chapter 14 outline
chapter 14 outline BIOL 1110
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This 5 page Class Notes was uploaded by Caitrín Hall on Tuesday January 26, 2016. The Class Notes belongs to BIOL 1110 at University of Connecticut taught by Bernard Goffinet in Summer 2015. Since its upload, it has received 12 views. For similar materials see Introduction to Botany in Biology at University of Connecticut.
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Date Created: 01/26/16
Reproduction, Meiosis, and Life Cycles 14.1 Sexual and asexual reproduction confer different advantages Sexual reproduction – fusion of male and female gametes to form zygote Gametes – single cells produced by adult organisms; specialized for mating Sperm – male gametes o Have flagella in seedless plants o Most lack flagella in seed plants—transmitted by tubes from germinating pollen grains to egg o Naked (wallless) o Produced by all plants Eggs – female gametes o Larger than sperm o Nonflagellete o Produced by all plants **Most protists produce gametes that look similar to each other** Zygotes – single cells that result from fusion of gametes; become adults under favorable conditions Meiosis – nuclear division process that reduces # of chromosomes by ½; required by eukaryotes in sexual reproduction Gets rid of deleterious genes faster (purging) than asexual; liberates beneficial genes Asexual reproduction Zygotes and meiosis lack Prokaryotic species Occurs rapidly, fewer resources, less energy than sexual 1 parent—don’t need partner All progeny are alike better chance of all surviving if environment is stable, homogenous, and similar to their parents’ More effective in harsh environments Many asexual evolved from sexual Arises when ability to reproduce sexually is lost o Ancestors could produce sexually or asexually Apomixis – reproduction exclusively by seeds formed asexually o Sexual reproduction is required for most plants to produce seeds Many produce sexually and asexually Phytophthora – protist that causes destruction of many crop plants Asexual cycle begins with sporangia – structures that produce and enclose spores – single-celled reproductive structures dispersed from parent (sexually or asexually) Tube-like body obtains food for growth/reproduction from leaves Food is used to produce branches with many sporangia that break off and are dispersed to other plant surfaces In asexual reproduction, each sporangium releases many genetically similar zoospores that disperse in water films and grow into new Phytophthora bodies In sexual reproduction, the hyphae produced by zoospores generate male and female gametes, and fertilization generates zygotes that are genetically diverse—contributes to protist’s ability to increase host range 14.2 Meiosis is essential to sexual reproduction Meiosis – cell division process associated with sexual reproduction Prevents buildup of chromosomes after sexual reproduction over many generations o Syngamy or fertilization – fusion of gametes during sexual mating; cytoplasm and nuclei combine o Haploid chromosome # = N (basic number of chromosomes present in gametes) o Diploid chromosome # = 2N (zygote) o Reduces chromosome number by half prior to gamete production Shuffles & recombines genetic info from each parent beneficial genetic variability o Separates homologous chromosomes – usually bear same numbers and types of genes (not always same DNA) o Each parent contributes one chromosome of each homologous pair o When cells undergo meiosis, chromosomes contributed by female parent mix with chromosomes contributed by male parent various combos of parental chromosomes 14.3 Meiosis resembles mitosis in some respects, but differs in important ways Features of meiosis Occurs in anthers and ovules 1 diploid cell (2N) undergoes 2 successive divisions to yield 4 haploid cells (4x1N) 4 haploid cells may be genetically different from one another and their parents DNA replication chromatids – identical copies of each chromosome that are attached by centromere until separation Meiosis I – separation of homologous chromosome pairs Homologous chromosomes form pairs that line up along the middle of the spindle and then separate (each homologue moves to opposite spindle pole Crossing over – chromosome segment exchange process o Homologous chromosomes paired in meiosis I can exchange segments to create new genetic combinations o Homologous chromosomes face each other at the first division o Genetic novelty o Chance determines how chromosomes align themselves during metaphase, how many chromosomes from male or female parent move to the same spindle pole during anaphase Nondisjunction – one or more homologous chromosomes do not separate normally at anaphase; results in abnormal meiotic products Meiosis II – chromatids are separated Two chromatids together constitute each chromosome Identical to meiosis I except cells resulting from meiosis have only half the number of chromosomes that were present before entering meiosis 14.4 Life cycles link one generation to the next Basic types of sexual life cycles: **Differ in the point at which meiosis occurs, relative importance of haploid/diploid, types of reproductive cells generate** Gametic o Animals & some algae o Meiosis occurs during production of gametes—represent only haploid stage o Primarily diploid organisms have 2 copies of necessary chromosomes, so if one is mutated, organism doesn’t suffer Alleles – variant gene; may become more useful than “normal” gene & cause evolution Zygotic o Fungi & most protists o Primarily haploid (only diploids are zygotes) Advantage: fast gamete production by mitosis; meiosis occurs during division of zygote—when conditions permit Disadvantage: no “spare genes” to mask mutation of an allele; only altered genes with little effect on organism’s survival are likely to be transmitted to next generation slow evolution Sporic = alternation of generations o Land plants & some algae o Meiosis produces spores instead of gametes o Mitosis produces gametes o Life cycle involves alternation of diploid (spore-producing spore generation) and haploid (gamete-producing gamete generation) Gametophyte generates eggs and sperms by mitosis Resulting zygote grows into plant sporophyte by mitosis until mature sporophytes produce spores by meiosis o Haploid gametophyte size has decreased while diploid sporophyte size has increased—reflects utility of homologous chromosome pairs Chapter Wrap-up Examine and Discuss Self Test 1. List several of the advantages and disadvantages that sexual reproduction confers on a species, and then do the same for asexual reproduction. 2. Why does meiosis always occur in the life cycle of any sexually reproducing organism; that is, why do sexual reproductionand meiosis always go together? What would happen if one occurred without the other? 4. Compare mitosis and meiosis, listing ways in which they are similar and ways in which they differ. 5. Draw diagrams representing gametic, zygotic, and sporic life cycles, showing where meiosis and mitosis occur. 6. Draw diagrams that explain the relationships among chromatids, chromosomes, and homologous chromosome pairs. How many total chromatids are present in a pair of homologous chromosomes at metaphase of meiosis I? 7. Draw a diagram of a pair of homologous chromosomes in the process of crossing-over, and then draw a diagram showing the four distinct types of chromatids that would occur in the products of meiosis II, as the result of one crossing over event. Use your diagrams to explain how crossing over contributes to genetic diversity. Applying Concepts 1. The sporophyte is the dominant plant body in both ferns and flowering plants, although both do produce gametophytes. Many fern gametophytes are about the size of your thumbnail and are green so can make their own food by photosynthesis. However, the gametophytes of flowering plants are much smaller, and are not green because protective sporophyte materials surround the gametophytes, which then don’t receive much light. How do you think flowering plant gametophytes get nutrients? 2. Growers can produce triploid plants, which are often robust and vigorous, but sterile. Suggest an explanation for this sterility in terms of the complications to meiosis that a triploid individual would experience.
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