Download Bigsby - Bio S - 5 - Reproduction and Development

Eukaryotic cells reproduce through
mitosis, and sex cells are produced
through meiosis. This lesson will
feature reproduction of organisms and
how you developed from a single cell
into the complex organism you are
today.
All living things, from the smallest
bacteria to the biggest and most
complex animals, reproduce. All pass
genetic information in the form of DNA.
Simple prokaryotic organisms only
reproduce asexually. They do so by a
process called binary fission .
Most animals reproduce sexually, but
there are exceptions. Among
eukaryotes, there are many modes of
reproduction, both asexual and sexual.
Asexual Reproduction
Asexual reproduction usually involves
production of genetically identical
offspring from a single parent. Meiosis
and fusion of two genetically unique
gametes to form a unique offspring
characterize sexual reproduction.
There are many different forms of
asexual reproduction.
For example, taking a cutting from a
plant and rooting it is a form of asexual
reproduction, because a single parent
is used to produce a clonal offspring.
A clone is simply a genetically identical
copy of an organism and can be
anything from a single cell to a giant
tree.
It all comes down to asexual
reproduction.
Usually asexual reproduction involves
an exact copy of DNA and production
of clonal offspring, but asexual
reproduction does not always produce
Many plants can reproduce both
sexually and asexually. When a
plant is produced asexually from a
cutting, the offspring is clonal and
is produced from a single parent.
The same plant might also
reproduce sexually. Sexual
reproduction in most plants occurs
through pollination. Self-pollination
of a flower is a form of sexual
reproduction, because the flower
has both male and female
reproductive structures. Each
structure produces unique
gametes that fuse via fertilization
to produce a unique offspring.
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theimage
toenlarge.
However, because those offspring are
produced via fusion of gametes and are
unique, it is still sexual reproduction.
Some animals can do the same thing.
Some species of lizards are all females
and have eggs that divide by mitosis. Bees
also commonly reproduce asexually.
Let's look at the most common forms of
asexual reproduction.
Forms of asexual reproduction include:
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mitosis and cytokinesis
binary fission
budding
fragmentation and regeneration
parthenogenesis and apomixis
Mitosis, followed by cytokinesis,
occurs in many single celled
eukaryotes. This is the way human
cells reproduce to allow growth and
repairs. In this type of cell division,
DNA replication is followed by mitosis,
which is nuclear division. Cytokinesis
is division of cytoplasm and its
organelles.
The nuclear membrane reforms in the
clonal daughter cells. In the case of
unicellular eukaryotes, the daughter
cells are new organisms.
Binary Fission
Binary fission is simpler than mitosis
and cytokinesis. As in all forms of
reproduction, DNA must first be
replicated. Bacteria have a single ring
chromosome, and their DNA is in a ring
form.
Most unicellular eukaryotes are
protists. Protists are a very diverse
group. While many reproduce
asexually via mitosis, others reproduce
the same way as prokaryotes.
Prokaryotes reproduce asexually by a
process called binary fission .
Following replication, the two rings of
DNA attach to adjacent sites on the cell
membrane. The cell stretches between
the bound rings of DNA by adding
phospholipids. Eventually, enough
membrane is produced to divide the
cell in half. This is a form of
cytokinesis. Two clonal daughter cells
Budding
Budding is a form of asexual
reproduction that occurs in eukaryotes.
Bacteria reproduce very quickly. The
process of binary fission takes only a
few hours. Mutation often occurs
during DNA replication, so offspring
are not always clonal. Mutation is the
main source of genetic variation in
organisms that reproduce asexually.
In yeast, a kind of unicellular fungus,
DNA replication and mitosis occur,
dividing the nucleus. After that, the
new nucleus begins to bulge from the
cell membrane. Eventually, the bulged
portion which contains the nucleus
and other necessary cellular elements
buds off from the original cell.
In hydra (a microscopic marine
animal) cell division and
differentiation produce a new,
tiny copy of the parent
organism. The offspring actually
grows on the side of the
parent, eventually buds off, and
floats away. Even though the
offspring is a multicellular
organism, this is still a form of
asexual reproduction because
the offspring is genetically
identical to the parent that
produced it.
Regeneration and Fragmentation
Most plants and some animals are capable of
regeneration. This is due to totipotency of
cells, the ability of some cells to develop into
any type of cell or tissue in the organism's
body. Totipotent cells can also develop into a
Fragmentation often comes before
regeneration. It involves the parent
organism breaking into pieces.
Afterward, each fragment grows into
a new individual.
Many plants such as cacti break apart
easily. In cacti, spines (hard and solid
leaves) serve as a method of
transporting fragments far away
from the parent body.
This is common in animals called
planarians. Planarians are simple
flatworms that live in wet
environments.
Each piece can produce roots and
grow into a whole new plant.
Organisms such as starfish can also
regenerate broken segments. This
process usually does not create a
whole new organism, but reproduces
parts of an organism by mitosis.
Although not many plants fragment
spontaneously, most are capable of
regeneration.
Asexual propagation (another word
for reproduction) is a common way of
growing plants. In fact, most of the
plants that can be bought at a local
nursery or garden center are
propagated asexually and not from
seeds. Seeds are the product of
sexual reproduction.
Most organisms that reproduce
sexually must reduce their number of
chromosomes by half via meiosis.
In general, meiosis and genetically
unique offspring characterize sexual
reproduction.
Asexual reproduction does not involve
meiosis and almost always produces
genetically identical offspring.
However, there are some organisms
that don't fit any clear definitions of
sexual versus asexual reproduction.
Parthenogenesis and Apomixis
Sometimes two haploid cells
fuse to form a unique diploid
organism, while at other times,
the whole organism is always
haploid or diploid. In some
cases, there are no males,
only females, which leaves an
unclear line between sexual
and asexual reproduction.
Parthenogenesis and apomixis involve
production of offspring from gametes, but not
through regular fertilization. Sometimes the
process may be:
• asexual
• sexual
• clonal
In bees, fertilized eggs become
females. Unfertilized eggs
become males. Most of the
females are sterile. The queen
bee is fertile and produces
haploid gametes by meiosis.
The males produce haploid
Clearly the female offspring are
produced by sexual reproduction, but
there are many questions regarding
the process by which male bees are
There are lizards that are all female.
They are also all diploid and do not
produce unique haploid cells. Their
young come from eggs, but the
offspring are genetically identical to the
parents. This is clearly asexual
reproduction and is called
parthenogenesis.
A similar process called apomixis
occurs in plants. Haploid eggs develop
into haploid seeds and haploid adults.
In other cases, two haploid cells
produced by the same parent cell fuse
to produce a diploid offspring. The
offspring are not clonal. As you can
see, there are exceptions to every rule.
Life Cycles
For organisms that definitely reproduce
sexually, meiosis occurs to produce
haploid gametes at some point during
their life cycle. Sexual reproducers
have one of three life distinct life
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Zygotic Life Cycle: The zygote is the
only diploid form of the organism. A
zygote is the cell produced when two
haploid gametes fuse to form a diploid
nucleus.
In fungi, the haploid cells are not male
or female. Sometimes they are labeled
+ and -. In some, the diploid cells are
called (n + n) cells. The main thing to
remember is this: The zygote never
reproduces via mitosis. It quickly
undergoes meiosis to produce haploid
cells. The haploid cells then undergo
mitosis to produce a dominant haploid
form. All sexually reproducing fungi
and some algae have zygotic life
Sporic Life Cycle: This is a true
alternation of generations, which
means that both the diploid and haploid
forms undergo mitosis to produce
multicellular forms. Some algae and all
plants have sporic life cycles.
In algae and primitive non-vascular
plants such as mosses, the haploid
form is dominant. That means that the
biggest and longest-lived form has half
the number of chromosomes. The
smaller and short-lived reproductive
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the image
toenlarge.
In seed plants, the diploid form is
dominant. The plants that we are most
familiar with are seed plants. In fact,
most of the plants in the world today
are seed plants.
The diploid form cannot live
independently of the haploid form. It is
simply a reproductive structure.
In ferns, both the diploid and haploid
forms are free living. That means they
do not have to be attached to a more
dominant form to survive. The diploid
form is dominant, but it actually grows
out of the haploid form. The diploid
form produces haploid spores via
meiosis. The spores are released and
undergo mitosis to produce a
free-living haploid form. The haploid
form produces sperm and eggs, which
unite to produce a zygote. The zygote
undergoes mitosis on the haploid form
In these plants, the diploid form is
dominant. The haploid form cannot live
on its own. Usually the haploid forms
contain only a few cells. In flowering
plants, both male and female gametes
are produced in structures on flowers.
The sperm and egg are single cells, but
they are produced by tiny multicellular
structures that are haploid.
When a sperm fertilizes an egg, a
zygote is produced. It will develop into
a diploid seed. The diploid seed will
grow into a diploid plant.
Click on the image to enlarge.
Gametic Life Cycle: The gamete is the
only haploid form of the organism, and
it is always unicellular. All animals that
reproduce sexually, including humans,
have gametic life cycles.
Click on the image to enlarge.
The main difference between sexual
and asexual reproduction concerns
whether or not genetic variation is
introduced. The diversity of life on
earth is thought to be the result of
billions of years of change in the
genetic code of organisms acted on by
natural selection. In prokaryotes,
almost all genetic variation is the result
of mutation, conjugation,
transformation, or transduction.
You are a diploid organism. Shortly
after fertilization, the zygote produced
began to reproduce via mitosis. Never
again would you be haploid. You do
produce haploid sex cells by meiosis.
There are no multicellular haploid forms
in humans or other animals. Human
gametes cannot survive for long
outside the body.
Overall, you can tell what kind of life
cycle a sexually reproducing organism
has by knowing when mitosis occurs.
In zygotic life cycles, only haploid cells
undergo mitosis. In gametic life cycles,
only diploid cells do. In sporic life
cycles, both do.
Conjugation
Bacteria often contain a bit of extra
DNA called a plasmid, a small ring that
carries extra genetic information that is
not vital to life. It helps bacteria perform
functions such as resisting antibiotics.
Antibiotics are the drugs doctors often
prescribe to stop bacterial infections.
Some bacteria have learned how to
fight those drugs. The information for
how to do that is carried in plasmids.
Bacteria can actually exchange
plasmids by producing a conjugation
tube.
Even though bacteria always
reproduce asexually, there is
sometimes a way they join together to
Conjugation is not really a form of
reproduction, because no new
organism is produced. It is simply a
Some simple eukaryotes such as
protists also exchange genetic
information via conjugation. No new
organisms are formed, only new
combinations of genetic information.
Even more are present in environments
such as soil. Exchange of genetic
information by transformation is
thought to occur often.
Transduction
Transformation
A living bacterium can actually get
genetic information from a dead one.
Transformation is the process by
which bacteria can take up fragments
of DNA from a surrounding
environment. Many different species of
bacteria usually live together. For
instance, there are trillions of different
bacterial species living in your
Mutation
As with all organisms, mutation can
occur during DNA replication. Because
DNA must always replicate before
cells divide, this is the ultimate source
of genetic variation in living things. For
two billion years, when only
prokaryotes existed, this was the way
that evolution happened.
Once sexual reproduction evolved,
diversity of life increased dramatically.
Sexual reproduction remarkably
speeds up genetic variation.
Transduction occurs when a virus
called a bacteriophage puts DNA into a
bacterial cell. Remember that viruses
are not living things and are simply
DNA or RNA surrounded by protein.
The genetic material that viruses put
into hosts is often taken into the host's
DNA. This is why viruses cause
disease.
When gametes are produced via
meiosis in sexually reproducing
organisms, genetic variation is
introduced in several ways. Crossing
over of homologous chromosomes
during prophase I leads to genetic
recombination. This produces new
chromosomes that have never existed
before.
Independent assortment of
homologous chromosomes during
anaphase I produces many unique
haploid gametes. Fusion of gametes
via fertilization creates even more
variety.
Asexual vs. Sexual
Asexual reproduction is favorable for
some organisms in several ways. It
allows very fast production of many
genetically identical offspring and it
does not require a lot of energy. These
are excellent characteristics to
possess if the organisms in question
are well adapted to a relatively stable
environment.
There is little need to introduce
variation if a current system works.
Asexual reproduction is also helpful
for some plants and animals under
stressful conditions, when mates or
Think about it. Sometimes a deadly
disease will be introduced into a
population or organisms. If all members
are genetically identical, the disease
will affect all of them equally. With
genetic variety, some members are
more likely to survive.
Genetic variation is the core of
evolution by natural selection. It is
thought to have produced the
marvelously adapted organisms on
earth today.
Sexual reproduction introduces genetic
diversity, a characteristic that is
usually better for a species. Diverse
populations are better able to handle
environmental changes, and it is less
likely that all members of a population
will die when changes do occur.
Embryonic Development - Differentiation
Earlier you learned about totipotent
cells called stem cells. You also
learned that five to seven days later a
process called differentiation begins.
Let's take a closer look at what
happens once stem cells begin to
differentiate in vertebrate animals
(animals with backbones) like humans.
The mass of cells produced after
fertilization and several days of
division is called a morula. It contains
about thirty-two cells and is actually no
bigger than the zygote.
The morula contains small cells tightly
packed together. These cells continue
to divide, but at this point, different
genes get switched on and off. This
leads to secretion of fluids and
formation of a hollow space in the
midst of the cell mass.
Eventually, a hollow ball of around five
hundred to two thousand cells is
formed. This happens around ten days
after fertilization. Inside, a mass of
cells at one end called a blastocyst
forms. It is destined to produce the
new offspring. The rest of the mass
will form the protective sac in which
the offspring will develop.
Next a process called gastrulation
occurs. Certain groups of cells move
inward within the hollow ball of cells.
In this process, the blastocyst will
differentiate into three distinctive germ
(seed) layers. They are called the
endoderm, the mesoderm, and the
ectoderm.
Remember thatendo means in. Ecto or
exo means out. Meso means middle.
The germ layers are named for the
kinds of tissue offspring will develop
into.
After about three weeks, a process
called neurulation begins. During this
stage, the rest of the body plan is
determined. From this point on, the
offspring is destined for its final form.
Eventually, the endoderm will develop
into the linings of the digestive and
respiratory systems. The ectoderm will
develop into the outer skin and the
nervous system. The mesoderm will
develop into the bones, muscles, and
outer parts of most of the body's
Further development and growth occur
until a new offspring is born. In most
animals, development continues long
after birth. You are very different from
the baby you once were. As you grow
older, you will be very different from
what you are now. These changes
are called development, and they
occur throughout life.
View the fetal development chart on
As you can see, very early on some
important things happen. This is why
stem cells are so versatile. Once the
process of differentiation begins, there
is no turning back. Only stem cells
have the ability to produce any part of
the body. Once they change, a
one-way process is set into motion. A
single cell develops into an embryo, a
fetus, a newborn, a child, and an adult.
Even adults change. An old person is
different from a young one. This is true
for almost all living things.
This lesson began with dividing cells
and ends with old age, an interesting
journey.