Advantages of AS
- Allows needed traits to be produced quickly and effectively.
- Creates the potential for added profitability.
- Creation of crops with a higher yield
- Production of crops with a shorter harvest time
- Creation of crops with higher resistance to pests and diseases
- Creation of new plant and animal varieties
Disadvantages of AS
- There can be unanticipated consequences of encouraging a specific trait.
- It lessens the amount of variation that exists in a gene pool.
- The quality of life of the organism being breed is often reduced
- poor traits also have the potential to be transferred because mutations are
spontaneous.
Angiosperms
- Angiosperms are flowering plants
that have flowers and produce seeds
enclosed within a carpel. - The carpel is the female
reproductive organ of a flower,
consisting of an ovary, a stigma, and
usually a style.
Spore
A dormant, reproductive cell formed by certain organisms..
Megasporangium
(female sporangia): produce megasporocytes that yield
megaspores.
Microsporangium
(male sporangia): produce microsporocytes that yield
microspores.
Sporophyte
A phase in the life cycle of certain plants and algae that starts
with the union of gametes. It is an asexual and usually diploid phase,
producing spores from which the gametophyte arises. It is the dominant
form in vascular plants, e.g., the frond of a fern.
Sporangium
The capsule structure belonging to many plants and fungi, in
which the reproductive spores are produced and stored
Alternation of Generations
- Alternation of generations is a type of life cycle in which subsequent generations of plants
alternate between diploid and haploid organisms. - The generations alternate between the sporophytes capable of creating spores and the
gametophytes, capable of creating gametes. - This type of life cycle is found in terrestrial plants and some algae.
Gametogenesis
as the biological process in which the formation of
gametes take place.
Male Gamete Formation in the Anther
- The formation of male gametes consist of two stages:
- Microsporogenesis
- Microgametogenesis
Gametogenesis in the Anther
Stamens are the male reproductive
organs of the flower.
* Stamen has two components: anther
and filament.
* The anther contains microsporangia.
* Each microsporangium contains
microspore mother cells or
microsporocytes.
Microsporogenesis
- Occurs in the microsporangium
- Microsporocytes (mother cells) are diploid cells
- Microsporocytes undergo meiosis to produce a tetrad of four haploid
microspores.
Microgametogenesis
- The single nucleus of each
microspore divides once by
mitosis and one of the two
daughter nuclei becomes a
generative nucleus and
accumulates little cytoplasm,
while the other becomes a
tube nucleus. - The generative nucleus
undergoes a single mitotic
division and forms two male
gametes or sperm cells. - These microspores form the pollen grain/ a
mature microgametophyte. - Pollens or microspores are very tiny round
structures. - After the formation, microspores or pollen grains
dry up and become powdery. The anther
becomes a dry structure and pollens are
liberated from the anther to the environment by
dehiscence of the anther. - Each pollen grain has two nuclei; one generative
nucleus and one tube nucleus
Megasporogenesis
- Megasporangium (the ovule) contains
megaspore mother cells. - Megaspore mother cells are diploid cells
(2n cells). These mother cells divide by
meiosis to produce haploid cells (n cells). - One mother cell divides by meiosis and
creates four haploid megaspores. This
process is known as megasporogenesis. - Megasporogenesis takes place within
the ovule. - In most plants, only one megaspore
develops into the megagametophyte and
the other three megaspores disintegrate.
Megagametogenesis
The surviving megaspore divides into eight nuclei
by consecutive mitotic divisions and produces
eight haploid daughter cells inside of the embryo
sac.
* 6 of the 8 nuclei migrate to opposing poles (3
each) while two nuclei remain at the center.
* The nuclei that remain at the center are known as
polar nuclei. These polar nuclei fuse to form the
secondary nucleus.
* The embryo sac with these eight nuclei represent
the mature megagametophyte.
Pollination
- Pollen a is powdery substance that serves to
move plant sperm to an egg. - A pollen grain has two cells: a tube cell and
a generative cell. The entire pollen grain is the
male gametophyte. - Pollination is the placement of pollen on the
stigma of the carpel. This pollen transfer can be
accomplished by:
o Wind
o Insects
o Built in mechanical discharge
o Other animals, including man
What happens after Pollination
- Once the pollen lands on the stigma, a
series of chemical reactions takes place
allowing the pollen grain’s tube
nucleus to begin producing a structure
called the pollen tube - The pollen tube digests its way down
the style toward the egg. - The pollen tube will work its way
through the style of the carpel and
touch the micropyle of the ovule.
Double Fertilization
- After penetrating the degenerated synergid, the pollen tube releases the two sperm into
the embryo sac - One sperm fuses with the egg and forms a zygote and the other fuses with the two polar
nuclei of the central cell and forms a triple fusion or endosperm, nucleus. - This is called double fertilization because the true fertilization (fusion of a sperm with an
egg) is accompanied by another fusion process (that of a sperm with the polar nuclei)
that resembles fertilization. - Fertilization can be defined as the fusion of the male gametes (pollen) with the female
gametes (ovum) to form a diploid zygote. - Double fertilization of this type is unique to angiosperms.
- The zygote now has a full complement of
chromosomes (i.e., it is diploid) - The endosperm nucleus has three sets of
chromosomes (triploid). - The endosperm nucleus divides mitotically to
form the endosperm of the seed, which is a
food-storage tissue utilized by the developing
embryo and the subsequent germinating seed. - The zygote undergoes a series of mitotic
divisions to form a multicellular,
undifferentiated embryo.
Seed
- An angiosperm seed consists of the:
1. embryo (2N) – this is the
immature sporophyte
2. endosperm (3N) which serves as
stored nutrients.
3. seed coat which protects the seed - The micropyle is the only opening into
the seed. It is through here that the
water will enter to start germination. - The seed contains stored food in the
form of seed leaves (cotyledons). Some
seeds contain one (monocots) while
others contain two seed leaves (dicots).
