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

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Life Cycles Meiosis and the Alternation of
Generations

• Chapter 12

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Life Cycles

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

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Alternation of Generations

• Refers to creation of both diploid and haploid
  bodies
• 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

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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

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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)

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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)
• Meiospore remains in ovule where it divides by
  mitosis
• Resulting cells divide 2 more times by mitosis to
  make a 7-celled female gametophyte

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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
  endosperm

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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

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

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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

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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

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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
  food

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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

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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

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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

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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

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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

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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

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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

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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
  gametophytes
• Only 1 set of chromosomes
• Every genes expression shows through in this
  phase

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Dominant Diploid Generation

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