Life Cycles: Meiosis and the
Alternation of Generations
Chapter 12
Life Cycles
• Transfer of genetic information from parent
to offspring
• Two types of reproduction
– Asexual reproduction
– Sexual reproduction
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
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)
• Meiospore remains in ovule where it divides by mitosis
– Resulting cells divide 2 more times by mitosis to make a 7celled 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
endosperm
Alternation of Generations
• Zygote divides mitotically
– Forms embryo within seed coat
– Small sporophyte that will become cherry tree
when seed germinates
Embryophytes
• 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
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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
food
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
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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
gametophytes
• Only 1 set of chromosomes
• Every gene’s expression shows through in this
phase
Dominant Diploid Generation
• Dominance
– Means sporophyte lives longer, is larger, is
more structurally complex, and is more
independent than gametophyte