EUKARYOTIC LIFE CYCLES
The simplest life cycles are asexual: prokaryotic (bacterial, archaeal)
and eukaryotic (protist, fungal) cells divide and separate
Bacillus cereus
Saccharomyces cerevisiae
The asexual “life cycle diagram” of a fungus might look like this:
Sexual life cycles involve gamete fusion and meiosis to form gametes
haploid and diploid phases, but the details differ
or “gametic”
Meiosis is a process for reducing
chromosome number from 2n to 1n
! Diploid nucleus
! Two chromosomes of each type
! One from each gamete in fertilization
! “Homologous chromosomes”
! Meiosis separates homologous chromosomes
! Result: one of each type of chromosome
meiosis
Diploid karyotype
fertilization
Haploid karyotype
Sexual life cycles involve gamete fusion and meiosis to form gametes
haploid and diploid phases, but the details differ
or “zygotic”
Sexual life cycles involve gamete fusion and meiosis to form gametes
haploid and diploid phases, but the details differ
“sporic” life cycle
A moss spends most of its life as a haploid organism (“gametophyte”): the diploid
stage (“sporophyte”) is multicellular but dependent
A fern has independent
multicellular haploid and
diploid stages, but the part
you normally see and that
lives longest is the
diploid stage
A pine tree is a diploid
organism, but it produces
male and female multicellular
haploid gametophytes. The
female gametophyte produces
an egg. The pollen grain is
dormant until it germinates
at the “micropylar” chamber
and grows to the egg to
deposit a sperm nucleus.
Summary
An asexual life cycle involves cells dividing solely by mitosis
(with some differentiation of the daughter cells possible); cells
can be either haploid or diploid
A sexual life cycle involves alternating fertilization and meiosis
events, resulting in alternating haploid and diploid phases
Animals have a diplontic life cycle; fungi have a haplontic life
cycle; plants have multicellular diploid and haploid forms
Next: the life cycle of a flowering plant