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cience fiction suggests humans have much to fear from clones. In fact, we eat,
live and work with natural clones every day!
Natural clones
Organisms grow and repair tissues by cell division.
This means that one cell will divide into two, then
the two into four, and so on. Each time, the genetic
material is duplicated, and then passed on to every
daughter cell. Cells divide at different rates, some
divide every eight hours, while others may only divide
every one hundred days or so. Tissues such as skin,
or the growing tips of plant roots, can grow rapidly
because their cells divide quickly.
What are natural clones?
Clones are animals or plants that are genetically
identical. Believe it or not, there are clones walking
amongst us! Identical twins are natural clones. They
are formed when one zygote, or fertilised egg, splits in
half and develops into two individuals (hence they are
called monozygotic twins).
Mitosis
The process involved in cell division is called mitosis.
This can be observed through a microscope in
dividing cells that have been stained.
Mitosis produces two offspring cells with the same
genetic content as the parent cell. Single-celled
organisms reproduce by mitosis. Many multi-cellular
organisms can also reproduce using mitosis alone.
This is called asexual reproduction, and the offspring
are clones.
The process of mitosis
Diploid mother
cell
Parent cell
before mitosis
begins
DNA replicates
so that each
chromosome is
made up of
two chromatids.
Chromosomes
are pulled
apart towards
new cells.
Two diploid
daughter
cells
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Science Alive for VELS Level 6
Many plants also produce clones. Some plants
such as strawberries and bananas grow suckers,
which then develop into adult plants. This type of
plant reproduction is called vegetative propagation.
Cuttings of leaves or stems can be planted in soft soil,
and roots will often form from the site of the cut.
Cloning is very useful in horticulture because it
guarantees that all the features of the parent plant
will be present in the offspring. A single apple
tree that produces delicious-tasting apples can be
multiplied millions of times, and this new variety of
apple can be released to the public. If the tree were
allowed to reproduce sexually, its array of advantages
could be lost, as first its alleles could recombine
during gamete formation, and then combine with
another set of alleles during pollination.
Asexual reproduction is much less common in
animals, but many types of invertebrates rely on
asexual reproduction at times. Sponges produce a
mass of specialised cells which are released into the
ocean and grow into new sponges. A new starfish
can regenerate from the broken-off arm of a parent.
Hydra grow a new individual from the parent,
and when it is sufficiently mature it breaks off and
continues life independently. Some frogs and lizards
reproduce by parthenogenesis. This is where eggs
are laid and hatch into new individuals without being
fertilised. Often the whole species is female.
These plant cuttings grow quickly if dipped in
hormone powder, which stimulates cell growth.
REMEMBER
1 Complete the following paragraph.
During mitosis one parent cell gives rise to
daughter cells that have the
number of chromosomes as the parent cell.
Mitosis involves one round of DNA
and
cell division.
2 Name the two main roles of mitosis.
3 What is a clone?
COMPARE
4 Compare and contrast mitosis and meiosis using
the following table:
Clone your cat
Mitosis
They say that cats have nine lives, but your
family cat will only live for around 17 years
— if it is lucky. After that, in the ordinary course
of events, it will die and be gone forever. But, what
if you could produce an exact copy of your cat,
genetically identical, with all its traits and features!
Well, it has already happened. On the death of her
cat Nicky, an American woman paid US$50 000
for scientists to produce ‘Little Nicky’, an exact
replica, or clone, of her adult cat. The replica is
the genetic equivalent of an identical twin, but
obviously not naturally produced, and was also
born many years after its twin.
No. of cell divisions
No. of DNA replications
No. of daughter cells produced
No. of chromosomes in daughter
cells compared with parent cell
For growth and development?
Yes/No
For reproduction? Yes/No
THINK
5 People who have been exposed to large doses of
radiation often develop cancer, and their children
may be born with abnormalities. Explain how
these devastating conditions develop, include
the terms meiosis, mitosis and mutation in your
explanation.
�learning
Meiosis
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8. Genetics
I CAN:
understand how identical twins are born
describe the cloning process
understand why many plants produce by mitosis.
9efoYWj
I
n Jurassic Park, scientists extract
dinosaur DNA from mosquitoes
trapped in amber and place it in
surrogate eggs. After incubation, healthy
baby dinosaurs emerge from their shells.
Simple, right?
for an adult, differentiated lung cell and so on, it
becomes a totipotent stem cell (a cell from which
a whole organism may develop). This egg, with its
donor nucleus, is then transplanted into a host animal
for gestation and birth.
Cell nucleus
Somatic (non-sex)
cells removed
from udder
Artificial cloning
Technology for cloning single genes and whole
animals has been developed. Cloning a whole animal
to produce an identical clone is difficult. So far, DNA
taken from a dead organism has never been used
successfully to produce a live clone, because DNA
degrades over time.
There are two main ways to clone whole organisms.
Multicelled
Uterus
1
Mating
3
Developing egg removed
4
Egg splits into single cells
Eggs implanted in surrogate mothers
Cloned calves
Nucleus
removed
Nucleus
retained
Enucleated egg
retained
Nucleus
placed in egg
Many
attempts
Cloned
sheep born
Egg implanted
in sheep
In 1970, John Gurdon successfully cloned a frog
and it grew as far as the tadpole stage. In 1997, Ian
Wilmut and colleagues produced ‘Dolly’, the cloned
sheep (after 277 attempts). Since the cloning of
Dolly, many mammals have been cloned by different
laboratories around the world. It has been noticed
that some cloned animals suffer from disorders and
they often die young.
5
Nuclear transfer cloning
Nuclear transfer cloning involves producing identical
copies of an adult animal. This form of cloning can
produce identical copies of animals with the same,
known qualities. In this type of cloning a somatic cell
(or non-sex cell) is taken from an adult. Its nucleus
is then carefully extracted and placed into a fertilised
egg, which has had its own nucleus removed or
destroyed. The remaining contents of the egg cell
have the effect of ‘rebooting’ the adult nucleus, so
that instead of just producing the proteins necessary
Nucleus
sucked out
Uterus
2
1
2
3
4
5
Sharp
glass pipette
Ovaries
Eggs removed
from ovaries
Reproductive cloning
Reproductive cloning involves separating the cells
of a developing embryo and implanting them into
different host ‘mothers’ so that multiple genetically
identical offspring are produced. This is an artificial
way of simulating what happens when identical twins
are produced. It is used to produce multiple births
from stud animals in agriculture.
Suction
tube
(ABOVE) The world’s first cloned
cat, CC (Carbon Copy), born
December 2001. (RIGHT) The
world’s first cloned dog, an
Afghan called Snuppy, born
April 2005 after 1095 attempts
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Science Alive for VELS Level 6
Therapeutic cloning
Therapeutic cloning is a form of somatic cell nuclear
transfer aimed at the artificial production of organs or
tissue types for ‘replacement parts’. The nucleus of an
adult cell is extracted and inserted into an egg cell, and
this then grows into a number of totipotent stem cells.
But, unlike in whole animal cloning, certain factors
are added so that the cells grow and develop into a
particular tissue or organ, which can be transplanted
back into the patient.
While this approach has not yet lead to the
successful treatment of any condition, research
scientists have produced nerve cells, insulin-producing
cells and others, which could one day be used to treat
diseases including Parkinson’s disease, Alzheimer’s
and diabetes.
This technology could enable scientists to produce
donor organs that would not be rejected after
transplant. It is even hoped that genetic modification
could lead to mass production of donor organs that
would be accepted by any patient.
Totipotent
containing a gene from another species
Transgenic
a non-sex cell
Somatic
a cell from which a whole organism may
develop
Gene cloning is the very widely used technique of copying a single gene rather than a whole organism.
The gene can be inserted into other cells to be massproduced. Important medicines, such as insulin for
diabetics, are produced using bacterial cells that have
the human insulin gene inserted into them. Gene
cloning is also used to produce genetically modified
organisms, such as crops and stock.
The gene is inserted into a
virus or a plasmid that can
infect bacteria. It is used to
get the DNA into
the host cell.
Genes are
cut out of
DNA using
restriction
enzymes that
cut through at
specific points.
The cells are cultivated in large vats and
supplied with the required nutrients and oxygen.
The host cells
are grown and
tested to make sure that
they have the gene and it
is producing the desired
product.
The desired
product is
collected and
purified.
BRAINSTORM
5 In small groups, discuss why you think the following
technologies are ethically acceptable or not:
(a) human cloning
(b) animal cloning
(c) stem cell research.
RESEARCH
6 Dolly only lived to the age of six before being put
down due to ill-health. Other cloned animals have
also experienced ill-health and a short life span.
Research this phenomenon.
(a) How much evidence is there that these clones had
ill-health because they were clones?
(b) What theories are there as to why cloning may
cause illness and abnormalities?
(c) If cloning does often cause health problems, does
this mean it should not be undertaken?
�learning
REMEMBER
1 Describe the following types of artificial cloning:
(a) reproductive cloning
(b) nuclear transfer cloning
(c) therapeutic cloning
(d) gene cloning
2 Which form of cloning is the easiest to do and
why?
3 Match the following words with their definitions:
Gene cloning
Scientists are undertaking research to combine
cloning with the transplantation of genes so that
they can produce genetically modified experimental
animals. Work is also underway to produce genetically
engineered sheep and goats that are like mini
‘biological factories’ to produce medicines and other
products in their milk.
THINK
4 Why is stem cell research considered controversial?
Make a list of the arguments for and against stem
cell research.
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8. Genetics
I CAN:
understand the difference between reproductive and
nuclear transfer cloning
describe experimental research like therapeutic cloning
understand the process of gene cloning.