chapter 22 p1

Question 1

Asexual reproduction

Answer 1

is a form of cloning and it results in offspring produced by mitosis and known as clones.
Clones are usually genetically identical to both the parent organism and to each other.

Question 2

Natural cloning:

Answer 2

• Vegetative propagation, or natural cloning, occurs in many species of flowering plants.
• A structure forms which develops into a fully differentiated new plant, which is genetically identical to the parent.
• The new plant may be propagated from the stem, leaf, bud, or root of the parent, depending on the type of plant, and it eventually becomes independent from its parent, for example, strawberries and spider plants.
• Vegetative propagation often involves perennating organs, which enables plants to survive adverse conditions.

Question 3

perennating organs

Answer 3

• These contain stored food from photosynthesis and can remain dormant in the soil.
• They are often not only a means of asexual reproduction, but also a way of surviving from one growing season to the next.

Question 4

Natural plant cloning occurs in:

Answer 4

bulbs
Runners
Rhizomes
Stem tubers

Question 5

bulbs

Answer 5

, for example, daffodil. The leaf bases swell with stored food from photosynthesis. Buds form internally which develop into new shoots and new plants in the next growing season.

Question 6

Runners

Answer 6

for example, a strawberry or spider plant. A lateral stem grows away from the parent plant and roots develop where the runner touches the ground. A new plant develops - the runner eventually withers away leaving the new individual independent.

Question 7

Rhizomes

Answer 7

for example, marram grass. A rhizome is a specialised horizontal stem running underground, often swollen with stored food. Buds develop and form new vertical shoots which become independent plants.

Question 8

Stem tubers

Answer 8

, for example, potato. The tip of an underground stem becomes swollen with stored food to form a tuber or storage organ. Buds on the storage organ develop to produce new shoots (e.g., the ‘eyes’ on a potato).

Question 9

Using natural clones in horticulture p1

Answer 9

Natural plant cloning is exploited in horticulture by farmers and gardeners to produce new plants.
Splitting up bulbs, removing young plants from runners, and cutting up rhizomes all increase plant numbers cheaply, and the new plants have exactly the same genetic characteristics as their parents.
It is also possible to take cuttings of many plants - short sections of stems are taken and planted either directly in the ground (e.g., sugar cane) or in pots, for example, pelargoniums.
Rooting hormone is often applied to the base of a cutting to encourage the growth of new roots.
Propagation from cuttings has several advantages over using seeds. It is much faster - the time from planting to cropping is much reduced.

Question 10

Using natural clones in horticulture p2

Answer 10

It also guarantees the quality of the plants.
By taking cuttings from good stock, the offspring will be genetically identical and will therefore crop well.
The main disadvantage is the lack of genetic variation in the offspring should any new disease or pest appear or if climate change occurs.
Many of the world’s most important food crops are propagated by cloning. Bananas, sugar cane, sweet potatoes, and cassava are all propagated from stem cuttings or rhizomes.
Coffee and tea bushes are also propagated from stem cuttings.

Question 11

Cloning sugar cane:

Answer 11

Sugar cane is an internationally important crop used to make sugar and manufacture biofuels.
It is one of the fastest growing crop plants in the world - the stems can grow 4-5 metres in 11 months if conditions are good - and it is usually propagated by cloning.
Short lengths of cane about 30 cm long, with three nodes, are cut and buried in a clear field in shallow trenches, covered with a thin layer of soil.
Per hectare, 10-25 000 lengths of stem are planted.

Question 12

Practical cloning:
- Many popular houseplants are propagated by taking cuttings.
- There are a number of points which increase the success rate of most cuttings:

Answer 12

Use a non-flowering stem
Make an oblique cut in the stem
Use hormone rooting powder
Reduce leaves to two or four
Keep cutting well watered
Cover the cutting with a plastic bag for a few days.

Question 13

Many plant cells are

Answer 13

totipotent - they can differentiate into all of the different types of cells in the plant.
Scientists have developed ways of using this property to produce huge numbers of identical clones from one desirable plant.

Question 14

Micropropagation

Answer 14

the process of making large numbers of genetically identical offspring from a single parent plant using tissue culture techniques.

Question 15

micropropagation is used to produce plants when a desirable plant:

Answer 15

does not readily produce seeds
doesn’t respond well to natural cloning
is very rare
has been genetically modified or selectively bred with difficulty
is required to be ‘pathogen-free’ by growers, for example, strawberries, bananas, and potatoes.

Question 16

There are a number of ways in which plants can be micropropagated.

Answer 16

One protocol, based on work done at the Royal Botanic Garden at Kew, uses sodium dichloroisocyanurate, the sterilising tablets used to make emergency drinking water and babies’ bottles safe.
This keeps the plant tissues sterile without being in a sterile lab so it is extremely useful for scientists in the field working with rare and endangered plant material - and also for use in schools.
Other protocols are more suited to industrial micropropagation where large sterilising units are available.

Question 17

The basic principles of micropropagation and tissue culture are as follows: p1

Answer 17

Take a small sample of tissue from the plant you want to clone - the meristem tissue from shoot tips and axial buds is often dissected out in sterile conditions to avoid contamination by fungi and bacteria. This tissue is usually virus-free.

The sample is sterilised, usually by immersing it in sterilising agents such as bleach, ethanol, or sodium dichloroisocyanurate.
The latter does not need to be rinsed off which means the tissue is more likely to remain sterile.
The material removed from the plant is called the explant.

Question 18

The basic principles of micropropagation and tissue culture are as follows: p2

Answer 18

The explant is placed in a sterile culture medium containing a balance of plant hormones (including auxins and cytokinins) which stimulate mitosis.
The cells proliferate, forming a mass of identical cells known as a callus.

The callus is divided up and individual cells or clumps from the callus are transferred to a new culture medium containing a different mixture of hormones and nutrients which stimulates the development of tiny, genetically identical plantlets.

The plantlets are potted into compost where they grow into small plants.

The young plants are planted out to grow and produce a crop.

Question 19

• The scale of micropropagation is

Answer 19

increasing.
- It now takes place in bioreactors, effectively making artificial embryo plants to be packaged in artificial seeds.

Question 20

plants that are largely produced by micropropagation

Answer 20

potatoes, sugar cane, bananas, cassava, strawberries, grapes, chrysanthemums, Douglas firs, and orchids.

Question 21

Arguments for micropropagation:

Answer 21

• allows for the rapid production of large numbers of plants with known genetic make-up which will yield good crops.
• Culturing meristem tissue produces disease-free plants.
• It makes it possible to produce viable numbers of plants after genetic modification of plant cells.
• It provides a way of producing very large numbers of new plants which are seedless and therefore sterile to meet consumer tastes (e.g., bananas and grapes).
• It provides a way of growing plants which are naturally relatively infertile or difficult to grow from seed (e.g., orchids).
• It provides a way of reliably increasing the numbers of rare or endangered plants.

Question 22

Arguments against micropropagation:

Answer 22

• It produces a monoculture - many plants which are genetically identical - so they are all susceptible to the same diseases or changes in growing conditions.
• It is a relatively expensive process and requires skilled workers.
• The explants and plantlets are vulnerable to infection by moulds and other diseases during the production process.
• If the source material is infected with a virus, all of the clones will also be infected.
• In some cases, large numbers of new plants are lost during the process

Question 23

Yes, we have no bananas…
p1

Answer 23

• A wild banana is full of hard seeds and it is virtually inedible.
• A mutation made them parthenocarpic which means they produce fruit without fertile seeds - which made them good to eat but also made them sterile.
• Scientists therefore think that since the dawn of agriculture, people cloned bananas using natural asexual reproduction to propagate the plants producing the seedless, tasty fruit.
• Sweet bananas are widely eaten in more economically developed countries, whilst plantains (cooking bananas) are a staple food in many less economically developed countries.
• In the early 20th century almost all of the sweet bananas eaten were the cultivar Gros Michel.

Question 24

Yes, we have no bananas…
p2

Answer 24

Then fungal Panama disease wiped them out in the major banana growing countries - none of the clones had any resistance and a new cultivar took over.
Cavendish bananas, while apparently not as tasty as Gros Michel bananas, are resistant to Panama disease.
But Cavendish bananas are also clones. Now another banana disease, Black Sigatoka, is destroying Cavendish plantations, and is also spreading to other cooking varieties of bananas.
New biotechnologies for example genetic engineering and micropropagation offer hope for the future.
Genetically engineered strains of bananas with resistance genes from the original wild fruit could be micropropagated and used to restock banana plantains across the whole growing region.

Question 25

Natural cloning is common in

Answer 25

invertebrate animals. Although it is less common in vertebrates, it still occurs in the form of twinning.

Question 26

Cloning in invertebrates:

Answer 26

Natural cloning in invertebrates can take several forms.
Some animals, such as starfish, can regenerate entire animals from fragments of the original if they are damaged.
Flatworms and sponges fragment and form new identical animals as part of their normal reproductive process, all clones of the original.
Hydra produce small ‘buds’ on the side of their body which develop into genetically identical clones.
In some insects, females can produce offspring without mating.
Scientists are increasingly finding differences between the mother and daughters, however, suggesting that as a result of high mutation rates the offspring are not true clones.

Question 27

Cloning in vertebrates:

Answer 27

The main form of vertebrate cloning is the formation of monozygotic twins (identical twins).
The early embryo splits to form two separate embryos.
No one is sure of the trigger which causes this to happen.
The frequency at which identical twins occur varies between species.
For example, domestic cattle rarely if ever produce identical twins naturally, while the incidence in natural human pregnancies is around 3 per 1000.
When monozygotic twins are born, although genetically identical, they may look different as a result of differences in their position and nutrition in the uterus.
Some female amphibians and reptiles will produce offspring when no male is available.
The offspring are often male rather than female, so they are not clones of their mother, yet all of their genetic material arises from her.

Question 28

Artificial clones in animals:

Answer 28

It is relatively easy to produce artificial clones of some invertebrates - liquidise a sponge or chop up a starfish and new animals will regenerate from most of the fragments.
It is much more difficult to produce artificial clones of vertebrates, especially mammals.
However, two methods are now used widely in the production of high-quality farm animals and in the development of genetically engineered animals for pharming.

Question 29

Artificial twinning:

Answer 29

• After an egg is fertilised, it divides to form a ball of cells.
• Each of these individual cells is totipotent - it has the potential to form an entire new animal.
• As the cells continue to divide, the embryo becomes a hollow ball of cells.
• Soon after this the embryo can no longer divide successfully.
• In natural twinning, an early embryo splits and two foetuses go on to develop from the two halves of the divided embryo.
• In artificial twinning the same thing happens, but the split in the early embryo is produced manually.
• In fact, the early embryo may be split into more than two pieces and results in a number of identical offspring.
• Artificial twinning, like embryo transfer which preceded it, is used by the farming community to produce the maximum offspring from particularly good dairy or beef cattle or sheep.

Question 30

The stages of artificial twinning in cattle can be summarised as follows:

Answer 30

• A cow with desirable traits is treated with hormones so she super-ovulates, releasing more mature ova than normal.
• The ova may be fertilised naturally, or by artificial insemination, by a bull with particularly good traits.
• The early embryos are gently flushed out of the uterus.
• Alternatively, the mature eggs are removed and fertilised by top-quality bull semen in the lab.
• Usually before or around day six, when the cells are still totipotent, the cells of the early embryo are split to produce several smaller embryos, each capable of growing on to form a healthy full-term calf.
• Each of the split embryos is grown in the lab for a few days to ensure all is well before it is implanted into a surrogate mother.
• Each embryo is implanted into a different mother as single pregnancies carry fewer risks than twin pregnancies.
• The embryos develop into foetuses and are born normally, so a number of identical cloned animals are produced by different mothers.

Question 31

artificial twinning in cows

Answer 31

Question 32

e.g:
artificial twinning in pigs

Answer 32

In pigs, a number of cloned embryos must be introduced into each mother pig.
This is because they naturally produce a litter of piglets, and the body may reject and reabsorb a single foetus.
This technology makes it possible to greatly increase the numbers of offspring produced by the animals with the best genetic stock.
Some of the embryos may be frozen.
This allows the success of a particular animal to be assessed and, if the stock is good, remaining identical embryos can be implanted and brought to term.

Question 33

Somatic cell nuclear transfer:

Answer 33

Artificial twinning clones an embryo.
However, it is now possible to clone an adult animal, by taking the nucleus from an adult somatic (body) cell and transferring it to an enucleated egg cell (an oocyte which has had the nucleus removed).
A tiny electric shock is used to fuse the egg and nucleus, stimulate the combined cell to divide, and form an embryo that is a clone of the original adult.
This process is known as somatic cell nuclear transfer (SCNT).
The first adult mammal to be cloned in this way was Dolly the sheep in 1996.
Since then scientists have cloned a wide range of species including mice, cows, horses, rabbits, cats, and dogs.
SCNT is simple in theory, although in practice there are many difficulties so the technique is still not widely used.

Question 34

As you can see in Figure 3, animals of different breeds are often used as the cell donor, the egg donor, and the surrogate mother to make it easier to identify the original animal at each stage. p1

Answer 34

1. The nucleus is removed from a somatic cell of an adult animal.
2. The nucleus is removed from a mature ovum harvested from a different female animal of the same species (it is enucleated).
3. The nucleus from the adult somatic cell is placed into the enucleated ovum and given a mild electric shock so it fuses and begins to divide.
   In some cases, the nucleus from the adult cell is not removed - it is simply placed next to the enucleated ovum and the two cells fuse (electrofusion) and begin to divide under the influence of the electric current.
4. The embryo that develops is transferred into the uterus of a third animal, where it develops to term.
5. The new animal is a clone of the animal from which the original somatic cell is derived, although the mitochondrial DNA will come from the egg cell.

Question 35

As you can see in Figure 3, animals of different breeds are often used as the cell donor, the egg donor, and the surrogate mother to make it easier to identify the original animal at each stage. p2

Answer 35

This process is also known as reproductive cloning, because live animals are the end result.
The cloned embryo can then be split to produce several identical clones.
There have been some problems with the animals produced by SCNT - Dolly the sheep had to be put down when she was only six years old because she suffered from arthritis and lung disease, usually seen in much older sheep.
However, scientists have improved the technique and whilst concerns about premature ageing in clones produced by SCNT persist, researchers in Japan have produced 581 clones from one original donor mouse, through 25 generations.
The mice in each generation were cloned to produce the next generation.
Furthermore, they all had babies naturally to prove they functioned normally. All of the mice had normal lifespans.
The same team has also produced SCNT clones from the bodies of mice which had been frozen for 16 years.
SCNT can be used in a number of ways. It is used in pharming - the production of animals which have been genetically engineered to produce therapeutic human proteins in their milk.
It can also be used to produce genetically modified (GM) animals which grow organs that have the potential to be used in human transplants.

Question 36

SCNT diagram

Answer 36

Question 37

Arguments for animal cloning:
p1

Answer 37

• Artificial twinning enables high-yielding farm animals to produce many more offspring than normal reproduction.
• Artificial twinning enables the success of a sire (the male animal) at passing on desirable genes to be determined.
  If the first cloned embryo results in a successful breeding animal, more identical animals can be reared from the remaining frozen clones.
  The use of meat from animals born to a cloned parent is now permitted in the US.
• SCNT enables GM embryos to be replicated and to develop, giving many embryos from one engineering procedure.

Question 38

Arguments for animal cloning:
p2

Answer 38

• It is an important process in pharming - the production of therapeutic human proteins in the milk of genetically engineered farm animals, such as sheep and goats.
• SCNT enables scientists to clone specific animals, for example, replacing specific pets or cloning top-class race horses.
  Pet cats and dogs have been cloned in the US at great expense.
• SCNT has the potential to enable rare, endangered, or even extinct animals to be reproduced.
  In theory, the nucleus from dried or frozen tissue could be transferred to the egg of a similar living species and used to produce clones of species that have been dead for a long time.

Question 39

Arguments against animal cloning: p1

Answer 39

• SCNT is a very inefficient process - in most animals it takes many eggs to produce a single cloned offspring.
• Many cloned animal embryos fail to develop and miscarry or produce malformed offspring.
• Many animals produced by cloning have shortened lifespans, although cloned mice have now been developed which live a normal two years.

Question 40

Arguments against animal cloning: p2

Answer 40

• SCNT has been relatively unsuccessful so far in increasing the populations of rare organisms or allowing extinct species to be brought back to life.
• For example, scientists have attempted to clone the gaur and the banteng - both extremely rare breeds of wild cattle.
• One gaur calf was born in 2001 and died within a couple of days.
• Two banteng calves were born in 2003 - one was deformed and euthanised, the other grew normally but its natural lifespan was halved.
• The idea of restoring extinct organisms is exciting but scientists are increasingly unconvinced that it will be possible by this method.