Transcript 6.2 Sexual Reproduction

Sexual Reproduction
Two Different Parents: Sexual Reproduction
Sexual reproduction occurs with two different parents which
is different from asexual reproduction which happens with
just one parent.
Sexual Reproduction: The Source of Variation
Overview of Sexual Reproduction
In sexual reproduction, a male provides sperm which
fertilizes one or more eggs of a female. To make an
offspring, one sperm fuses with one egg to form a fertilized
egg or zygote.
The Chromosome Number in Sexual Reproduction
Each reproductive cell or gamete (sperm and egg) has one
set of chromosomes (n). When they fuse to form a zygote,
they make a cell with two sets of chromosomes (2n).
Egg Cells Control What Sperms May Fertilize Them
In each species (kind) of organism, their eggs make special
proteins and sugars on their cell membranes. For a sperm to
be allowed into an egg cell, its proteins and sugars must
match the proteins and sugars of the egg cell.
Urchin Egg Cells Recognize Their Own Type of Sperm
Two species of sea urchin, green and purple live in the same
environment. These sea urchins release their sperm and
eggs into the surrounding water. Sperm of purple sea
urchins is not recognized by eggs of green sea urchins.
Mating
Mating is the process by which a male places sperm near the
eggs of a female of the same species.
Internal Fertilization
Internal fertilization is when sperm are placed inside the body
of a female.
External Fertilization
External fertilization is when the sperm and eggs are released
outside the bodies of organisms to meet in the outside
environment rather than inside one of the parents bodies.
Sexual Reproduction in Plants
Many plants use sexual reproduction through cones or
flowers. Male reproductive cells are formed from the pollen
of plants and the egg cells are located in cones or flowers.
Pollination
In conifers and flowering plants, male reproductive cells
(pollen) fertilize eggs (in cones or in flower ovaries). The
transfer of pollen from anthers (male organs) to the pistil
(female organs) is called pollination.
Pollination and Double Fertilization
When pollen lands on a pistil, it grows a tube down through
the pistil towards the ovule in the ovary. As it grows, it
produces two sperms. One sperm fertilizes the endosperm
nuclei in the ovule and the other sperm fertilizes the egg
nucleus in the ovule. This is a double fertilization.
The Importance of Double Fertilization
The sperm and
endosperm nuclei
develop into the food
source of a seed. The
sperm fusing with the
egg develops into a tiny
embryo plant in the seed
What Flower Parts Develop Into
The ovary of a flower becomes a fruit. The ovules inside the
flower ovary develop into the seeds. Each ovule has
endosperm nuclei that become (when fertilized) the food
source in the seed and an egg which develops (when
fertilized) into a tiny embryo plant.
Examples of Flowers and Fruits
Examples of Flowers and Fruits
Human Development After Fertilization
The fertilized egg (zygote) divides 4 or 5 times until it forms a
ball of cells called the morula. The cells in the morula are
stem cells which can change into any kind of cell. The
morula continues to divide and forms a hollow ball of cells
called the blastula.
Human Development From Blastula to Gastrula
The hollow ball of cells (blastula) begins to form an
indentation which becomes a two cell layer ball of cells called
the gastrula.
From Two Cell Layers to Three
The gastrula then develops outgrowths of cells that move to
make a layer between the inner and outer layer. Now the
embryo has three cell layers. The middle layer is called
mesoderm, the outer layer is called ectoderm and the inner
layer is called endoderm.
Our Bodies Come From An Embryo With 3 Layers
The three layers all human bodies develop from are
ectoderm, mesoderm, endoderm.
Which Body Parts Come From Each Layer?
The endoderm layer
becomes the lining of
the digestive system.
The mesoderm layer
becomes our skeleton,
muscles, kidneys and
blood.
The ectoderm layer
becomes the skin and
nervous system.
Differentiation
As cells divide throughout the embryo, they start to change
their shapes and take on special jobs – they become
different. This process of becoming a specialized cell in both
structure (shape) and function (jobs performed) is called
differentiation.
The Placenta Nourishes the Fetus
The embryo and fetus develops an umbilical cord attached to an
organ called the placenta. The placenta is attached to the inside
of a mother’s womb (uterus) and transfers food and oxygen to
the fetus as well as taking wastes away like carbon dioxide and
ammonia. The mother supplies the placenta with food and
oxygen and also takes the wastes of the fetus away.
Human Development in Trimester One
A trimester is three months of time. During the first 3 months
(12 weeks), a human develops from a zygote to a blastula to
an embryo and then into a fetus. A fetus’ heart forms in 3-4
weeks as does its brain and spinal cord. All organs are
formed by 12 weeks, 3 months.
8 weeks (2 months)
The Second Trimester (Months 4-6)
A fetus grows rapidly during the second trimester. By 5
months, the mother can feel the baby moving and the baby’s
eyelids open. By month 6, the baby can survive outside the
mother with specialized care.
Birth at Around 9
Months
After about 9 months of
development, a baby
passes through its
mother’s pelvis. When
the baby’s chest is
exposed to air, the baby
begins to breathe using
its own lungs.
End of Presentation
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