Strictly speaking, cloning is the creation of a genetic copy of a sequence of DNA or of the entire genome of an organism. In the latter sense, cloning occurs naturally in the birth of identical twins and other multiples. But cloning can also be done artificially in the laboratory via embryo twinning or splitting: SCNT involves transferring the nucleus of a somatic cell into an oocyte from which the nucleus and thus most of the DNA has been removed.
According to the Telegrapha response to one grant application she made said: This concept of evolution inspired what is now recognized as her most notable idea, the notion that the eukaryotic mitochondrion — the power plant of the cell — was acquired by virtue of an endosymbiotic event.
Endosymbiotic theory essentially maintains that mitochondria arose by virtue of a symbiotic union of prokaryote cells. The nearest living relative to the mitochondrion is thought to be the alpha-proteobacteria Rickettsia Emelyanov, ; Andersson et al. Chloroplasts are also thought to have arisen in a similar manner from the photosynthetic cyanobacteria.
In NovemberI drew attention to a paper in Nature by Nick Lane and Bill Martin, who showed that the prokaryote-to-eukaryote transition was effectively impossible without the energy demands, pertinent to the biggest event of gene manufacture in the history of life on earth, being met by the mitochondrial processes of oxidative phosphorylation and the electron transport chain.
The bacterial cell alone could not meet these energy demands. The evidence that is typically offered for endosymbiotic theory includes the following: Mitochondria possess a circular genome lacking in introns and independent from the nuclear DNA in which transcription is coupled to translation, characteristic of bacterial DNA.
There are also some other notable similarities. Mitochondria divide and replicate independently of host cell division and do so in a manner akin to binary fission, possessing homologues of the bacterial division protein FtsZ Kiefel et al. They are enclosed by a double-membrane.
Mitochondria and bacteria are of a similar size and shape. Circular Mitochondrial Genome As noted, one of the core arguments for endosymbiosis points to its circular genome. What is often not noted, however, are the cases where eukaryotic mitochondria have linear genomes with eukaryotic telomeres Rycovska et al.
Indeed, two strains of the same species of yeast differ with respect to the linearity or circularity of their mitochondrial genome Fukuhara et al.
In the case of linear chromosomes, the DNA polymerase enzymes are unable to replicate right to the end of the chromosome. Unless there is a mechanism for circumventing this, it will result in the chromosomes shortening after each round of replication in eukaryotes, the enzyme telomerase attaches extra DNA to the chromosomal ends.
This means that the transition from genome circularity to linearity — a fete in itself given the changes that have to be made to the mode of replication — must happen in concert with the evolution of a mechanism to prevent progressive chromosomal shortening.
Such an evolutionary transition is far from trivial. The telomeric regions of mitochondrial chromosomes do not seem to have a direct phylogenetic relation since they use other proteins and mechanisms than nuclear telomeres.
Thus, it is difficult to deduce evolutionary pathways purely based on phylogenetic data on telomeres and mechanisms for end replication. Furthermore, mitochondrial genes often do possess introns Lang et al.
These are particularly prevalent in the mtDNA of fungi and plants.
The mitochondrial genetic code may also be slightly different from that of bacteria Jukes and Osawa, While this is true, in at least some respects, there are also important differences. For example, many of the key components are of eukaryotic origin and replication beginning at the Displacement D- loop Fish et al.
Double Membrane It is frequently asserted that the double membrane of mitochondria provides evidence for its endosymbiotic origin. There are, however, important differences between bacterial and mitochondrial membranes.
Albert de Roos observesThe bacterial membrane is one of the basic characteristics that distinguish bacteria from eukaryotes, see some examples here. In order for mitochondria to resemble bacterial membranes, they should share characteristics such as a cell wall with peptidoglycan and lipopolysaccharides, gram-staining and antibiotic sensitivity.Moral Arguments in Favour of Human Cloning Dr.
Josephus J. Brimah School of Education Njala University Sierra Leone in the research, prevention and cure of diseases, which make the lives of some of the sick absolutely miserable.
his DNA lodged in a different cytoplasm with its own mitochondria DNA and then given birth to.
Moral Arguments in Favour of Human Cloning Dr. Josephus J. Brimah School of Education Njala University Sierra Leone in the research, prevention and cure of diseases, which make the lives of some of the sick absolutely miserable.
his DNA lodged in a different cytoplasm with its own mitochondria DNA and then given birth to. Circular Mitochondrial Genome As noted, one of the core arguments for endosymbiosis points to its circular genome.
What is often not noted, however, are the cases where eukaryotic mitochondria have linear genomes with eukaryotic telomeres (Rycovska et al., ; Nosek et al., ; Fukuhara et al., ). Scientists at Great Ormond Street and University College London say that just 80 per cent of mitochondrial disease is actually caused by the mutation in the mitochondria and more time should be spent looking for genetic faults in the nucleus.
Research on mitochondrial replacement therapy (MRT) holds the promise of helping women who have, or are at risk of transmitting, mitochondrial disease, but has recently been blocked by the Food and Drug Administration (FDA).
Thus, several critical ethical and policy questions arise. Mitochondrial. Mitochondria exist in large numbers per cell.
Therefore, the strength of natural selection on individual mtDNAs for their contribution to cellular fitness is weak whereas the strength of selection in favor of mtDNAs that increase their own replication without regard for cellular functions is strong.