What is a Genome??

Life is specified by genomes. Every organism, including humans, has a genome that contains all of the biological information needed to build and maintain a living example of that organism. The biological information contained in a genome is encoded in its deoxyribonucleic acid (DNA) and is divided into discrete units called genes. Genes code for proteins that attach to the genome at the appropriate positions and switch on a series of reactions called gene expression.

Why we study Mitochondrial??

There are many diseases caused by mutations in mitochondrial DNA (mtDNA). Because the mitochondria produce energy in cells, symptoms of mitochondrial diseases often involve degeneration or functional failure of tissue. For example, mtDNA mutations have been identified in some forms of diabetes, deafness, and certain inherited heart diseases. In addition, mutations in mtDNA are able to accumulate throughout an individual's lifetime. This is different from mutations in nuclear DNA, which has sophisticated repair mechanisms to limit the accumulation of mutations. Mitochondrial DNA mutations can also concentrate in the mitochondria of specific tissues. A variety of deadly diseases are attributable to a large number of accumulated mutations in mitochondria. There is even a theory, the Mitochondrial Theory of Aging, that suggests that accumulation of mutations in mitochondria contributes to, or drives, the aging process. These defects are associated with Parkinson's and Alzheimer's disease, although it is not known whether the defects actually cause or are a direct result of the diseases. However, evidence suggests that the mutations contribute to the progression of both diseases.

Why is there a separate Mitochondrial Genome??

The energy-conversion process that takes place in the mitochondria takes place aerobically, in the presence of oxygen. Other energy conversion processes in the cell take place anaerobically, or without oxygen. The independent aerobic function of these organelles is thought to have evolved from bacteria that lived inside of other simple organisms in a mutually beneficial, or symbiotic, relationship, providing them with aerobic capacity. Through the process of evolution, these tiny organisms became incorporated into the cell, and their genetic systems and cellular functions became integrated to form a single functioning cellular unit. Because mitochondria have their own DNA, RNA, and ribosomes, this scenario is quite possible. This theory is also supported by the existence of a eukaryotic organism, called the amoeba, which lacks mitochondria. Therefore, amoeba must always have a symbiotic relationship with an aerobic bacterium.

Organelle DNA

Not all genetic information is found in nuclear DNA. Both plants and animals have an organelle—a "little organ" within the cell— called the mitochondrion. Each mitochondrion has its own set of genes. Plants also have a second organelle, the chloroplast, which also has its own DNA. Cells often have multiple mitochondria, particularly cells requiring lots of energy, such as active muscle cells. This is because mitochondria are responsible for converting the energy stored in macromolecules into a form usable by the cell, namely, the adenosine triphosphate (ATP) molecule. Thus, they are often referred to as the power generators of the cell.

Unlike nuclear DNA (the DNA found within the nucleus of a cell), half of which comes from our mother and half from our father, mitochondrial DNA is only inherited from our mother. This is because mitochondria are only found in the female gametes or "eggs" of sexually reproducing animals, not in the male gamete, or sperm. Mitochondrial DNA also does not recombine; there is no shuffling of genes from one generation to the other, as there is with nuclear genes.

The four DNA bases

Each DNA base is made up of the sugar 2'-deoxyribose linked to a phosphate group and one of the four bases depicted above: adenine (top left), cytosine (top right), guanine (bottom left), and thymine (bottom right).

A DNA chain, also called a strand, has a sense of direction, in which one end is chemically different than the other. The so-called 5' end terminates in a 5' phosphate group (-PO4); the 3' end terminates in a 3' hydroxyl group (-OH). This is important because DNA strands are always synthesized in the 5' to 3' direction.

The DNA that constitutes a gene is a double-stranded molecule consisting of two chains running in opposite directions. The chemical nature of the bases in double-stranded DNA creates a slight twisting force that gives DNA its characteristic gently coiled structure, known as the double helix. The two strands are connected to each other by chemical pairing of each base on one strand to a specific partner on the other strand. Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Thus, A-T and G-C base pairs are said to be complementary. This complementary base pairing is what makes DNA a suitable molecule for carrying our genetic information—one strand of DNA can act as a template to direct the synthesis of a complementary strand. In this way, the information in a DNA sequence is readily copied and passed on to the next generation of cells.

Physical structure of a Human Genome

Nuclear DNA

Inside each of our cells lies a nucleus, a membrane-bounded region that provides a sanctuary for genetic information. The nucleus contains long strands of DNA that encode this genetic information. A DNA chain is made up of four chemical bases: adenine (A) and guanine (G), which are called purines, and cytosine (C) and thymine (T), referred to as pyrimidines. Each base has a slightly different composition, or combination of oxygen, carbon, nitrogen, and hydrogen. In a DNA chain, every base is attached to a sugar molecule (deoxyribose) and a phosphate molecule, resulting in a nucleic acid or nucleotide. Individual nucleotides are linked through the phosphate group, and it is the precise order, or sequence, of nucleotides that determines the product made from that gene.

What is a Gene therapy???

A gene is a linear sequence of DNA that codes for a particular protein. On rare occasions, usually during the division of the cell, the nucleotide sequence (the order of the DNA base pairs) of a gene can get jumbled up and mutated, so that the resultant protein is faulty. Such a mutation event is the root cause of genetic diseases such as cystic fibrosis, adenosine deaminase (ADA) deficiency and sickle-cell anaemia. For example, people who suffer from cystic fibrosis produce a faulty cellular transport protein called cystic fibrosis transmembrane conductance regulator, which results in the build-up of mucous in their lungs.

The earliest applications of gene therapy were based on the principle that a disease is caused by a faulty gene (or combination of genes), and if such genes can be replaced with ‘correct’ versions, the disease might be controlled, prevented or cured. Gene therapy is being applied to many different genetic diseases, both congenital (since birth) and acquired. However, most diseases involve multiple genetic factors (they are polygenic). Until the precise involvement of different genes (their regulation and expression) in the disease process and the proteins they encode is established, gene therapy is most likely to be clinically effective as a preventative or curative treatment for single-gene defects such as ADA deficiency, familial hypercholesterolaemia. and cystic fibrosis. Several clinical trials employing gene therapy protocols have already been completed, with some success in patients who have cystic fibrosis and ADA deficiency, although the effectiveness of the protocols was not as dramatic as first envisaged, mainly owing to the inefficiency of the gene transfer vectors that were used.

Originally known as ‘genetic replacement therapy’ during the early 1980s, ‘gene therapy’ has now outgrown its original definition and is applied to all manner of protocols that involve an element of gene transfer, either in vivo or ex vivo, and not necessarily a gene that is known to cause a disease. In vivo gene transfer is the introduction of genes to cells at the site they are found in the body, for example to skin cells on an arm, or to lung epithelial cells following inhalation of the gene transfer vector. Ex vivo gene transfer is the transfer of genes into viable cells that have been temporarily removed from the patient and are then returned following treatment (e.g. bone marrow cells). Gene therapy can be subdivided into somatic cell gene transfer (that is transfer to normal diploid cells), which is the focus of this review, and germline gene transfer (transfer to haploid sperm or egg cells of the reproductive system). The ethical issues associated with germline gene therapy are far more complex than those surrounding somatic cell gene transfer, because the genes are transferred not only to treated individuals but also to their progeny. Germline gene therapy is being widely used for the production of transgenic animals for research, and increasingly for agriculture and biotechnology, but the long-term effects of each transferred gene in animals will need to be carefully monitored and analysed, as well as the significance of any residual vector DNA if applicable. The benefits that the use of germline gene therapy in humans could bring are significant. The development of serious and distressing inherited genetic diseases could be prevented before birth and eliminated in subsequent generations. However, because of the potential for abuse and eugenics, gene therapy in humans needs to be widely discussed and the associated safety issues evaluated before this approach can be used for the treatment of diseases.