DNA contains the genetic information that is the basis for living functions including growth, reproduction and evolution. This information is held in segments of the DNA called genes that may span in size from scores of DNA base-pairs to many thousands of base-pairs. In eukaryotes (organisms such as plants, yeasts and animals whose cells have a nucleus) DNA usually occurs as several large, linear chromosomes, each of which may contain hundreds or thousands of genes. Prokaryotes (organisms such as common bacteria) generally have a single large circular chromosome, but often possess other miniature chromosomes called plasmids. The set of chromosomes in a cell makes up its genome; the human genome has about three billion base pairs of DNA arranged into 46 chromosomes and contains 20-25,000 genes.
There are many interactions that happen between DNA and other molecules to coordinate its functions. When cells divide, the genetic information must be duplicated to produce two daughter copies of DNA in a process called DNA replication. When a cell uses the information in a gene, the DNA sequence is copied into a complementary single strand of RNA in a process called transcription. Of the transcribed sequences, some are used to directly make a matching protein sequence by a process called translation (meaning translation from a nucleic acid polymer to an amino acid polymer). The other transcribed RNA sequences may have regulatory, structural or catalytic roles. This article introduces some of the functions and interactions that characterize the DNA molecules in cells, and touches on some of the more technological uses for this molecule.
Genes Our understanding of the various ways in which genes play a role in cells has been continually revised throughout the history of genetics, starting from abstract concepts of inheritable particles whose composition was unknown. This has led to several modern different definitions of a gene. One of the most straightforward ways to define a gene is simply as a segment of DNA that is transcribed into RNA - that is - the gene is a unit of transcription. This definition encompasses genes for non-translated RNAs, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), as well as messenger RNA (mRNA) which is used for encoding the sequences of proteins.
A second approach is to define a gene as a region of DNA that encodes a single polypeptide. By this definition, any particular mRNA transcription unit can cover more than one gene, and thus a mRNA can carry regions encoding one or more polypeptides. Such a multi-genic transcription unit is called an operon.
Other definitions include consideration of genes as units of biological function. This definition can include sites on DNA that are not transcribed, such as DNA sites at which regulatory and catalytically active proteins concerned with gene regulation and expression are located. Examples of such sites (loci, sing. locus) are promoters and operators. (Locus is a genetic term very similar in meaning to gene, and which refers to a site or region on a chromosome concerned with a particular function or trait.)
All of the cells in our body contain essentially the same DNA, with a few exceptions; red blood cells for example do not have a nucleus and contain no DNA. However although two cells may carry identical DNA, this does not make them identical, because the two cells may have different patterns of gene expression; only some genes will be active in each cell, and the level of activity varies between cells, and this is what makes different cell types different. The "level" of gene expression (for a given gene) is used sometimes to refer to the amount of mRNA made by the cell, and sometimes to refer to the amount of protein produced.
Every human has essentially the same genes, but has slightly different DNA; on average the DNA of two individuals differs at about three million bases. These differences are very rarely in the protein-coding sequences of genes, but some affect how particular genes are regulated — they may affect exactly where in the body a gene is expressed, how intensely it is expressed, or how expression is regulated by other genes or by environmental factors; these slight differences help to make every human being unique. By comparison, the genome of our closest living relative, the chimpanzee, differs from the human genome at about 30 million bases.
Genomes In eukaryotes, DNA is located mainly in the cell nucleus (there are also small amounts in mitochondria and chloroplasts). In prokaryotes, the DNA is in an irregularly shaped body in the cytoplasm called the nucleoid. The DNA is usually in linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The human genome has about three billion base pairs of DNA arranged into 46 chromosomes, and contains 20-25,000 genes , the simple nematode C elegans has almost as many genes (more than 19,000).
In many species, only a small fraction of the genome encodes protein: only about 1.5% of the human genome consists of protein-coding exons, while over 50% consists of non-coding repetitive sequences. Some have concluded that much of human DNA is "junk DNA" because most of the non-coding elements appear to have no function, Some other vertebrates, including the puffer fish Fugu have very much more compact genomes, and (for multicellular organisms) there seems to be no consistent relationship between the size of the genome and the complexity of the organism . Some non-coding DNA sequences are now known to have a structural role in chromosomes. In particular, telomeres and centromeres contain few genes, but are important for the function and stability of chromosomes. An abundant form of non-coding DNA in humans are pseudogenes, which are copies of genes that have been disabled by mutation; these are usually just molecular 'fossils', but they can provide the raw genetic material for new genes.
A recent challenge to the long-standing view that the human genome consists of relatively few genes along with a vast amount of "junk DNA" comes from the ENCyclopedia Of DNA Elements (ENCODE) consortium. Their survey of the human genome shows that most of the DNA is transcribed into molecules of RNA. This broad pattern of transcription was unexpected, but whether these transcribed (but not translated) elements have any biological function is not yet clear.
Forensics Forensic scientists can use DNA in blood, semen, skin, saliva or hair to match samples collected at a crime scene to samples taken from possible suspects. This process is called genetic fingerprinting or more formally, DNA profiling. In DNA profiling, the lengths of variable sections of repetitive DNA (such as short tandem repeats and minisatellites) are compared between people. This is usually very reliable for identifying the source of a sample, but identification can be complicated if the samples that are collected include DNA from several people. DNA profiling was developed in 1984 by British geneticist Sir Alec Jeffreys, and first used in forensic science to convict Colin Pitchfork (and to clear the prime suspect) in the 1988 Enderby murders case. People convicted of certain types of crimes may be required to provide a sample of DNA for a database. This has helped investigators solve old cases where only a DNA sample was obtained from the scene. DNA profiling can also be used to identify victims of mass casualty incidents.
Bioinformatics Bioinformatics involves the analysis of DNA sequence data; DNA from hundreds of different organisms has now been partially sequenced, and this information is stored in massive databases. The development of techniques to store and search DNA sequences has led to many applications in computer science. String-searching or 'matching' algorithms, which identify a given sequence of letters inside a larger sequence of letters, are used to search for specific sequences of nucleotides. The related problem of sequence alignment aims to identify homologous sequences; these are sequences that are very similar but not identical. When two different genes in an organism have very similar sequences, this is evidence that, at some stage in evolution, a single gene was duplicated, and the sequences subsequently diverged (under different selection pressures) by incorporating different mutations. Identifying such holologies can give valuable clues about the likely function of novel genes. Similarly, identifying homologies between genes in different organisms can be used to reconstruct the evolutionary relationships between organisms. Data sets representing entire genomes' worth of DNA sequences, such as those produced by the Human Genome Project, are difficult to use without annotations, which label the locations of genes and regulatory elements on each chromosome. Regions of DNA sequence that have patterns that are characteristic of protein- or RNA-coding genes can be identified by gene finding algorithms, allowing researchers to predict the presence of particular gene products in an organism.
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