Reproduction
Reproduction is the process by which organisms produce offspring of their own
kind. Organisms can reproduce asexually or sexually. In asexual reproduction, only
one parent is required. Sexual reproduction involves the union of two reproductive
cells to form a new organism. A special kind of cell division produces these
reproductive cells.
Asexual Reproduction
The production of offspring without the union of special sex cells is known as
asexual reproduction. There is only one parent. Each of the offspring is an exact copy
of its parents because it receives copies of the parent’s chromosomes. An organism
that comes from one parent and is genetically identical to it is called a clone.
Many organisms reproduce asexually. The process can occur in a number of
different ways. The result, however, is always the same—one or more offspring
genetically identical to the parent.
The simplest form of asexual reproduction
occurs when a unicellular organism splits into two and
forms two new organisms. Bacteria, algae, and
protozoa commonly reproduce this way. This type of
reproduction is called fission. If the two resulting cells
are of equal size, the process is called binary fission.
The picture to the right shows a bacterium in
undergoing binary fission.
The growth of a small reproductive fragment
called a bud, from a larger parent organism is called
budding. In unicellular organism budding occurs when a parent cell forms a small bud
or knob by means of unequal cell
division. In this kind of fission, the nuclear
material is evenly divided, but the
cytoplasm is not. Thus, the bud has a
complete set of genetic instructions but
only a small amount of cytoplasm. The
bud may separate from the parent cell,
synthesize cytoplasm, and grow to be an
exact copy of the parent cell. Budding
in yeast cells is shown to the left. The
arrows are pointing to just some of the
budding yeast cells in the picture.
In multicellular animals, buds form as outgrowths from the parent’s body. The
cells in these buds divide, and the buds increase in size while
still attached to the parent. Eventually they fall off the parent
organism. They then develop into full-sized organism identical
to their parents. The picture to the right shows a hydra
budding.
Multicellular organism may also reproduce by
regeneration of body parts. This is commonly seen in the little
worm called planaria. Planaria,
as seen to the left, if cut into
two, each of the two pieces
has the ability to regenerate the
missing parts. It will even work if
you cut it into more than two
pieces. The only requirement is that some of the worm’s
primitive nervous system is contained in the piece and it
will regenerate.
Many organisms such as
molds reproduce asexually by
means of special reproductive
cells called spores. A hard coat
often protects spores. This enables them to survive
unfavorable environmental conditions. Spores are set free
from the parent and can form new organisms when
conditions are right. The picture to the left shows the fungus
that causes athlete’s foot. The spores are being pointed to.
Asexual reproduction in plants is called vegetative
propagation. In nature many plants reproduce themselves.
Some plants, like strawberries pictured below and ferns have stems that run along the
ground. These runners can take root and produce new plants. The eyes of potatoes
will grow new potato plants when planted.
Leaves and stems of many plants will form
roots when placed in water or a rooting
medium. Vegetative propagation is an
important technique used in agriculture to
make sure that the offspring of a desired
plant will be exactly like the parent.
Conjugation
Sometimes, simple organisms undergo a mating process called conjugation.
Organisms that undergo conjugation are bacteria,
some protists, and some fungi. This process is not like
mating in higher organisms that reproduce sexually.
During conjugation, DNA is passed from one strain of
organism to another strain of the same species. In
bacteria, DNA is passed through pili that grow out of
the donor bacterium and attach to the recipient
bacteria, as seen in the picture to the right.
In protists such as paramecia, conjugation
occurs when two paramecia become joined at their
oral grooves (where they take in food) by a bridge-like
structure. DNA material is exchanged after a series of
changes.
In fungi such as bread mold, conjugation occurs
when two different mating strains that look alike are
close together. Their “rootlike” structures of
attachment form short club shaped structures that
grow toward each other. When they touch, the nuclei from each type fuse in pairs
making a new organism. This can be seen in the picture to the left.
Conjugation often results in the transfer of operating genes from one bacterium
to another. Traits such as drug resistance in bacteria are transferred in this way.
Conjugation provides genetic variations that could lead to an increased chance for
species’ survival since it’s mixing up the genes in the population.
Sexual Reproduction
Sexual reproduction involves the fusion or joining of two special cells. These
sexual reproductive cells are called gametes. The fusion of two gametes is a process
known as fertilization. The cell produced by fertilization is called a zygote. The zygote
develops into the new organism by means of mitotic cell divisions. Gametes, the
special reproductive cells, are produced by a different kind of cell division.
A gamete from the female parent is known as an ova or egg. The male parent
produces gametes called sperm. Eggs and sperm differ from body cells in the number
of chromosomes they contain. Gametes have only half the normal number of
chromosomes. Cells with half the number of chromosomes are called haploid cells.
All other cells that have the full number of chromosomes are said to be diploid. Body
cells are all diploid. Diploid cells have to have an even number of chromosomes.
Meiosis
Not all cells reproduce by mitosis. A different type of cell reproduction is
important to the passing on of traits to offspring.
An Introduction to Meiosis
Besides body cells, most living things also have sex cells. Sex cells, or to use their
fancy name gametes, are special reproductive cells produced in sex organs. Sperm
are sex cells made by the male. An egg is a sex cell made by the female. Another
name for egg is ovum. Sex cells are made during meiosis. Meiosis is a kind of cell
reproduction that forms eggs and sperm that have half of the chromosomes that the
body cells do.
To explain how a cell goes through meiosis, let’s look at a cell with four
chromosomes. These four chromosomes make up 2 matching pairs as seen in the cell
at the top of the picture below.
In meiosis, a cell divides twice. When the cell divides the first time, each
chromosome in a pair moves away from its partner. Each chromosome of a pair goes
to a different cell. The sister chromatids stay joined together. The two cells then divide
again.
Look at the picture above. How many chromosomes are in each of the final
four cells? The number of chromosomes in each cell is one-half of the original number.
The original cell started with four chromosomes. Each chromosome had doubled to
make the X looking thing. Each new cell has only two chromosomes, or exactly half of
the original number. Lets look at the process of meiosis in more detail to see how the
chromosome number was halved.
The Steps of Meiosis
The steps of meiosis have very familiar names since they are the same as in
mitosis. The things that happen to the chromosomes are very similar as well. Because
there are two cell divisions in meiosis, all of the steps that happen in mitosis happen
then they happen again. It could get kind of confusing as to which step you are
talking about so the first set of steps are called interphase I, prophase I, metaphase I,
anaphase I, and telophase I. The second time the cell divides it uses the same steps
but this time they are called interphase II, prophase II, metaphase II, anaphase II, and
telophase II. Follow along in the diagram below as each step of meiosis is described.
Just before meiosis begins, in interphase I, the chromosomes double just like they do in
interphase of mitosis.
Prophase I
1. Sister chromatids shorten and thicken.
2. The nuclear membrane begins to break down.
3. Centrioles begin to move away for one another and fiber form.
4. Matching chromosomes now come together to form pairs.
5. Matching pairs wrap around each other and trade pieces (crossing over).
Note: steps 1-4 are the same as in prophase of mitosis. Step 5 is unique to meiosis.
Metaphase I
1. The centrioles have moved to opposite ends of the cell.
2. Sister chromatids are lined up in the middle of the cell.
3. Sister chromatids become attached to the fibers.
Note: Each chromosome is made up of sister chromatids, which are hooked together
by a centromere. The pairs of chromosomes are not hooked together.
Anaphase I
1. Fibers begin moving the matching chromosomes apart. The sister chromatids
remain joined in this step. In mitosis they separate.
Telophase I
1. The chromosomes are on opposite sides of the cell and the cell membrane
pinches the cell into two.
2. The cytoplasm and organelles are divided.
3. Two new cells have formed. Each cell has two chromosomes (2 X’s).
Interphase II
1. Usually very short.
2. Centrioles double.
This is the beginning of the second division. The remaining steps happen in both of the
new cells that were just made. The movement of the chromatids is the same as in
mitosis.
Prophase II
1. The centrioles move apart and fibers form between them.
2. Fibers attached to the middle of each of the sister chromatids.
Metaphase II
1. The sister chromatids line up in the center of the cell.
Anaphase II
1. Each chromatid in the chromosome begins to be pulled to the opposite side of
the cell.
Telophase II
1. Sister chromatids are on opposite sides of the cell.
2. Nuclear membrane forms around the chromosomes.
3. Cell membranes begin to pinch each cell in two along the center. The
cytoplasm is divided between the cells. The four new cells are sex cells or
gametes.
Remember that in our cell we started with four chromosomes.
We now have two chromosomes in each of four cells. In a
normal cell, there may be many more chromosomes. But, at
the end of meiosis, there will always be half the original
number of chromosomes in each of four gametes.
Sperm, Eggs and Fertilization
The picture to the left shows a human egg and sperm
cells. How are the cells alike? How are they different? The cells are alike in four ways.
1. Both are gametes.
2. Both formed during meiosis
3. Each as half the number of chromosomes found in body cells.
4. In humans, both cells begin to develop by meiosis at puberty. Puberty is the
stage in life when a person begins to develop sex cells. It takes place
between the ages of 10 and 15 most of the time.
How are sperm and eggs different? As you can see from the picture on the previous
page, eggs are much larger than sperm. Sperm have tails to help them swim, eggs do
not. Sperm form in the testes, eggs form in the ovary. In animals, testes are male sex
organs that produce sperm. In animals and some plants, ovaries are the female sex
organs.
During meiosis, each original male cell becomes four sperm just like in the
diagram of meiosis. Meiosis in males occurs all the time form the beginning of puberty.
Now look at how many eggs
are formed in the diagram below.
The main difference between male
and female meiosis comes when
the cells are dividing. The male
cells divide equally. The female
cells do not. Because the female
cell has the responsibility for
carrying all the food and
organelles the new organism will
need to live, the cell divisions
cannot be equal. The female cell
can’t keep the chromosomes so it
has to throw them away but it
doesn’t want to lose anything else.
What happens is unequal
cell division. One large cell is
formed and one smaller cell. This
smaller cell is called a polar body.
Since the polar body doesn’t have much cytoplasm and no organelles, it soon dies.
The large cell goes on to the second part of meiosis. When the large cell is ready to
divide in telophase II, it divides unequally again. This makes a very nice large ovum
and a very small polar body. The polar body does not develop and soon dies. The
result is a nice big ovum that has almost all of the food and organelles it began meiosis
with. An egg or ovum is formed once a month from the onset of puberty in females.
In animals, when sperm and egg join, the chromosomes from each cell also
come together. The new organism has a complete set of chromosomes in each one
of its body cells. Half the chromosomes in the organism come from the father, and
half come from the mother.
Sexual reproduction in plants
Flowering plants make our world beautiful to live in. Flowers come in a huge
variety of colors, shapes and sizes. But, plants don’t make flowers for our pleasure.
Flowers are important to the survival of the plant species. Without flowers, many plants
wouldn’t be able to reproduce sexually.
Flowers and sexual Reproduction
In the diagram to the left, you can see a section through one kind of flower.
Find the parts on
the diagram as
you read the
statements
describing them.
In most flowers,
some flower
parts are showy.
These parts are
neither male nor
female. In most
flowers some
parts are female
and other parts
are male. If you
find male and
female parts,
the plant must
reproduce sexually. Yes, plants have sex too.
Flowers that have both male and female parts are called complete flowers. An
example of a complete flower is a tulip. Some flowers have just female part or the
male parts. These are called incomplete flowers. An example of an incomplete flower
is a holly. There are male and female holly bushes. If you have just female bushes, you
will never get any bright red holly berries on any of them. The gametes from the male
bush’s flowers must fertilize the female gametes found in the female bush’s flowers.
The sepals and petals are neither male nor female. The sepals are found
around the base of the flower and are usually green, leaf like parts that protect the
flower when it is a bud. The petals are more involved in the reproduction process.
Petals are often brightly colored and scented parts of a flower. Their function is to
attract something (an insect or bird) so it can help with the pollination. Both sepals
and petals are special modified leaves.
The male reproductive organ in the flower is called the stamen. Usually there
are many stamens in each flower. Each stamen has a long stalk called a filament with
a saclike part at its top. The saclike part is called the anther. The anther makes the
pollen. Each grain of pollen contains male gametes (sperm cells).
The female flower reproductive organ is called a pistil. Often there is only one
pistil in the center of the flower. Each pistil has a large, round, ovary at its base.
Ovules are small, round parts found in the ovary. The ovules contain the egg cells of a
seed plant. Above the ovary is a stalk like structure called a style with a sticky end.
The sticky end is called a stigma. The stigma is sticky so the pollen will stick to it when it
get put there by the wind, an insect, or a bird.
If you have ever looked closely at a flower, such as a dandelion, a rose, a
carnation, or a daffodil, you may have noticed that none of these has the structure of
our typical flower in the picture on the previous page. Some flowers don’t have
sepals, some have petals that are fused to form a tube, some have many tiny flowers
all clustered together in a head, and dome don’t have any stamens. Flowers, just like
all other organisms, show an enormous range of variation.
Pollination and Fertilization
You know that in sexual reproduction, an egg and a sperm must join for
fertilization. How does pollen get from the stamens of a plant to the pistil? Then how
do sperm cells inside the pollen reach the eggs within the ovules?
Pollen is transported from the stamens to pistils by a process called pollination.
Pollination is the transfer of pollen from the male parts of a seed plant to the female
plant. Sometime this is a relatively short trip, sometimes this is a greater distance.
In flowering plants, the design of the flower is very important in pollination. Many
flowers with colorful petals have a strong scent that attracts insects. Insects climb over
the flowers and pick up the pollen on their bodies. Many insects, such as bees, collect
pollen for food. Some flowers also make a sugary chemical called nectar that bees
and birds also use as food. Pollen catches on the hairs and feathers of their bodies
while they feed on the nectar.
Some plants don’t have large, colorful petals or attractive scents. These plants
usually rely on wind to move the pollen from one plant to another. Grasses and many
trees have this type of flower. Many trees, such as maples and willows have long,
hanging flowers that are yellow or green in color. Their flowers are not attractive to
insects. The pollen formed in these flowers is blown around by the wind. So remember
in the spring when your car is covered with yellow pollen, its really plant sperm.
Pollen is often carried by insects, birds, or the wind between different plants of
the same
species. This is
called crosspollination.
Cross-pollination
is when pollen
from the stamen
of one flower is
carried to the
pistil of another
flower on a
different plant.
Self pollination is when pollen moves from the stamen of one flower to the pistil of the
same flower or of another flower on the same plant.
How does sperm inside a pollen grain reach the egg inside the ovary? Follow
the steps in the figure to the left, as they are decribed.
1. A pollen grain lands on the sticky tip of the pistil. It must somehow reach the
egg within the ovule.
2. The pollen grain begins to grow a tube into the stalk of the pistil. This pollen tube
grows all the way down to the ovary. Inside each pollen grain are three nuclei.
One nucleus leads the way down the tube. The other two nuclei are sperm
nuclei and these follow the first nucleus.
3. The tube reaches the ovule. A sperm nucleus inside the tube can now pass into
the ovule and join the ebb. Fertilization takes place. The other sperm from the
pollen tube joins with another nucleus in the ovule to form a food supply for the
new seed.
4. A new plant or embryo now grows inside the ovule. As the ovule grows, it
becomes a seed.