Transcript 6.2 Sexual Reproduction

BC Science 9:
p. 189-223
Sexual Reproduction

Sexual reproduction brings non-identical gametes
together to form a new organism – it occurs in three
stages:
1. Mating – the process by which
gametes are brought together at
the same place and same time.
2. Fertilization – the process by
which egg and sperm join to
form a zygote (ie. a new
organism).
3. Development – the process by
which an embryo develops as an
organism.
Fertilization Requirements

In order for fertilization to produce a successful
developing embryo, certain conditions must be met:
1. Embryo must have enough
nutrients.
2. Temperature must not be too
cold or too hot.
3. There must be enough moisture
so that the embryo does not dry
out.
4. Embryo must be protected from
predators and items in the
environment that can potentially
harm it.
External Fertilization
In external fertilization, sperm and egg join
outside of the body.
 Advantages:

 Very little energy required to mate.
 Large numbers of offspring produced.
 Offspring can be spread widely in the environment (ie.
less competition between each other and parents).

Disadvantages:
 Many gametes will not survive.
 Many eggs will not be fertilized.
 Offspring are often not protected by parents, so many
of them die.
Internal Fertilization
In internal fertilization, sperm and egg join
inside the parents and the embryo is nourished
inside the mother.
 Advantages:

 Embryo is protected from predators.
 Offspring more likely to survive due to protection.

Disadvantages:
 More energy required to find a mate.
 Fewer zygotes produced, resulting in less
offspring.
 More energy required to raise and care for
offspring.
Pollination
Most plants transfer male gametes as pollen.
 Pollen can be carried by wind or by other organisms
(eg. bees).

Embryonic Development

Embryonic development is the early development of
an organism (eg. the first 2 months in humans).

The stages include:
 A ball of cells called a morula by the end of


1.
2.
3.
the first week.
Morula becomes hollow by the end of the
second week and becomes a blastula.
In the next stage, the embryo is known as a
gastrula and develops three distinct layers.
Ectoderm (Outside)
Mesoderm (Middle)
Endoderm (Inside)
Embryonic Development
Fetal Development
The cell layers now differentiate into the
organs and tissues of the baby.
 Fetal development is divided into three
trimesters.
 First Trimester (0-12 weeks)

 Organ systems and bones begin to form.

Second Trimester (12-24 weeks)
 Rapid growth and movement begin.

Third Trimester (24+ weeks)
 Continued growth, especially of the brain.
 Fat begins to deposit at 32 weeks to keep the
baby warm at birth.
Advantages and Disadvantages
of Sexual Reproduction