Find my unique collection of essays on ‘Nuclear Cloning’ especially written for school and college students.
Essay on Nuclear Cloning
Essay Contents:
- Essay on the Meaning of Nuclear Cloning
- Essay on the Methodology of Nuclear Cloning
- Essay on the Applications of Nuclear Cloning
- Essay on the Advantages of Nuclear Cloning
- Essay on the Limitations of Nuclear Cloning
Essay # 1. Meaning of Nuclear Cloning:
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The production of nuclear clones is a multi-step process that essentially generates an entire organism from the nuclear deoxyribonucleic acid (DNA) of a single donor cell using a technique known as nuclear transfer (NT). The basic methodology was first developed in amphibians in the 1950s and was used to investigate nuclear totipotency in differentiated cell populations. In livestock species, undifferentiated embryonic blastomeres were first used successfully in sheep, cattle and pigs.
In more recent times, embryonic NT has been extended in mice to include the use of other undifferentiated cell types including embryonic stem cells derived from the inner cell mass of blastocysts. Conversely, the use of more differentiated cell types obtained from either embryos, foetuses or most significantly adult animals, as in the case of ‘Dolly’ the sheep, overturned a dogma in biology concerning nuclear totipotency from adult cells and has opened new opportunities and directions in research. This has been termed somatic cell NT to distinguish it from embryonic NT.
Essay # 2. Methodology of Nuclear Cloning:
The nuclear cloning process comprises a sequence of five main steps, which are summarised below:
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1. Oocytes, commonly obtained by aspirating follicles on ovaries collected from commercially slaughtered cows, are matured in vitro and then enucleated. This process involves the physical removal of the metaphase chromosomes (which incorporate the oocytes’ own nuclear DNA) and the extruded first polar body, using finely controlled micro-surgical instruments.
Thus, the nuclear genetic material of the oocyte is removed, resulting in what is termed a cytoplast (a cell containing only cytoplasmic material). The mitochondrial DNA within the cytoplasm of the oocyte remains present.
2. With conventional NT methods, a single donor cell is injected underneath the outer zona pellucida and adjacent to the cytoplast membrane. The donor cells can come from a variety of sources representing different degrees of cellular differentiation. For instance, donor cells could be embryonic blastomeres, cell lines such as embryonic stem cells, or primary cultures derived from biopsies obtained from selected adults.
3. The cytoplast and the donor cell are then fused together utilising a direct current electrical field. Thus, the genetic information contained within the nucleus of the donor cell enters the cytoplast. This is the essence of the term ‘nuclear transfer’, whereby the genetic information from the oocyte is removed and is replaced with that from the donor cell.
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Immediately following reconstruction the donor nucleus has the opportunity to be reprogrammed following molecular interactions between factors present in the oocyte cytoplasm and the donor chromatin.
4. The reconstructed 1-cell embryos are artificially activated using either specific chemical signals or electrical pulses, in order to initiate embryonic development.
5. Following activation, the reconstructed embryos are cultured in vitro in a chemically-defined medium, in the case of cattle, for seven days. After this time, embryos that have developed into blastocysts of suitable quality (that is, embryos comprising around 120 cells) are transferred to the uteri of recipient females that have a synchronised oestrus cycle, where some may develop to term and result in viable cloned animals.
The NT-derived animals are not strictly true clones and possess greater differences than naturally occurring monozygotic twins.
Compared to the donor animal, they might for instance, possess:
i. Different mitochondrial DNA derived from the recipient oocyte.
ii. Point mutations or other chromosomal rearrangements in the genomic QNA of individual donor cells used for NT – alternative patterns of X-chromosome inactivation in females.
iii. Various other epigenetic alterations arising from in vitro culture (of the donor cells and/or reconstructed embryos) or perturbations from the NT process.
iv. Differences that occur as a result of environmental influences from the oocyte cytoplasm, the maternal uterus in the surrogate female, or the post-natal environment.
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In addition to the heritability of a trait, all these extra variables contribute to potential variations in phenotype (and also genotype in some cases) within a clonal family (a set of nuclear clones derived from the same source of donor cells) and deviations from the original donor animal.
Essay # 3. Applications of Nuclear Cloning:
There are several important issues to be addressed before commercial opportunities for cloning in livestock agriculture can be realised. Firstly, there are significant animal welfare concerns limiting the acceptability and applicability of the technology in its current form.
There needs to be confidence in the long-term health status of cloned livestock and in that of subsequent generations. There are issues surrounding the safety of food products derived from clones and their offspring. Regulatory agencies in a number of countries are presently addressing these issues.
There needs to be ongoing assessment and modelling of the technology to identify where it is best suited, i.e. to which farming system.
Some applications of cloning technology for agriculture and medicine are briefly discussed below. Some will not be realised until well into the future:
i. Rapid Multiplication of Desired Livestock:
Cloning could enable the rapid dissemination of superior genotypes from nucleus breeding flocks and herds, directly to commercial farmers. Genotypes could be provided that are ideally suited for specific product characteristics, disease resistance, or environmental conditions.
Cloning could be extremely useful in multiplying outstanding F1 crossbred animals, or composite breeds, to maximise the benefits of both heterosis and potential uniformity within the clonal family. These genetic gains could be achieved through the controlled release of selected lines of elite live animals or cloned embryos.
More appropriately, given that cloning is not particularly efficient at present, a niche opportunity exists in the production of small numbers of cloned animals with superior genetics for breeding.
These could be clones of performance tested animals, especially sires. This would be particularly relevant in the sheep and beef industries, where cloned sires could be used in widespread natural mating to provide an effective means of disseminating their superior genetics. This could be used as a substitute for artificial insemination, which in these more extensive industries is often expensive and inconvenient.
ii. Animal Conservation:
Cloning can be used along with other forms of assisted reproduction to help preserve indigenous breeds of livestock, which have production traits and adaptability to local environments that should not be lost from the global gene pool. In some situations, inter-species NT and embryo transfer may be used to aid the conservation of some exotic species.
At the very least, it is appropriate to consider the cryopreservation of somatic cells from these endangered animals as insurance against further losses in diversity.
iii. Research Models:
Sets of cloned livestock animals could be effectively used to reduce genetic variability and reduce the numbers of animals needed for some experimental studies. This could be conducted on a larger scale than is currently possible with naturally occurring genetically identical twins.
Lambs cloned from sheep selected either for resistance or susceptibility to nematode worms will be useful in studies aimed at discovering novel genes and regulatory pathways in immunology.
iv. Human Cell-Based Therapies:
There are also direct applications of NT technology in human medicine; principally therapeutic cloning as opposed to human reproductive cloning. Patients with particular diseases or disorders in tissues that neither repair nor replace themselves effectively (as occurs, for example, in insulin-dependent diabetes, muscular dystrophy, spinal cord injury, certain cancers and various neurological disorders, including Parkinson’s disease) could potentially generate their own immunologically compatible cells for transplantation, which would offer lifelong treatment without tissue rejection.
Initially, this approach could employ human NT and embryonic stem cells, the use of which is controversial. In the longer term, however, fundamental understanding of reprogramming will enable one cell type to be directly trans-differentiated into another cell type specifically required for cell-based therapy.
v. Cloning for Transgenic Applications:
A significant application of NT is to clone animals from cells that have been genetically modified in order to produce transgenic livestock. Even acknowledging the current problems with NT, the cloning route is more efficient than conventional pronuclear injection of DNA, where typically less than 1% of injected zygotes develop into transgenic animal.
Essay # 4. Advantages of Nuclear Cloning:
Additional advantages of the NT and cell-mediated transgenic approach include the ability to:
1. Introduce, functionally delete or subtly modify genes of interest.
2. Screen cells for the specific genetic modification before producing the transgenic animal.
3. Introduce a specific transgene into a desired genetic background of the chosen sex (particularly important for agricultural traits).
4. Produce embryos or offspring that are all transgenic and where none should be mosaic (with a mixture of transgenic and non- transgenic cells in the same organism).
5. Produce small herds from each cell line in the first generation, rather than individual founder animals that need to be subsequently bred.
Nevertheless, whilst the present NT technology is able to produce a few founder transgenic animals, currently it is desirable to use assisted sexual reproduction thereafter, to further multiply animals and to circumvent potential epigenetic aberrations in the cloned generation.
The most efficient means of introducing a transgene into the wider livestock population is through artificial insemination. Ideally, the sire should be homozygous for the desired trait so that all progeny receive a copy of the transgene. Animal industries may choose to annually introduce the transgene on a new genetic background using cell lines derived from the most recently selected progeny-tested sires.
Depending upon the particular genes that are manipulated, there are a wide variety of potential uses for genetically modified livestock in both biomedicine and agriculture.
Examples include the following:
a. Human pharmaceutical proteins (harvested from the milk of livestock).
b. Pig organs for xenotransplantation.
c. Models for human genetic diseases (such as for cystic fibrosis).
d. Various agricultural applications aimed at improving the quality or quantity of food or fibre products, reducing environmental pollution and improving animal disease resistance.
For agricultural transgenics, functional genomics will contribute greatly to the understanding of the genes that influence livestock production traits and provide the knowledge to accurately modify the appropriate genes to generate new and desired animal products in the future.
Essay # 5. Limitations of Nuclear Cloning:
If an acceptable and safe nuclear cloning technology that has wide applicability is to be developed, solutions to the cloning abnormalities must be found. It is desirable that the health and wellbeing of cloned animals should be equal to non-clones and that any deficit should be minimised and thoroughly justified in terms of the benefits expected from the application. Achieving this goal will only result from improvements in the efficiency of the cloning process. This is the prime focus of many international groups presently working in this field.
Improvements will probably come from modifications to the basic NT manipulation procedure, the choice of an appropriate type of donor cell and recipient cytoplast, embryo culture media formulations and greater fundamental understanding and control of reprogramming.
The use of molecular markers to screen new protocols in nuclear cloning using either DNA microarrays or candidate gene approaches and to identify viable embryos before transfer to recipient females, will be vital tools for capturing the potential opportunities of this technology.