote (2n) fertilization multicelled sporophyte (2n) DIPLOID meiosis HAPLOID gametes (n) meiosis multicelled gametophyte (n) spores (n) meiosis a Plant life cycle Fig. 9.8a, p.146 meiosis zygote (2n) fertilization multicelled body (2n) DIPLOID meiosis HAPLOID gametes (n) b Animal life cycle Fig. 9.8b, p.146 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 Sperm Formation in Animals Egg Formation in Animals Key Concepts: SEXUAL REPRODUCTION IN LIFE CYCLES In animals, gametes form by different mechanisms in males and females In most plants, spore formation and other events intervene between meiosis and gamete formation Comparing Mitosis and Meiosis Both mitosis and meiosis require spindles 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 Comparing Mitosis and Meiosis Key Concepts: MITOSIS AND MEIOSIS COMPARED Recent molecular evidence suggests that meiosis originated through mechanisms that already existed for mitosis and, before that, for repairing damaged DNA Animation: Comparing mitosis and meiosis Animation: Crossing over Animation: Egg formation Animation: Generalized life cycles Animation: Meiosis I and II Animation: Meiosis step-by-step Animation: Random alignment Animation: Sperm formation