580 Chapter 17 NEL
mature treeLodgepole Pine Life Cyclefertilization meiosisDIPLOIDHAPLOIDzygotesporophytesporesgametophytesgametesmitosismitosisFigure 10
Lodgepole pine life cycle. The
diploid cells formed at fertilization
undergo mitosis to form the
multicelled sporophyte (the tree).
The haploid stage starts when
meiosis produces spores. These
undergo mitosis to form a
multicellular gametophyte, which is
contained in the cones.
DIPLOIDHAPLOIDfertilization meiosisDIPLOIDHAPLOIDzygotemulticelled
bodyadult
personHuman Life Cycle
body cellsgamete-
producing cellsgametesmitosisFigure 11
Human life cycle. The diploid cells
formed at fertilization undergo
mitosis to form the multicelled body.
The haploid stage is the single-
celled gametes.
- Meiosis involves the formation of sex cells or gametes. All gametes produced by
meiosis have haploid chromosome numbers. - Cells undergoing meiosis pass through two divisions.
- Homologous chromosomes are similar in shape, size, gene arrangement, and
gene information. - Crossing over is the exchange of genetic material between homologous
chromosomes that occurs during meiosis.
SUMMARY Meiosis
Two Styles of Life Cycle
Some species undergo both sexual
and asexual life cycles. For example,
the spider plant can reproduce by
seeds (sexual reproduction) or by
runners (asexual reproduction).
Aphid females reproduce asexually
when the environment is stable, and
sexually when the environment
changes. Similarly, the male drones
in a honey bee colony are produced
by asexual reproduction, but the
female workers and the queens are
products of sexual reproduction.
DID YOUKNOW??