Download 5.2.1 Cloning in Plants and Animals

OCR A2 F215 CLONING IN PLANTS AND ANIMALS
Specification:
a) Outline the differences between reproductive and non-reproductive
cloning
b) Describe the production of natural clones in plants using the example
of vegetative propagation in elm trees
c) Describe the production of artificial clones of plants from tissue
culture
d) Discuss the advantages and disadvantages of plant cloning in
agriculture
e) Describe how artificial clones of animals can be produced
f) Discuss the advantages and disadvantages of cloning animals
DEFINITION OF A CLONE

A clone is a group of genetically identical genes, cells or whole
organisms

Genetically identical means that individuals in the clone have
identical DNA with identical chromosome numbers

Cloning is a natural process for growth and asexual reproduction

Natural cloning of cells involves : DNA replication in cells, mitosis
in eukaryotic cells (or binary fission in prokaryotic cells) followed by
cell division

Cloning can also be achieved artificially

To produce identical DNA copies during cloning, it is assumed that no
mutation occurs during the process
REPRODUCTIVE AND NON-REPRODUCTIVE CLONING

Reproductive cloning is the cloning of new organisms with the
same genotype of the existing organism

Non-reproductive cloning is the production of genetically identical
cells, tissues and organs, so that damaged tissues and organs can
be replaced. Some examples are the regeneration of cardiac muscle
tissue after a heart attack and the repair of the spinal cord in those
paralysed by a broken neck/back
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NATURAL CLONING IN PLANTS
Natural cloning in plants is called asexual reproduction or vegetative
propagation. This occurs in a number of different ways. Some examples are
given below. Examples (1) and (2) are for interest only. Example (3) is on the
specification
1. Growing potato plants from potato tubers
Potato tubers are the swollen underground stems that we eat as the
vegetable. Each tuber is able to produce shoots and roots and will
grow into a new potato plant if planted in soil
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2. Growing strawberry plants from strawberry runners
After producing strawberry fruits, the strawberry plant sends out
horizontal stems along the ground, called runners.
New plants grow at the nodes, along the stem. Initially, the new plant
obtains its nutrients from the parent plant. When the new plant has
leaves and can produce its own organic nutrients, the runner dies back
3. Vegetative propagation of elm trees by root suckers

The English Elm (Ulmus procera) is one of our native trees that
reproduces asexually by vegetative propagation

English Elms developed from a variety of elm brought to Britain
from Rome in the first century A.D.

Although English Elms produce flowers and fruits, the fruits
contain no seeds and therefore new Elm trees cannot be
produced by sexual reproduction

Instead, Elm trees reproduce by developing structures known as
suckers from their roots. These are shoots that grow from
meristem tissue below ground level, usually from roots

Each root sucker can grow into a new tree

All English Elm trees are clones of each other

English Elms are genetically isolated from other Elm varieties
because they cannot interbreed with them. English Elms can
only reproduce asexually
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Dutch Elm Disease and Devastation of the English Elm Population
in Britain

In 1967, a new virulent strain of an Elm disease fungus arrived
in Great Britain on imported timber

Beetles that lived under the bark of elm trees spread the fungus

The saws used to cut down dead branches were not sterilised
after use. When the saws were used to prune healthy trees,
these trees were infected

Approximately 25 million elm trees, most of the English Elm
population, died within a few years of the arrival of this fungus

The dying Elm trees responded by producing root suckers that
grew into new trees

Unfortunately, the new trees were clones of the original and
being genetically identical to the parent tree, also lacked
resistant genes to the disease. There was no variation on which
natural selection could act. The new Elm trees did not survive
and died from Dutch Elm disease also
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Why Dutch Elm Disease led to such a Rapid Loss of English Elm Trees

All trees were genetically identical

Therefore, all trees were susceptible to this disease; none were
resistant

Beetles are the vector of the disease and can fly from infected tree to
healthy tree to spread the disease

The beetles only live on the target tree, that is the English Elm

Since the trees only reproduce asexually from root suckers, new trees
are very close together in clonal patches

Tree surgery (cutting out infected branches) was used as a control
measure. However, these attempts at control contributed to the
spread of the disease. As more trees became infected then more
tree surgery was necessary and more saws/surgery equipment were
contaminated

Dead leaves left on the ground also carry the fungus and are a source
of infection for young trees
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ADVANTAGES OF ASEXUAL REPRODUCTION

It is a rapid method of reproduction since only one parent is required

Large numbers of offspring can be produced to rapidly colonise an
area

All offspring are genetically identical to the parent and can
therefore thrive in their existing environment
DISADVANTAGES OF ASEXUAL REPRODUCTION

Offspring have no genetic variation and will have the same less
favourable features as the parent

If the environmental conditions change (as they did for the elm
trees) the cloned organisms may not be able to evolve and adapt to
survive
ARTIFICIAL CLONING IN PLANTS
Farmers and plant growers have been propagating plants artificially for many
years. Note that methods (1) and (2) below are for interest only. Method (3)
is on the specification.
(1)Taking Cuttings
Stem, leaf or root cuttings are taken to clone grape vines and other
crop plants. This method is used on a large scale in some plant
nurseries
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(2) Grafting
A shoot section of a woody plant with desirable features (eg a fruit tree
or rosebush) is joined to an already growing root stock. The grafted
plant grows into a fruit tree/rosebush that is genetically identical to the
parent plant that provided the shoot section with desirable features.
The rootstock is different genetically
(3) Artificial Propagation in plants using Tissue Culture (on the
specification)
Definition of Tissue Culture:
The production of plants from a small amount of plant tissue that is initially
grown in or on a sterile nutrient medium
Features of the Use of Tissue Culture

This technique can be used to produce very large numbers of
genetically identical plants from a small amount of plant tissue from just
one, or very few parent plants with desired features

Tissue culture is used to produce plants that do not naturally reproduce
asexually or are difficult to grow from seeds

It is used for the production of more expensive and rare plants such as
orchids

It can be carried out at any time of year, in any country as it is started
in a laboratory. Plants are only transferred into soil in glasshouses
when they are well developed
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Procedure for Tissue Culture
1) Small separate groups of meristematic cells are removed from a
plant. These groups of meristematic cells are called explants
2) The explants are dipped into a bleach solution to sterilise the surface
of the cells
3) The explants are placed in a sterile aerated nutrient growth medium
solution containing:





glucose (energy source)
amino acids
vitamins
inorganic ions such as K+, Mg2+ , PO4- NO3plant growth substances such as auxin and cytokinin to
stimulate cell growth and division
4) The undifferentiated cells in the explants divide repeatedly by mitosis
to produce separate masses of undifferentiated/totipotent cells each
called a callus
5) Each callus can be subdivided into smaller groups of callus cells that
can be transferred onto sterile agar jelly containing a mixture of plant
growth substances that cause the development of shoots and then
roots. This technique is called sub-culturing
6) Inducing root or shoot growth involves changing the plant hormone
ratio. Root growth requires a high auxin to cytokinin ratio (100 auxin to
1 cytokinin). Shoot growth requires 4 auxin to 1 cytokinin
7) All the procedures described require aseptic conditions to
prevent growth of bacteria, fungi and algae in the culture media
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8) When the plants are large enough, they can be transferred to a less
controlled environment and grown in soil in a greenhouse to grow
further
ADVANTAGES AND DISADVANTAGES OF PLANT CLONING IN AGRICULTURE
Advantages

A quick method to produce a large number of offspring plants

Disease- free stock can be produced

All plants have the same features/phenotype eg grow at the same
rate/grow to the same height

Can be used to reproduce infertile plants

Can reproduce plants that are difficult to grow from seed
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
Can produce plants from calluses that have been genetically
modified

Production is not determined by the seasons. Tissue culture
techniques can be carried out in any season and anywhere in the world

Small plantlets can be transported easily (including air freighted) and
grown in a small space

The techniques can be used to culture rare species to save them from
extinction
Disadvantages

Tissue culture is expensive and labour intensive

Whole batches can fail if there is microbial contamination

There is no or very little genetic variation

Therefore, all offspring are susceptible to the same diseases, pests
and changing environmental conditions such as none of the plants are
drought resistant
ARTIFICIAL CLONING IN ANIMALS
Reproductive Cloning – the production of whole new animals by cloning
techniques
The techniques of reproductive cloning are commonly used in the selective
breeding of animals, such as dairy cattle and sheep
Two Methods of Cloning Animals
1) Splitting Early Embryos (16-32 undifferentiated cells)

A female with the desired features is treated with hormones so that she
super-ovulates (produces many egg cells at the same time)

The eggs are collected and fertilised in vitro (in a petri dish) by sperm,
from a desirable male

Cells from the embryos produced are separated into several groups.
Each group of cells can produce a new genetically identical individual

Each embryo can be implanted into a surrogate mother. These
surrogate mothers are less valuable cows in terms of desirable
features. These cows will have been treated with reproductive
hormones to ensure that their uteruses are ready to receive an embryo
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
The embryos develop normally and are born in the usual way.

The female with the desirable features, from which the embryos were
developed, is not put at risk during pregnancy and is available for more
super-ovulation

All the calves are clones of each other, but not clones of their surrogate
mothers or of either parent that provided the gametes. Why not?
2) Somatic Nuclear Transfer – using enucleated eggs
This method was used to produce Dolly the sheep, the first mammal to be
produced in this way by cloning

A nucleus from a body cell (somatic cell) of one adult mammal, is
inserted into an enucleated egg cell (an egg cell with its nucleus
previously removed) of another adult female of the same species,
using electric shock stimulation. This is called somatic nuclear
transfer

The diploid cell produced is grown on in vitro to produce an early
embryo
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
This early embryo consisting of undifferentiated cells, can be divided
into several smaller groups of embryonic cells

Each small embryo can be placed into a surrogate mother’s uterus to
complete development. In this way, several offspring can be produced
using several surrogate mothers. The offspring are clones of each
other and of the parent that donated the original somatic cell
Label the haploid and diploid nuclei in this diagram. Indicate where
electric shock treatment is used. Add diagrams to show how several
cloned offspring could be produced from one embryo. Indicate where
meiosis and mitosis occur.
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Table of Advantages and Disadvantages of Cloning Animals by
Reproductive Cloning (compared with traditional breeding techniques)
ADVANTAGES
The number of animals with the
required characteristics is increased
(if a question indicates details of
these characteristics, be specific in
your answer and refer to them)
DISADVANTAGES
Animals are genetically identical –
and unable to adapt to changing
conditions in their environment.
These animals are therefore more
susceptible to disease
The reproductive rate of a desired
animal is increased – many offspring
produced in a short time
Cloning success rate is very poor
The technique can be used to breed
endangered animals, without
requiring a fertile female of the
species.
The technique is labour intensive and
needs more technology – more
expensive
The technique can be used to
produce transgenic animals
(genetically modified )
Cloned animals may have a shorter
life span.
Dolly was not particularly healthy and
died at a relatively young age.
The body cell used for the nuclear
transfer technique is from an older
sheep. Its DNA therefore is
chronologically older than the newly
born offspring sheep. DNA does
degrade as it ages and cells have a
limited capacity for cell division (the
Hayflick limit)
Animals are being denied their natural
instincts and behaviour – ethical
objections. (note that in a recent
examination question of this topic –
reference to these ethical objections
was not on the mark scheme)
Examples include transgenic cows
that produce milk with human factor
IX and transgenic goats that produce
milk with a spider silk protein used to
produce bullet – proof vests
Avoids the risks of mating males and
females (physical damage and
transfer of disease)
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Non- Reproductive Cloning in Animals – the production of genetically
identical animal cells and tissues
Uses of Non-Reproductive Animal Cloning
Laboratory testing of cells
Cells from humans and other animals are cultured in vitro

to test the effects of new drugs

to investigate the involvement of genes or external factors in the
development of diseases such as cancer or diabetes
In gene therapy
Research in this area remains experimental. It involves incorporating a
dominant allele into cells of a patient that lack the allele and suffer a disease,
as a consequence. The genetically modified cells are cloned in tissue culture
to produce a mass of cells that are transferred into the patient’s body. This is
covered in the F215 pack on Genomes and Gene Technologies
Use of stem cells to treat diseases
Stem cells are undifferentiated cells that are able to undergo mitosis to form
cells that can become differentiated and specialised.
It is hoped that stem cells can be used to treat:

Insulin-dependent diabetes, by replacing β cells in the Islets of
Langerhans

Parkinson’s disease by replacing dopamine-producing cells in the
brain.
The problem is obtaining stem cells, since there are very few true stem cells
in adults (most of their stem cells cannot differentiate into any cell type).
A useful source is very young embryos where most or all of the cells are
undifferentiated and able to form any cell type in the body. These embryonic
stem cells are said to be totipotent (able to form any cell type in the body) or
pluripotent (able to form most cell types in the body)
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Table of Advantages and Disadvantages of Non-Reproductive Cloning
ADVANTAGES
DISADVANTAGES
Tissues/organs will be genetically
identical to the individual and will not
be rejected
Ethical objections to using human
embryonic cells in this way. The
embryo is a potential human and has
rights
Avoids the reliance of waiting for an
organ transplant
No understanding of how cloned
tissues/organs will behave over time
Cloned cells are totipotent
(undifferentiated) and can be used to
produce any cell type
Religious objections to cloning
techniques
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