Science 9.1.7 Reproduction Background Reading
Science Focus 9 Textbook Reading pages 26 to 36
Asexual Reproduction
Asexual reproduction occurs when young organisms are produced by just one parent.
All the offspring have the same genetic material (genes) as each other and as the parent.
They are said to be genetically identical organisms. Such a group of genetically identical
organisms is called a clone.
Asexual reproduction is normal among plants and single-celled organisms. It is less
common in animals except for simple types of organisms such as sponges that are found
growing in tropical seas. However insects, such as the aphid, are capable of asexual
reproduction at certain times of the year.
Advantages of Asexual Reproduction
It is fast and only requires one “parent.”
Only one parent is involved, but this
parent, to which the young are genetically
identical, is able to grow and survive
successfully.
The parent may supply food, water, and
other materials that the offspring need to
survive.
Rapid spreading may enable the species to
crowd out competitors.
No outside agents, e.g., to carry pollen or
spread seeds, are required.
Disadvantages of Asexual Reproduction
All offspring are genetically identical. This
reduces the variation in the population as a
whole. This makes it less likely that the
population will survive if environmental
conditions change.
Offspring may be less vigorous (not as well
adapted to survive).
Overcrowding may occur which will result
in competition for food, space, and water.
Among the common types of asexual reproduction are binary fission, budding, spore
formation, and various forms of vegetative reproduction in flowering plants.
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Binary Fission
Binary fission means splitting in two. It is a common method of reproduction in simple,
single-celled organisms such as an amoeba (an amoeba is a single-celled organism that is
commonly found in pond water). Binary fission also occurs in organisms such as
bacteria.
Step 1. The organism (amoeba) grows to its maximum size.
Step 2. The nucleus reproduces to form two complete and identical daughter nuclei – one
for each new cell.
Step 3. The cytoplasm is divided and the parent cell splits into identical daughter cells.
Budding
Budding occurs in yeast, a fungus which consists of a single cell. A bulge called a “bud”
forms on the yeast cell. The nucleus of the yeast divides and a daughter nucleus goes into
the bud. The bud gets bigger until it eventually separates to form a daughter cell. If
growth conditions are good, small buds may form on the large ones before the have
broken off. In this way, chains of yeast cells are form.
Spore Formation
Spore formation is the usual method of reproduction fro fungi including bread mould.
The body, or mycelium, is made of a tangled mass of fine threads called hyphae. The
hyphae spread through a suitable food source (such as bread), digesting and absorbing it.
Eventually, some hyphae grow upwards out of the food. Swellings called sporangia
develop at the top of these. Spores form inside the sporangia. When the sporangia bursts
open, these tough little cells are released and carried off into the air and water. They can
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survive very harsh conditions until they chance to land on some moist food. The spores
quickly germinate sending hyphae into the food and so forming new mycelium.
Vegetative Reproduction in Some Plants
When flowering plants reproduce sexually, seeds are formed. Plants formed from a seed
are often very vigorous and show a great deal of variety. However, asexual reproduction,
usually called vegetative reproduction, is common among flowering plants. This kind of
reproduction allows plants to spread much more quickly than by seed. Also, while young
seedlings are weak and vulnerable, young plants produced by vegetative reproduction can
often rely on food and water from the parent plant. On the other hand, like all offspring
produced asexually, such plants are all genetically identical (clones) and this lack of
variety can be a source of weakness for the species.
Runners
Runners are long, thin stems which grow along the ground from the parent plant. Roots
and buds appear at the ends of the runner and new plants develop from these areas.
Eventually, the runner rots, leaving each new plant on it’s own. Since each parent plant
produces a number of runners, a dense patch of plants can form, crowding out the
competition. Strawberries are plants that reproduce asexually by producing runners.
Tubers
Tubers are food storage organs of plants like a potato. During the summer, tubers form at
the ends of underground stems as they swell up with food (mainly starch) made by the
leaves (photosynthesis). Being stems, tubers have buds on them, commonly called
“eyes.” After the winter, these buds use the stored food to produce new shoots. New
plants, and finally more tubers grow from these areas.
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Rhizomes
Rhizomes, like tubers, are underground stems swollen with stored food. However, unlike
tubers, the food is stored all along the stem. Each spring, the terminal (end) bud uses
some food to send up new flowers and leaves. Food from these leaves is then sent back
for storage. Rhizomes last for years, but when the older plants eventually die, this leaves
the younger branches as separate plants.
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Bulbs
Bulbs consist of a central bud surrounded by thick, fleshy leaves swollen with stored
food. In spring, the bud uses the stored food to develop into green leaves and flowers
above the ground. These green foliage leaves now make food which the send
underground to their bases, causing them to swell. The bulbs have become next year’s
fleshy, food-storing leaves.
Artificial Propagation of Plants
Plant growers can make use of vegetative reproduction by plants to propagate (spread) a
useful variety very quickly. For example, by cutting tubers or rhizomes, and by break off
the daughter bulbs, plant growers can produce new plants that are genetically identical.
This is an advantage to the plant grower because they can produce identical plants to sell.
Cuttings
Cuttings are usually shoots cut from a desired plant. When planted in moist soil, they
grow roots and form a new plant. Geraniums are often spread by cutting.
Grafting
Grafting is used by commercial growers to get the most out of woody plants such as fruit
trees or rose bushes. Grafting allows new varieties of fruits or flowers to be supplied
quickly and abundantly to the market. Also, many commercial varieties of plants are to
delicate to survive and grafting provides them with a tough, disease-resistant root system.
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Sexual Reproduction
Sexual reproduction involves not one, but two parent organisms. The parents produce
special cells for reproduction. These reproductive cells are called gametes or sex cells.
Most species produce two distinct types of gametes—a male gamete usually called a
sperm cell or a female gamete usually called an egg cell.
During sexual reproduction, the male sex cell fertilizes the female sex cell. This process
is called fertilization. The new cell which is formed is called a zygote. The zygote, with
its complete set of genes, can now grow into a new organism. Have of its genetic material
(genes) have come from one parent and the other half of its genetic material (genes) have
come from its other parent. The young organism will be similar to both parents but it can
never be identical to either parent.
Advantages of Sexual Reproduction
Leads to increased variation in the
population. Increased variation, in turn,
provides the species with a better
opportunity of surviving changing
environmental conditions.
Offspring are stronger and more vigorous
(better adapted for survival).
Disadvantages of Sexual Reproduction
It is usually slower than asexual
reproduction.
Sexual reproduction requires two parents.
Young may be vulnerable during their early
days, weeks, or even years.
Outside agents (pollen or seeds) may be
required.
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Fertilization in Animals
Apart from some very simple species, all animals use only sexual reproduction to
produce offspring. In sexual reproduction there are two parent organisms—a male and a
female. The two parents produce special cells for reproduction. These are known as sex
cells or gametes. The gametes made by the male are called sperm. The sex cells
produced by females are called eggs.
The nucleus of a cell contains a number of chromosomes. These chromosomes carry the
genes, or instructions, which the cell needs in order to do its job in the organism.
Fertilization of the egg occurs when one sperm cell fuses to and then penetrates the egg
cell. This new cell is called a zygote.
External Fertilization
Sexual reproduction by animals is not possible without water. The sperms must have
water to swim in to reach the eggs. The eggs, both before and after fertilization, must be
kept moist or they will dry up and die. This is not a problem for animals that live in water
or return to water to reproduce. They simply release their gametes (egg and sperm) into
the water around them and let fertilization take place. This is called external fertilization.
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Internal Fertilization
Land animals cannot use external fertilization. If they did, their eggs would dry up and
die as soon as they were laid. Also, their sperms, with nothing to swim in, would not be
able to reach the eggs to fertilize them. For land animals, the only suitable place for the
eggs to be fertilized is inside the body of the female. This is called internal fertilization.
The advantage of internal fertilization are:
1. there is a moist environment for the sperm to swim and to keep the egg alive;
2. sperms are placed inside the female, there is a better chance that they will find and
fertilize the eggs; and
3. the eggs can be better protected and so are more likely to survive.
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Sexual Reproduction in Plants
The flower is the reproductive structure of flowering plants. Its job is to produce the male
and female gametes and them brought together so that fertilization can take place. This
results in the production of seeds from which the young plants will grow. Flowers come
in a huge variety of shapes, sizes, and colours and each kind of flower has it own
particular way of bringing about successful reproduction. The diagram shows the
different parts of the flower.
Pollination
The male gametes of a flowering plant are called pollen grains. Pollen is made on the
anther of the flower. When the anther bursts open, the pollen is released.
Pollination is the transfer of pollen from the male anthers to the female stigma. To help
them catch the pollen, stigmas may be sticky, feathery, or covered in fine hairs. During
pollination the pollen is usually transferred from the anthers of one flower to the stigmas
of a second flower. This is called cross-pollination.
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Insects, such as bees, are responsible for transferring the pollen from flower to flower.
Grasses and pine trees release large quantities of pollen into the air. The wind transfers
this pollen to other flowers.
Fertilization
Fertilization and pollination are two quite separate events in a flower’s life. After
pollination, a flower’s stigma may be covered with pollen. However, the female egg cells
are at the end of the carpel, inside the ovary. Ovaries contain the undeveloped seeds. The
eggs are contained in an embryo sac. For fertilization to take place, this egg cell and the
pollen must somehow get together. To make this possible, the stigma produces
chemicals, including sugar, which cause a sort of “root”, called the pollen tube, to grown
out of the pollen. The pollen tube grows down the style into the ovary and enters an ovule
through a hole. It penetrates the embryo sac and its male nucleus fuses with the egg’s
female nucleus.
Fertilization has now taken place and the ovule can begin to develop into a seed. The
fertilized egg cell, or zygote, becomes the embryo plant and the other parts of the ovule
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develop into the seed’s food store and protective coat. The ovary begins to change. It will
develop into a fruit, such as an apple, which will help to spread the seeds around.
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