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