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Life Cycles: Meiosis and the Alternation of Generations Chapter 12 – PowerPoint PPT presentation

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Title: Life%20Cycles:%20Meiosis%20and%20the%20Alternation%20of%20Generations

Life Cycles Meiosis and the Alternation of
  • Chapter 12

Life Cycles
  • Transfer of genetic information from parent to
  • Two types of reproduction
  • Asexual reproduction
  • Sexual reproduction

Alternation of Generations
  • Refers to creation of both diploid and haploid
  • Cherry tree life cycle
  • Cherry tree
  • Diploid part of life cycle
  • Referred to as sporophyte
  • Makes reproductive units called spores
  • Spores one celled reproductive unit that can
    develop into new plant without mating with
    another organism

Alternation of Generations
  • Forms two kinds of spores
  • One kind develops into male haploid plant that
    makes gametes called sperm cells
  • Other kind develops into female haploid plant
    that makes a gamete called an egg

Alternation of Generations
  • Male gametophyte formation occurs in pollen sacs
    of anthers
  • Meiospores (produced by meiosis) divide by
    mitosis to form male gametophyte ? pollen grain
  • Pollen grains released from anther
  • Pollen reaches stigma of female flower part
  • Pollen grain grows pollen tube
  • Contains two sperm nuclei (male gametophyte is
    now mature)

Alternation of Generations
  • Female gametophyte formation occurs in ovary
  • Chambers of ovary lined with ovules
  • Single ovule undergoes meiosis, produces 4
    haploid cells
  • 3 of the 4 cells degenerate
  • 1 remaining cell matures into female spore
  • Meiospore remains in ovule where it divides by
  • Resulting cells divide 2 more times by mitosis to
    make a 7-celled female gametophyte

Alternation of Generations
  • Pollination
  • Transfer of pollen to tip of pistil
  • Pollen tube reaches egg
  • One sperm fuses with egg to form zygote
  • Plasmogamy ? fusion of cytoplasmic contents
  • Karyogamy ? fusion of nuclei
  • Other sperm fuses with polar nuclei to form

Alternation of Generations
  • Zygote divides mitotically
  • Forms embryo within seed coat
  • Small sporophyte that will become cherry tree
    when seed germinates

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  • Embryophytes
  • Plants that shelter their offspring as embryos
    within parental body

Sexual Cycles
  • Sexual reproductive cycles can be of two types
  • Heterosporic
  • Makes two kinds of spores and gametophytes
  • One spore produced in large numbers
  • Small enough to be carried far away
  • One spore too heavy to travel
  • Contains plenty of food

Sexual Cycles
  • Homosporic
  • Makes one kind of spore and gametophyte
  • Spores too small to travel far
  • Most mosses and plants such as ferns
  • Plants not important in our food supply except as
    emergency foods

Comparison of Heterospory and Homospory
  • Heterospory
  • Makes 2 kinds of spores and gametophytes
  • 1 spore produced in large numbers and small
    enough to be carried far away, other spore too
    heavy to travel far but contains plenty of food
  • Seeds produced are part of our basic food supply
  • Homospory
  • Makes 1 kind of spore and gametophyte
  • Spores too small to travel far
  • Only important in human food supply as emergency

Types of Life Cycles
  • Zygotic or gametic life cycle
  • Life cycle that lacks sporophyte
  • No multicellular 2n stage
  • Example Chlamydomonas (green alga)
  • Sporic life cycle
  • Life cycle that includes alternating sporophyte
    and gametophyte bodies
  • All embryophytes, mosses

Zygotic of Gametic Life Cycle
  • Gametophytes
  • Single, motile cells with haploid nucleus
  • Genetically exist as plus or minus mating types
  • Gametophyte nucleus occasionally undergoes
    mitosis and produces haploid spores
  • Parent cell bursts
  • Releases spores that develop into new gametophyte
    generation cell

Zygotic of Gametic Life Cycle
  • Plus and minus mating types can mate
  • Plasmogamy and karyogamy occur
  • Results in 2n zygote
  • Zygote eventually undergoes meiosis
  • Releases haploid cells
  • Each cell matures into either a plus or minus
    gametophyte generation cell

Gametic Life Cycle
  • Example Fucus (brown alga)
  • Begins with multicellular sporophyte
  • Large and complex
  • Within body cavities of sporophyte
  • Cells enlarge, become sporangia, nuclei of cells
    undergo meiosis
  • 1 type of sporangium produces large meiospores
  • Other type of sporangium produces small meiospores

Gametic Life Cycle
  • Large meiospore differentiates into female
    gametophyte (egg)
  • Smaller meiospores differentiate into male
    gametophytes (sperm)
  • Gametes released into surf in large numbers
  • Eggs from one parent and sperm from another
    parent fuse
  • Egg and sperm from same plant not attracted to
    each other

Gametic Life Cycle
  • Plasmogamy and karyogamy occur
  • Zygote begins to divide and grows into sporophyte
  • Sporophyte enlarges, sinks to bottom, attaches to
    rock, grows into maturity
  • Only haploid phase is a single-celled gamete
  • No multicellular gamete generation in a gametic
    life cycle

Dominant Diploid Generation
  • Gametic and zygotic life cycles common among
    algae but absent from any more advanced plants
  • Sporic life cycles are rule among complex
    terrestrial plants
  • Increasing dominance by sporophyte in groups more
    recent in fossil record

Dominant Diploid Generation
  • Diploid condition
  • Permits recessive genes to be carried along from
    generation to generation
  • Could be valuable to species future
  • No recessive genes in haploid cells of
  • Only 1 set of chromosomes
  • Every genes expression shows through in this

Dominant Diploid Generation
  • Dominance
  • Means sporophyte lives longer, is larger, is more
    structurally complex, and is more independent
    than gametophyte