Cloning mammals

Animals can undergo natural and artificial cloning.

Human identical twins are an example of natural animal clones.

Suggest why human identical twins do not look exactly identical.

Komodo dragons are large lizards.

Male Komodo dragons have two Z chromosomes. Female Komodo dragons have one Z chromosome and one W chromosome.

A reproductively isolated female Komodo dragon can reproduce without a male using a process called parthenogenesis. All of the offspring are males.

This process is outlined in the diagram.

The offspring are not the same sex as the parent.

Identify two other reasons why parthenogenesis in Komodo dragons is not an example of cloning.

Suggest why female mammals cannot produce male offspring by parthenogenesis.

Animals can be cloned artificially using embryo splitting.

This is the procedure for cloning by embryo splitting:

1. An egg from a female is fertilised by sperm from a male.

2. The zygote divides to form an embryo.

3. After a few days, the developing embryo is split into several smaller embryos.

4. After a few days, each smaller embryo is implanted into a surrogate mother.

5. Each embryo grows into a fetus and is born normally.

State the property of the cells in the embryo that allows cloning to be successful.

Embryos from stage 3 above are sometimes frozen for later use.

State one advantage to agriculture of freezing embryos from stage 3.

Animals can be cloned artificially using a technique called somatic cell nuclear transfer (SCNT).

SCNT can be used to increase the population of an endangered species.

Przewalski’s horse, Equus przewalskii, is a species of horse related to the domestic horse, Equus caballus. The common ancestor of both horse species is thought to have lived 45 000 years ago.

Przewalski’s horse became extinct in the wild in the 1960s. Captive breeding from 12 individuals has increased the population and some have since been reintroduced to the wild.

The genetic diversity of all modern Przewalski’s horses is very low.

In 2020 and 2022, SCNT using frozen cells from an individual that died 42 years previously has led to the birth of two cloned Przewalski’s horses which appear to be free of health problems.

Outline how genetic diversity in a species such as Przewalski’s horse can be measured.

State why SCNT is more useful than embryo splitting when increasing the population of endangered species.

Suggest one reason why the health of the cloned Przewalski’s horses is better than that of Dolly the sheep, the first mammal to be cloned by SCNT.

Many organisms undergo asexual reproduction by mitosis.

State two other roles of mitosis in organisms.

Plants reproduce asexually by mitosis.

Explain why mitosis, and not meiosis, is used for asexual reproduction in plants.

Bacteria reproduce asexually by a process called binary fission.

Suggest why bacteria are unable to reproduce asexually by mitosis.

During meiosis, the independent assortment of chromosomes leads to the formation of genetically different gametes.

Gametes of the western clawed frog contain 10 chromosomes.

Calculate the number of genetically different gametes resulting from the independent assortment of chromosomes that would be produced by an individual frog.

The sea sponge, Aplysina aerophoba, and the zebra shark, Stegostoma fasciatum, are both animals.

A. aerophoba does not have an internal circulatory system. Instead, it filters food and oxygen from the surrounding water, as shown in Fig. 4.1.

Suggest why A. aerophoba does not need a circulatory system.

A diagram of the circulatory system of S. fasciatum is shown in Fig. 4.2.

Describe the type of circulatory system that S. fasciatum has.

Both A. aerophoba and S. fasciatum reproduce sexually, but under particular conditions they are both able to reproduce asexually.

In asexual reproduction in A. aerophoba:

• clumps of diploid cells detach from the body of the sponge

• the cells reattach to a surface and grow into new, adult sponges.

In asexual reproduction in S. fasciatum:

• meiosis occurs in a female

• two of the haploid cells produced by meiosis fuse to form a diploid cell

• the diploid cell develops into a new shark.

A student stated, ‘When they reproduce asexually, both animals produce clones of themselves.’.

Evaluate the student’s statement.

* Humans can produce artificial clones of animals.

Describe two methods for producing artificial clones of animals.

Additional answer space if required.

Valves control the flow of blood through the heart.

Complete the table below to show the roles of two valves in the heart.

A ventricular septal defect (VSD) is a hole in the septum of the heart.

The diagram below shows a heart with VSD.

Describe and explain why people with VSD can easily become tired.

Creatine kinase (CK) is an enzyme that catalyses reactions in heart muscle. High levels of CK in the blood indicate that a person may have had a heart attack.

Suggest why high levels of CK in the blood indicate that a person may have had a heart attack.

Mice are often used in laboratory studies to research treatments for heart conditions.

These mice are often clones.

Suggest one reason why clones are used in these studies.

Corals are a group of animals that usually live on the sea bed close to the surface of the water.

Many corals can reproduce both sexually and asexually.

Which of the following statements about asexually-produced coral offspring is not true?

A All offspring produced from an individual organism will be genetically identical.

B If a change in the environment harms one of the offspring produced from an individual organism it will probably harm them all.

C Meiosis occurred in order to produce the offspring.

D The offspring will tend to thrive if conditions are similar to those present when the parent organism reproduced.

Your answer

It is possible to clone animals using a technique called somatic cell nuclear transfer (SCNT).

The most well-known example of this was the cloning of Dolly the sheep in 1996.

Thirty years before Dolly the sheep, successful cloning of an animal was carried out using a frog, Xenopus laevis.

Frogs lay eggs in water. These eggs then develop and hatch into swimming tadpoles. When the tadpoles grow to a certain size they develop into adult frogs.

The cloning process is outlined in Fig. 19.1.

Describe what is happening at stage A and suggest a practical procedure that could allow this to occur.

Identify a key difference between the processes between stages A and C and the cloning of Dolly the sheep.

The frog produced by the process in Fig. 19.1 is not a complete clone of the donor tadpole.

Suggest why the cloned frog might not be considered a complete clone of the donor tadpole.

Suggest why the x-axis does not show the age of the donor nuclei.

Dolly the sheep suffered health problems throughout her life and died at an early age.

The donor nucleus that was used to create Dolly came from a sheep that was already five years old. The normal lifespan of a domestic sheep is ten years.

A student concluded that Dolly’s health problems were caused by the stage of development of the sheep that provided the donor nucleus.

List three reasons why the information in Fig. 19.2 does not support the student’s conclusion.

One measure of the success of cloning procedures is the number of pregnancies that result in live births.

Table 19 shows information from the work of many scientists about the success of SCNT in four different species.

Calculate the percentage of pregnancies that resulted in live births in goats and mice.

Compiling results from different scientists can have problems as their investigations may not have been controlled in the same way.

List three factors that should have been controlled when compiling the data to include in Table 19.