Cloning (HSC SSCE Biology): Revision Notes

Cloning

Cloning represents a significant advancement in genetic technology, offering a more precise alternative to traditional selective breeding methods. Unlike selective breeding, which relies on unpredictable outcomes when crossing organisms, cloning allows scientists to create exact genetic copies with complete certainty.

What is cloning?

Cloning is the process of creating a genetically identical copy of an organism or gene. This technology eliminates the uncertainty inherent in selective breeding by producing organisms or genetic material that are exact replicas of the original. The key difference is that whilst selective breeding combines genes from two parents with unpredictable results, cloning produces organisms with identical DNA to a single source.

Types of cloning

There are two primary forms of cloning used in modern biotechnology, each serving different purposes in research and agriculture.

Gene cloning

Gene cloning operates at the cellular level, focusing on creating multiple identical copies of a specific gene rather than an entire organism. Scientists use this technique when they need large quantities of a particular gene for research purposes or biotechnological applications, such as producing insulin on an industrial scale.

The technique is essential for genetic engineering because researchers require numerous copies of genes to study their function, create genetically modified organisms, or manufacture important proteins for medical use.

Whole-organism cloning

Whole-organism cloning, also known as reproductive cloning, creates an entire organism that is genetically identical to another. This process uses somatic cells (body cells, not sex cells) from a mature organism as the genetic source. Because the offspring develops from a single parent's genetic material without fertilisation, this represents an artificial form of asexual reproduction and is classified as a reproductive technology.

Other specialised forms include cell cloning for unicellular organisms and molecular cloning used in recombinant DNA research, but these are less commonly discussed in basic biology.

Gene cloning process

The gene cloning process involves carefully extracting a desired gene from one organism and inserting it into another, where it can be replicated many times. This technique has revolutionised biotechnology by enabling mass production of important proteins and hormones.

The process follows four main steps:

Step 1: Gene extraction - Scientists identify and remove the desired section of DNA from the source organism using restriction enzymes. These specialised enzymes, naturally produced by bacteria, act like molecular scissors to cut DNA at specific sequences.

Step 2: Insertion into vector - The extracted gene is joined to a vector DNA molecule, typically a plasmid (a small circular piece of DNA found in bacteria). This joining process, called ligation, uses ligase enzymes to chemically bond the gene to the plasmid, creating what scientists call recombinant DNA.

Step 3: Transformation - The modified plasmid carrying the desired gene is introduced into a host cell, usually a bacterium. This introduction process is called transformation. The host cell accepts the plasmid into its cytoplasm.

Step 4: Replication - As the host cell divides and multiplies, it copies both its own DNA and the vector DNA containing the desired gene. This results in numerous bacterial cells, each carrying copies of the target gene, effectively amplifying the gene many times over.

Polymerase chain reaction (PCR)

Polymerase chain reaction represents an alternative approach to gene cloning that occurs in vitro (in a test tube) rather than inside living organisms. PCR has become widely used in research laboratories because it can rapidly amplify specific DNA sequences, creating millions of copies from a tiny sample. The technique finds applications in forensic science, medical diagnostics, and evolutionary research.

The PCR process requires several components: the original DNA sample, short DNA sequences called primers that mark the region to be copied, free nucleotides ($C$, $G$, $A$, $T$) to build new DNA strands, and a special heat-resistant enzyme called Taq polymerase. These components are placed in a thermal cycler machine that repeatedly changes temperature to drive the reaction.

PCR operates through three distinct stages that repeat in cycles:

Denaturing ($95°C$) - The high temperature breaks the hydrogen bonds holding the two DNA strands together, separating them into single strands. This exposes the genetic sequence that needs to be copied.

Annealing ($55°C$) - At this lower temperature, the primers attach to complementary sequences on the single DNA strands, marking the specific region to be amplified.

Extension ($72°C$) - The Taq polymerase enzyme works at this temperature, using the free nucleotides to build new complementary DNA strands, starting from the primers and extending along the template.

Each complete cycle doubles the amount of target DNA, so after $20$ cycles, the original sequence has been amplified over a million times. After $30$ cycles, amplification exceeds a billion copies.

Whole-organism cloning

The development of whole-organism cloning marked a revolutionary moment in biotechnology. The birth of Dolly the sheep on 5 July 1996 at the Roslin Institute in Edinburgh, Scotland, represented the first time scientists successfully cloned a mammal from an adult cell. What made this achievement particularly remarkable was that Dolly developed from cells taken from the udder of a six-year-old sheep, not from an embryo.

Whilst embryo-splitting to create identical twins occurs naturally and can be replicated relatively easily in laboratories, cloning from adult cells presents significant technical challenges. The main difficulty lies in the fact that adult cells have already undergone differentiation – they have become specialised for specific functions. During this specialisation, certain genes, particularly those controlling early development, become permanently 'switched off'. To clone from adult cells, scientists must reprogram these differentiated cells, reactivating the developmental genes that have been shut down.

Examples of cloned organisms

Following the success with Dolly, which required approximately 276 attempts, scientists have cloned numerous other mammals. Tetra, a rhesus macaque born in $2000$, became the first primate to be cloned, achieved through embryo-splitting rather than adult cell cloning.

In $2003$, Prometea became the first cloned horse, born in Italy. However, cloning remains controversial in horse racing, where international rules prohibit artificial insemination and fertility treatments for breeding, making it unlikely that cloned horses will be accepted in competitive racing.

Agriculture represents the most common application for animal cloning. Japanese supermarkets stock beef from cloned cattle, whilst cloned plants like seedless grapes are consumed worldwide, demonstrating how cloning has already entered commercial food production.

Somatic cell nuclear transfer (SCNT)

The technique used to create Dolly, known as somatic cell nuclear transfer or SCNT, requires three different animals to produce one clone: a nucleus donor, an egg donor, and a surrogate mother.

Ian Wilmut and his research team at the Roslin Institute developed the following SCNT procedure:

Cell extraction and preparation - Cells were removed from the mammary gland (udder) of a six-year-old ewe, designated as sheep $1$. These cells were deliberately starved of nutrients to halt their division cycle, preparing them for the cloning process.

Enucleation - An unfertilised egg cell was taken from a second sheep (sheep $2$). The nucleus of this egg, containing the egg donor's genetic material, was carefully removed using a micropipette. This process is called enucleation, leaving an empty egg cell with cytoplasm but no nuclear DNA.

Cell fusion - The complete udder cell from sheep $1$ (containing its nucleus) was injected into the enucleated egg from sheep $2$. Scientists then applied an electric shock to the combined cells. This electrical stimulus caused the cell membranes to fuse together and triggered the cell to begin dividing, just as a fertilised egg would.

Embryo development and implantation - The fused cell began dividing through mitosis, developing into an embryo. When the embryo reached the appropriate stage, it was implanted into the uterus of a third sheep (sheep $3$), who served as the surrogate mother.

Birth and genetic identity - After a normal five-month pregnancy, the surrogate gave birth to Dolly. Critically, Dolly was genetically identical to sheep 1 (the nucleus donor), not to the egg donor or surrogate mother. This is why Dolly was considered a genetic twin of sheep $1$ rather than her offspring – they shared identical nuclear DNA.

Artificial embryo twinning

An alternative and less expensive cloning technique involves artificial embryo twinning. Shortly after an egg is fertilised naturally, but before the embryonic cells begin specialising, scientists split the early embryo into two separate groups of cells in the laboratory. Each group is then implanted into a surrogate mother, where they develop independently. Because both embryos originated from the same fertilised egg before cell differentiation occurred, the resulting offspring are genetically identical twins.

This technique is extensively used in cattle breeding because it is more economical than SCNT and has a higher success rate, allowing farmers to produce multiple copies of animals with desirable characteristics.

Plant cloning

Plant cloning has a much longer history than animal cloning and remains common practice in horticulture. The simplest method involves taking a cutting from an existing plant and encouraging it to develop roots and grow into a new, independent plant. Both the original plant and the new plant are genetically identical because they share the same DNA. This technique, called plant propagation, has been used for centuries, with grape vine propagation dating back to early European civilisation. Plant propagation offers several advantages: it preserves desirable varieties exactly and often produces mature plants faster than growing from seeds.

Tissue culture, also known as micropropagation, represents a more sophisticated plant cloning technique. Scientists remove small pieces of plant tissue and culture them in sterile conditions on solid growth medium or in nutrient-rich liquid broth. Under carefully controlled conditions, these tissue samples develop into complete plants that are genetically identical to the parent. This method allows mass production of plants with specific characteristics.

Ethical considerations in cloning

Cloning raises important ethical questions that society must address. It is essential to recognise that ethical concerns vary depending on which type of cloning is being considered, as gene cloning and whole-organism cloning have quite different implications. Most ethical debates centre on whole-organism and potential human cloning rather than gene or cell cloning.

Animal welfare concerns - Many people already have concerns about how animals are treated in large-scale farming operations. Cloning animals for food production could potentially intensify these welfare issues, as cloned animals may experience health problems and the process itself can be stressful.

Human cloning potential - The same techniques used to clone animals could theoretically be applied to humans. This possibility raises profound moral questions about human identity and individuality, as well as serious legal questions about the rights and status of clones. Religious communities also express concerns about human cloning conflicting with their beliefs about the sanctity of human life.

Religious objections - Some religious groups argue that cloning represents humans "playing God" by artificially creating life. They contend that this crosses ethical boundaries and interferes with natural processes that should remain beyond human control.

Health risks - Cloned animals often experience unforeseen health complications. Dolly the sheep, for instance, developed arthritis and lung disease at a relatively young age. Scientists still don't fully understand all the health risks associated with cloning, making it difficult to predict long-term outcomes.

Economic accessibility - Reproductive cloning remains an extremely expensive procedure due to its low success rate and technical complexity. This high cost means that only wealthy individuals and organisations can access the technology, raising questions about fairness and equal access to potentially beneficial treatments.

Impact on genetic diversity

Cloning significantly affects the genetic composition of populations, with both advantages and disadvantages that must be carefully considered.

Advantages of cloning

In agricultural contexts, cloning serves as a powerful form of selective breeding once breeders have identified an "ideal" hybrid organism. The primary advantage lies in eliminating uncertainty – breeders can precisely control which characteristics are reproduced rather than hoping favourable gene combinations appear in offspring. This precision makes cloning more reliable than traditional selective breeding methods.

Cloning also offers efficiency benefits. Desired characteristics can be reproduced rapidly and in large numbers, significantly reducing the time required to establish a population with specific traits. This speed is particularly valuable in commercial agriculture where quick production of high-quality livestock or crops is economically important.

Disadvantages and risks

However, cloning creates serious concerns about genetic diversity. Organisms produced through reproductive cloning derive from only one parent, representing an artificial form of asexual reproduction. This means every clone has identical DNA to its parent. When farmers or breeders clone only a few superior parent organisms to produce many offspring, genetic diversity within the population decreases dramatically.

This reduction in genetic diversity creates vulnerability. If all members of a cloned population are genetically identical, they share the same susceptibilities to diseases and environmental stresses. A pathogen that can infect one individual can potentially infect the entire population equally effectively. Similarly, if environmental conditions change suddenly, the entire population may lack the genetic variation needed to adapt and survive.

Through continued cloning and selection, the cloned organisms gradually become predominant in the population whilst natural genetic combinations that weren't selected disappear. This concentrates specific gene combinations within the population, effectively reducing the gene pool. In nature, evolution conserves essential genes – for example, genes coding for cytochromes (proteins crucial for cellular respiration) are preserved because organisms cannot survive without them. However, artificial selection through cloning may eliminate genetic variation that could prove valuable under different circumstances.

The fundamental risk is that genetically uniform populations lack resilience. Natural populations maintain genetic diversity as a form of insurance against unpredictable threats. Cloning, whilst offering short-term advantages in productivity and consistency, potentially compromises long-term survival by creating populations that cannot adapt to new challenges.