alternation of generation

Examples of alternation of generation in the following topics:

• Life Cycles of Sexually Reproducing Organisms

  • The main categories of sexual life cycles in eukaryotic organisms are: diploid-dominant, haploid-dominant, and alternation of generations.
  • There are three main categories of life cycles in eukaryotic organisms: diploid-dominant, haploid-dominant, and alternation of generations.
  • Species with alternation of generations have both haploid and diploid multicellular organisms as part of their life cycle.
  • Although all plants utilize some version of the alternation of generations, the relative size of the sporophyte and the gametophyte and the relationship between them vary greatly.
  • The germ cells are generated early in the development of the zygote.

• Seedless Vascular Plants

  • More than 260,000 species of tracheophytes represent more than 90 percent of the earth's vegetation.
  • Although seedless vascular plants have evolved to spread to all types of habitats, they still depend on water during fertilization, as the sperm must swim on a layer of moisture to reach the egg.
  • The life cycle of seedless vascular plants is an alternation of generations, where the diploid sporophyte alternates with the haploid gametophyte phase.
  • Throughout plant evolution, there is a clear reversal of roles in the dominant phase of the life cycle.
  • This life cycle of a fern shows alternation of generations with a dominant sporophyte stage.

• Liverworts and Hornworts

  • Thus, the life cycle of liverworts follows the pattern of alternation of generations .
  • The life cycle of hornworts also follows the general pattern of alternation of generations and has a similar life cycle to liverworts.
  • The haploid spores germinate and produce the next generation of gametophytes .
  • The life cycle of a typical liverwort follows the pattern of alternation of generations.
  • Both follow the pattern of alternation of generations.

• Plant Adaptations to Life on Land

  • Four major adaptations are found in all terrestrial plants: the alternation of generations, a sporangium in which the spores are formed, a gametangium that produces haploid cells, and apical meristem tissue in roots and shoots.
  • Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages (n represents the number of copies of chromosomes) .
  • Most plants exhibit alternation of generations, which is described as haplodiplodontic.
  • This distinguishing feature of land plants gave the group its alternate name of embryophytes.
  • Plants exhibit an alternation of generations between a 1n gametophyte and 2n sporophyte.

• Mosses

  • These structures are precursors of roots.
  • They originate from the base of the gametophyte, but are not the major route for the absorption of water and minerals.
  • The moss life cycle follows the pattern of alternation of generations .
  • A structure called a peristome increases the spread of spores after the tip of the capsule falls off at dispersal.
  • The alternation of generations cycle begins when the gametophyte germinates from a haploid spore and forms a protonema.

• Chromalveolata: Stramenopiles

  • The unifying feature of this group is the presence of a textured, or "hairy," flagellum.
  • Their characteristic gold color results from their extensive use of carotenoids, a group of photosynthetic pigments that are generally yellow or orange in color.
  • A variety of algal life cycles exists, but the most complex is alternation of generations in which both haploid and diploid stages involve multicellularity.
  • In humans, haploid gametes produced by meiosis (sperm and egg) combine in fertilization to generate a diploid zygote that undergoes many rounds of mitosis to produce a multicellular embryo and then a fetus.
  • Terrestrial plants also have evolved alternation of generations.

• Basidiomycota: The Club Fungi

  • The lifecycle of basidiomycetes includes alternation of generations .
  • Spores are generally produced through sexual reproduction, rather than asexual reproduction.
  • The haploid nuclei migrate into basidiospores, which germinate and generate monokaryotic hyphae.
  • Eventually, the secondary mycelium generates a basidiocarp, which is a fruiting body that protrudes from the ground; this is what we think of as a mushroom.
  • The lifecycle of a basidiomycete alternates generation with a prolonged stage in which two nuclei (dikaryon) are present in the hyphae.

• Characteristics of Fungi

  • While scientists have identified about 100,000 species of fungi, this is only a fraction of the 1.5 million species of fungus probably present on earth.
  • Some fungal organisms multiply only asexually, whereas others undergo both asexual reproduction and sexual reproduction with alternation of generations.
  • The roots of the plant connect with the underground parts of the fungus forming mycorrhizae.
  • Through mycorrhizae, the fungus and plant exchange nutrients and water, greatly aiding the survival of both species Alternatively, lichens are an association between a fungus and its photosynthetic partner (usually an alga).
  • In humans, fungal infections are generally considered challenging to treat.

• Alternatives to Dominance and Recessiveness

  • Mendel's experiments with pea plants suggested that: (1) two "units" or alleles exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype.
  • An example of codominance is the MN blood groups of humans.
  • Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type.
  • Example of a mutant allele interfering with the function of a wild-type gene
  • Discuss incomplete dominance, codominance, and multiple alleles as alternatives to dominance and recessiveness

• RNA Splicing

  • Alternative splicing is a process that occurs during gene expression and allows for the production of multiple proteins (protein isoforms) from a single gene coding.
  • This results in what is called alternative splicing .
  • The pattern of splicing and production of alternatively-spliced messenger RNA is controlled by the binding of regulatory proteins (trans-acting proteins that contain the genes) to cis-acting sites that are found on the pre-RNA.
  • Proteins that are translated from alternatively-spliced messenger RNAs differ in the sequence of their amino acids which results in altered function of the protein.
  • Alternative splicing is a common process that occurs in eukaryotes; most of the multi-exonic genes in humans are spliced alternatively.