Micropropagation clones plants from explants
Micropropagation (tissue culture) grows many plants from small pieces of tissue called explants, in vitro. A sterilised explant from a chosen parent is grown on sterile nutrient medium with plant hormones; cells divide by mitosis into a callus then plantlets, which move to compost. Because only mitosis is used, every plant is genetically identical to the parent, a clone with no genetic variation. Uses: rapidly mass-producing valuable, disease-free or rare plants.
Mammal cloning by nuclear transfer (Dolly)
Nuclear transfer (Dolly the sheep): take a diploid nucleus from a body (somatic) cell; remove the egg's nucleus (enucleate it); insert the diploid nucleus; an electric shock triggers mitosis, forming an embryo; implant the embryo into a surrogate uterus. The offspring is a clone of the donor. Key words: diploid, body cell, embryo (not zygote), mitosis (not meiosis). 5.20B: cloning a transgenic donor gives clones that make a human protein in milk.
Cloning: uniformity gains, diversity risks
Advantages: cloning quickly makes many organisms of a known, desirable genotype (e.g. high yield, disease resistance), aids conservation, and gives reliable protein from transgenic clones. Disadvantages: all clones share one genome, so diversity is very low; a new disease could wipe out the whole population, with no variation to evolve resistance. Ethical concerns: animal welfare and high nuclear-transfer failure rates. Evaluate = both sides plus a verdict.
Drawn from real examiner reports.
Say genetically identical, not identical
A clone is not just "identical"; it is genetically identical, with the same nuclear DNA. Answers saying only "identical" or "a copy of the parent" lose the mark. Use "genetically identical to the parent", or "no genetic variation / same genes / same DNA". Environment can make clones look different, so "genetically" is the word that earns the mark.
June 2024 Paper 1BR Q2(c): "identical" alone was not credited; candidates needed "genetically identical / no genetic variation / clones".
Use a diploid body-cell nucleus, not a sperm
The transferred nucleus must be diploid and from a body (somatic) cell, because it holds the full chromosome set. A sperm nucleus is wrong: it is haploid and carries a different individual's DNA, so it would not clone the donor. Do not write "haploid" or "gamete". A body-cell nucleus already has the complete genome to copy.
June 2024 Paper 2B Q1(d)(ii): candidates failed to state the nucleus is diploid, or used a sperm nucleus.
Embryo not zygote; mitosis not meiosis
After the electric shock the egg divides to form an embryo, not a zygote: a zygote forms only by fertilisation (fusion of gametes), and no fertilisation occurs in cloning. The division is mitosis, which keeps the diploid number; meiosis would halve it and could not build a diploid embryo. Writing "zygote" or "meiosis" loses two easy marks.
June 2024 Paper 2B Q1(d)(ii) and Nov 2024 Paper 2B Q5: "meiosis" for mitosis and "zygote" for embryo were repeatedly flagged.
Transfer the nucleus, not "the genes"
In nuclear transfer the whole nucleus is moved into the enucleated egg, so write "nucleus", not "genes" or "DNA". "The genes are transferred" loses the mark, because the entire nucleus, with its full chromosome set, is inserted. Also name every stage: do not omit the embryo or the implantation into a surrogate uterus.
Nov 2024 Paper 2B Q5: candidates wrote "genes" instead of "nuclei"; the embryo and surrogate stages were sometimes omitted.
Cloning is not GM or selective breeding
Keep three techniques apart. Cloning copies an existing genome by mitosis, adding no new gene. Genetic modification inserts a gene from a different species using enzymes and a vector, changing the genome. Selective breeding chooses parents over many generations, shuffling existing alleles. On a cloning question, do not describe enzymes, vectors or breeding.
June 2024 Paper 2B Q1(d)(ii): candidates confused mammalian cloning with genetic engineering, gaining no credit.
Two cloning methods: do not mix them
Both micropropagation and nuclear transfer make clones, but differently. Micropropagation grows plant explants on nutrient medium: no egg, no nucleus transfer, no surrogate. Nuclear transfer puts a diploid body-cell nucleus into an enucleated egg and needs a surrogate. Describe the method the question asks about; a plant method scores nothing on an animal-cloning question.
June 2024 Paper 2B Q1(d)(ii): candidates confused mammalian cloning with micropropagation.
Answer "evaluate cloning" in three moves
Cloning evaluate questions take three moves: two advantages with a reason, two disadvantages with a mechanism (one genome means all clones are equally hit by a new disease), then a conclusion weighing them. No verdict drops the top mark.
Describe stages as separate, ordered marks
Marks in "describe the stages" come one per stage, so write a numbered sequence. For nuclear transfer hit each: diploid body-cell nucleus, enucleated egg, nucleus inserted, shock and mitosis, embryo, surrogate uterus. Merged or missing stages cost marks.
Use exact vocabulary; the marks are words
This topic is graded on precise terms: genetically identical, explant, diploid, body cell, enucleated, embryo, mitosis, surrogate. A near-synonym (identical, zygote, meiosis, genes) usually scores zero. Use the specification word, not an everyday paraphrase.
Scope: statements 5.17B–5.20B (Paper 2B only — all are extended, emboldened "B" content).
Clone — an organism that is genetically identical to its parent; produced by asexual reproduction (or nuclear transfer). The nuclear deoxyribonucleic acid (DNA) is identical; the word "genetically" is required for full marks.
Micropropagation (tissue culture) — the production of large numbers of genetically identical plants by growing small pieces of tissue (explants) on a sterile nutrient medium under laboratory (in vitro) conditions.
Explant — a small piece of tissue cut from a selected parent plant to begin the tissue-culture process.
Enucleated egg cell — an egg cell from which the nucleus has been removed, so that the egg's own genetic material is absent.
Transgenic animal — an animal whose genome contains genetic material (a gene) from a different species (the word "different species" is required by the mark scheme — see sub-topic 5c).
Define the term clone.
Cloning produces organisms that are genetically identical to their parent.
(a) State the type of reproduction involved in cloning. (1 mark)
(b) Name the type of cell division that produces genetically identical cells. (1 mark)
