Questions on cloning in plants and animals

Question 1

Evaluate the use of therapeutic cloning, rather than reproductive cloning, as a way of obtaining stem cells to treat a patient.

Accepted Answer

Indicative content

for: the two share every step until the blastocyst stage, and in therapeutic cloning the embryo is never implanted, so no animal or person is born and the welfare problems of reproductive cloning do not arise
for: the stem cells carry the patient's own nuclear DNA and so the patient's own antigens, so they are not rejected by the patient's immune system
for: the cells obtained are pluripotent, so a far wider range of tissues can be produced from them than from adult stem cells taken from the patient
against: a human embryo is created and then destroyed, which some hold to be the ending of a human life whatever the benefit to the patient
against: human egg cells are needed in quantity, and collecting them requires hormone treatment and a procedure that carries a risk to the donor; success rates are also low
a judgement is stated and supported by the points made, for example that induced pluripotent cells now offer patient-matched pluripotent cells with no egg and no embryo; credit either conclusion where the reasoning leads to it

Levels

Level 3 (5–6 marks) the advantages are tied to therapeutic cloning in particular, the patient's own antigens or no birth, at least two objections are explained, and the judgement is weighed against an alternative such as induced pluripotent cells
Level 2 (3–4 marks) advantages and objections are both given, but the answer treats cloning in general rather than therapeutic against reproductive, or the judgement is stated without reasons
Level 1 (1–2 marks) one or two points, such as that the cells are not rejected or that an embryo is destroyed, with no attempt to weigh them and no judgement

Question 2

Describe how a large number of genetically identical plants can be produced from one parent plant by micropropagation.

Accepted Answer

B1 an explant, a small piece of tissue such as a shoot tip or a node, is cut from the chosen parent plant B1 the surface of the explant is sterilised, because any bacteria or fungal spores on it would outgrow the plant tissue on the medium B1 the explant is placed on sterile nutrient medium containing sugar, mineral ions and plant hormones, and its cells divide to form a callus of undifferentiated cells B1 the callus is divided repeatedly, and the ratio of cytokinin to auxin in the medium is used to make the pieces grow shoots and then roots B1 the plantlets are acclimatised, being moved gradually from the sterile medium to compost at high humidity; every division has been mitosis, so all the plants are genetically identical to the parent

Question 3

Discuss the commercial advantages and disadvantages of using micropropagation to produce a crop, compared with growing the same crop from seed.

Accepted Answer

B1 advantage: micropropagation produces very large numbers of genetically identical plants quickly, at any time of year, from a single valuable parent B1 advantage: it produces uniform plants that mature together, which suits a commercial grower supplying a market that wants a standard size and timing B1 disadvantage: the sterile facilities, culture media and skilled labour needed make it far more costly per plant than growing from seed B1 disadvantage: not every species forms a callus and regenerates successfully, so the technique does not work for every crop, and a contamination discovered late costs the whole batch B1 on balance, micropropagation is justified commercially where uniformity, speed or freedom from disease outweigh its higher cost, for example in high-value ornamentals or virus-free seed stock, but not for a low-value crop that grows easily from seed

Question 4

Describe how a cloned mammal is produced from an adult body cell by somatic cell nuclear transfer.

Accepted Answer

B1 an egg cell is taken from a donor female and enucleated, its nucleus being removed and its cytoplasm left intact B1 a somatic cell, such as a skin or mammary gland cell, is taken from the animal to be cloned, and it or its nucleus is placed next to the enucleated egg B1 the two are fused using a pulse of electricity, which also stimulates the reconstructed cell to begin dividing, and factors in the egg cytoplasm reprogramme the transferred nucleus B1 the cell divides by mitosis, the embryo is cultured to the blastocyst stage and is implanted into a surrogate mother that carries the pregnancy

Question 5

One callus is divided into 8 pieces every 4 weeks, and each piece grows into a callus of the same size before the next division. Calculate the number of calluses present after 12 weeks.

Accepted Answer

M1 12 weeks allows three rounds of division, one every 4 weeks M1 1 multiplied by 8 three times, which is 8³ A1 512 calluses are present after 12 weeks

Question 6

A field planted entirely with micropropagated potato plants is more likely to be lost to a new strain of a fungal pathogen than a field grown from seed. Explain why.

Accepted Answer

B1 the micropropagated plants are clones, produced by mitosis from one parent, so they are genetically identical to one another B1 there is therefore no variation between them in resistance to the pathogen, whereas plants grown from seed carry new combinations of alleles produced by meiosis and fertilisation B1 a strain able to infect one plant can infect every plant in the field, so the whole crop can be lost, and no resistant individuals survive to be selected or bred from

Question 7

A calf produced by nuclear transfer is not identical in every respect to the animal that donated the nucleus. Explain why.

Accepted Answer

B1 only the nuclear DNA comes from the donor animal, because only the nucleus was transferred B1 the mitochondria, and so the mitochondrial DNA, came with the cytoplasm of the egg cell and belong to the egg donor B1 the reprogramming of the transferred nucleus is not perfect, so the pattern of gene expression differs, and the calf develops in a different uterus and grows up in different conditions, which affect the phenotype

Question 8

Describe how a cutting is used to produce a new plant that is a clone of its parent.

Accepted Answer

B1 a piece of stem is cut from the parent plant just below a node, and the lower leaves are removed B1 the cut end is dipped in rooting powder containing auxin, and the cutting is pushed into moist compost and kept humid B1 cells near the cut divide and roots develop, producing a new plant genetically identical to the parent because every division has been mitosis

Question 9

Compare cuttings with micropropagation as methods of cloning a plant, referring to the scale each can achieve and the cost of each.

Accepted Answer

B1 a cutting needs only a knife and rooting powder, and is cheap, whereas micropropagation needs sterile facilities, culture media and trained staff, and is far more costly per plant B1 one parent plant yields at most a few dozen cuttings in a season, whereas micropropagation can produce thousands of genetically identical plantlets, repeatedly, from one parent B1 micropropagation can also be started from meristem tissue to give stock free of a virus the parent carries, which taking cuttings cannot do because a cutting carries the parent's vascular tissue and any virus in it

Question 10

Growers who want disease-free stock of a valuable potato variety start their tissue cultures from the extreme tip of a shoot, even though a larger explant would grow away faster. Suggest why the shoot tip is used.

Accepted Answer

B1 viruses spread through a plant in its vascular tissue, and the cells at the very tip of the shoot meristem have no vascular tissue supplying them yet B1 those cells are therefore often free of a virus that has infected the rest of the plant B1 an explant taken from just the tip can give rise to a whole population of virus-free plants, which is worth the slower start because no other propagation method can clear the virus

Question 11

Identical twins arise when an early embryo splits naturally into two. Explain why the resulting twins are clones of each other but not clones of either parent.

Accepted Answer

B1 the twins are genetically identical to each other because both arose from the same zygote by mitosis, which produces genetically identical daughter cells B1 they are not clones of either parent because the zygote they came from was produced by fertilisation, combining alleles from both parents in a new combination B1 that combination of alleles is not identical to either parent's genotype, so neither twin is a clone of the mother or of the father

Question 12

Compare artificial embryo splitting with somatic cell nuclear transfer as ways of cloning cattle, referring to whose phenotype is copied and what limitation this creates.

Accepted Answer

B1 embryo splitting copies an embryo produced by fertilising two chosen parents, so it copies a new genotype whose phenotype is not yet known, whereas nuclear transfer copies an adult animal whose phenotype has already proved itself B1 in embryo splitting the resulting calves' qualities can only be estimated from both parents, whereas in nuclear transfer every calf carries the same nuclear DNA as the donor animal, so the genotype behind its known qualities is copied exactly B1 embryo splitting is the more established technique with a higher success rate, whereas nuclear transfer has a lower success rate and a greater welfare cost, since many reconstructed pregnancies fail or produce unhealthy offspring

Question 13

In a female cat, one of the two X chromosomes in each cell of the early embryo is inactivated, and the same X stays inactive in all of that cell's descendants. Which X is inactivated is decided at random, separately in each cell. A gene for orange or black coat colour is on the X chromosome. A female tortoiseshell cat, heterozygous for this gene, is cloned. The clone's nuclear DNA is identical to the donor's, but its patches of orange and black are arranged differently. Suggest why the two cats' coat patterns differ.

Accepted Answer

B1 because the donor is heterozygous, each patch of skin is orange or black according to which allele is on the active X in the cells that produced it; the allele on the inactive X is not expressed B1 the clone developed from its own embryo, in which the choice of X was made again at random, cell by cell, so the donor's pattern was not copied B1 the two cats therefore express different alleles in different patches, so the same genotype gives a different phenotype through which genes are switched off, not through any difference in DNA sequence

Question 14

State two structures by which a plant reproduces asexually in nature, and name a plant that uses each one.

Accepted Answer

B1 any one of runner or stolon, rhizome, stem tuber, bulb or leaf plantlet, with a correct plant such as strawberry, couch grass, potato, onion or Kalanchoe B1 a second correct structure from that list, with a correct plant named for it

Question 15

State the species of the first mammal cloned from an adult cell using somatic cell nuclear transfer, and state the general trend in success rates for reproductive cloning since then.

Accepted Answer

B1 the first mammal cloned from an adult cell was a sheep, born in 1996 B1 success rates have improved in the decades since but remain low