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Biology - Cloning (AQA Syllabus A)

Genetic variation and its control is one of the topics studied in GCSE Science. This is the third of four quizzes on the topic and it looks at sexual and asexual reproduction and cloning.

There are two forms of reproduction: sexual and asexual. During asexual reproduction there is no fusion of gametes (sex cells) and only one individual is needed as the parent. Therefore, unlike sexual reproduction, in asexual reproduction there is no mixing of genetic information and so no genetic variation in the offspring. These genetically identical individuals are known as clones.

Humans have been cloning plants for thousands of years - taking cuttings produces new plants that are genetically identical to the original parent plant. Single celled organisms like bacteria and amoeba create clones of themselves as they reproduce by binary fission or budding.

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Creating clones of animals is not an easy task. The first animal clone, a frog, was created in 1958. Fish were cloned in the 1960s and 1970s but it wasn't until the 1990s that mammals were cloned. The most famous is perhaps Dolly the sheep. The reason for that is that she was cloned from an adult cell, rather than the cells of an embryo.

There are two ways of cloning animals. Embryo transplants involve splitting apart cells from a developing animal embryo before they become specialised. The identical embryos are then implanted into host mothers to develop. The cells are fertilised naturally and no internal changes are made to the cells.

For adult cell cloning the nucleus is removed from an unfertilised egg cell. The nucleus from an adult body cell, e.g. a skin cell, is then inserted into the egg cell. An electric shock is given which causes the egg cell to begin to divide to form embryo cells. These embryo cells contain the same genetic information as the adult skin cell. When the embryo has developed sufficiently, it is inserted into the womb of an adult female host.

Science fiction writers have for many years explored the possibility of the cloning of people which could theoretically be done by either method. But this raises lots of moral and ethical issues - you should have discussed some of them in class.

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  1. What is cloning?
    Cloning describes a number of different asexual reproduction methods that can be used to produce genetically identical copies of an organism
  2. Which of the following is not true about plant tissue culture?
    All of the plants produced by tissue culture are genetically the same as their parent plant and each other. Any variation will be environmental
  3. Why does asexual reproduction produce genetically identical offspring?
    Only one individual is needed as the parent so there is no mixing of genetic information, therefore no genetic variation in the offspring
  4. As well as leaves, spider plants grow long branches with lots of plantlets (miniature spider plants) attached. What is the name of this form of asexual reproduction?
    Tubers and rhizomes grow underground and, most of the time, you don't see them unless you dig the plant up. Runners, or stolons, grow above ground and you can easily see them. Mexican hat plants clone themselves by growing plantlets around the edges of their leaves. These drop off and grow in the soil around the original plant
  5. During tissue culture, why are plant cells washed in antiseptic before placing them into the culture medium?
    The usual culture medium is agar gel which is extremely good food for bacteria and fungi so they need to be killed off first
  6. Why are plants easier to clone than animals?
    Plant cells are amazing. Many of them have the ability to regenerate a whole plant
  7. When cloning animals from adult cells, which part of the cells is involved?
    The nucleus from any cell of the animal to be cloned is used to replace the nucleus of an egg cell from a different animal
  8. When cloning animals from adult cells, what must one do to the egg cell to make it start to grow?
    The egg cell has to be prompted to start growing as it has not been fertilised
  9. When cloning mammals from adult cells, after the nucleus has been replaced and growth has started, where must the egg cell be placed so the clone can develop?
    When it has started to grow, the egg cell will soon become an embryo. The only place that a mammal embryo can develop properly is inside the uterus (womb) of a female of the same type of animal
  10. Another way of cloning animals is by carrying out an embryo transplant. The growing cells of the embryo taken from a pregnant animal can be separated and grown as genetically similar individuals. But at what stage can this be done?
    After fertilisation, the egg cell begins to grow, dividing repeatedly to form a ball of new cells. These are stem cells which have the ability to form any cell in the body. When the cells have started to differentiate becoming limbs and other organs, it is too late to clone using this method

Author: Kev Woodward

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