DNA、Chromosome and Gene
Deoxyribonucleic acid (DNA) is a molecule that carries the genetic information used in the growth, development, functioning and reproduction of all known living organisms. Human genomic DNA molecules are formed in a double helix with two biopolymer strands coiled around each other. Each nucleotide is composed of one of four nucleobases, cytosine (C), guanine (G), adenine (A), or thymine (T), and a sugar called deoxyribose and a phosphate group. Usually, DNA is not existing on its own, but rather is structured in long strands which are wrapped around protein complexes called nucleosomes. Such packaged and organised structure is called “chromosome”, most eukaryotic cells have a set of chromosomes (46 in humans) with the genetic material spread amongst them.
A gene is a locus (or region) of DNA which is made up of nucleotides and is the molecular unit of heredity. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits. To understand the human genome, scientists started an international scientific research project with the goal of determining the sequence of nucleotide base pairs which make up human DNA, and of identifying and mapping all of the genes of the human genome from both a physical and a functional standpoint - The Human Genome Project (HGP) - in 1990.
The research and application of molecular genetics
The HGP was a 13-year-long, publicly funded project initiated in 1990 with the objective of determining the DNA sequence of the entire euchromatic human genome. It remains the world's largest collaborative biological project and most of the government-sponsored sequencing was performed in twenty universities and research centres in the United States, the United Kingdom, Japan, France, Germany, Canada, and China. The HGP was declared complete in April 2003 and the cost was roughly at $3 billion. Although the main sequencing phase of the HGP has been completed, studies of DNA variation continue in the International HapMap Project, whose goal is to identify patterns of single-nucleotide polymorphism (SNP) groups.
Any two unrelated people share about 99.5% of their DNA sequence, their genomes differ at specific nucleotide locations. Such sites are known as SNPs, and each of the possible resulting gene forms is called an allele where each variation is present to some appreciable degree within a population (e.g. > 1%). In general, where natural selection is acting and 'fixing' the allele (eliminating other variants) of the SNP that constitutes the most favourable genetic adaptation. There are variations between human populations, so a SNP allele that is common in one geographical or ethnic group may be much rarer in another. Recent studies showed that variations in the DNA sequences of humans can affect how humans develop diseases and respond to pathogens, chemicals, drugs, vaccines, and other agents. SNPs are also critical for personalised medicine.