Alternation of Generations
Plants, algae, and some protists have a life cycle that regularly alter-
nates between a haploid phase and a diploid phase. As shown in
Figure 8,in plants, the diploid phase in the life cycle that produces
spores is called a (SPOH ruh fiet).Spore-forming cells in
the sporophyte undergo meiosis to produce spores. A is a
haploid reproductive cell produced by meiosis that is capable of
developing into an adult without fusing with another cell. Thus,
unlike a gamete, a spore gives rise to a multicellular individual
called a gametophyte (guh MEET uh fiet) without joining with
another cell.
In the life cycle of a plant, the is the haploid phase
that produces gametes by mitosis. The gametophyte produces gametes
that fuse and give rise to the diploid phase. Thus, the sporophyte and
gametophyte generations take turns, or alternate, in the life cycle.
In moss, for example, haploid
spores develop in a capsule at the tip
of the sporophyte “stalk.” When the
lid of the capsule pops off, the spores
scatter. The spores germinate by
mitosis and eventually form sexually
mature gametophytes. The male
gametophytes release sperm which
swim through a film of moisture to
the eggs in the female gametophyte.
The diploid zygote develops as a
sporophyte within the gametophyte
and the life cycle continues.
It is important not to lose sight of
the basic similarity of all three types
of sexual life cycles. All three involve
an alternation of haploid and diploid
phases. The three types of sexual life
cycles differ from each other only in
which phases become multicellular.gametophytesporesporophytennnn2 n2 nn nSpore-
forming
cellZygoteSporesMitosisMeiosisFertilizationSporophyte Gametophytes2 nGametesn
nDiploid (2n) Haploid (n)
Alternation of generations154 CHAPTER 7Meiosis and Sexual Reproduction
Identifythe type of reproduction that results in
offspring that are genetically identical to their
parent.Describetwo different types of eukaryotic
asexual reproduction.Comparethe haploid life cycle found in
Chlamydomonaswith a diploid life cycle.Summarizethe process of alternation of
generations.Critical ThinkingEvaluating Information
Evaluate the significance of mutations and repair of
mutations to the evolution of sexual reproduction.The amount of genetic
variation in offspring is greatest in organisms
that reproduce 6E
Asexually through meiosis.
Bsexually through fission.
Casexually through mitosis.
Dasexually through budding.TAKS Test PrepTAKS Test PrepSection 2 Review
Some organisms, such as
roses, have a life cycle that
alternates between diploid and
haploid phases.
Figure 8 Alternation of
generations
6C4B6E6E