EUKARYOTIC LIFE CYCLES - University of California, Davis · EUKARYOTIC LIFE CYCLES . The simplest...
Transcript of EUKARYOTIC LIFE CYCLES - University of California, Davis · EUKARYOTIC LIFE CYCLES . The simplest...
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 gameteshaploid and diploid phases, but the details differ
or “gametic”
Meiosis is a process for reducingchromosome 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
Diploid karyotype
meiosis
fertilizationHaploid karyotype
Sexual life cycles involve gamete fusion and meiosis to form gameteshaploid and diploid phases, but the details differ
or “zygotic”
Sexual life cycles involve gamete fusion and meiosis to form gameteshaploid 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 independentmulticellular haploid and diploid stages, but the partyou normally see and thatlives longest is thediploid stage
A pine tree is a diploid organism, but it producesmale and female multicellular haploid gametophytes. Thefemale gametophyte produces an egg. The pollen grain isdormant until it germinatesat the “micropylar” chamberand grows to the egg todeposit a sperm nucleus.
Summary
An asexual life cycle involves cells dividing solely by mitosis(with some differentiation of the daughter cells possible); cellscan be either haploid or diploid
A sexual life cycle involves alternating fertilization and meiosisevents, resulting in alternating haploid and diploid phases
Animals have a diplontic life cycle; fungi have a haplontic lifecycle; plants have multicellular diploid and haploid forms
Next: the life cycle of a flowering plant