Life Cycles:
Meiosis and the Alternation of Generations
Life Cycles Transfer Genetic Information
Plant reproduction can be either asexual or sexual reproduction. Both transfer
genetic information from parent to offspring. Some species can do both.
Asexual Reproduction – Any
type of reproduction NOT involving the
union of gametes (sex cells) or meiosis.
Each generation is genetically identical to
the previous generation. This type of
reproduction requires only one parent in a
sense. Clone- a group of genetically
identical individual organisms produced
asexually. Plants can clone themselves or
reproduce asexually by vegetative
propagation. In this process, a piece of the
vegetative part of a plant, the root, stem or
leaf, produces an entirely new plant
genetically identical to the parent plant.
Trees can be clones of each other and share
the same root system. Examples are
grafting, cuttings, bulbs and runners. Plant
Examples: Wandering Jew & Banana.
Trembling Aspens probably
connected underground.
Asexual Reproduction Transfers Unchanged
Genetic Information through Mitosis.
Asexual reproduction requires mitosis. An Individual cell divides into two
cells, each with the same number of chromosomes as the parent cell.
Similarly, one 2n cell (diploid) yields two 2n cells (diploid). During this
type of cell division, the chromosomes duplicate during interphase, then
mitosis (involving nuclear contents) begins.
Advantages of Asexual
Reproduction
- Requires only a single parent.
- Individuals are often adapted to
local environment.
- Asexual Reproduction can
happen quickly.
- No energy required for flower
production.
Disadvantages of Asexual Reproduction
- Genetic diversity remains fixed (Can’t adapt to environment); Genetic diversity is
essential for adaptation (genetic change).
Sexual Reproduction – Reproduction that requires meiosis
and fertilization for a complete life cycle. This involves the union of egg
and sperm. Egg and sperm are gametes. Each generation is a
combination of genetics provided by the previous generation. Before
union of gametes, the number of chromosomes are reduced during
meiosis so offspring have the same number of chromosome as parents.
Meiosis - a type of cell division occurring in sexual reproduction in
which two rounds of division convert one diploid (2n) cell to four
haploid (n) cells, effecting a segregation of homologous chromosomes.
Sexual Reproduction continued –
Humans have 23 chromosomes in one set. Humans have 2 sets (46
chromosomes) in their body. An egg or sperm has 23 chromosomes (1
set).
Cotton has 10 chromosome in a set.Cotton has 2 sets (20 chromosomes)
in its plant body. An egg or sperm has 10 chromosomes (1 set).
Haploid or n cells have one chromosome set. Gametes (Sex cells,
which are egg and sperm) are haploid.
When egg and sperm join, a zygote is formed.
n +
n
=
2n
Diploid or 2n cells have two chromosome sets. Zygotes and
regular plant body cells are diploid.
The two copies of a given chromosome in a diploid cell are said to be
homologous. This pair carries the same information (same genes).
Thus, a gamete of a cotton plant has 10 unpaired chromosomes and a
zygote has 10 chromosome pairs.
Alternation of Generations
Most plants alternate from n to 2n generations.
Zygote
Mitosis
2n
Sporophyte is 2n & is the diploid
plant body. Gametophyte is n &
produces & holds the gametes.
Diploid
Sporophyte
(2n)
2n
Fertilization
Meiosis
n
n
Gametes
n
n
n
n
Haploid
Gametophyte
(n)
n
n
Mitosis
Haploid
Spores
(n)
Life Cycle of a Cherry Tree (Typical Flowering Plant Alternation of Generations)
When the pollen tube reaches an egg, one sperms fuses with the egg to make
a zygote, which divides mitotically to from an embryo within a seed coat. An
embryo is a small sporophyte, and the large cherry tree is still a sporophyte.
Plants Vary in the Details of Their Life Cycles
Cone-bearing plants, ferns, mosses and algae differ in their life cycles when
compared to a cherry tree. Below is generalized life cycle of plants. It is
basically the same as two slides ago but explained a little differently.
Generalized life cycle of
organisms in the plant
kingdom & plant-related
relatives. No single
species has all the steps
shown here. The terms
used in this diagram are
basic, & many synonyms
exist for variations.
Karyogamy is when
nuclei fuse.
Plasmogamy is when
cytoplasmic contents
fuse.
Note: Gametangia are
cells or organs that has
haploid cells that
differentiate into
gametes.
Life Cycles
SPORIC – life cycle which includes alternating sporophyte
and gametophyte bodies. The sporophyte & gametophyte
bodies are multicellular. All embryophytes (Trees, Flowers,
Ferns, Mosses) have sporic life cycles. These are the more
complex (advanced) plants.
2 Different Types of Sporic Life Cycles
1) Heterosporic – has 2 kinds of spores & 2
gametophytes. The male spore is often lightweight &
can easily travel. The female spore often has much
stored energy and is immobile. Example: flowering
plants and conifers.
2) Homosporic – has 1 kind of spore & 1 gametophyte.
Therefore, male and female spores are together in one
structure. Example: Seedless plants such as moss.
Note: Some ferns are homosporic & other ferns are heterosporic.
Life Cycles
ZYGOTIC – A life cycle in which the only diploid phase is the singlecelled zygote. The zygote does NOT divide & does NOT proceed into a
multicellular stage. NO sporophyte present. NO multicellular 2n phase.
However, it is multicellular during the haploid stage.
Life Cycle of
Freshwater Green Algae
(Kingdom Protista)
Note:
Only one
multicellular
generation.
Life Cycles
GAMETIC – A life cycle in which only the gametes are haploid. All other
phases are diploid. NO multicellular gametophyte generation is present.
However, a multicellular sporophyte is present.
Life Cycle of Intertidal
Brown Algae, Rockweed,
(Kingdom Protista)
Note:
Only one
multicellular
generation.
The Diploid Generation Has Become Dominant Over Evolutionary Time
Dominant
means larger,
more
complex, and
having a
longer life
span.
BIO 141 Botany with Laboratory
•
This product is sponsored by a grant awarded under the President’s Community-Based Job Training Grants as implemented by the U.S. Department of
Labor’s Employment and Training Administration. The information contained in this product was created by a grantee organization and does not
necessarily reflect the official position of the U.S. Department of Labor. All references to non-governmental companies or organizations, their services,
products, or resources are offered for informational purposes and should not be construed as an endorsement by the Department of Labor. This product
is copyrighted by the institution that created it and is intended for individual organizational, non-commercial use only.